Bibliography - Thomas R Knutson

1. Garner, Stephen T., Isaac Held, Thomas R Knutson, and Joseph J Sirutis, in press: The roles of wind shear and thermodynamic stability in past and projected changes of Atlantic tropical-cyclone activity. Journal of Climate. 2/09.
   [ Abstract ]

   Atlantic tropical-cyclone activity has trended upward in recent decades. The increase coincides with favorable changes in local sea-surface temperature and other environmental indices, principally vertical shear and thermodynamic stability. The relative importance of these environmental factors has not been firmly established. A recent study using a high-resolution dynamical downscaling model has captured both the trend and interannual variations in Atlantic storm frequency with considerable fidelity. In the present work, this downscaling framework is used to assess the importance of the large-scale thermodynamic environment relative to other factors influencing Atlantic tropical storms. Separate assessments are done for the recent multi-decadal trend (1980 to 2006) and a model-projected global-warming environment for the late 21st century. For the multi-decadal trend, changes in the seasonal-mean thermodynamic environment (sea-surface temperature and atmospheric temperature profile at fixed relative humidity) account for most of the observed increase in tropical-cyclone frequency, with other seasonal-mean changes (including vertical shear) having a smaller combined effect. In contrast, the model’s projected reduction in Atlantic tropical-cyclone activity in the warm-climate scenario appears driven mostly by increased seasonal-mean vertical shear in the western Atlantic and Caribbean, rather than changes in the SST and thermodynamic profile.

2. Landsea, C, G A Vecchi, L Bengtsson, and Thomas R Knutson, in press: Increases in Atlantic tropical cyclone counts since the late 1800s are likely due to advances in observations and analyses. Journal of Climate. 2/09.
   [ Abstract ]

   Records of Atlantic basin tropical cyclones since the late-19th Century indicate a very large upward trend in storm frequency. This increase in documented cyclones has been previously interpreted as connected to alterations in climate conditions, in particular to those resulting from anthropogenic climate change. However, improvements in observing and recording practices provide an alternative interpretation for these changes, with recent studies suggesting that the number of “missed” tropical cyclones may have sufficient to explain a large part of the recorded increase in storm counts. Of note is that much of the recorded increase in cyclones has been in those that were short-lived and weak. This study, through analysis of numerical model output and observational data, explores the influence of tropical cyclone duration on modeled and observed changes in cyclone frequency. In a global climate modeling framework there is an extremely large sensitivity of tropical cyclone counts to the duration threshold required for a cyclone to be “counted”—much larger than the model’s response of Atlantic tropical cyclone frequency to 21st century greenhouse warming. In the observational record, the increase in cyclone frequency since the late 19th Century is due to an increase in very short-lived tropical cyclones, whose occurrence increased from about once per year in the late-19th/early-20th Century to about five per year since about 2000. Improvements in the quantity and quality of observations along with enhanced analytical techniques allow National Hurricane Center forecasters to better detect initial tropical cyclone formation (and thus incorporate very short-lived systems into the tropical cyclone database) with much more confidence than earlier eras. Finally, a sampling study based upon the distribution of ship observations provides quantitative estimates of the frequency of “missed” tropical cyclones once the very short-lived systems are removed. Upon adding these estimated numbers of missed tropical cyclones to the time series of moderate to long-lived systems, no significant trend in Atlantic tropical cyclones remains, though substantial multi-decadal variability is still present. Such lack of significant increasing trends in the Atlantic tropical cyclone frequency record is consistent with recent simulations of anthropogenic greenhouse warming influence on 21st century Atlantic tropical storm frequency.

3. Chen, C-T, and Thomas R Knutson, 2008: On the verification and comparison of extreme rainfall indices from climate models. Journal of Climate, 21(7), doi:10.1175/2007JCLI1494.1.
   [ Abstract ]

   The interpretation of model precipitation output (e.g., as a gridpoint estimate versus as an areal mean) has a large impact on the evaluation and comparison of simulated daily extreme rainfall indices from climate models. It is first argued that interpretation as a gridpoint estimate (i.e., corresponding to station data) is incorrect. The impacts of this interpretation versus the areal mean interpretation in the context of rainfall extremes are then illustrated. A high-resolution (0.25° × 0.25° grid) daily observed precipitation dataset for the United States [from Climate Prediction Center (CPC)] is used as idealized perfect model gridded data. Both 30-yr return levels of daily precipitation (P₃₀) and a simple daily intensity index are substantially reduced in these data when estimated at coarser resolution compared to the estimation at finer resolution. The reduction of P₃₀ averaged over the conterminous United States is about 9%, 15%, 28%, 33%, and 43% when the data were first interpolated to 0.5° × 0.5°, 1° × 1°, 2° × 2°, 3° × 3°, and 4° × 4° grid boxes, respectively, before the calculation of extremes. The differences resulting from the point estimate versus areal mean interpretation are sensitive to both the data grid size and to the particular extreme rainfall index analyzed. The differences are not as sensitive to the magnitude and regional distribution of the indices. Almost all Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models underestimate U.S. mean P₃₀ if it is compared directly with P₃₀ estimated from the high-resolution CPC daily rainfall observation. On the other hand, if CPC daily data are first interpolated to various model resolutions before calculating the P₃₀ (a more correct procedure in our view), about half of the models show good agreement with observations while most of the remaining models tend to overestimate the mean intensity of heavy rainfall events. A further implication of interpreting model precipitation output as an areal mean is that use of either simple multimodel ensemble averages of extreme rainfall or of intermodel variability measures of extreme rainfall to assess the common characteristics and range of uncertainties in current climate models is not appropriate if simulated extreme rainfall is analyzed at a model’s native resolution. Owing to the large sensitivity to the assumption used, the authors recommend that for analysis of precipitation extremes, investigators interpret model precipitation output as an area average as opposed to a point estimate and then ensure that various analysis steps remain consistent with that interpretation.

