Bibliography - Ronald J Stouffer

Meehl, G A., Ronald J Stouffer, and Keith W Dixon, et al., in press: Decadal Prediction: Can it be skillful?. Bulletin of the American Meteorological Society. 3/09.
[ Abstract ]
A new field of study, “decadal prediction”, is emerging in climate science. Decadal prediction lies between seasonal/interannual forecasting and longer term climate change projections, and focuses on time-evolving regional climate conditions over the next 10-30 years. Numerous assessments of climate information user needs have identified this timescale as being important to infrastructure planners, water resource managers, and many others. It is central to the information portfolio required to adapt effectively to and through climatic changes. At least three factors influence timeevolving regional climate at the decadal timescale: 1) climate change commitment (further warming as the coupled climate system comes into adjustment with increases of greenhouse gases that have already occurred), 2) external forcing, particularly from future increases of greenhouse gases and recovery of the ozone hole, and 3) internallygenerated variability. Some decadal prediction skill has been demonstrated to arise from the first two of these factors, and there is evidence that initialized coupled climate models can capture mechanisms of internally-generated decadal climate variations, thus increasing predictive skill globally and particularly regionally. Several methods have been proposed for initializing global coupled climate models for decadal predictions, all of which involve global time-evolving three-dimensional ocean data, including temperature and salinity. An experimental framework to address decadal predictability/prediction is described in this paper and has been incorporated into the coordinated CMIP5 experiments, some of which will be assessed for the IPCC AR5. These experiments will likely guide work in this emerging field over the next five years.
Yin, J, Ronald J Stouffer, and M Schlesinger, April 2009: Model projections of rapid sea-level rise on the northeast coast of the United States. Nature Geoscience, 2(4), doi:10.1038/NGEO462.
[ Abstract ]
Human-induced climate change is expected to cause sea-level rise globally as well as regionally. An analysis of state-of-the-art climate models indicates that the northeastern US coast is particularly likely to experience substantial rises in regional sea level as a result of the projected slowdown of the Atlantic meridional overturning circulation.
Milly, P C., J Betancourt, M Falkenmark, R M Hirsch, Z W Kundzewicz, D Lettenmaier, and Ronald J Stouffer, 2008: Stationarity is dead: Whither water management? Science, 319(5863), 573-574.
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Yin, J, M Schlesinger, and Ronald J Stouffer, in press: Rapid Sea Level Rise Threatens the Northeast Coast of the United States. Nature. 8/08.
[ Abstract ]
Human-induced climate change could cause sea level rise (SLR) and slow down the Atlantic meridional overturning circulation (MOC)1. Low-lying Florida and western Europe are often evoked as the vulnerable regions to these changes2-4. Here based on a state-of-the-art climate model, we report a rapid SLR on the northeast coast of the U.S. during the 21st century. The rapid SLR is projected under various IPCC greenhouse-gas (GHG) emission scenarios including low, medium and high rates. A dynamic (relative to the geoid) SLR induced by the weakening of the MOC is imposed on the global steric (density-dependent) and eustatic (mass-dependent) SLRs, thereby greatly enhancing the SLR rate on the northeast coast of the U.S. The future changes of sea level and ocean circulation therefore pose a great and imminent threat to this heavily populated area.
AchutaRao, K M., M Ishii, B D Santer, P J Gleckler, K E Taylor, D W Pierce, Ronald J Stouffer, and T M L Wigley, June 2007: Simulated and observed variability in ocean temperature and heat content. Proceedings of the National Academy of Sciences, 104(26), doi:10.1073/pnas.0611375104.
[ Abstract ]
Observations show both a pronounced increase in ocean heat content (OHC) over the second half of the 20th century and substantial OHC variability on interannual-to-decadal time scales. Although climate models are able to simulate overall changes in OHC, they are generally thought to underestimate the amplitude of OHC variability. Using simulations of 20th century climate performed with 13 numerical models, we demonstrate that the apparent discrepancy between modeled and observed variability is largely explained by accounting for changes in observational coverage and instrumentation and by including the effects of volcanic eruptions. Our work does not support the recent claim that the 0- to 700-m layer of the global ocean experienced a substantial OHC decrease over the 2003 to 2005 time period. We show that the 2003-2005 cooling is largely an artifact of a systematic change in the observing system, with the deployment of Argo floats reducing a warm bias in the original observing system.
Balaji, Ventakramani, Thomas L Delworth, Robert W Hallberg, Hiram Levy II, Ronald J Stouffer, and Michael Winton, 2007: Toward a new generation of ice sheet models. EOS, 88(52), 578-579.
Came, R E., W B Curry, D W Oppo, Anthony J Broccoli, Ronald J Stouffer, and J Lynch-Stieglitz, 2007: North Atlantic intermediate depth variability during the Younger Dryas: Evidence from Benthic Foraminiferal Mg/Ca and the GFDL R30 Coupled Climate Model In Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series 173, Washington, DC, American Geophysical Union, 247-263.
[ Abstract ]
Two new records of paired benthic foraminiferal Mg/Ca and 18 O from two low latitude western Atlantic sediment cores—one taken from within the Florida Current and the other from the Little Bahama Bank — provide insights into the spatial distribution of intermediate depth temperature and salinity variability during the deglaciation. During the Younger Dryas cold event, both temperature and salinity increased at the Florida Current site and decreased at the Little Bahama Bank site. The temperature increase within the Florida Current is consistent with a reduction in the strength of the northward-moving surface return flow of the Atlantic meridional overturning circulation; the temperature decrease at the Little Bahama Bank is consistent with a cooling of high latitude North Atlantic surface waters. To test the possibility that a freshening of the surface North Atlantic caused the paleoceanographic changes during the Younger Dryas, the Geophysical Fluid Dynamics Laboratory (GFDL) R30 coupled ocean-atmosphere general circulation model was forced using a North Atlantic freshwater perturbation of 0.1 Sv for a period of 100 years. The freshwater flux causes an overall reduction in the Atlantic overturning from 25 Sv to 13 Sv. However, at ~1,100 m water depth, ventilation increases, causing decreases in both temperature and salinity throughout much of the intermediate depth, open-ocean North Atlantic. At the western boundary, intermediate depth temperatures and salinities increase due to weakened overturning, and also due to an increase in runoff from the Amazon River, which causes a surface stability and a decrease in the upwelling of colder, deeper waters.
Findell, Kirsten L., Elena Shevliakova, P C D Milly, and Ronald J Stouffer, July 2007: Modeled impact of anthropogenic land cover change on climate. Journal of Climate, 20(14), doi:10.1175/JCLI4185.1.
[ Abstract ]
Equilibrium experiments with the Geophysical Fluid Dynamics Laboratory’s climate model are used to investigate the impact of anthropogenic land cover change on climate. Regions of altered land cover include large portions of Europe, India, eastern China, and the eastern United States. Smaller areas of change are present in various tropical regions. This study focuses on the impacts of biophysical changes associated with the land cover change (albedo, root and stomatal properties, roughness length), which is almost exclusively a conversion from forest to grassland in the model; the effects of irrigation or other water management practices and the effects of atmospheric carbon dioxide changes associated with land cover conversion are not included in these experiments. The model suggests that observed land cover changes have little or no impact on globally averaged climatic variables (e.g., 2-m air temperature is 0.008 K warmer in a simulation with 1990 land cover compared to a simulation with potential natural vegetation cover). Differences in the annual mean climatic fields analyzed did not exhibit global field significance. Within some of the regions of land cover change, however, there are relatively large changes of many surface climatic variables. These changes are highly significant locally in the annual mean and in most months of the year in eastern Europe and northern India. They can be explained mainly as direct and indirect consequences of model-prescribed increases in surface albedo, decreases in rooting depth, and changes of stomatal control that accompany deforestation.
Manabe, Syukuro, and Ronald J Stouffer, 2007: Role of ocean in global warming. Journal of the Meteorological Society of Japan, 85B, 385-403.
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Based upon the results obtained from coupled ocean-atmosphere models of various complexities, this review explores the role of ocean in global warming. It shows that ocean can play a major role in delaying global warming and shaping its geographical distribution. It is very encouraging that many features of simulated change of the climate system have begun to agree with observation. However, it has been difficult to confirm the apparent agreement because the density and frequency of the observation are insufficient in many oceanic regions of the world, in particular, in the Circumpolar Ocean of the Southern Hemisphere. It is therefore essential to intensify our effort to monitor not only at the surface but also in the subsurface layers of oceans.
Meehl, G A., C Covey, Thomas L Delworth, M Latif, B McAveney, J F B Mitchell, Ronald J Stouffer, and K E Taylor, 2007: The WCRP CMIP3 multimodel dataset: A new era in climate change research. Bulletin of the American Meteorological Society, 88(9), doi:10.1175/BAMS-88-9-1383.
[ Abstract ]
A coordinated set of global coupled climate model [atmosphere–ocean general circulation model (AOGCM)] experiments for twentieth- and twenty-first-century climate, as well as several climate change commitment and other experiments, was run by 16 modeling groups from 11 countries with 23 models for assessment in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Since the assessment was completed, output from another model has been added to the dataset, so the participation is now 17 groups from 12 countries with 24 models. This effort, as well as the subsequent analysis phase, was organized by the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Working Group on Coupled Models (WGCM) Climate Simulation Panel, and constitutes the third phase of the Coupled Model Intercomparison Project (CMIP3). The dataset is called the WCRP CMIP3 multimodel dataset, and represents the largest and most comprehensive international global coupled climate model experiment and multimodel analysis effort ever attempted. As of March 2007, the Program for Climate Model Diagnostics and Intercomparison (PCMDI) has collected, archived, and served roughly 32 TB of model data. With oversight from the panel, the multimodel data were made openly available from PCMDI for analysis and academic applications. Over 171 TB of data had been downloaded among the more than 1000 registered users to date. Over 200 journal articles, based in part on the dataset, have been published so far. Though initially aimed at the IPCC AR4, this unique and valuable resource will continue to be maintained for at least the next several years. Never before has such an extensive set of climate model simulations been made available to the international climate science community for study. The ready access to the multimodel dataset opens up these types of model analyses to researchers, including students, who previously could not obtain state-of-the-art climate model output, and thus represents a new era in climate change research. As a direct consequence, these ongoing studies are increasing the body of knowledge regarding our understanding of how the climate system currently works, and how it may change in the future.
Randall, D A., and Ronald J Stouffer, et al., 2007: Climate models and their evaluation In Climate Change 2007: The Physical Science Basis, Cambridge, UK, Cambridge University Press, 589-662.
Solomon, S, D Qin, M Manning, V Ramaswamy, and Ronald J Stouffer, et al., 2007: Technical summary In Climate Change 2007: The Physical Science Basis, Cambridge, UK, Cambridge University Press, 19-92.
Stouffer, Ronald J., D Seidov, and B J Haupt, 2007: Climate response to external sources of freshwater: North Atlantic versus the Southern Ocean. Journal of Climate, 20(3), doi:10.1175/JCLI4015.1.
[ Abstract ]
The response of an atmosphere–ocean general circulation model (AOGCM) to perturbations of freshwater fluxes across the sea surface in the North Atlantic and Southern Ocean is investigated. The purpose of this study is to investigate aspects of the so-called bipolar seesaw where one hemisphere warms and the other cools and vice versa due to changes in the ocean meridional overturning. The experimental design is idealized where 1 Sv (1 Sv 10⁶ m³ s⁻¹) of freshwater is added to the ocean surface for 100 model years and then removed. In one case, the freshwater perturbation is located in the Atlantic Ocean from 50° to 70°N. In the second case, it is located south of 60°S in the Southern Ocean. In the case where the North Atlantic surface waters are freshened, the Atlantic thermohaline circulation (THC) and associated northward oceanic heat transport weaken. In the Antarctic surface freshening case, the Atlantic THC is mainly unchanged with a slight weakening toward the end of the integration. This weakening is associated with the spreading of the fresh sea surface anomaly from the Southern Ocean into the rest of the World Ocean. There are two mechanisms that may be responsible for such weakening of the Atlantic THC. First is that the sea surface salinity (SSS) contrast between the North Atlantic and North Pacific is reduced. And, second, when freshwater from the Southern Ocean reaches the high latitudes of the North Atlantic Ocean, it hinders the sinking of the surface waters, leading to the weakening of the THC. The spreading of the fresh SSS anomaly from the Southern Ocean into the surface waters worldwide was not seen in earlier experiments. Given the geography and climatology of the Southern Hemisphere where the climatological surface winds push the surface waters northward away from the Antarctic continent, it seems likely that the spreading of the fresh surface water anomaly could occur in the real world. A remarkable symmetry between the two freshwater perturbation experiments in the surface air temperature (SAT) response can be seen. In both cases, the hemisphere with the freshwater perturbation cools, while the opposite hemisphere warms slightly. In the zonally averaged SAT figures, both the magnitude and the pattern of the anomalies look similar between the two cases. The oceanic response, on the other hand, is very different for the two freshwater cases, as noted above for the spreading of the SSS anomaly and the associated THC response. If the differences between the atmospheric and oceanic responses apply to the real world, then the interpretation of paleodata may need to be revisited. To arrive at a correct interpretation, it matters whether or not the evidence is mainly of atmospheric or oceanic origin. Also, given the sensitivity of the results to the exact details of the freshwater perturbation locations, especially in the Southern Hemisphere, a more realistic scenario must be constructed to explore these questions.
Stouffer, Ronald J., and Richard T Wetherald, 2007: Changes of Variability in Response to Increasing Greenhouse Gases. Part I: Temperature. Journal of Climate, 20(21), 5455-5467.