4. Knutson, Thomas R., and Robert E Tuleya, May 2008: Tropical cyclones and climate change: Revisiting recent studies at GFDL In Climate Extremes and Society, Diaz, H.F. and R.J. Murnane, Eds., New York, NY, Cambridge University Press, 120-144.
   [ Abstract ]

   In this chapter, we revisit two recent studies performed at the Geophysical Fluid Dynamics Laboratory (GFDL), with a focus on issues relevant to tropical cyclones and climate change. The first study was a model-based assessment of twentieth-century regional surface temperature trends. The tropical Atlantic Main Development Region (MDR) for hurricane activity was found to have warmed by several tenths of a degree Celsius over the twentieth century. Coupled model historical simulations using current best estimates of radiative forcing suggest that the century-scale warming trend in the MDR may contain a significant contribution from anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations. The results further suggest that the low-frequency variability in the MDR, apart from the trend, may contain substantial contributions from both radiative forcing (natural and anthropogenic) and internally generated climate variability. The second study used the GFDL huyrricane model, in an idealized setting, to simulate the impact of a pronounced CO2-induced warming on hurricane intensities and precipitation. A 1.75°C warming increases the intensities of hurricanes in the model by 5.8% in terms of surface wind speeds, 14% in terms of central pressure fall, or about one half category on the Saffir-Simpson Hurricane Scale. A revised storm-core accumulated (six-hour) rainfall measure shows a 21.6% increase under high CO2 conditions. Our simulated storm intensities are substantially less sensitive to sea surface temperature (SST) changes than recently reported historical observational trends are - a difference we are not able to completely reconcile at this time.

5. Knutson, Thomas R., Joseph J Sirutis, Stephen T Garner, G A Vecchi, and Isaac Held, 2008: Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions. Nature Geoscience, 1(6), 359-364.
   [ Abstract PDF ]

   Increasing sea surface temperatures in the tropical Atlantic Ocean and measures of Atlantic hurricane activity have been reported to be strongly correlated since at least 1950 (refs 1, 2, 3, 4, 5), raising concerns that future greenhouse-gas-induced warming6 could lead to pronounced increases in hurricane activity. Models that explicitly simulate hurricanes are needed to study the influence of warming ocean temperatures on Atlantic hurricane activity, complementing empirical approaches. Our regional climate model of the Atlantic basin reproduces the observed rise in hurricane counts between 1980 and 2006, along with much of the interannual variability, when forced with observed sea surface temperatures and atmospheric conditions7. Here we assess, in our model system7, the changes in large-scale climate that are projected to occur by the end of the twenty-first century by an ensemble of global climate models8, and find that Atlantic hurricane and tropical storm frequencies are reduced. At the same time, near-storm rainfall rates increase substantially. Our results do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations.

6. Knutson, Thomas R., C Landsea, and K A Emanuel, in press: Tropical cyclones and climate change: A review. In Global Perspectives on Tropical Cyclones: From Science to Mitigation, Singapore, World Scientific Publishing Company. 7/08.
   [ Abstract ]

   A review of the science on the relationship between climate change and tropical cyclones (TCs) is presented. Topics include changes in aspects of tropical climate that are relevant to TC activity; observed trends and low-frequency variability of TC activity; paleoclimate proxy studies; theoretical and modeling studies; future projections; roadblocks to resolution of key issues; and recommendations for making future progress.

7. Vecchi, G A., and Thomas R Knutson, January 2008: On estimates of historical North Atlantic tropical cyclone activity. Journal of Climate, 21(14), 3580-3600.
   [ Abstract PDF ]

   In this study, an estimate of the expected number of Atlantic tropical cyclones (TCs) that were missed by the observing system in the presatellite era (between 1878 and 1965) is developed. The significance of trends in both number and duration since 1878 is assessed and these results are related to estimated changes in sea surface temperature (SST) over the “main development region” (“MDR”). The sensitivity of the estimate of missed TCs to underlying assumptions is examined. According to the base case adjustment used in this study, the annual number of TCs has exhibited multidecadal variability that has strongly covaried with multidecadal variations in MDR SST, as has been noted previously. However, the linear trend in TC counts (1878–2006) is notably smaller than the linear trend in MDR SST, when both time series are normalized to have the same variance in their 5-yr running mean series. Using the base case adjustment for missed TCs leads to an 1878–2006 trend in the number of TCs that is weakly positive, though not statistically significant, with p ~ 0.2. The estimated trend for 1900–2006 is highly significant (+~ 4.2 storms century⁻¹) according to the results of this study. The 1900–2006 trend is strongly influenced by a minimum in 1910–30, perhaps artificially enhancing significance, whereas the 1878–2006 trend depends critically on high values in the late 1800s, where uncertainties are larger than during the 1900s. The trend in average TC duration (1878–2006) is negative and highly significant. Thus, the evidence for a significant increase in Atlantic storm activity over the most recent 125 yr is mixed, even though MDR SST has warmed significantly. The decreasing duration result is unexpected and merits additional exploration; duration statistics are more uncertain than those of storm counts. As TC formation, development, and track depend on a number of environmental factors, of which regional SST is only one, much work remains to be done to clarify the relationship between anthropogenic climate warming, the large-scale tropical environment, and Atlantic TC activity.