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This study documents the temperature variance change in two different versions of a coupled ocean–atmosphere general circulation model forced with estimates of future increases of greenhouse gas (GHG) and aerosol concentrations. The variance changes are examined using an ensemble of 8 transient integrations for the older model version and 10 transient integrations for the newer one. Monthly and annual data are used to compute the mean and variance changes. Emphasis is placed upon computing and analyzing the variance changes for the middle of the twenty-first century and compared with those found in a control integration. The large-scale variance of lower-tropospheric temperature (including surface air temperature) generally decreases in high latitudes particularly during fall due to a delayed onset of sea ice as the climate warms. Sea ice acts to insolate the atmosphere from the much larger heat capacity of the ocean. Therefore, the near-surface temperature variance tends to be larger over the sea ice–covered regions, than the nearby ice-free regions. The near-surface temperature variance also decreases during the winter and spring due to a general reduction in the extent of sea ice during winter and spring. Changes in storminess were also examined and were found to have relatively little effect upon the reduction of temperature variance. Generally small changes of surface air temperature variance occurred in low and midlatitudes over both land and oceanic areas year-round. An exception to this was a general reduction of variance in the equatorial Pacific Ocean for the newer model. Small increases in the surface air temperature variance occur in mid- to high latitudes during the summer months, suggesting the possibility of more frequent and longer-lasting heat waves in response to increasing GHGs.
Timmermann, A, Y Okumura, S I An, A C Clement, B Dong, E Guilyardi, A Hu, J H Jungclaus, M Renold, T F Stocker, Ronald J Stouffer, R Sutton, S Xie, and J Yin, 2007: The influence of a weakening of the Atlantic Meridional Overturning Circulation on ENSO. Journal of Climate, 20(19), doi:10.1175/JCLI4283.1.
[ Abstract ]
The influences of a substantial weakening of the Atlantic meridional overturning circulation (AMOC) on the tropical Pacific climate mean state, the annual cycle, and ENSO variability are studied using five different coupled general circulation models (CGCMs). In the CGCMs, a substantial weakening of the AMOC is induced by adding freshwater flux forcing in the northern North Atlantic. In response, the well-known surface temperature dipole in the low-latitude Atlantic is established, which reorganizes the large-scale tropical atmospheric circulation by increasing the northeasterly trade winds. This leads to a southward shift of the intertropical convergence zone (ITCZ) in the tropical Atlantic and also the eastern tropical Pacific. Because of evaporative fluxes, mixing, and changes in Ekman divergence, a meridional temperature anomaly is generated in the northeastern tropical Pacific, which leads to the development of a meridionally symmetric thermal background state. In four out of five CGCMs this leads to a substantial weakening of the annual cycle in the eastern equatorial Pacific and a subsequent intensification of ENSO variability due to nonlinear interactions. In one of the CGCM simulations, an ENSO intensification occurs as a result of a zonal mean thermocline shoaling. Analysis suggests that the atmospheric circulation changes forced by tropical Atlantic SSTs can easily influence the large-scale atmospheric circulation and hence tropical eastern Pacific climate. Furthermore, it is concluded that the existence of the present-day tropical Pacific cold tongue complex and the annual cycle in the eastern equatorial Pacific are partly controlled by the strength of the AMOC. The results may have important implications for the interpretation of global multidecadal variability and paleo-proxy data.
Yin, J, and Ronald J Stouffer, 2007: Comparison of the Stability of the Atlantic Thermohaline Circulation in Two Coupled Atmosphere–Ocean General Circulation Models. Journal of Climate, 20(17), doi:10.1175/JCLI4256.1.
[ Abstract ]
Two coupled atmosphere–ocean general circulation models developed at GFDL show differing stability properties of the Atlantic thermohaline circulation (THC) in the Coupled Model Intercomparison Project/Paleoclimate Modeling Intercomparison Project (CMIP/PMIP) coordinated “water-hosing” experiment. In contrast to the R30 model in which the “off” state of the THC is stable, it is unstable in the CM2.1. This discrepancy has also been found among other climate models. Here a comprehensive analysis is performed to investigate the causes for the differing behaviors of the THC. In agreement with previous work, it is found that the different stability of the THC is closely related to the simulation of a reversed thermohaline circulation (RTHC) and the atmospheric feedback. After the shutdown of the THC, the RTHC is well developed and stable in R30. It transports freshwater into the subtropical North Atlantic, preventing the recovery of the salinity and stabilizing the off mode of the THC. The flux adjustment is a large term in the water budget of the Atlantic Ocean. In contrast, the RTHC is weak and unstable in CM2.1. The atmospheric feedback associated with the southward shift of the Atlantic ITCZ is much more significant. The oceanic freshwater convergence into the subtropical North Atlantic cannot completely compensate for the evaporation, leading to the recovery of the THC in CM2.1. The rapid salinity recovery in the subtropical North Atlantic excites large-scale baroclinic eddies, which propagate northward into the Nordic seas and Irminger Sea. As the large-scale eddies reach the high latitudes of the North Atlantic, the oceanic deep convection restarts. The differences in the southward propagation of the salinity and temperature anomalies from the hosing perturbation region in R30 and CM2.1, and associated different development of a reversed meridional density gradient in the upper South Atlantic, are the cause of the differences in the behavior of the RTHC. The present study sheds light on important physical and dynamical processes in simulating the dynamical behavior of the THC.
Broccoli, Anthony J., K A Dahl, and Ronald J Stouffer, January 2006: Response of the ITCZ to Northern Hemisphere cooling. Geophysical Research Letters, 33(1), L01702, doi:10.1029/2005GL024546.
[ Abstract ]
Climate simulations, using models with different levels of complexity, indicate that the north-south position of the intertropical convergence zone (ITCZ) responds to changes in interhemispheric temperature contrast. Paleoclimate data on a variety of timescales suggest a similar behavior, with southward displacements of the ITCZ and associated changes in tropical atmospheric circulation during cold periods in the Northern Hemisphere. To identify a mechanism by which ITCZ displacements can be forced from the extratropics, we use a climate model with idealized geography and a simple slab ocean. We cool the northern extratropics and warm the southern extratropics to represent the asymmetric temperature changes associated with glacial-interglacial and millennial-scale climate variability. A southward shift in the ITCZ occurs, along with changes in the trade winds and an asymmetric response of the Hadley circulation. Changes in atmospheric heat exchange between the tropics and midlatitudes are the likely cause of this response, suggesting that this mechanism may play an important role in ITCZ displacements on timescales from decadal to glacial-interglacial.
Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, Ventakramani Balaji, J A Beesley, W F Cooke, Keith W Dixon, John Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen Griffies, Rich Gudgel, Matthew J Harrison, Isaac Held, Richard S Hemler, Larry Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, A R Langenhorst, H C Lee, Shian-Jiann Lin, Jian Lu, S Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), doi:10.1175/JCLI3629.1.
[ Abstract ]
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments. The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic. Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/). Manuscript received 8 December 2004, in final form 18 March 2005
Gnanadesikan, Anand, Keith W Dixon, Stephen Griffies, Ventakramani Balaji, M Barreiro, J A Beesley, W F Cooke, Thomas L Delworth, R Gerdes, Matthew J Harrison, Isaac Held, William J Hurlin, H C Lee, Z Liang, G Nong, Ronald C Pacanowski, Anthony Rosati, J L Russell, Bonita L Samuels, Qian Song, Michael J Spelman, Ronald J Stouffer, C Sweeney, G A Vecchi, Michael Winton, Andrew T Wittenberg, Fanrong Zeng, Rong Zhang, and John Dunne, 2006: GFDL's CM2 Global Coupled Climate Models. Part II: The baseline ocean simulation. Journal of Climate, 19(5), doi:10.1175/JCLI3630.1.
[ Abstract ]
The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.
Gnanadesikan, Anand, and Ronald J Stouffer, 2006: Diagnosing atmosphere-ocean general circulation model errors relevant to the terrestrial biosphere using the Köppen climate classification. Geophysical Research Letters, 33, L22701, doi:10.1029/2006GL028098.
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Coupled atmosphere-ocean-land-sea ice climate models (AOGCMs) are often tuned using physical variables like temperature and precipitation with the goal of minimizing properties such as the root-mean-square error. As the community moves towards modeling the earth system, it is important to note that not all biases have equivalent impacts on biology. Bioclimatic classification systems provide means of filtering model errors so as to bring out those impacts that may be particularly important for the terrestrial biosphere. We examine one such diagnostic, the classic system of Köppen, and show that it can provide an “early warning” of which model biases are likely to produce serious biases in the land biosphere. Moreover, it provides a rough evaluation criterion for the performance of dynamic vegetation models. State-of-the art AOGCMs fail to capture the correct Köppen zone in about 20–30% of the land area excluding Antarctica, and misassign a similar fraction to the wrong subzone.
Hewitt, C, Anthony J Broccoli, M Crucifix, J M Gregory, J F B Mitchell, and Ronald J Stouffer, 2006: The effect of a large freshwater perturbation on the Glacial North Atlantic Ocean using a Coupled General Circulation Model. Journal of Climate, 19(17), doi:10.1175/JCLI3867.1.
[ Abstract ]
The commonly held view of the condition in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the "strength of the overturning circulation" is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation. Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors' model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project's reconstruction of glacial sea surface temperature and sea ice extent.
Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, Isaac Held, Jian Lu, V Ramaswamy, M Daniel Schwarzkopf, G Stenchikov, and Ronald J Stouffer, 2006: Assessment of Twentieth-Century regional surface temperature trends using the GFDL CM2 coupled models. Journal of Climate, 19(9), doi:10.1175/JCLI3709.1.
[ Abstract ]
Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing. Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observations contribute substantially to an underprediction of the observed warming over northern Asia. In the all-forcing and natural-only forcing runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s. The simulations support previous findings that twentieth-century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-twentieth-century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/western Pacific warm pool during the past half-century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multidecadal variability.
Pitman, A J., and Ronald J Stouffer, 2006: Abrupt change in climate and climate models. Hydrology and Earth System Sciences, 10(6), 903-912.
[ Abstract PDF ]
First, we review the evidence that abrupt climate changes have occurred in the past and then demonstrate that climate models have developing capacity to simulate many of these changes. In particular, the processes by which changes in the ocean circulation drive abrupt changes appear to be captured by climate models to a degree that is encouraging. The evidence that past changes in the ocean have driven abrupt change in terrestrial systems is also convincing, but these processes are only just beginning to be included in climate models. Second, we explore the likelihood that climate models can capture those abrupt changes in climate that may occur in the future due to the enhanced greenhouse effect. We note that existing evidence indicates that a major collapse of the thermohaline circulation seems unlikely in the 21st century, although very recent evidence suggests that a weakening may already be underway. We have confidence that current climate models can capture a weakening, but a collapse in the 21st century of the thermohaline circulation is not projected by climate models. Worrying evidence of instability in terrestrial carbon, from observations and modelling studies, is beginning to accumulate. Current climate models used by the Intergovernmental Panel on Climate Change for the 4th Assessment Report do not include these terrestrial carbon processes. We therefore can not make statements with any confidence regarding these changes. At present, the scale of the terrestrial carbon feedback is believed to be small enough that it does not significantly affect projections of warming during the first half of the 21st century. However, the uncertainties in how biological systems will respond to warming are sufficiently large to undermine confidence in this belief and point us to areas requiring significant additional work.
Russell, J L., Keith W Dixon, Anand Gnanadesikan, Ronald J Stouffer, and J Robert Toggweiler, 2006: The Southern Hemisphere westerlies in a warming orld: Propping open the door to the deep ocean. Journal of Climate, 19(24), doi:10.1175/JCLI3984.1.
[ Abstract ]
A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.
Russell, J L., Ronald J Stouffer, and Keith W Dixon, 2006: Intercomparison of the Southern Ocean Circulations in IPCC Coupled Model Control Simulations. Journal of Climate, 19(18), doi:10.1175/JCLI3869.1.
[ Abstract ]
The analyses presented here focus on the Southern Ocean as simulated in a set of global coupled climate model control experiments conducted by several international climate modeling groups. Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from it being the region where most of the transformation of the World Ocean’s water masses occurs. In climate change experiments that simulate greenhouse gas–induced warming, Southern Ocean air–sea heat fluxes and three-dimensional circulation patterns make it a region where much of the future oceanic heat storage takes place, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. Factors such as the Southern Ocean’s wind forcing, heat, and salt budgets are linked to the structure and transport of the ACC in ways that have not been expressed clearly in the literature. These links are explored here in a coupled model context by analyzing a sizable suite of preindustrial control experiments associated with the forthcoming Intergovernmental Panel on Climate Change’s Fourth Assessment Report. A framework is developed that uses measures of coupled model simulation characteristics, primarily those related to the Southern Ocean wind forcing and water mass properties, to allow one to categorize, and to some extent predict, which models do better or worse at simulating the Southern Ocean and why. Hopefully, this framework will also lead to increased understanding of the ocean’s response to climate changes.
Stenchikov, G, K P Hamilton, Ronald J Stouffer, A Robock, V Ramaswamy, B D Santer, and H F Graf, et al., 2006: Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models. Journal of Geophysical Research, 111, D07107, doi:10.1029/2005JD006286.
[ Abstract ]
Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860–1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.
Stouffer, Ronald J., Thomas L Delworth, Keith W Dixon, Rich Gudgel, Isaac Held, Richard S Hemler, Thomas R Knutson, M Daniel Schwarzkopf, Michael J Spelman, Michael Winton, Anthony J Broccoli, H C Lee, Fanrong Zeng, and Brian J Soden, 2006: GFDL's CM2 Global Coupled Climate Models. Part IV: Idealized Climate Response. Journal of Climate, 19(5), doi:10.1175/JCLI3632.1.
[ Abstract ]
The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented. Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR). The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).
Stouffer, Ronald J., Keith W Dixon, Michael J Spelman, William J Hurlin, J Yin, J M Gregory, A J Weaver, M Eby, G M Flato, D Y Robitaille, H Hasumi, A Oka, A Hu, J H Jungclaus, I V Kamenkovich, A Levermann, M Montoya, S Murakami, S Nawrath, W R Peltier, G Vettoretti, A P Sokolov, and S L Weber, 2006: Investigating the Causes of the Response of the Thermohaline Circulation to Past and Future Climate Changes. Journal of Climate, 19(8), doi:10.1175/JCLI3689.11.
[ Abstract ]
The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
Stouffer, Ronald J., J L Russell, and Michael J Spelman, 2006: Importance of oceanic heat uptake in transient climate change. Geophysical Research Letters, 33(17), L17704, doi:10.1029/2006GL027242.
[ Abstract ]
The impact of the differences in the oceanic heat uptake and storage on the transient response to changes in radiative forcing is investigated using two newly developed coupled atmosphere-ocean models. In spite of its larger equilibrium climate sensitivity, one model (CM2.1) has smaller transient globally averaged surface air temperature (SAT) response than is found in the second model (CM2.0). The differences in the SAT response become larger as radiative forcing increases and the time scales become longer. The smaller transient SAT response in CM2.1 is due to its larger oceanic heat uptake. The heat storage differences between the two models also increase with time and larger rates of radiative forcing. The larger oceanic heat uptake in CM2.1 can be traced to differences in the Southern Ocean heat uptake and is related to a more realistic Southern Ocean simulation in the control integration.
Vinnikov, K Y., N C Grody, A Robock, Ronald J Stouffer, P Jones, and M D Goldberg, 2006: Temperature trends at the surface and in the troposphere. Journal of Geophysical Research, 111, D03106, doi:10.1029/2005JD006392.
[ Abstract ]
This paper incorporates the latest improvements in intersatellite calibration, along with a new statistical technique, to determine the diurnal and seasonal cycles and climatic trends of 1978–2004 tropospheric temperature using Microwave Sounding Unit measurements. We also compare the latitudinal distribution of temperature trends from the surface and troposphere with each other and with model simulations for the past 26 years. The observations at the surface and in the troposphere are consistent with climate model simulations. At middle and high latitudes in the Northern Hemisphere, the zonally averaged temperature at the surface increased faster than in the troposphere while at low latitudes of both hemispheres the temperature increased more slowly at the surface than in the troposphere. The resulting global averaged tropospheric trend is +0.20 K/10 yr, with a standard error of 0.05 K/10 yr, which compares very well with the trend obtained from surface reports.
Dahl, K A., Anthony J Broccoli, and Ronald J Stouffer, 2005: Assessing the role of North Atlantic freshwater forcing in millennial scale climate variability: a tropical Atlantic perspective. Climate Dynamics, 24(4), doi:10.1007/s00382-004-0499-5.
[ Abstract ]
This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.
Gregory, J M., Keith W Dixon, Ronald J Stouffer, A J Weaver, E Driesschaert, M Eby, T Fichefet, H Hasumi, A Hu, J H Jungclaus, I V Kamenkovich, A Levermann, M Montoya, S Murakami, S Nawrath, A Oka, A P Sokolov, and R B Thorpe, 2005: A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration. Geophysical Research Letters, 32, L12703, doi:10.1029/2005GL023209.
[ Abstract ]
As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic thermohaline circulation (THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.
Meehl, G A., C Covey, B McAveney, M Latif, and Ronald J Stouffer, 2005: Overview of the coupled model intercomparison project. Bulletin of the American Meteorological Society, 86(1), doi:10.1175/BAMS-86-1-89.
Seidov, D, Ronald J Stouffer, and B J Haupt, 2005: Is there a simple bi-polar ocean seesaw. Global & Planetary Change, 49(1-2), doi:10.1016/j.gloplacha.2005.05.00.
[ Abstract ]
Using an atmosphere–ocean coupled model, the climate response to an idealized freshwater input into the Southern Ocean is studied. In response to the freshwater input, the surface waters around Antarctica freshen and cool. As the addition of freshwater continues, the fresh, surface anomalies spread throughout the world ocean in contrast to ocean-only experiments and North Atlantic experiments using coupled models. Because of the fundamental difference in altering sea surface salinity (SSS) from the two sources (northern hemisphere and southern hemisphere), a bi-polar seesaw fails to develop in the ocean, at least in our coupled atmosphere–ocean experiments. Control ocean-only experiments with mixed boundary conditions and similar short-term southern freshwater impacts match the results of the coupled experiments. Based on these experiments, we argue that the concept of ocean bi-polar seesaw should be taken with some caveats.
Braganza, K, D J Karoly, A C Hirst, P Stott, Ronald J Stouffer, and S F B Tett, 2004: Simple indices of global climate variability and change Part II: attribution of climate change during the twentieth century. Climate Dynamics, 22(8), doi:10.1007/s00382-004-0413-1.
[ Abstract ]
Five simple indices of surface temperature are used to investigate the influence of anthropogenic and natural (solar irradiance and volcanic aerosol) forcing on observed climate change during the twentieth century. These indices are based on spatial fingerprints of climate change and include the global-mean surface temperature, the land-ocean temperature contrast, the magnitude of the annual cycle in surface temperature over land, the Northern Hemisphere meridional temperature gradient and the hemispheric temperature contrast. The indices contain information independent of variations in global-mean temperature for unforced climate variations and hence, considered collectively, they are more useful in an attribution study than global mean surface temperature alone. Observed linear trends over 1950–1999 in all the indices except the hemispheric temperature contrast are significantly larger than simulated changes due to internal variability or natural (solar and volcanic aerosol) forcings and are consistent with simulated changes due to anthropogenic (greenhouse gas and sulfate aerosol) forcing. The combined, relative influence of these different forcings on observed trends during the twentieth century is investigated using linear regression of the observed and simulated responses of the indices. It is found that anthropogenic forcing accounts for almost all of the observed changes in surface temperature during 1946–1995. We found that early twentieth century changes (1896–1945) in global mean temperature can be explained by a combination of anthropogenic and natural forcing, as well as internal climate variability. Estimates of scaling factors that weight the amplitude of model simulated signals to corresponding observed changes using a combined normalized index are similar to those calculated using more complex, optimal fingerprint techniques.
Manabe, Syukuro, Richard T Wetherald, P C D Milly, Thomas L Delworth, and Ronald J Stouffer, 2004: Century-scale change in water availability: CO2-quadrupling experiment. Climatic Change, 64(1-2), 59-76.
[ Abstract PDF ]
It has been suggested that, unless a major effort is made, the atmospheric concentration of carbon dioxide may rise above four times the pre-industrial level in a few centuries. Here we use a coupled atmosphere-ocean-land model to explore the response of the global water cycle to such a large increase in carbon dioxide, focusing on river discharge and soil moisture. Our results suggest that water is going to be more plentiful in those regions of the world that are already `water-rich'. However, water stresses will increase significantly in regions and seasons that are already relatively dry. This could pose a very challenging problem for water-resource management around the world. For soil moisture, our results indicate reductions during much of the year in many semi-arid regions of the world, such as the southwestern region of North America, the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. In some of these regions, soil moisture values are reduced by almost a factor of two during the dry season. The drying in semi-arid regions is likely to induce the outward expansion of deserts to the surrounding regions. Over extensive regions of both the Eurasian and North American continents in high and middle latitudes, soil moisture decreases in summer but increases in winter, in contrast to the situation in semi-arid regions. For river discharge, our results indicate an average increase of ~ 15% during the next few centuries. The discharges from Arctic rivers such as the Mackenzie and Ob' increase by much larger fractions. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra also increase considerably. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller.
Sarmiento, Jorge L., Richard D Slater, R T Barber, L Bopp, S C Doney, A C Hirst, J Kieypas, R Matear, U Mikolajewicz, Patrick Monfray, V Soldatov, S A Spall, and Ronald J Stouffer, 2004: Response of ocean ecosystems to climate warming. Global Biogeochemical Cycles, 18, GB3003, doi:10.1029/2003/GB002134.
[ Abstract ]
We examine six different coupled climate model simulations to determine the ocean biological response to climate warming between the beginning of the industrial revolution and 2050. We use vertical velocity, maximum winter mixed layer depth, and sea ice cover to define six biomes. Climate warming leads to a contraction of the highly productive marginal sea ice biome by 42% in the Northern Hemisphere and 17% in the Southern Hemisphere, and leads to an expansion of the low productivity permanently stratified subtropical gyre biome by 4.0% in the Northern Hemisphere and 9.4% in the Southern Hemisphere. In between these, the subpolar gyre biome expands by 16% in the Northern Hemisphere and 7% in the Southern Hemisphere, and the seasonally stratified subtropical gyre contracts by 11% in both hemispheres. The low-latitude (mostly coastal) upwelling biome area changes only modestly. Vertical stratification increases, which would be expected to decrease nutrient supply everywhere, but increase the growing season length in high latitudes. We use satellite ocean color and climatological observations to develop an empirical model for predicting chlorophyll from the physical properties of the global warming simulations. Four features stand out in the response to global warming: (1) a drop in chlorophyll in the North Pacific due primarily to retreat of the marginal sea ice biome, (2) a tendency toward an increase in chlorophyll in the North Atlantic due to a complex combination of factors, (3) an increase in chlorophyll in the Southern Ocean due primarily to the retreat of and changes at the northern boundary of the marginal sea ice zone, and (4) a tendency toward a decrease in chlorophyll adjacent to the Antarctic continent due primarily to freshening within the marginal sea ice zone. We use three different primary production algorithms to estimate the response of primary production to climate warming based on our estimated chlorophyll concentrations. The three algorithms give a global increase in primary production of 0.7% at the low end to 8.1% at the high end, with very large regional differences. The main cause of both the response to warming and the variation between algorithms is the temperature sensitivity of the primary production algorithms. We also show results for the period between the industrial revolution and 2050 and 2090.
Stouffer, Ronald J., 2004: Time scales of climate response. Journal of Climate, 17(1), 209-217.
[ Abstract PDF ]
A coupled atmosphere-ocean general circulation model (AOGCM) is integrated to a near-equilibrium state with the normal, half-normal, and twice-normal amounts of carbon dioxide in the atmosphere. Most of the ocean below the surface layers achieves 70% of the total response almost twice as fast when the changes in radiative forcing are cooling as compared to the case when they are warming the climate system. In the cooling case, the time to achieve 70% of the equilibrium response in the midoceanic depths is about 500-1000 yr. In the warming case, this response time is 1300-1700 yr. In the Pacific Ocean and the bottom half of the Atlantic Ocean basins, the response is similar to the global response in that the cooling case results in a shorter response time scale. In the upper half of the Atlantic basin, the cooling response time scales are somewhat longer than in the warming case due to changes in the oceanic thermohaline circulation. In the oceanic surface mixed layer and atmosphere, the response time scale is closely coupled. In the Southern Hemisphere, the near-surface response time is slightly faster in the cooling case. However in the Northern Hemisphere, the near-surface response times are faster in the warming case by more than 500 yr at times during the integrations. In the Northern Hemisphere, both the cooling and warming cases have much shorter response time scales than found in the Southern Hemisphere. Oceanic mixing of heat is the key in determining these time scales. It is shown that the model's simulation of present-day radiocarbon and chlorofluorocarbon (CFC) distributions compares favorably to the observations indicating that the quantitative time scales may be realistic.
Stouffer, Ronald J., A J Weaver, and M Eby, 2004: A method for obtaining pre-twentieth century initial conditions for use in climate change studies. Climate Dynamics, 23, 327-339.
[ Abstract PDF ]
A method is proposed to initialise coupled atmosphere-ocean general circulation models (AOGCMs) developed to study climate change on multicentury time scales. The method assumes that current generation AOGCMs are developed and evaluated using present-day radiative forcing and near present day oceanic initial conditions. To find pre-twentieth century initial conditions, we propose that the radiative forcing be run backwards in time from the present to the desired starting date. The model should then be run for 3–5 centuries with the radiative forcing held constant at the desired date. In our tests, instantaneously switching to pre-twentieth century radiative forcing did not save computational time. When a sufficiently stable pretwentieth century condition is achieved, the coupled system can be integrated forward to the present and into the future. This method is a first step toward the standardization of AOGCM initialization and suggests a framework for AOGCM initialization for the first time. It provides an internally consistent set of pre-twentieth century initial conditions, although they will vary from model to model. Furthermore, it is likely that this method will yield a fairly realistic present-day climate in transient climate change experiments of the twentieth century, if the model biases are not too large. The main disadvantage of the method is that it is fairly computationally expensive in that it requires an additional 4–6 centuries of model integration before starting historical twentieth century integrations. However, the relative cost of this technique diminishes as more simulations are conducted using the oceanic initial condition obtained using our method.
Braconnot, P, S Joussaume, S Harrison, C Hewitt, P Valdes, G Ramstein, Ronald J Stouffer, B Otto-Bliesner, and K E Taylor, 2003: The second phase of the Paleoclimate Modeling Intercomparison Project (PMIPII). Clivar Exchanges, 8(4), 19-20.
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Braganza, K, D J Karoly, A C Hirst, M E Mann, P Stott, Ronald J Stouffer, and S F B Tett, 2003: Simple indices of global climate variability and change: Part I - variability and correlation structure. Climate Dynamics, 20(5), 491-502.