8. Donner, S D., Thomas R Knutson, and M Oppenheimer, March 2007: Model-based assessment of the role of human-induced climate change in the 2005 Caribbean coral bleaching event. Proceedings of the National Academy of Sciences, 104(13), doi:10.1073/pnas.0610122104.
   [ Abstract ]

   Episodes of mass coral bleaching around the world in recent decades have been attributed to periods of anomalously warm ocean temperatures. In 2005, the sea surface temperature (SST) anomaly in the tropical North Atlantic that may have contributed to the strong hurricane season caused widespread coral bleaching in the Eastern Caribbean. Here, we use two global climate models   to evaluate the contribution of natural climate variability and anthropogenic forcing to the thermal stress that caused the 2005 coral bleaching event. Historical temperature data and simulations for the 1870–2000 period show that the observed warming in the region is unlikely to be due to unforced climate variability alone. Simulation of background climate variability suggests that anthropogenic warming may have increased the probability of occurrence of significant thermal stress events for corals in this region by an order of magnitude. Under scenarios of future greenhouse gas emissions, mass coral bleaching in the Eastern Caribbean may become a biannual event in 20–30 years. However, if corals and their symbionts can adapt by 1–1.5°C, such mass bleaching events may not begin to recur at potentially harmful intervals until the latter half of the century. The delay could enable more time to alter the path of greenhouse gas emissions, although long-term "committed warming" even after stabilization of atmospheric CO₂ levels may still represent an additional long-term threat to corals.

9. Frappier, A, Thomas R Knutson, K-B Liu, and K A Emanuel, 2007: Perspective: coordinating paleoclimate research on tropical cyclones with hurricane-climate theory and modelling. Tellus A, 59(4), 529-537.
   [ Abstract PDF ]

   Extending the meteorological record back in time can offer critical data for assessing tropical cyclone-climate links. While paleotempestology, the study of ancient storms, can provide a more realistic view of past ‘worst case scenarios’, future environmental conditions may have no analogues in the paleoclimate record. The primary value in paleotempestology proxy records arises from their ability to quantify climate–tropical cyclone interactions by sampling tropical cyclone activity during pre-historic periods with a wider range of different climates. New paleotempestology proxies are just beginning to be applied, encouraging new collaboration between the paleo and tropical cyclone dynamics communities. The aim of this paper is to point out some paths toward closer coordination by outlining target needs of the tropical cyclone theory and modelling community and potential contributions of the paleotempestology community. We review recent advances in paleotempestology, summarize the range of types and quality of paleodata generation, and identify future research opportunities for paleotempestology, tropical cyclone dynamics and climate change impacts and attribution communities.

10. Knutson, Thomas R., Joseph J Sirutis, Stephen T Garner, Isaac Held, and Robert E Tuleya, 2007: Simulation of the Recent Multidecadal Increase of Atlantic Hurricane Activity Using an 18-km-Grid Regional Model. Bulletin of the American Meteorological Society, 88(10), doi:10.1175/BAMS-88-10-1549.
    [ Abstract ]

    In this study, a new modeling framework for simulating Atlantic hurricane activity is introduced. The model is an 18-km-grid nonhydrostatic regional model, run over observed specified SSTs and nudged toward observed time-varying large-scale atmospheric conditions (Atlantic domain wavenumbers 0–2) derived from the National Centers for Environmental Prediction (NCEP) reanalyses. Using this “perfect large-scale model” approach for 27 recent August–October seasons (1980–2006), it is found that the model successfully reproduces the observed multidecadal increase in numbers of Atlantic hurricanes and several other tropical cyclone (TC) indices over this period. The correlation of simulated versus observed hurricane activity by year varies from 0.87 for basin-wide hurricane counts to 0.41 for U.S. landfalling hurricanes. For tropical storm count, accumulated cyclone energy, and TC power dissipation indices the correlation is 0.75, for major hurricanes the correlation is 0.69, and for U.S. landfalling tropical storms, the correlation is 0.57. The model occasionally simulates hurricanes intensities of up to category 4 (942 mb) in terms of central pressure, although the surface winds (< 47 m s^-1 ) do not exceed category-2 intensity. On interannual time scales, the model reproduces the observed ENSO-Atlantic hurricane covariation reasonably well. Some notable aspects of the highly contrasting 2005 and 2006 seasons are well reproduced, although the simulated activity during the 2006 core season was excessive. The authors conclude that the model appears to be a useful tool for exploring mechanisms of hurricane variability in the Atlantic (e.g., shear versus potential intensity contributions). The model may be capable of making useful simulations/projections of pre-1980 or twentieth-century Atlantic hurricane activity. However, the reliability of these projections will depend on obtaining reliable large-scale atmospheric and SST conditions from sources external to the model.

11. Shepherd, J M., and Thomas R Knutson, 2007: The current debate on the linkage between global warming and hurricanes. Geography Compass, 1(1), doi:10.1111/j.1749-8198.2006.00002.
    [ Abstract ]

    Following Hurricane Katrina and the parade of storms that affected the conterminous United States in 2004–2005, the apparent recent increase in intense hurricane activity in the Atlantic basin, and the reported increases in recent decades in some hurricane intensity and duration measures in several basins have received considerable attention. An important ongoing avenue of investigation in the climate and meteorology research communities is to determine the relative roles of anthropogenic forcing (i.e., global warming) and natural variability in producing the observed recent increases in hurricane frequency in the Atlantic, as well as the reported increases of tropical cyclone activity measures in several other ocean basins. A survey of the existing literature shows that many types of data have been used to describe hurricane intensity, and not all records are of sufficient length to reliably identify historical trends. Additionally, there are concerns among researchers about possible effects of data inhomogeneities on the reported trends. Much of the current debate has focused on the relative roles of sea-surface temperatures or large-scale potential intensity versus the role of other environmental factors such as vertical wind shear in causing observed changes in hurricane statistics. Significantly more research – from observations, theory, and modeling – is needed to resolve the current debate around global warming and hurricanes.

12. 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

13. Findell, Kirsten L., Thomas R Knutson, and P C D Milly, 2006: Weak simulated extratropical responses to complete tropical deforestation. Journal of Climate, 19(12), 2835-2850.
    [ Abstract PDF ]

    The Geophysical Fluid Dynamics Laboratory atmosphere–land model version 2 (AM2/LM2) coupled to a 50-m-thick slab ocean model has been used to investigate remote responses to tropical deforestation. Magnitudes and significance of differences between a control run and a deforested run are assessed through comparisons of 50-yr time series, accounting for autocorrelation and field significance. Complete conversion of the broadleaf evergreen forests of South America, central Africa, and the islands of Oceania to grasslands leads to highly significant local responses. In addition, a broad but mild warming is seen throughout the tropical troposphere (<0.2°C between 700 and 150 mb), significant in northern spring and summer. However, the simulation results show very little statistically significant response beyond the Tropics. There are no significant differences in any hydroclimatic variables (e.g., precipitation, soil moisture, evaporation) in either the northern or the southern extratropics. Small but statistically significant local differences in some geopotential height and wind fields are present in the southeastern Pacific Ocean. Use of the same statistical tests on two 50-yr segments of the control run show that the small but significant extratropical differences between the deforested run and the control run are similar in magnitude and area to the differences between nonoverlapping segments of the control run. These simulations suggest that extratropical responses to complete tropical deforestation are unlikely to be distinguishable from natural climate variability.

14. 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.

15. 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).

16. Webb, M J., C A Senior, D M H Sexton, W J Ingram, K D Williams, M A Ringer, B McAveney, R Colman, Brian J Soden, Rich Gudgel, Thomas R Knutson, S Emori, T Ogura, V Tsushima, N Andronova, B Li, I Musat, S Bony, and K E Taylor, 2006: On the contribution of local feedback mechanisms to the range of climate sensitivity in two GCM ensembles. Climate Dynamics, 27(1), doi:10.1007/s00382-006-0111-2.
    [ Abstract ]

    Global and local feedback analysis techniques have been applied to two ensembles of mixed layer equilibrium CO2 doubling climate change experiments, from the CFMIP (Cloud Feedback Model Intercomparison Project) and QUMP (Quantifying Uncertainty in Model Predictions) projects. Neither of these new ensembles shows evidence of a statistically significant change in the ensemble mean or variance in global mean climate sensitivity when compared with the results from the mixed layer models quoted in the Third Assessment Report of the IPCC. Global mean feedback analysis of these two ensembles confirms the large contribution made by inter-model differences in cloud feedbacks to those in climate sensitivity in earlier studies; net cloud feedbacks are responsible for 66% of the inter-model variance in the total feedback in the CFMIP ensemble and 85% in the QUMP ensemble. The ensemble mean global feedback components are all statistically indistinguishable between the two ensembles, except for the clear-sky shortwave feedback which is stronger in the CFMIP ensemble. While ensemble variances of the shortwave cloud feedback and both clear-sky feedback terms are larger in CFMIP, there is considerable overlap in the cloud feedback ranges; QUMP spans 80% or more of the CFMIP ranges in longwave and shortwave cloud feedback. We introduce a local cloud feedback classification system which distinguishes different types of cloud feedbacks on the basis of the relative strengths of their longwave and shortwave components, and interpret these in terms of responses of different cloud types diagnosed by the International Satellite Cloud Climatology Project simulator. In the CFMIP ensemble, areas where low-top cloud changes constitute the largest cloud response are responsible for 59% of the contribution from cloud feedback to the variance in the total feedback. A similar figure is found for the QUMP ensemble. Areas of positive low cloud feedback (associated with reductions in low level cloud amount) contribute most to this figure in the CFMIP ensemble, while areas of negative cloud feedback (associated with increases in low level cloud amount and optical thickness) contribute most in QUMP. Classes associated with high-top cloud feedbacks are responsible for 33 and 20% of the cloud feedback contribution in CFMIP and QUMP, respectively, while classes where no particular cloud type stands out are responsible for 8 and 21%.

17. Williams, K D., M A Ringer, C A Senior, M J Webb, B McAveney, N Andronova, S Bony, J-L Dufresne, S Emori, Rich Gudgel, Thomas R Knutson, B Li, K Lo, I Musat, J Wegner, A Slingo, and J F B Mitchell, 2006: Evaluation of a component of the cloud response to climate change in an intercomparison of climate models. Climate Dynamics, 26(2-3), doi:10.1007/s00382-005-0067-7.
    [ Abstract ]

    Most of the uncertainty in the climate sensitivity of contemporary general circulation models (GCMs) is believed to be connected with differences in the simulated radiative feedback from clouds. Traditional methods of evaluating clouds in GCMs compare time–mean geographical cloud fields or aspects of present-day cloud variability, with observational data. In both cases a hypothetical assumption is made that the quantity evaluated is relevant for the mean climate change response. Nine GCMs (atmosphere models coupled to mixed-layer ocean models) from the CFMIP and CMIP model comparison projects are used in this study to demonstrate a common relationship between the mean cloud response to climate change and present-day variability. Although atmosphere–mixed-layer ocean models are used here, the results are found to be equally applicable to transient coupled model simulations. When changes in cloud radiative forcing (CRF) are composited by changes in vertical velocity and saturated lower tropospheric stability, a component of the local mean climate change response can be related to present-day variability in all of the GCMs. This suggests that the relationship is not model specific and might be relevant in the real world. In this case, evaluation within the proposed compositing framework is a direct evaluation of a component of the cloud response to climate change. None of the models studied are found to be clearly superior or deficient when evaluated, but a couple appear to perform well on several relevant metrics. Whilst some broad similarities can be identified between the 60°N–60°S mean change in CRF to increased CO2 and that predicted from present-day variability, the two cannot be quantitatively constrained based on changes in vertical velocity and stability alone. Hence other processes also contribute to the global mean cloud response to climate change.