[ Abstract PDF ]
Some simple indices are used to describe global climate variability in observational data and climate model simulations. The indices are surface temperature based and include the global-mean, the land-ocean contrast, the meridional gradient, the interhemispheric contrast, and the magnitude of the annual cycle. These indices contain information independent of the variations of the global-mean temperature for unforced climate variations. They also represent the main features of the modelled surface temperature response to increasing greenhouse gases in the atmosphere. Hence, they should have a coherent response for greenhouse climate change. On interannual and decadal time scales, the variability and correlation structure of the indices from long control climate model simulations compare well with those from detrended instrumental observations for the twentieth century and proxy based climate reconstructions for 1700-1900. The indices provide a simple but effective way to evaluate global-scale climate variability in control climate model simulations. On decadal time scales, the observed correlation structure between the indices during the twentieth century shows significant differences from the detrended observations and control model simulations. These changes are consistent with forced climate variations in greenhouse climate change simulations. This suggests that the changes in the correlation structure between these indices can be used as an indicator of climate change.
Broccoli, Anthony J., Keith W Dixon, Thomas L Delworth, Thomas R Knutson, Ronald J Stouffer, and Fanrong Zeng, 2003: Twentieth-century temperature and precipitation trends in ensemble climate simulations including natural and anthropogenic forcing. Journal of Geophysical Research, 108(D24), 4798, doi:10.1029/2003JD003812.
[ Abstract PDF ]
We present results from a series of ensemble integrations of a global coupled atmosphere-ocean model for the period 1865-1997. Each ensemble consists of three integrations initialized from different points in a long-running GFDL R30 coupled model control simulation. The first ensemble includes time-varying forcing from greenhouse gases only. In the remaining three ensembles, forcings from anthropogenic sulfate aerosols, solar variability, and volcanic aerosols in the stratosphere are added progressively, such that the fourth ensemble uses all four of these forcings. The effects of anthropogenic sulfate aerosols are represented by changes in surface albedo, and the effects of volcanic aerosols are represented by latitude-dependent perturbations in incident solar radiation. Comparisons with observations reveal that the addition of the natural forcings (solar and volcanic) improves the simulation of global multidecadal trends in temperature, precipitation, and ocean heat content. Solar and volcanic forcings are important contributors to early twentieth century warming. Volcanic forcing reduces the warming simulated for the late twentieth century. Interdecadal variations in global mean surface air temperature from the ensemble of experiments with all four forcings are very similar to observed variations during most of the twentieth century. The improved agreement of simulated and observed temperature trends when natural climate forcings are included supports the climatic importance of variations in radiative forcing during the twentieth century.
Dixon, Keith W., Thomas L Delworth, Thomas R Knutson, Michael J Spelman, and Ronald J Stouffer, 2003: A comparison of climate change simulations produced by two GFDL coupled climate models. Global & Planetary Change, 37(1-2), 81-102.
[ Abstract PDF ]
The transient responses of two versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model to a climate change forcing scenario are examined. The same computer codes were used to construct the atmosphere, ocean, sea ice and land surface components of the two models, and they employ the same types of sub-grid-scale parameterization schemes. The two model versions differ primarily, but not solely, in their spatial resolution. Comparisons are made of results from six coarse-resolution R15 climate change experiments and three medium-resolution R30 experiments in which levels of greenhouse gases (GHGs) and sulfate aerosols are specified to change over time. The two model versions yield similar global mean surface air temperature responses until the second half of the 21st century, after which the R15 model exhibits a somewhat larger response. Polar amplification of the Northern Hemisphere's warming signal is more pronounced in the R15 model, in part due to the R15's cooler control climate, which allows for larger snow and ice albedo positive feedbacks. Both models project a substantial weakening of the North Atlantic overturning circulation and a large reduction in the volume of Arctic sea ice to occur in the 21st century. Relative to their respective control integrations, there is a greater reduction of Arctic sea ice in the R15 experiments than in the R30 simulations as the climate system warms. The globally averaged annual mean precipitation rate is simulated to increase over time, with both model versions projecting an increase of about 8% to occur by the decade of the 2080s. While the global mean precipitation response is quite similar in the two models, regional differences exist, with the R30 model displaying larger increases in equatorial regions.
Hegerl, G, G A Meehl, C Covey, M Latif, B McAveney, and Ronald J Stouffer, 2003: 20C3M: CMIP collecting data from 20th century coupled model simulations. Clivar Exchanges, 26, 1-4.
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Hewitt, C, Ronald J Stouffer, Anthony J Broccoli, J F B Mitchell, and P Valdes, 2003: The effect of ocean dynamics in a coupled GCM simulation of the Last Glacial Maximum. Climate Dynamics, 20(2/3), 203-218.
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General circulation models (GCMs) of the climate system are powerful tools for understanding and predicting climate change. The last glacial maximum (LGM) provides an extreme test of the model's ability to simulate a change of climate, and allows us to increase our understanding of mechanisms of climate change. We have used a coupled high resolution ocean-atmosphere GCM (HadCM3) to simulate the equilibrium climate at the LGM. The effect of ocean dynamics is investigated by carrying out a parallel experiment replacing the dynamic three-dimensional ocean GCM with a static thermodynamic mixed-layer ocean model. Changes to the ocean circulation, and feedbacks between the ocean, atmosphere and sea ice have an important influence on the surface response, and are discussed. The coupled model produces an intensified thermohaline circulation and an increase in the amount of heat transported northward by the Atlantic Ocean equatorward of 55°N, which is at odds with the interpretation of some proxy records. Such changes, which the thermodynamic mixed-layer ocean model cannot produce, have a large impact around the North Atlantic region, and are discussed in the study.
Meehl, G A., C Covey, M Latif, B McAveney, and Ronald J Stouffer, 2003: Coupled model intercomparison project. GEWEX News, 13(1), 10-11.
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Rutherford, S, M E Mann, Thomas L Delworth, and Ronald J Stouffer, 2003: Climate field reconstruction under stationary and nonstationary forcing. Journal of Climate, 16(3), 462-479.
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The fidelity of climate reconstructions employing covariance-based calibration techniques is tested with varying levels of sparseness of available data during intervals of relatively constant (stationary) and increasing (nonstationary) forcing. These tests employ a regularized expectation-maximization algorithm using surface temperature data from both the instrumental record and coupled ocean-atmosphere model integrations. The results indicate that if radiative forcing is relatively constant over a data-rich calibration period and increases over a data-sparse reconstruction period, the imputed temperatures in the reconstruction period may be biased and may underestimate the true temperature trend. However, if radiative forcing is stationary over a data-sparse reconstruction period and increases over a data-rich calibration period, the imputed values in the reconstruction period are nearly unbiased. These results indicate that using the data-rich part of the twentieth-century instrumental record (which contains an increasing temperature trend plausibly associated with increasing radiative forcing) for calibration does not significantly bias reconstructions of prior climate.
Stouffer, Ronald J., and Syukuro Manabe, 2003: Equilibrium response of thermohaline circulation to large changes in atmospheric CO₂ concentration. Climate Dynamics, 20(7/8), 759-773.
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This study evaluates the equilibrium response of a coupled ocean-atmosphere model to the doubling, quadrupling, and halving of CO₂ concentration in the atmosphere. Special emphasis in the study is placed upon the response of the thermohaline circulation in the Atlantic Ocean to the changes in CO₂ concentration of the atmosphere. The simulated intensity of the thermohaline circulation (THC) is similar among three quasi-equilibrium states with the standard, double the standard, and quadruple the standard amounts of CO₂ concentration in the atmosphere. When the model atmosphere has half the standard concentration of CO₂, however, the THC is very weak and shallow in the Atlantic Ocean. Below a depth of 3 km, the model oceans maintain very thick layer of cold bottom water with temperature close to -2 °C, preventing the deeper penetration of the THC in the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere, sea ice extends beyond the Antarctic Polar front, almost entirely covering the regions of deepwater ventilation. In addition to the active mode of the THC, there exists another stable mode of the THC for the standard, possibly double the standard (not yet confirmed), and quadruple the standard concentration of atmospheric carbon dioxide. This second mode is characterized by the weak, reverse overturning circulation over the entire Atlantic basin, and has no ventilation of the entire subsurface water in the North Atlantic Ocean. At one half the standard CO₂ concentration, however, the intensity of the first mode is so weak that it is not certain whether there are two distinct stable modes or not. The paleoceanographic implications of the results obtained here are discussed as they relate to the signatures of the Cenozoic changes in the oceans.
Delworth, Thomas L., Ronald J Stouffer, Keith W Dixon, Michael J Spelman, Thomas R Knutson, Anthony J Broccoli, P J Kushner, and Richard T Wetherald, 2002: Review of simulations of climate variability and change with the GFDL R30 coupled climate model. Climate Dynamics, 19(7), 555-574.
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A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near milennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.
Gregory, J M., Ronald J Stouffer, S C B Raper, P Stott, and P Rayner, 2002: An Observationally based estimate of the climate sensitivity. Journal of Climate, 15(22), 3117-3121.
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A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5th percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analyses of observed interior-ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthropogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late nineteenth century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulation model, it provides an independent observationally based constraint on this important parameter of the climate system.
Harrison, S, P Braconnot, S Joussaume, C Hewitt, and Ronald J Stouffer, 2002: Comparison of Palaeoclimate simulations enhances confidence in models. EOS, 83(40), 447.
Mahlman, Jerry D., and Ronald J Stouffer, 2002: Projection of future changes in climate In Encyclopedia of Global Environmental Change, Vol. I, Chichester, UK, John Wiley & Sons, 126-139.
Raper, S C., J M Gregory, and Ronald J Stouffer, 2002: The role of climate sensitivity and ocean heat uptake on AOGCM transient temperature response. Journal of Climate, 15(1), 124-130.
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The role of climate sensitivity and ocean heat uptake in determining the range of climate model response is investigated in the second phase of the Coupled Model Intercomparison Project (CMIP2) AOGCM results. The fraction of equilibrium warming that is realized at any one time is less in those models with higher climate sensitivity, leading to a reduction in the temperature response range at the time of CO2 doubling [transient climate response (TCR) range]. The range is reduced by a further 15% because of an apparent relationship between climate sensitivity and the efficiency of ocean heat uptake. Some possible physical causes for this relationship are suggested.
Church, J A., J M Gregory, P Huybrechts, M Kuhn, K Lambeck, M T Nhuan, D Qin, P L Woodworth, O A Anisimov, F O Bryan, A Cazenave, Keith W Dixon, B B Fitzharris, G M Flato, A Ganopolski, V Gornitz, J A Lowe, A Noda, J M Oberhuber, S P O'Farrell, A Ohmura, M Oppenheimer, W R Peltier, S C B Raper, C Ritz, G Russell, E Schlosser, C K Shum, T F Stocker, Ronald J Stouffer, R S W van der Wal, R Voss, E C Wiebe, M Wild, D J Wingham, and H J Zwally, 2001: Changes in sea level In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 640-693.
Cubasch, U, G A Meehl, G J Boer, Ronald J Stouffer, M R Dix, A Noda, C A Senior, S C B Raper, K S Yap, A Abe-Ouchi, S Brinkop, M Claussen, M Collins, J Evans, I Fischer-Bruns, J C Fyfe, A Ganopolski, J M Gregory, Z-Z Hu, F Joos, Thomas R Knutson, R Knutti, C Landsea, L Mearns, P C D Milly, J F B Mitchell, T Nozawa, H Paeth, J Räisänen, R Sausen, S Smith, T F Stocker, A Timmermann, U Ulbrich, A J Weaver, J Wegner, P Whetton, T M L Wigley, Michael Winton, and F Zwiers, 2001: Projections of future climate change In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 526-582.
Gregory, J M., J A Church, G J Boer, Keith W Dixon, G M Flato, D R Jackett, J A Lowe, S P O'Farrell, M M Rienecker, G Russell, Ronald J Stouffer, and Michael Winton, 2001: Comparison of results from several AOGCMs for global and regional sea-level change 1900-2100. Climate Dynamics, 18(3/4), 225-240.
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Sea-level rise is an important aspect of climate change because of its impact on society and ecosystems. Here we present an intercomparison of results from ten coupled atmosphere-ocean general circulation models (AOGCMs) for sea-level changes simulated for the twentieth century and projected to occur during the twenty first century in experiments following scenario 1892a for greenhouse gases and sulphate aerosols. The model results suggest that the rate of sea-level rise due to thermal expansion of sea water has increased during the twentieth century, but the small set of tide gauges with long records might not be adequate to detect this acceleration. The rate of sea-level rise due to thermal expansion continues to increase throughout the twenty first century, and the projected total is consequently larger than in the twentieth century; for 1990-2090 it amounts to 0.20-0.37 m. This wide range results from systematic uncertainty in modeling of climate change and of heat uptake by the ocean. The AOGCMs agree that sea-level rise is expected to be geographically non-uniform, with some regions experiencing as much as twice the global average, and others practically zero, but they do not agree about the geographical pattern. The lack of agreement indicates that we cannot currently have confidence in projections of local sea-level changes, and reveals a need for detailed analysis and intercomparison in order to understand and reduce the disagreements.
Hall, A, and Ronald J Stouffer, 2001: An abrupt climate event in a coupled ocean-atmosphere simulation without external forcing. Nature, 409(6817), 171-174.
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Temperature reconstructions from the North Atlantic region indicate frequent abrupt and severe climate fluctuations during the last glacial and Holocene periods. The driving forces for these events are unclear and coupled atmosphere-ocean models of global circulation have only simulated such events by inserting large amounts of fresh water into the northern North Atlantic Ocean. Here we report a drastic cooling event in a 15,000-yr simulation of global circulation with present-day climate conditions without the use of such external forcing. In our simulation, the annual average surface temperature near southern Greenland spontaneously fell 6-10 standard deviations below its mean value for a period of 30-40 yrs. The event was triggered by a persistent northwesterly wind that transported large amounts of buoyant cold and fresh water into the northern North Atlantic Ocean. Oceanic convection shut down in response to this flow, concentrating the entire cooling of the northern North Atlantic by the colder atmosphere in the uppermost ocean layer. Given the similarity between our simulation and observed records of rapid cooling events, our results indicate that internal atmospheric variability alone could have generated the extreme climate disruptions in this region.