18. Held, Isaac, Thomas L Delworth, Jian Lu, Kirsten L Findell, and Thomas R Knutson, 2005: Simulation of Sahel drought in the 20th and 21st centuries. Proceedings of the National Academy of Sciences, 102(50), doi:10.1073/pnas.0509057102.
    [ Abstract ]

    The Sahel, the transition zone between the Saharan desert and the rainforests of Central Africa and the Guinean Coast, experienced a severe drying trend from the 1950s to the 1980s, from which there has been partial recovery. Continuation of either the drying trend or the more recent ameliorating trend would have far-ranging implications for the economy and ecology of the region. Coupled atmosphere/ocean climate models being used to simulate the future climate have had difficulty simulating Sahel rainfall variations comparable to those observed, thus calling into question their ability to predict future climate change in this region. We describe simulations using a new global climate model that capture several aspects of the 20th century rainfall record in the Sahel. An ensemble mean over eight realizations shows a drying trend in the second half of the century of nearly half of the observed amplitude. Individual realizations can be found that display striking similarity to the observed time series and drying pattern, consistent with the hypothesis that the observations are a superposition of an externally forced trend and internal variability. The drying trend in the ensemble mean of the model simulations is attributable to anthropogenic forcing, partly to an increase in aerosol loading and partly to an increase in greenhouse gases. The model projects a drier Sahel in the future, due primarily to increasing greenhouse gases.

19. Knutson, Thomas R., and Robert E Tuleya, 2005: Reply. Journal of Climate, 18(23), doi:10.1175/JCLI3593.1.
    [ Abstract ]

    A response is made to the comments of Michaels et al. concerning a recent study by the authors. Even after considering Michaels et al.'s comments, the authors stand behind the conclusions of the original study. In contrast to Michaels et al., who exclusively emphasize uncertainties that lead to smaller future changes, uncertainties are noted that could lead to either smaller or larger changes in future intensities of hurricanes than those summarized in the original study, with accompanying smaller or larger societal impacts.

20. Knutson, Thomas R., and Robert E Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17(18), 3477-3495.
    [ Abstract PDF ]

    Previous studies have found that idealized hurricanes, simulated under warmer, high-CO2 conditions, are more intense and have higher precipitation rates than under present-day conditions. The present study explores the sensitivity of this result to the choice of climate model used to define the CO2-warmed environment and to the choice of convective parameterization used in the nested regional model that simulates the hurricanes. Approximately 1300 five-day idealized simulations are performed using a higher-resolution version of the GFDL hurricane prediction system (grid spacing as fine as 9 km, with 42 levels). All storms were embedded in a uniform 5 m s−1 easterly background flow. The large-scale thermodynamic boundary conditions for the experiments— atmospheric temperature and moisture profiles and SSTs—are derived from nine different Coupled Model Intercomparison Project (CMIP2+) climate models. The CO2-induced SST changes from the global climate models, based on 80-yr linear trends from +1% yr−1 CO2 increase experiments, range from about +0.8° to +2.4°C in the three tropical storm basins studied. Four different moist convection parameterizations are tested in the hurricane model, including the use of no convective parameterization in the highest resolution inner grid. Nearly all combinations of climate model boundary conditions and hurricane model convection schemes show a CO2-induced increase in both storm intensity and near-storm precipitation rates. The aggregate results, averaged across all experiments, indicate a 14% increase in central pressure fall, a 6% increase in maximum surface wind speed, and an 18% increase in average precipitation rate within 100 km of the storm center. The fractional change in precipitation is more sensitive to the choice of convective parameterization than is the fractional change of intensity. Current hurricane potential intensity theories, applied to the climate model environments, yield an average increase of intensity (pressure fall) of 8% (Emanuel) to 16% (Holland) for the high-CO2 environments. Convective available potential energy (CAPE) is 21% higher on average in the high-CO2 environments. One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas–induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.

21. 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.

22. 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.

23. Davey, M K., M Huddleston, K R Sperber, P Braconnot, F O Bryan, D Chen, R Colman, C Cooper, U Cubasch, P Delecluse, D G DeWitt, L Fairhead, G M Flato, C Tony Gordon, T Hogan, M Ji, M Kimoto, A Kitoh, Thomas R Knutson, M Latif, H Le Treut, T Li, Syukuro Manabe, C R Mechoso, G A Meehl, S B Power, E Roeckner, L Terray, A Vintzileos, R Voss, B Wang, W M Washington, I Yoshikawa, J-Y Yu, S Yukimoto, and S E Zebiak, 2002: A study of coupled model climatology and variability in tropical ocean regions. Climate Dynamics, 18(5), 403-420.
    [ Abstract PDF ]

    We describe the behavior of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose; nevertheless several comon features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind sress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with interannual Niño 3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress.

24. 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.
    [ Abstract PDF ]

    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.

25. Tuleya, Robert E., and Thomas R Knutson, 2002: Impact of climate change on tropical cyclones In Atmosphere-Ocean Interactions, Vol. 1, Southampton, UK, WIT Press, 293-312.
    [ Abstract ]

    One of the possible impacts of global warming is on tropical cyclones, on their formation, track, intensity and decay rates. One of the consequences of global warming appears to be not only an increase in sea surface temperature, but more importantly a potential increase in the overall energy flux at the tropical ocean surface. Theoretical considerations imply that this increased surface disequilibrium may lead to more intense tropical storms. Three-dimensional numerical modeling is another approach to evaluating these potential consequences. Since global models are still rather limited in simulating mesoscale storm structure, this paper describes a regional modeling approach utilizing a multiple nested technique which has already been shown to be practical in operational forecasts. These 3-D model results confirm theoretical methods that indicate an increase of 3 to 10% in maximum wind speeds for a CO2 tropical SST warming of ~2.5°C. Perhaps more importantly, model results indicate a 20 to 30% increase in hurricane-related precipitation. Furthermore, the resulting increases in intensity and precipitation appear to be qualitatively insensitive to changes in convective parameterization. This paper emphasizes the impact of global warming on storm intensity and precipitation. The question of the possible impact on tropical storm frequency and track is still problematic.