Hewitt, C, Anthony J Broccoli, J F B Mitchell, and Ronald J Stouffer, 2001: A coupled model study of the last glacial maximum: Was part of the North Atlantic relatively warm? Geophysical Research Letters, 28(8), 1571-1574.
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A coupled ocean-atmosphere general circulation model is used to simulate the climates of today and the last glacial maximum (LGM). The model, which does not require artificial flux adjustments, produces a pattern of cooling at the LGM that is broadly consistent with the findings from simpler models and paleoclimatic data. However, changes to the ocean circulation produce anomalously warm LGM surface conditions over parts of the North Atlantic, seemingly at odds with paleoceanographic data. The thermohaline circulation is intensified for several centuries, as is the northward heat transport in the Atlantic equatorward of 55°N, but this may be a transient result. Mechanisms that lead to this response are discussed.
Manabe, Syukuro, Thomas R Knutson, Ronald J Stouffer, and Thomas L Delworth, 2001: Exploring natural and anthropogenic variation of climate. Quarterly Journal of the Royal Meteorological Society, 127(571), 1-24.
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This lecture discusses the low-frequency variability of surface temperature using a coupled ocean-atmosphere-land-surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the coupled model.
McAveney, B, Anthony J Broccoli, Keith W Dixon, and Ronald J Stouffer, et al., 2001: Model evaluation In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 472-523.
Moore, B, W L Gates, L J Mata, A Underdal, and Ronald J Stouffer, 2001: Advancing our understanding In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 770-785.
Stone, D A., A J Weaver, and Ronald J Stouffer, 2001: Projections of climate change onto modes of atmospheric variability. Journal of Climate, 14(17), 3551-3565.
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Two possible interpretations of forced climate change view it as projecting, either linearly or nonlinearly, onto the dominant modes of variability of the climate system. An evaluation of these two interpretations is performed using annual mean sea level pressure (SLP) and surface air temperature (SAT) fields obtained from integrations of the Geophysical Fluid Dynamics Laboratory coupled general circulation model forced with varying concentrations of greenhouse gases. The dominant modes of SLP both represent much of the total variability and remain important in warmer climates. With SAT, however, the dominant modes are often related to variations in the sea-ice edge and so do not remain important once the ice has retreated; those unrelated to sea ice remain dominant in the warmer climates but represent smaller fractions of the total variability. In general, climate change tends to project most strongly onto the more dominant modes. The change in SLP projects partially onto the top two modes in the Northern Hemisphere, reflecting both an overall decrease in hemispheric SLP as well as the pattern of change. In the Southern Hemisphere the change projects negligibly onto the dominant patterns between equilibrium climates but very strongly onto the Antarctic oscillation-like mode in the transient integrations. Changes in SAT project partially onto the dominant modes but relate more to the mean warming rather than the pattern of change. In general, the change projects most strongly onto the more dominant modes. In all SLP domains, the projection of climate change overwhelmingly manifests itself as a linear translation in the mode, consistent with the linear interpretation. In SAT domains related to sea-ice variability, the projection reflects an increased tendency toward ice-free regimes, consistent with the nonlinear perspective; however this nonlinear projection represents only a small portion of the overall climate change.
Wetherald, Richard T., Ronald J Stouffer, and Keith W Dixon, 2001: Committed warming and its implications for climate change. Geophysical Research Letters, 28(8), 1535-1538.
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Time lags between changes in radiative forcing and the resulting simulated climate responses are investigated in a set of transient climate change experiments. Both surface air temperature (SAT) and soil moisture responses are examined. Results suggest that if the radiative forcing is held fixed at today's levels, the global mean SAT will rise an additional 1.0K before equilibrating. This unrealized warming commitment is larger than the 0.6K warming observed since 1990. The coupled atmosphere-ocean GCM's transient SAT response for the year 2000 is estimated to be similar to its equilibration response to 1980 radiative forcings - a lag of ~20 years. Both the time lag and the warming commitment are projected to increase in the future, and depend on the model's climate sensitivity, oceanic heat uptake, and the forcing scenario. These results imply that much of the warming due to current greenhouse gas levels is yet to be realized.
Covey, C, A Abe-Ouchi, and Ronald J Stouffer, et al., 2000: The seasonal cycle in coupled ocean-atmosphere general circulation models. Climate Dynamics, 16, 775-787.
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We examine the seasonal cycle of near-surface air temperature simulated by 17 coupled ocean-atmosphere general circulation models participating in the Coupled Model Intercomparison Project (CMIP). Nine of the models use ad hoc "flux adjustment" at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux adjusted models separately and examine the behavior of each class. When averaged over all of the flux-adjusted model simulations, near-surface air temperature falls within 2 K of observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ~6 K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ~5 K) temperature in areas of elevated terrain. The corresponding average over non-flux -adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January to measure seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ~30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ~5 K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle is a necessary condition for confidence in their prediction of long-term climatic changes (such as global warming), but it is not a sufficient condition unless the seasonal cycle and long-term changes involve similar climatic processes. To test this possible connection, we compare seasonal cycle amplitude with equilibrium warming under doubled atmospheric carbon dioxide for the models in our data base. A small but positive correlation exists between these two quantities. This result is predicted by a simple conceptual model of the climate system, and it is consistent with other modeling experience, which indicates that the seasonal cycle depends only weakly on climate sensitivity.
Manabe, Syukuro, and Ronald J Stouffer, 2000: Study of abrupt climate change by a coupled ocean-atmosphere model. Quaternary Science Reviews, 19(1-5), 285-299.
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This study examines the responses of the simulated modern climate of a coupled ocean-atmosphere model to the discharge of freshwater into the North Atlantic Ocean. Two numerical experiments were conducted. In the first numerical experiment in which freshwater is discharged into high North Atlantic latitudes over the period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens. This weakening reduces surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic Continent of the Southern Hemisphere. Upon termination of the water discharge at the 500th year, the THC begins to reintensify, gaining its original intensity in a few hundred years. As a result, the climate of the northern North Atlantic and surrounding regions resumes its original distribution. However, in the Pacific sector of the Circumpolar Ocean of the Southern Hemisphere, the initial cooling and recovery of surface air temperature is delayed by a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces a multidecadal variation in the intensities of the THC and convective activities, which generate large multidecadal fluctuations of both sea surface temperature and salinity in the northern North Atlantic. Such oscillation yields almost abrupt changes of climate with rapid rise and fall of surface temperature in a few decades. In the second experiment, in which the same amount of freshwater is discharged into the subtropical North Atlantic over the period of 500 years, the THC and climate evolve in a manner of qualitatively similar to the first experiment. However, the magnitude of the THC response is 4-5 times smaller. It appears that freshwater is much less effective in weakening the THC if it is discharged outside high North Atlantic latitudes. The results from numerical experiments conducted earlier indicate that the intensity of the THC could also weaken in response to a future increase of atmospheric CO2, thereby moderating the CO2-induced warming over the northern North Atlantic and surrounding regions.
Meehl, G A., G J Boer, C Covey, M Latif, and Ronald J Stouffer, 2000: The Coupled Model Intercomparison Project (CMIP). Bulletin of the American Meteorological Society, 81(2), 313-318.
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The Coupled Model Intercomparison Project (CMIP) was established to study and intercompare climate simulations made with coupled ocean-atmosphere-cryosphere-land GCMs. There are two main phases (CMIP1 and CMIP2), which study, respectively, 1) the ability of models to simulate current climate, and 2) model simulations of climate change due to an idealized change in forcing (a 1% per year CO2 increase). Results from a number of CMIP projects were reported at the first CMIP Workshop held in Melbourne, Australia, in October 1998. Some recent advances in global coupled modeling related to CMIP were also reported. Presentations were based on preliminary unpublished results. Key outcomes from the workshop were that 1) many observed aspects of climate variability are simulated in global coupled models including the North Atlantic oscillation and its linkages to North Atlantic SSTs, El Niño-like events, and monsoon interannual variability; 2) the amplitude of both high- and low-frequency global mean surface temperature variability in many global coupled models is less than that observed, with the former due in part to simulated ENSO in the models being generally weaker than observed, and the latter likely to be at least partially due to the uncertainty in the estimates of past radiative forcing; 3) an El Niño-like pattern in the mean SST response with greater surface warming in the eastern equatorial Pacific than the western equatorial Pacific is found by a number of models in global warming climate change experiments, but other models have a more spatially uniform or even a La Niña-like, response; 4) flux adjustment, by definition, improves the simulation of mean present-day climate over oceans, does not guarantee a drift-free climate, but can produce a stable base state in some models to enable very long term (1000 yr and longer) integrations- in these models it does not appear to have a major effect on model processes or model responses to increasing CO2; and 5) recent multicentury integrations show that a stable surface climate can be attained without flux adjustment (though still with some systematic simulation errors).
Stouffer, Ronald J., G Hegerl, and S F B Tett, 2000: A comparison of surface air temperature variability in three 1000-yr coupled ocean-atmosphere model integrations. Journal of Climate, 13(3), 513-537.
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This study compares the variability of surface air temperature in three long coupled ocean-atmosphere general circulation model integrations. It is shown that the annual mean climatology of the surface air temperatures (SAT) in all three models is realistic and the linear trends over the 1000-yr integrations are small over most areas of the globe. Second, although there are notable differences among the models, the models' SAT variability is fairly realistic on annual to decadal timescales, both in terms of the geographical distribution and of the global mean values. A notable exception is the poor simulation of observed tropical Pacific variability. In the HadCM2 model, the tropical variability is overestimated, while in the GFDL and HAM3L models, it is underestimated. Also, the ENSO-related spectral peak in the globally averaged observed SAT differs from that in any of the models. The relatively low resolution required to integrate models for long time periods inhibits the successful simulation of the variability in this region. On timescales longer than a few decades, the largest variance in the models is generally located near sea ice margins in high latitudes, which are also regions of deep oceanic convection and variability related to variations in the thermohaline circulation. However, the exact geographical location of these maxima varies from model to model. The preferred patterns of interdecadal variability that are common to all three coupled models can be isolated by computing empirical othrogonal functions (EOFs) of all model data simultaneously using the common EOF technique. A comparison of the variance each model associated with these common EOF patterns shows that the models generally agree on the most prominent patterns of variability. However, the amplitudes of the dominant modes of variability differ to some extent between the models and between the models and observations. For example, two of the models have a mode with relatively large values of the same sign over most of the Northern Hemisphere midlatitudes. This mode has been shown to be relevant for the separation of the temperature response pattern due to sulfate aerosol forcing from the response to greenhouse gas forcing. This indicates that the results of the detection of climate change and its attribution to different external forcings may differ when unperturbed climate variability in surface air temperature is estimated using different coupled models. Assuming that the simulation of variability of the global mean SAT is as realistic on longer timescales as it is for the shorter timescales, then the observed warming of more than 0.5 K of the SAT in the last 110 yrs. is not likely to be due to internally generated variability of the coupled atmosphere-ocean-sea ice system. Instead, the warming is likely to be due to changes in the radiative forcing of the climate system, such as the forcing associated with increases in greenhouse gases.
Vinnikov, K Y., A Robock, Ronald J Stouffer, J Walsh, C L Parkinson, D J Cavalieri, J F B Mitchell, D Garrett, and V F Zakharov, 2000: Technical comment on "Northern Hemisphere Sea Ice Extent". Science, 288, 927a.
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von Storch, J-S, P Müller, Ronald J Stouffer, R Voss, and S F B Tett, 2000: Variability of deep-ocean mass transport: spectral shapes and spatial scales. Journal of Climate, 13(11), 1916-1935.
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This paper studies the variability of deep-ocean mass transport using four 1000-yr integrations performed with coupled general circulation models. Statistics describing the spectral and spatial features are considered. It is shown that these features depend crucially on the time-mean state. For the transport of tropical and subtropical water masses in three of the integrations, the spectral levels continually increase with decreasing frequency and do not show isolated peaks at low frequencies. The slope of the low-frequency spectrum (in a log-log plot) changes with increasing depth. It has values of about 0 near the surface, about -1 at intermediate depth, and about -2 at or near the bottom. The result indicates that the maximal memory timescale for deep-ocean mass transport is longer than a few centuries. The situation is different in the fourth integration, which has a different mean circulation pattern. In this case, the low-frequency spectrum is more or less flat in the tropical and subtropical oceans below 2000-3000 m, indicating weak low-frequency variations. The dominant spatial covariance structures describe an anomalous recirculation of intermediate water masses, which is confined to a large extent to each ocean basin. The spatial scale of the dominant modes is therefore smaller than that of the time-mean circulation.
Covey, C, and Ronald J Stouffer, et al., 1999: The Seasonal Cycle in Coupled Ocean-Atmosphere General Circulation Models, PCMDI Report No. 51 - UCRL-JC-133438, Livermore, CA: University of California, Lawrence Livermore National Laboratory, 28 pp.
[ Abstract ]
We examine the seasonal cycle of near-surface air temperature simulated by 17 modern coupled ocean-atmosphere general circulation models. Nine of the models use ad hoc "flux adjustment" at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux adjusted models separately and examine the behavior of each class. Near-surface air temperatures averaged over all flux-adjusted models fall within 2°K of the observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ~6 K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ~5 K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January as a measure of seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ~30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ~5 K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle has been cited as demonstrating their ability to respond properly to external forcing, which supports their use for prediction of climatic changes such as anthropogenic greenhouse warming. To test this connection, we compare seasonal cycle amplitudes with equilibrium warmings under doubled atmospheric carbon dioxide for the models in our data base. A small positive correlation exists between these two quantities, as expected from a simple conceptual model of the climate system.