26. 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.
27. Knutson, Thomas R., Robert E Tuleya, W Shen, and I Ginis, 2001: Impact of CO₂-induced warming on hurricane intensities simulated in a hurricane model with ocean coupling. Journal of Climate, 14(11), 2458-2468.
    [ Abstract PDF ]

    This study explores how a carbon dioxide (CO₂) warming-induced enhancement of hurricane intensity could be altered by the inclusion of hurricane-ocean coupling. Simulations are performed using a coupled version of the Geophysical Fluid Dynamics Laboratory hurricane prediction system in an idealized setting with highly simplified background flow fields. The large-scale atmospheric boundary conditions for these high-resolution experiments (atmospheric temperature and moisture profiles and moisture profiles and SSTs) are derived from control and high-CO₂ climatologies obtained from a low-resolution (R30) global coupled ocean-atmosphere climate model. The high-CO₂ conditions are obtained from years 71-120 of a transient +1% yr ^-1 CO₂-increase experiment with the global model. The CO₂-induced SST changes from the global climate model range from +2.2° to +2.7°C in the six tropical storm basins studied. In the storm simulations, ocean coupling significantly reduces the intensity of simulated tropical cyclones, in accord with previous studies. However, the net impact of ocean coupling on the simulated CO₂ warming-induced intensification of tropical cyclones is relatively minor. For both coupled and uncoupled simulations, the percentage increase in maximum surface wind speeds averages about 5%-6% over the six basins and varies from about 3% to 10% across the different basins. Both coupled and uncoupled simulations also show strong increases of near-storm precipitation under high-CO₂ climate conditions, relative to control (present day) conditions.

28. Latif, M, Thomas R Knutson, and Syukuro Manabe, et al., 2001: ENSIP: The El Niño simulation intercomparison project. Climate Dynamics, 18(3-4), 255-276.
    [ Abstract PDF ]

    Abstract An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.

29. 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.
    [ Abstract PDF ]

    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.

30. Barnett, T P., G Hegerl, Thomas R Knutson, and S F B Tett, 2000: Uncertainty levels in predicted patterns of anthropogenic climate change. Journal of Geophysical Research, 105(D12), 15525-15542.
    [ Abstract PDF ]

    This paper investigates the uncertainties in different model estimates of an expected anthropogenic signal in the near-surface air temperature field. We first consider nine coupled global climate models (CGCMs) forced by CO2 increasing at the rate of 1%/yr. Averaged over years 71-80 of their integrations, the approximate time of CO2 doubling, the models produce a global mean temperature change that agrees to within about 25% of the nine model average. However, the spatial patterns of change can be rather different. This is likely to be due to different representations of various physical processes in the respective models, especially those associated with land and sea ice processes. We next analyzed 11 different runs from three different CGCMs, each forced by observed/projected greenhouse gases (GHG) and estimated direct sulfate aerosol effects. Concentrating on the patterns of trend of near-surface air temperature change over the period 1945–1995, we found that the raw individual model simulations often bore little resemblance to each other or to the observations. This was due partially to large magnitude, small-scale spatial noise that characterized all the model runs, a feature resulting mainly from internal model variability. Heavy spatial smoothing and ensemble averaging improved the intermodel agreement. The existence of substantial differences between different realizations of an ensemble produced by identical forcing almost requires that detection and attribution work be done with ensembles of scenario runs, as single runs can be misleading. Application of recent detection and attribution methods, coupled with ensemble averaging, produced a reasonably consistent match between model predictions of expected patterns of temperature trends due to a combination of GHG and direct sulfate aerosols and those observed. This statement is provisional since the runs studied here did not include other anthropogenic pollutants thought to be important (e.g., indirect sulfate aerosol effects, tropospheric ozone) nor do they include natural forcing mechanisms (volcanoes, solar variability). Our results demonstrate the need to use different estimates of the anthropogenic fingerprint in detection studies. Different models give different estimates of these fingerprints, and we do not currently know which is most correct. Further, the intramodel uncertainty in both the fingerprints and, particularly, the scenario runs can be relatively large. In short, simulation, detection, and attribution of an anthropogenic signal is a job requiring multiple inputs from a diverse set of climate models.

31. Delworth, Thomas L., and Thomas R Knutson, 2000: Simulation of early 20th Century global warming. Science, 287(5461), 2246-2250.
    [ Abstract PDF ]

    The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model's climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.

32. 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.
    [ PDF ]
33. Delworth, Thomas L., Jerry D Mahlman, and Thomas R Knutson, 1999: Changes in heat index associated with CO₂ -induced global warming. Climatic Change, 43(2), 369-386.
    [ Abstract PDF ]

    Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO₂), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765-2065 is used to force the model, and a third (fourth) integration in which atmospheric CO₂ increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.

34. 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.
    [ Abstract PDF ]

    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.

35. Knutson, Thomas R., and Robert E Tuleya, 1999: Increased hurricane intensities with CO₂ -induced global warming as simulated using the GFDL hurricane prediction system. Climate Dynamics, 15, 503-519.
    [ Abstract PDF ]

    The impact of CO₂ -induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO₂ increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO₂ conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO₂ storms, with SSTs warmer by about 2.2° C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3-7 m/s (5%-11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO₂ sample. In terms of storm tracks, the high CO₂ sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO₂ storm than for the control,and the high CO₂ storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.