Delworth, Thomas L., Anthony J Broccoli, Keith W Dixon, Isaac Held, Thomas R Knutson, P J Kushner, Michael J Spelman, Ronald J Stouffer, K Y Vinnikov, and Richard T Wetherald, 1999: Coupled climate modelling at GFDL: Recent accomplishments and future plans. Clivar Exchanges, 4(4), 15-20.
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Dixon, Keith W., Thomas L Delworth, Michael J Spelman, and Ronald J Stouffer, 1999: The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM climate change experiment. Geophysical Research Letters, 26(17), 2749-2752.
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The mechanism by which the model-simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi-century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.
Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, and Ronald J Stouffer, 1999: Model assessment of regional surface temperature trends (1949-1997). Journal of Geophysical Research, 104(D24), 30,981-30,996.
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Analyses are conducted to assess whether simulated trends in SST and land surface air temperature from two versions of a coupled ocean-atmosphere model are consistent with the geographical distribution of observed trends over the period 1949-1997. The simulated trends are derived from model experiments with both constant and time-varying radiative forcing. The models analyed are low-resolution (R15, ~4º) and medium-resolution (R30, ~2º) versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Internal climate variability is estimated from long control integrations of the models with no change of external forcing. The radiatively forced trends are based on ensembles of integrations using estimated past concentrations of greenhouse gases and direct effects of anthropogenic sulfate aerosols (G+S). For the regional assessment, the observed trends at each grid point with adequate temporal coverage during 1949-1997 are first compared with the R15 and R30 model unforced internal variability. Nearly 50% of the analyzed areas have observed warming trends exceeding the 95th percentile of trends from the control simulations. These results suggest that regional warming trends over much of the globe during 1949-1997 are very unlikely to have occurred due to internal climate variability alone and suggest a role for a sustained positive thermal forcing such as increasing greenhouse gases. The observed trends are then compared with the trend distributions obtained by combining the ensemble mean G+S forced trends with the internal variability "trend" distributions from the control runs. Better agreement is found between the ensemble mean G+S trends and the observed trends than between the model internal variability alone and the observed trends. However, the G+S trends are still significantly different from the observed trends over about 30% of the areas analyzed. Reasons for these regional inconsistencies between the simulated and the observed trends include possible deficiencies in (1) specified radiative forcings, (2) simulated responses to specified radiative forcings, (3) simulation of internal climate variability, or (4) observed temperature records.
Manabe, Syukuro, and Ronald J Stouffer, 1999: Are two modes of thermohaline circulation stable? Tellus A, 51A(3), 400-411.
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The standard version of the coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory (GFDL) of NOAA has at least two stable equilibria. One has a realistic and active thermohaline circulation (THC) with sinking regions in the northern North Atlantic Ocean. The other has a reverse THC with extremely weak upwelling in the North Atlantic and sinking in the Circumpolar Ocean of the Southern Hemisphere. Although the model has the seasonal variation of insolation, the structure of these two stable equilibria are very similar to those of a previous GFDL model without the seasonal variation. It is noted that the inactive mode of the reverse THC mentioned above is not a stable equilibrium for another version of the same coupled model which has a large coefficient of vertical subgrid scale diffusion. Although the reverse THC cell was produced in the Atlantic Ocean by a massive discharge of freshwater, it began to transform back to the original direct THC as soon as the freshwater discharge was terminated. It appears that there is a critical value of diffusivity, above which two stable equilibria do not exist. Based upon paleoceanographic evidence, we suggest that the stable state of the reverse THC mentioned above did not prevail during the cold periods of Younger Dryas event which occurred during the last deglacial period. Instead, it is likely that the THC weakened temporarily, but reintensified before it reached the state of the reverse THC with no deep water formation in the North Atlantic Ocean.
Manabe, Syukuro, and Ronald J Stouffer, 1999: The role of thermohaline circulation in climate. Tellus, 51A-B(1), 91-109.
[ Abstract PDF ]
This article discusses the role of the THC in climate, based upon the results of several numerical experiments which use a coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of NOAA, USA. The first part of the article explores the mechanism which is responsible for the abrupt climate change such as the Younger Dryas event using the coupled model. In response to the freshwater discharge into high north Atlantic latitudes over a period of 500 years, the THC in the Atlantic Ocean weakens, reducing surface air temperature over the northern north Atlantic Ocean, the Scandinavian Peninsula, and the circumpolar ocean and Antarctic Continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces the multidecadal fluctuation in the intensity of the THC, which generates the almost abrupt change of climate. It is noted that similar but much weaker oscillation of the THC is also evident in the control integration of the coupled model without freshwater forcing. The irregular oscillation of the THC mentioned above appears to be related to the fluctuation of the Subarctic Gyre and associated east Greenland current, yielding the evolution of the surface salinity anomaly which resembles that of "great salinity anomaly". The second part of this article describes the response of a coupled ocean-atmosphere model to the doubling and quadrupling of atmospheric carbon dioxide over centuries time-scale. In one integration, the CO₂ concentration increases by 1%/year (compounded) until it reaches 4 x the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO₂ concentration also increases at the rate of 1%/year until it reaches 2 x the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO₂-quadrupling integration is the gradual disappearance of thermohaline circulation in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the north Atlantic by the 250 years of the integration and remains very weak until the 900th year. However, it begins to restore the original intensity by the 1600th year. In the CO₂-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years, but almost recovers its original intensity by the 500th year. The weakening of the THC moderates temporarily the greenhouse warming over the north Atlantic Ocean and its vicinity. In both numerical experiments described above, the initial weakening of the THC results from the capping of oceanic surface by relatively fresh, low-density water, which surpresses the convective cooling of water in the sinking region of the THC.
Stouffer, Ronald J., and Syukuro Manabe, 1999: Response of a coupled ocean-atmosphere model to increasing atmospheric carbon dioxide: Sensitivity to the rate of increase. Journal of Climate, 12(8), 2224-2237.
[ Abstract PDF ]
The influence of differing rates of increase of the atmospheric CO₂ concentration on the climatic response is investigated using a coupled ocean-atmosphere model. Five transient integrations are performed, each using a different constant exponential rate of CO₂ increase ranging from 4% yr^-1 to 0.25% yr^-1. By the time of CO₂ doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally more than 35% of the equilibrium response. The land-sea contrast in the warming, which is evident in the equilibrium results, is larger in all the transient experiments. The land-sea difference in the response increases with the rate of increase in atmospheric CO₂ concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO₂ concentration in all the transient integrations, confirming earlier work. The results also indicate that the slower the rate of increase, the larger the weakening of the THC by the time of doubling. Two of the transient experiments are continued beyond the time of CO₂ doubling with the CO₂ concentration maintained at that level. The amount of weakening of the THC after the CO₂ stops increasing is smaller in the experiment with the slower rate of CO₂ increase, indicating that the coupled system has more time to adjust to the forcing when the rate of CO₂ increase is slower. After a period of slow overturning, the THC gradually recovers and eventually regains the intensity found in the control integration, so that the equilibrium THC is very similar in the control and doubled CO₂ integrations. Considering only the sea level changes due to the thermal expansion of seawater, the integration with the slowest rate of increase in CO₂ concentration (i.e., 0.25% yr^-1 ) has the largest globally averaged sea level rise by the time of CO₂ doubling (about 42 cm). However, only a relatively small fraction of the equilibrium sea level rise of 1.9 m is realized by the time of doubling in all the transient integrations. This implies that sea level continues to rise long after the CO₂ concentration stops increasing, as the warm anomaly penetrates deeper into the ocean.
Vinnikov, K Y., and Ronald J Stouffer, et al., 1999: Global warming and Northern Hemisphere sea ice extent. Science, 286(5446), 1934-1937.
[ Abstract PDF ]
Surface and satellite-based observations show a decrease in Northern Hemisphere sea ice extent during the past 46 years. A comparison of these trends to control and transient integrations (forced by observed greenhouse gases and tropospheric sulfate aerosols) from the Geophysical Fluid Dynamics Laboratory and Hadley Centre climate models reveals that the observed decrease in Northern Hemisphere sea ice extent agrees with the transient simulations, and both trends are much larger than would be expected from natural climate variations. From long-term control runs of climate models, it was found that the probability of the observed trends resulting from natural climate variability, assuming that the models' natural variability is similar to that found in nature, is less than 2 percent for the 1978-98 sea ice trends and less than 0.1 percent for the 1953-98 sea ice trends. Both models used here project continued decreases in sea ice thickness and extent throughout the next century.
Sarmiento, Jorge L., T M C Hughes, Ronald J Stouffer, and Syukuro Manabe, 1998: Simulated response of the ocean carbon cycle to anthropogenic climate warming. Nature, 393(6682), 245-249.
[ Abstract PDF ]
A 1995 report of the Intergovernmental Panel on Climate Change provides a set of illustrative anthropogenic CO2 emission models leading to stabilization of atmospheric CO2 concentrations ranging from 350 to 1,000 p.p.m. (refs 1 - 4). Ocean carbon-cycle models used in calculating these scenarios assume that oceanic circulation and biology remain unchanged through time. Here we examine the importance of this assumption by using a coupled atmosphere-ocean model of global warming for the period 1765 to 2065. We find a large potential modification to the ocean carbon sink in a vast region of the Southern Ocean where increased rainfall leads to surface freshening and increased stratification. The increased stratification reduces the downward flux of carbon and the loss of heat to the atmosphere, both of which decrease the oceanic uptake of anthropogenic CO2 relative to a constant-climate control scenario. Changes in the formation, transport and cycling of biological material may counteract the reduced uptake, but the response of the biological community to the climate change is difficult to predict on present understanding. Our simulation suggests that such physical and biological changes might already be occurring, and that they could substantially affect the ocean carbon sink over the next few decades.
Stouffer, Ronald J., and Keith W Dixon, 1998: Initialization of coupled models for use in climate studies: A review In Research Activities in Atmospheric and Oceanic Modelling, WMO/TD No. 865, Geneva, Switzerland, World Meteorological Organization, I.1-I.8.
Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1997: Multidecadal climate variability in the Greenland Sea and surrounding regions: a coupled model simulation. Geophysical Research Letters, 24(3), 257-260.
[ Abstract PDF ]
Pronounced oscillations of ocean temperature and salinity occur in the Greenland Sea in a 2000 year integration of a coupled ocean-atmosphere model. The oscillations, involving both the surface and subsurface ocean layers, have a timescale of approximately 40-80 years, and are associated with fluctuations in the intensity of the East Greenland Current. The Greenland Sea temperature and salinity variations are preceded by large-scale changes in near-surface salinity in the Arctic, which appear to propagate out of the Arctic through the East Greenland Current. These anomalies then propagate around the subpolar gyre into the Labrador Sea and the central North Atlantic. These oscillations are coherent with previously identified multi-decadal fluctuations in the intensity of the North Atlantic thermohaline circulation. The oscillations in the Greenland Sea are related to atmospheric variability. Negative (cold) anomalies of surface air temperature are associated with negative (cold) sea surface temperature (SST) anomalies in the Greenland Sea, with amplitudes up to 2°C near Greenland declining to several tenths of a degree C over northwestern Europe. The cold SST anomalies and intensified East Greenland Current are also associated with enhanced northerly winds over the Greenland Sea.
Haywood, J M., Ronald J Stouffer, Richard T Wetherald, Syukuro Manabe, and V Ramaswamy, 1997: Transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations. Geophysical Research Letters, 24(11), 1335-1338.
[ Abstract PDF ]
This study investigates changes in surface air temperature (SAT), hydrology and the thermohaline circulation due to the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and thermohaline circulation are also briefly discussed.
Manabe, Syukuro, and Ronald J Stouffer, 1997: Climate variability of a coupled ocean-atmosphere-land surface model: Implication for the detection of global warming (Walter Orr Roberts Lecture). Bulletin of the American Meteorological Society, 78(6), 1177-1185.
[ Abstract PDF ]
This lecture evaluates the low-frequency variability of surface air temperature that was obtained from a 1000-yr integration of a coupled ocean-atmosphere-land surface model. The model simulates reasonably well the variability of local and global mean surface air temperature (SAT) at decadal timescales. The physical mechanisms responsible for this variability are explored. Based upon an analysis of the time series of the simulated global mean SAT, it is indicated that the warming trend of ~0.5 degrees C century-1 since the end of the last century was not generated internally through the interaction among the atmosphere, ocean, and land surface. Instead, it appears to have been induced by a sustained change in the thermal forcing such as that resulting from changes in atmospheric greenhouse gas concentration, solar irradiance, and aerosol loading.
Manabe, Syukuro, and Ronald J Stouffer, 1997: Coupled ocean-atmosphere model response to freshwater input: Comparison to Younger Dryas event. Paleoceanography, 12(2), 321-336.
[ Abstract PDF Supplemental ]
This study explores the responses of a coupled ocean-atmosphere model to the discharge of freshwater into the North Atlantic Ocean. In the first numerical experiment in which freshwater is discharged into high North Atlantic latitudes over a period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens, reducing surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. With the exception of the Pacific sector of the Circumpolar Ocean of the southern hemisphere, where the surface air temperature recovery is delayed, the climate of the northern North Atlantic and surrounding regions rapidly resumes its original distribution. The evolution of the ocean-atmosphere system described above resembles the Younger Dryas event as inferred from the comprehensive analysis of ice cores and deep-sea and lake sediments. In the second experiment, in which the same amount of freshwater is discharged into the subtropical North Atlantic again over a period of 500 years, the THC and climate evolve in a manner qualitatively similar to the first experiment. However, the magnitude of the THC response is 4-5 times smaller. It appears that freshwater is much less effective in weakening the THC if it were discharged outside high North Atlantic latitudes.
Meehl, G A., G J Boer, C Covey, M Latif, and Ronald J Stouffer, 1997: Intercomparison makes for a better climate model. EOS, 78(41), 445-446, 451.
Dixon, Keith W., J L Bullister, R H Gammon, and Ronald J Stouffer, 1996: Examining a coupled climate model using CFC-11 as an ocean tracer. Geophysical Research Letters, 23(15), 1957-1960.
[ Abstract PDF ]
Anthropogenic CFC-11 dissolved in seawater is used to analyze ocean ventilation simulated in a global coupled air-sea model. Modeled CFC-11 distributions are compared to observations gathered on three Southern Hemisphere research cruises. The total amount of CFC-11 absorbed by the model's Southern Ocean is realistic, though some notable differences in the vertical structure exist. Observed and simulated CFC-11 distributions are qualitatively consistent with the coupled model's predictions that the ocean may delay greenhouse gas-induced warming of surface air temperatures at high southern latitudes. The sensitivity of model-predicted CFC-11 levels in the deep Southern Ocean to the choice of gas exchange parameterization suggests that quantitative assessments of model performance based upon simulated CFC-11 distributions can be limited by air-sea gas flux uncertainties in areas of rapid ocean ventilation. Such sensitivities can complicate the quantitative aspects of CFC-11 comparisons between models and observations, and between different models.
Kattenberg, A, and Ronald J Stouffer, et al., 1996: Climate models - projections of future climate In Climate Change 1995: The Science of Climate Change, Cambridge, UK, Cambridge University Press, 289-357.
Manabe, Syukuro, and Ronald J Stouffer, 1996: Low-frequency variability of surface air temperature in a 1000-year integration of a coupled atmosphere-ocean-land surface model. Journal of Climate, 9(2), 376-393.
[ Abstract PDF ]
This study analyzes the variability of surface air temperature (SAT) and sea surface temperature (SST) obtained from a 1000-yr. integration of a coupled atmosphere-ocean-land surface model, which consists of general circulation models of the atmosphere and oceans and a heat and water budget model of land surface. It also explores the role of oceans in maintaining the variability of SAT by comparing the long-term integration of the coupled model with those of two simpler models. They are 1) a "mixed layer model," that is, the general circulation model of the atmosphere combined with a simple slab model of the mixed layer ocean, and 2) a "fixed SST model," that is, the same atmosphere model overlying seasonally varying, prescribed SST. With the exception of the tropical Pacific, both the coupled and mixed layer models are capable of approximately simulating the standard deviations of observed annual and 5-yr. mean anomalies of local SAT. The standard deviation tends to be larger over continents than over oceans, in agreement with the observations. Over most continental regions, the standard deviations of annual, 5-yr. and 25-yr. mean SATs in the fixed SST model are slightly less than but comparable to the corresponding standard deviations in the coupled model, suggesting that a major fraction of low-frequency local SAT variability over continents of the coupled model is generated in situ. Over the continents of both the coupled and the mixed layer models, the spectral density of local SAT is nearly independent of frequency. On the other hand, the spectral density of local SAT over most of the oceans of both models increases very gradually with decreasing frequency apparently influenced by the thermal inertia of mixed layer oceans. However, both SST and SAT spectra in the coupled model are substantially different from those in the mixed layer near the Denmark Strait and in some regions of the circumpolar ocean of the Southern Hemisphere where water mixes very deeply. In these regions, both SST and SAT are much more persistent in the coupled than in the mixed layer models, and their spectral densities are much larger at multi-decadal and/or centennial timescales. It appears significant that not only the coupled model but also the mixed layer model without ocean currents can approximately simulate the power spectrum of observed, global mean SAT at decadal to interdecadal time scales. However, neither model generates a sustained, long-term warming trend of significant magnitude such as that observed since the end of the last century.
Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1996: A search for human influences on the thermal structure of the atmosphere. Nature, 382(6586), 39-46.
[ Abstract PDF ]
The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability.
Stouffer, Ronald J., and Syukuro Manabe, 1996: The role of the oceans in the variability of surface air temperature as found in a 1000 year integration of a coupled atmosphere-ocean model In Proceedings of the Workshop on Dynamics and Statistics of Secular Climate Variations, Calverton, MD, Center for Ocean-Land-Atmosphere Studies, Report 26, 27-31.
Vinnikov, K Y., A Robock, Ronald J Stouffer, and Syukuro Manabe, 1996: Vertical patterns of free and forced climate variations. Geophysical Research Letters, 23(14), 1801-1804.
[ Abstract PDF ]
Observations of the vertical structure of atmospheric temperature changes over the past three decades show that while the global-average lower atmosphere has warmed, the upper troposphere and lower stratosphere have cooled. While these changes may be due to observed anthropogenic increases of greenhouse gases, decreases of lower stratospheric ozone, and increases of tropospheric aerosols, the changes may also have been caused by natural unforced internal fluctuations of the climate system. Here we use the results of a 1000-year simulation from a mathematical model of the coupled ocean- atmosphere-land system performed without any changes in external forcing, so that we may consider its variations as a surrogate for free, internally-generated, natural fluctuations of the climate system. When the global mean surface air temperature is warm in the model, the lower troposphere, upper troposphere and lower stratosphere are also warm over most of the Earth, in contrast to the observations of the last three decades and to model simulations of the forced climate response due to increased greenhouse gases. The observed temperature change of the past three decades is therefore unlikely to have been caused solely by natural internal variations of the climate system, thereby strengthening the argument that these changes can at least partly be attributed to anthropogenic activities.
Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1995: North Atlantic Interdecadal variability in a coupled model In Natural Climate Variability on Decade-to-Century Time Scales, Washington, DC, National Academy Press, 432-439; 440-441.
[ Abstract ]
A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 40 to 50 years. The fluctuations appear to be driven by density anomalies in the sinking region of the thermohaline circulation combined with much smaller density anomalies of opposite sign in the broad, rising region. Anomalies of sea surface temperature associated with this oscillation induce surface air temperature anomalies over the northern North Atlantic, the Arctic, and northwestern Europe. The spatial pattern of sea surface temperature anomalies bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic.
Manabe, Syukuro, and Ronald J Stouffer, 1995: Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean. Nature, 378, 165-167.
[ Abstract PDF ]
Temperature records from Greenland ice cores suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean-atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean-atmosphere system recorded in ice and deep-sea cores. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and it neighbourhood, but is relatively small in the rest of the world.
Manabe, Syukuro, Ronald J Stouffer, and Michael J Spelman, 1995: Interaction between polar climate and global warming In Fourth Conference on Polar Meteorology and Oceanography, Boston, MA, American Meteorological Society, J1-J9.
Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1995: A Search for Human Influences on the Thermal Structure of the Atmosphere, Program for Climate Model Diagnosis and Intercomparison, PCMDI Report No. 27, UCRL-ID-121956: Lawrence Livermore, CA, 26 pp.
[ Abstract ]
Recent studies have shown that patterns of near-surface temperature change due to combined forcing by CO and anthropogenic sulfate aerosols are easier to identify in the observations than signals due to changes in CO alone (Santer et al., 1995; Mitchell et al., 1995a). Here we extend this work to the vertical structure of atmospheric temperature changes, and additionally consider the possible effects of stratospheric ozone reduction. We compare modelled and observed patterns over the lower troposphere to the lower stratosphere (850 to 50 hPa) and over the low- to mid-troposphere (850 to 500 hPa). In both regions there are strong similarities between observed changes and model-predicted signals. Over 850 to 50 hPa similarities are evident both in CO-only signals and in signals that incorporate the added effects of sulfate aerosols and stratospheric ozone reduction. These similarities are due largely to a common pattern of stratospheric cooling and tropospheric warming in the observations and model experiments. Including the effects of stratospheric ozone reduction results in a more realistic height for the transition between stratospheric cooling and results in a more realistic height for the transition between stratospheric cooling and tropospheric warming. In the low- to mid-troposphere the observations are in better agreement with the temperature-change patterns due to combined forcing than with the CO-only pattern. This is the result of hemispheric-scale temperature-change contrasts that are common to the observations and the combined forcing signal but absent in the CO-only case. The levels of model-versus-observed pattern similarity in both atmospheric regions increase over the period 1963 to 1987. If model estimates of natural internal variability are realistic, it is likely that these trends in pattern similarity are partially due to human activities.
Karoly, D J., J A Cohen, G A Meehl, J F B Mitchell, Abraham H Oort, Ronald J Stouffer, and Richard T Wetherald, 1994: An example of fingerprint detection of greenhouse climate change. Climate Dynamics, 10, 97-105.
[ Abstract ]
As an example of the technique of fingerprint detection of greenhouse climate change, a multi-variate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO₂ concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period. These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El Niño-Southern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here.
Manabe, Syukuro, and Ronald J Stouffer, 1994: Multiple-century response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide. Journal of Climate, 7(1), 5-23.
[ Abstract PDF ]
To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the CO₂ concentration increases by 1% yr^-1 (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO₂ concentration also increases at the rate of 1% yr^-1 until it reaches twice the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO₂-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the first 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere. In the CO₂-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by thegradual increase in surface salinity that accompanies the intensification of the thermohaline circulation. During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future change.
Manabe, Syukuro, Ronald J Stouffer, and Michael J Spelman, 1994: Response of a coupled ocean-atmosphere model to increasing atmospheric carbon dioxide. Ambio, 23(1), 44-49.
[ Abstract PDF ]
This study investigates the response of a climate model to a 1% per year increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system, with a global computational domain, smoothed geography, and seasonal variation of insolation. The simulated increase of sea-surface temperature is very slow in the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. Extending this work, we investigated the transient responses of the coupled model to the doubling and quadrupling of atmospheric CO₂, over the period of several centuries. During the entire 500-yr. period of the experiment, the global mean surface air temperature increases almost 3.5°C when CO₂ is doubled, and 7°C when it is quadrupled. In the latter experiment, the thermal structure and dynamics of the model oceans undergo drastic changes, such as cessation of the thermohaline circulation in most of the model oceans, and substantial deepening of the thermocline, especially in the North Atlantic. These changes prevent the ventilation of the deeper layer of the oceans and, if they occurred in reality, could have a profound impact on the carbon cycle and biogeochemistry of the coupled ocean-atmosphere system.
Stouffer, Ronald J., Syukuro Manabe, and K Y Vinnikov, 1994: Model assessment of the role of natural variability in recent global warming. Nature, 367, 634-636.
[ Abstract ]
Since the late nineteenth century, the global mean surface air temperature has been increasing at the rate of about 0.5°C per century, but our poor understanding of low-frequency natural climate variability has made it very difficult to determine whether the observed warming trend is attributable to the enhanced greenhouse effect associated with increased atmospheric concentrations of greenhouse gases. Here we evaluate the observed warming trend using a 1,000-year time series of global temperature obtained from a mathematical model of the coupled ocean-atmosphere-land system. We find that the model approximately reproduces the magnitude of the annual to interdecadal variation in global mean surface air temperature. But throughout the simulated time series no temperature change as large as 0.5°C per century is sustained for more than a few decades. Assuming that the model is realistic, these results suggest that the observed trend is not a natural feature of the interaction between the atmosphere and oceans. Instead, it may have been induced by a sustained change in the thermal forcing, such as that resulting from changes in atmospheric greenhouse gas concentrations and aerosol loading.
Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1993: Interdecadal variations of the thermohaline circulation in a coupled ocean-atmosphere model. Journal of Climate, 6(11), 1993-2011.
[ Abstract PDF ]
A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of sea surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.
Gates, W L., U Cubasch, G A Meehl, J F B Mitchell, and Ronald J Stouffer, 1993: An Intercomparison of Selected Features of the Control Climates Simulated by Coupled Ocean-Atmosphere General Circulation Models, WCRP-82 WMO/TD No. 574, Geneva, Switzerland: World Meteorological Organization, 46 pp.
Manabe, Syukuro, and Ronald J Stouffer, 1993: Century-scale effects of increased atmospheric CO₂ on the ocean-atmosphere system. Nature, 364(6434), 215-218.
[ Abstract PDF ]
Several studies have addressed the likely effects of CO₂-induced climate change over the coming decades, but the longer-term effects have received less attention. Yet these effects could be very significant, as persistent increases in global mean temperatures may ultimately influence the large-scale processes in the coupled ocean-atmosphere system that are thought to play a central part in determining global climate. The thermohaline circulation is one such process-Broecker has argued that it may have undergone abrupt changes in response to rising temperatures and ice-sheet melting at the end of the last glacial period. Here we use a coupled ocean-atmosphere climate model to study the evolution of the world's climate over the next few centuries, driven by doubling and quadrupling of the concentration of atmospheric CO₂. We find that the global mean surface air temperature increases by about 3.5 and 7°C, respectively, over 500 years, and that sea-level rise owing to thermal expansion alone is about 1 and 2 m respectively (ice-sheet melting could make these values much larger). The thermal and dynamical structure of the oceans changes markedly in the quadrupled-CO₂ climate-in particular, the ocean settles into a new stable state in which the thermohaline circulation has ceased entirely and the thermocline deepens substantially. These changes prevent the ventilation of the deep ocean and could have a profound impact on the carbon cycle and biogeochemistry of the coupled system.
Manabe, Syukuro, and Ronald J Stouffer, 1993: Reply. Journal of Climate, 6(1), 178-179.
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Manabe, Syukuro, and Ronald J Stouffer, 1992: Multiple-century response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide. Journal of Climate, 7(1), 5-23.