36. Knutson, Thomas R., and Syukuro Manabe, 1998: Model assessment of decadal variability and trends in the Tropical Pacific Ocean. Journal of Climate, 11(9), 2273-2296.
    [ Abstract PDF ]

    In this report, global coupled ocean-atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in Pacific Ocean SSTs over the past century. The leading mode of internally generated decadal (>7 yr) variability in the model resembles the observed decadal variability in terms of pattern and amplitude. In the model, the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and ENSO timescales, suggesting that the decadal variability has a similar "delayed oscillator" mechanism to that on the ENSO timescale. The westward phase propagation of the heat content anomalies, however, is slower and centered slightly farther from the equator (~12° vs 9° N) for the decadal variability. Cool SST anomalies in the midlatitude North Pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies. An index of observed SST over a broad triangular region of the tropical and subtropical Pacific indicates a warming rate of +0.41°C (100 yr)-1 since 1900, +1.2°C (100 yr)-1 since 1949, and +2.9°C (100 yr)-1 since 1971. All three warming trends are highly unusual in terms of their duration, with occurrence rates of less than 0.5% in a 2000-yr simulation of internal climate variability using a low-resolution model. The most unusual is the trend since 1900 (96-yr duration); the longest simulated duration of a trend of this magnitude is 85 yr. This suggests that the observed trends are not entirely attributable to natural (internal) variability alone, although natural variability could potentially account for much of the observed trends. To quantitatively explore the possible role of greenhouse gases and aerosols in the observed warming trends, two simulations (using different initial conditions) of twentieth-century climate change due to these two radiative forcings were analyzed. These simulate an accelerated warming trend [~2°C (100 yr)-1] in the triangular Pacific region beginning around the 1960s and suggest that nearly all of the recent warming in the region could be attributable to such a thermal forcing. In summary, the authors' model results indicate that the observed warming trend in the eastern tropical Pacific is not likely to be solely attributable to internal (natural) climate variability. Instead, it is likely that a sustained thermal forcing, such as the increase of greenhouse gases in the atmosphere, has been at least partly responsible for the observed warming.

37. Knutson, Thomas R., and Syukuro Manabe, 1998: Model assessment of decadal variability and trends in the tropical Pacific Ocean In The Ninth Symposium on Global Change Studies and Namias Symposium on the Status and Prospects for Climate Prediction, Boston, MA, American Meteorological Society, 216-219.
38. Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1998: Simulated increase of hurricane intensities in a CO2-warmed climate. Science, 279(5353), 1018-1020.
    [ Abstract PDF ]

    Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character of these powerful storms could change in response to greenhouse gas-induced global warming. The impact of climate warming on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO2 conditions. More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model. For a sea surface temperature warming of about 2.2°C, the simulations yielded hurricanes that were more intense by 3 to 7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.

39. Knutson, Thomas R., Syukuro Manabe, and D Gu, 1997: Simulated ENSO in a global coupled ocean-atmosphere model: Multidecadal amplitude modulation and CO₂ sensitivity. Journal of Climate, 10(1), 138-161.
    [ Abstract PDF ]

    An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr. experiments with a low-resolution (R15) global coupled ocean-atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO's life cycle is qualitatively similar to the "delayed oscillator" ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different mutidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from Delta 18O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2-7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean-atmosphere system. In two 1000-yr CO₂ sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO₂. This decreased variability is due in part to CO₂-induced changes in the model's time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO₂. The frequency of ENSO in the model does not appear to be strongly influenced by increased CO2. Since the multidecadal fluctuations in the model ENSO's amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO₂, the results suggest that the impact of increased CO₂ on ENSO is unlikely to be clearly distinguishable from the climate system "noise" in the near future - unless ENSO is substantially more sensitive to increased CO₂ than indicated in the present study.

40. Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1997: Exploring the sensitivity of hurricane intensity to CO2-induced global warming using the GFDL Hurricane Prediction System In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 587-588.
41. Knutson, Thomas R., and Syukuro Manabe, 1995: Time-mean response over the tropical Pacific to increased CO2 in a coupled ocean-atmosphere model. Journal of Climate, 8(9), 2181-2199.
    [ Abstract PDF ]

    The time-mean response over the tropical Pacific region to a quadrupling of CO₂ is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4 degrees - 5 degrees C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with CO₂ increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase. Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with a quadrupling of CO₂ is surprising in light of the increased time-mean condensation heating over the "warm pool" region. Three aspects of the model response are important for interpreting this result. 1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. 2) The dynamical cooling term, - omega delta theta/ delta p, is enhanced due to increased time-mean static stability ( - delta theta/delta p). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making - delta theta/ delta p larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 3) The weaker surface zonal winds and wind stress in the equatorial Pacific are consistent with the reduced zonal SST gradient. The SST gradient is damped by the west-east differential in evaporative surface cooling (with greater evaporative cooling in the west than in the east). This evaporative damping increases with increasing temperature, owing to the temperature dependence of saturation mixing ratios, which leads to a reduction in the SST gradient in the warmer climate.

42. Knutson, Thomas R., and Syukuro Manabe, 1994: Impact of increased CO₂ on simulated ENSO-like phenomena. Geophysical Research Letters, 21(21), 2295-2298.
    [ Abstract PDF ]

    The impact of a CO₂-induced global warming on ENSO-like fluctuations in a global coupled ocean-atmosphere GCM is analyzed using two multi-century experiments. In the 4xCO₂ experiment, CO₂ increases by a factor of four in the first 140 years and then remains constant at 4xCO₂ for another 360 years; in the control experiment, CO₂ remains constant at 1xCO₂ for 1000 years. The standard deviation of tropical Pacific SST fluctuations (7°N-7°S, 173°E-120°W; 2 to 15 year timescales) is 24% lower in the 4xCO₂ experiment than in the control experiment; for the model's Southern Oscillation Index, a 19% decrease occurs, whereas for central tropical Pacific rainfall, a 3% increase occurs. An important feature of the control simulation is the internally generated modulation of variability on a multi-century timescale, which is comparable in magnitude to the changes occurring with 4xCO₂. We conclude that despite an order 5 K warming of the tropical Pacific, and order 50% increase in time-mean atmospheric water vapor under 4xCO₂ conditions, ENSO-like SST fluctuations in the coupled model do not intensify, but rather decrease slightly in amplitude.