[ Abstract PDF ]
To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the CO₂ concentration increases by 1% yr^-1 (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO₂ concentration also increases at the rate of 1% yr^-1 until it reaches twice the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO₂-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the first 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere. In the CO₂-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by the gradual increase in surface salinity that accompanies the intensification of the thermohaline circulation. During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future change.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Response of a coupled ocean-atmosphere-land surface model to a gradual increase of atmospheric carbon dioxide In The Global Role of Tropical Rainfall, Hampton, Virginia, Deepak Publishing, 93-103.
[ Abstract ]
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal variation of insolation. It is found that the simulated warming of sea surface temperature is very slow over the northern North Atlantic and the circumpolar ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies. The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model, but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basins is increased markedly. In qualitative agreement with the results of equilibrium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1992: Transient response of a coupled ocean-atmosphere-land surface model to increasing atmospheric carbon dioxide In Advances in Theoretical Hydrology: A Tribute to Jim Dooge, The Netherlands, Elsevier Science Publishers, 159-173.
[ Abstract ]
This study investigates the response of a climate model to a gradual increase of atmospheric carbon dioxide. The model is a general circulation model of the coupled ocean-atmosphere-land surface system with a global computational domain, smoothed geography, and seasonal veriation of insolation. It is found that the simulated increase of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where the vertical mixing of water penetrates very deeply and the rate of deep water formation is relatively fast. With the exception of these two regions identified above, the distribution of the change in surface temperature of the model is qualitatively similar to the equilibrium response of an atmospheric-mixed layer ocean model, which has been the subject of many previous studies. In most of the Northern Hemisphere, the seasonal dependence of surface air temperature change is also similar to the equilibrium response. For example, the temperature increase is at a maximum over the Arctic Ocean and its surroundings in the late fall and winter, whereas it is at a minimum in summer. However, the increase of surface air temperature and its seasonal variation is very small in the Circumpolar Ocean of the Southern Hemisphere and the northern North Atlantic. The increase of atmospheric carbon dioxide affects not only the thermal structure of the coupled model but also its hydrologic cycle. For example, the global mean rates of both precipitation and evaporation increase. The increase in evaporation rate is particularly large in low latitudes and decreases with increasing latitudes. On the other hand, the increase in the precipitation rate is substantial in high latitudes due to the increased penetration of warm, moisture-rich air into high latitudes. Thus, the rate of runoff in the subarctic basin increases markedly. In qualitative agreement with the results of equilibium response studies, soil moisture is reduced in summer over extensive regions of the middle and high latitudes, such as the North American Great Plains, Western Europe, Northern Canada, and Siberia.
Bryan, Kirk, and Ronald J Stouffer, 1991: A note on Bjerknes' hypothesis for North Atlantic variability. Journal of Marine Systems, 1, 229-241.
[ Abstract PDF ]
On decadal time-scales the historical surface temperature record over land in the Northern Hemisphere is dominated by polar amplified variations. These variations are coherent with SST anomalies concentrated in the Northwest Atlantic, but extending with lesser amplitude in the North Pacific as well. Bjerknes suggested that multi-year SST anomalies in the subpolar North Atlantic were due to irregular changes in the intensity of the thermohaline circulation. In support of the Bjerknes hypothesis there is evidence that winter overturning in the Labrador Sea was suppressed for a brief period from 1967-1969 by a cap of relative fresh water at the surface. Cause and effect are unclear, but this event was associated with a marked cooling of the entire Northern Hemisphere. The difference in SST averaged over the Northern Hemisphere oceans and SST averages over the Southern Hemisphere oceans from the equator to 40°S is coherent with Sahel summer rainfall on decadal time scales. Empirical evidence is supported by numerical experiments with the British Meteorological Office atmospheric climate model which simulate augmented monsoonal rainfall in the Sahel region of Africa in response to realistic warm SST anomalies in the Northwest Atlantic. A coupled ocean-atmosphere global model exhibits two equilibrium climate states. One has an active thermohaline circulation in the North Atlantic and the other does not. The two climate states provide an extreme example which illustrates the type of large scale air-sea interaction Bjerknes visualized as a mechanism for North Atlantic climate variability on decadal time-scales.
MacCracken, M, Syukuro Manabe, and Ronald J Stouffer, 1991: Working Group 2: A critical appraisal of model simulations In Greenhouse-Gas-Induced Climatic Change: A Critical Appraisal of Simulations and Observations, The Netherlands, Elsevier Science Publishers, 583-591.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1991: Transient responses of a coupled ocean-atmosphere-land surface model to gradual changes of atmospheric CO₂ In Global Change, Proceedings of the first Demetra meeting held at Chianciano Terme, Italy from 28 to 31 October 1991, Environment and Quality of Life, EUR 15158 EN, Directorate-General Science, Research and Development, European Commission, 82-93.
Manabe, Syukuro, Ronald J Stouffer, Michael J Spelman, and Kirk Bryan, 1991: Transient responses of a coupled ocean-atmosphere model to gradual changes of atmospheric CO₂. Part I: Annual mean response. Journal of Climate, 4(8), 785-818.
[ Abstract PDF ]
This study investigates the response of a climate model to a gradual increase or decrease of atmospheric carbon dioxide. The model is a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal variation of insolation. To offset the bias of the coupled model toward settling into an unrealistic state, the fluxes of heat and water at the ocean-atmosphere interface are adjusted by amounts that vary with season and geography but do not change from one year to the next. Starting from a quasi-equilibrium climate, three numerical time integrations of the coupled model are performed with gradually increasing, constant, and gradually decreasing concentrations of atmospheric carbon dioxide. It is noted that the simulated response of sea surface temperature is very slow over the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere where vertical mixing of water penetrates very deply. However, in most of the Northern Hemisphere and low latitudes of the Southern Hemisphere, the distribution of the change in surface air temperature of the model at the time of doubling (or halving) of atmospheric carbon dioxide resembles the equilibrium response of an atmospheric-mixed layer ocean model to CO₂ doubling (or halving). For example, the rise of annual mean surface air temperature in response to the gradual increase of atmospheric carbon dioxide increases with latitudes in the Northern Hemisphere and is larger over continents than oceans. When time-dependent response of the model oceans to the increase of atmospheric carbon dioxide is compared with the corresponding response to the CO₂ reduction at an identical rate, the penetration of the cold anomaly in the latter case is significantly deeper than that of the warm anomaly in the former case. The lack of symmetry in the penetration depth of a thermal anomaly between the two cases is associated with the difference in static stability, which is due mainly to the change in the vertical distribution of salinity in high latitudes and temperature changes in middle and low latitudes. Despite the difference in penetration depth and accordingly, the effective thermal inertia of the oceans between two experiments, the time-dependent response of the global mean surface air temperature in the CO₂ reduction experiment is similar in magnitude to the corresponding response in the CO₂ growth experiment. In the former experiment with a colder climate, snow and sea ice with high surface albedo cover a much larger area, thereby enhancing their positive feedback effect upon surface air temperature. On the other hand, surface cooling is reduced due to the larger effective thermal inertia of the oceans. Because of the compensation between these two effects, the magnitude of surface air temperature response turned out to be similar between the two experiments.
Stouffer, Ronald J., Syukuro Manabe, and Kirk Bryan, 1991: Climatic response to a gradual increase of atmospheric carbon dioxide In Greenhouse-Gas-Induced Climatic Change: A Critical Appraisal of Simulations and Observations, The Netherlands, Elsevier Science Publishers, 129-136.
[ Abstract ]
The transient response of a coupled ocean-atmosphere model to an increase of carbon dioxide has been the subject of several studies (Bryan et al., 1982; Spelman and Manabe, 1984; Bryan and Spelman, 1985; Schlesinger and Jiang, 1988; Schlesinger et al., 1985; Bryan et al., 1988; Manabe et al., 1990; Washington and Meehl, 1989). The models used in these studies explicitly incorporate the effect of heat transport by ocean currents and are different from the model used by Hansen et al. (1988). Here we evaluate the climatic influence of increasing atmospheric carbon dioxide using a coupled model recently developed at the NOAA Geophysical Fluid Dynamics Laboratory. The model response exhibits a marked and unexpected interhemispheric asymmetry. In the circumpolar ocean of the Southern Hemisphere, a region of deep vertical mixing, the increase of surface air temperature is very slow. In the Northern Hemisphere of the model, the rise of surface air temperature is faster and increases with latitude, with the exception of the northern North Atlantic, where it is relatively slow because of the weakening of the thermohaline circulation.
Stouffer, Ronald J., Syukuro Manabe, and Kirk Bryan, 1989: Interhemispheric asymmetry in climate response to a gradual increase of atmospheric CO₂. Nature, 342, 660-662.
[ Abstract PDF ]
The transient response of a coupled ocean-atmosphere model to an increase of atmospheric carbon dioxide has been the subject of several studies. The models used in these studies explicitly incorporate the effect of heat transport by ocean currents and are different from the model used by Hansen et al. Here we evaluate the climatic influence of increasing atmospheric carbon dioxide using a coupled model recently developed at the NOAA Geophysical Fluid Dynamics Laboratory. The model response exhibits a marked and unexpected interhemispheric asymmetry. In the circumpolar ocean of the Southern Hemisphere, a region of deep vertical mixing, the increase of surface air temperature is very slow. In the Northern Hemisphere of the model, the warming of surface air is faster and increases with latitude, with the exception of the northern North Atlantic, where it is relatively slow because of the weakening of the thermohaline circulation.
Manabe, Syukuro, and Ronald J Stouffer, 1988: Two stable equilibria of a coupled ocean-atmosphere model. Journal of Climate, 1(9), 841-866.
[ Abstract PDF ]
Two stable equilibria have been obtained from a global model of the coupled ocean-atmosphere system developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The model used for this study consists of general circulation models of the atmosphere and the world oceans and a simple model of land surface. Starting from two different initial conditions, "asynchronous" time integrations of the coupled model, under identical boundary conditions, lead to two stable equilibria. In one equilibrium, the North Atlantic Ocean has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there is no thermohaline circulation, and an intense halocline exists in the surface layer at high latitudes. In both integrations, the air-sea exchange of water is adjusted to remove a systematic bias of the model that supresses the thermohaline circulation in the North Atlantic. Nevertheless, these results raise the intriguing possibility that the coupled system may have at least two equilibria. They also suggest that the thermohaline overturning in the North Atlantic is mainly responsible for making the surface salinity of the northern North Atlantic higher than that of the northern North Pacific. Finally, a discussion is made on the paleoclimatic implications of these results for the large and abrupt transition between the Allerod and Younger Dryas events which occurred about 11,000 years ago.
Manabe, Syukuro, and Ronald J Stouffer, 1982: Seasonal and latitudinal variation of the CO2-induced change in a climate of an atmosphere-mixed-layer ocean model. In Carbon Dioxide Effects Research and Assessment, DOE/CONF-8106214, UC-11, Washington, DC, U.S. Dept. of Energy, 79-94.
Manabe, Syukuro, Richard T Wetherald, and Ronald J Stouffer, 1981: Summer dryness due to an increase of atmospheric CO₂ concentration. Climatic Change, 3, 347-386.
[ Abstract ]
To investigate the hydrologic changes of climate in response to an increase of CO₂-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat land surface. The second model has a global computational domain with realistic geography. The third model is identical to the second model except that it has a higher computational resolution. In each numerical experiment, the CO₂ -induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times the normal concentration of carbon dioxide in the air. It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and high latitudes in response to the increase of the CO₂ -concentration in air. This CO₂-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases.
Manabe, Syukuro, and Ronald J Stouffer, 1980: Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere. Journal of Geophysical Research, 85(C10), 5529-5554.
[ Abstract ]
This study investigates the response of a global model of the climate to the quadrupling of the CO₂ concentration in the atmosphere. The model consists of (1) a general circulation model of the atmosphere, (2) a heat and water balance model of the continents, and (3) a simple mixed layer model of the oceans. It has a global computational domain and realistic geography. For the computation of radiative transfer, the seasonal variation of insolation is imposed at the top of the model atmosphere, and the fixed distribution of cloud cover is prescribed as a function of latitude and of height. It is found that with some exceptions, the model succeeds in reproducing the large-scale characteristics of seasonal and geographical variation of the observed atmospheric temperature. The climatic effect of a CO₂ increase is determined by comparing statistical equilibrium states of the model atmosphere with a normal concentration and with a 4 times the normal concentration of CO₂ in the air. It is found that the warming of the model atmosphere resulting from the CO₂increase has significant seasonal and latitudinal variation. Because of the absence of an albedo feedback mechanism, the warming over the Antarctic continent is somewhat less than the warming in high latitudes of the northern hemisphere. Over the Arctic Ocean and its surroundings, the warming is much larger in winter than summer, thereby reducing the amplitude of seasonal temperature variation. It is concluded that this seasonal asymmetry in the warming results from the reduction in the coverage and thickness of the sea ice. The warming of the model atmosphere results in an enrichment of the moisture content in the air and an increase in the poleward moisture transport. The additional moisture is picked up from the tropical ocean and is brought to high latitudes where both precipitation and runoff increase throught the year. Further, the time of rapid snowmelt and maximum runoff becomes earlier.
Manabe, Syukuro, and Ronald J Stouffer, 1979: A CO₂-climate sensitivity study with a mathematical model of the global climate. Nature, 282(5738), 491-493.
[ Abstract PDF ]
An increase in the CO₂-content of the atmosphere resulting from man's activity could have a significant effect on the climate in the near future. We describe here some new results from a study of the response of a mathematical model of the climate to an increase in the CO₂-content of the air.

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Bibliography - Ronald J Stouffer