43. Knutson, Thomas R., and Syukuro Manabe, 1994: Impact of increasing CO₂ on the Walker circulation and ENSO-like phenomena in a coupled ocean-atmosphere model In The Sixth Conference on Climate Variations, Boston, MA, American Meteorological Society, 80-81.
44. Knutson, Thomas R., and K M Weickmann, 1987: 30-60 day atmospheric oscillations: Composite life cycles of convection and circulation anomalies. Monthly Weather Review, 115(7), 1407-1436.
    [ Abstract PDF ]

    Life cycles of the 30-60-day atmospheric oscillation were examined by compositing 30-60-day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing long-wave radiation (OLR) for the years 1979-1984. Separate composite life cycles were constructed for the May-Oct. and Nov.-April seasons by using empirical orthogonal function analysis of the large-scale divergent wind field (250-mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields. Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown by using sequences of composite maps. Good agreement exists between areas of upper air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South America than over the Indian and western Pacific oceans. Anomalies of OLR are nearly negligible over cooler tropical sea surfaces. Fluctuations in summer monsoon region convection are influenced by the global-scale eastward-moving wave. The oscillation's vertical structure varies with latitude. In the Tropics, upper level and lower level tropospheric wind anomalies are about 180° out of phase. Poleward of about 20°, there is no pronounced phase shift between levels. In tropical and subtropical latitudes, analysis of the nondivergent circulation composites at 250 mb reveals cyclones to the east of the convection and anticyclones alongside or west of the convection. While convection anomalies are most pronounced in the summer hemisphere Tropics, the tropical and subtropical features are most prominent in the winter hemisphere. There is some evidence of symmetry of cyclonic and anticyclonic circulations about the equator. A subset of the composite extratropical vector wind fields were statistically significant (95% level) at 850 and 250 mb in the winter hemisphere (25-85° latitude), based upon a Monte Carlo simulation. During the Nov.-April season, the East Asian jet is retracted toward Asia when positive 30-60-day convection anomalies are occurring over the equatorial Indian Ocean. The eastward shift of convection into the western and central Pacific is accompanied by a series of circulation features over northern Asia and an eastward extension of the East Asian jet. During the May-Oct. season, the shift of large-scale tropical convection anomalies from the Indian Ocean and Indian monsoon regions to the tropical western Pacific is followed (10-15 days later) by the occurrence of strengthened westerlies over southern Australia. In contrast, the extratropical "response" in the summer hemisphere for both the May-Oct. and Nov.-April seasons was not statistically significant.

45. Knutson, Thomas R., K M Weickmann, and J E Kutzbach, 1986: Global-scale intraseasonal oscillations of outgoing longwave radiation and 250 mb zonal wind during Northern Hemisphere summer. Monthly Weather Review, 114(3), 605-623.
    [ Abstract PDF ]

    Intraseasonal fluctuations of satellite-based observations of outgoing longwave radiation (OLR) and NMC analyses of 250 mb zonal wind (U250) are described based on global data from nine Northern Hemisphere summers (May- October). Cross-spectral analysis of the 28-72 day spectral band is used to establish statistically significant relationships for the entire data period. Hovmoller diagrams are used to examine individual events and to estimate the oscillation's time scale and propagation characteristics. Intraseasonal OLR fluctuations have their greatest amplitude in the Indian monsoon region and north of the equator in the western tropical Pacific. These two regions have out-of-phase fluctuations and appear to be linked by OLR anomalies propagating eastward (at 3-6 m s -1) along the equator between 50 degrees and 160 degrees E. The OLR oscillation has a preferred time scale of 30-60 days during May-October, based on a sample of more than 30 events. The initiation near the equator of northward-propagating (1-2 m s- 1) OLR anomalies in the Indian monsoon region is also associated with the eastward-propagating equatorial OLR anomalies. The U250 intraseasonal fluctuations have a prominent zonal wavenumber-one structure throughout the tropics with the exception of the Northern Hemisphere tropics over the Atlantic, Africa, and the Indian monsoon region. The U250 anomalies propagate eastward along 0 degrees - 10 degrees S at about 6 m s^{- 1} from 40 degrees to 160 degrees E and at about 15 m s^{- 1} from 160 degrees E to 0 degrees W. These longitudinal changes in the oscillation's ground speed may be due in part to longitudinal changes in the zonal wind basic state. The 28-72 day U250 anomalies along 30 degrees S (50 degrees S) are out of phase (in phase) with the tropical U250 anomalies over most of the Pacific and Indian Ocean sectors. The phase relationships between tropical OLR and U250 anomalies seem dynamically consistent, generally showing 250 mb u-component divergence flanking regions of convection. Although the eastward propagation of OLR anomalies along 5 degrees N-5 degrees S is not continuous around the globe, areas of significant coherence scattered throughout the tropics exhibit a zonal wavenumber-one phase structure. In these remote regions, OLR anomalies may be dynamically linked by an eastward-propagating tropical circulation feature.

46. Miyakoda, Kikuro, Joseph J Sirutis, and Thomas R Knutson, 1986: Experimental 30-day forecasting at GFDL In Workshop on Predictability in the Medium and Extended Range, ECMWF, 17-50.
    [ Abstract ]

    This is the report of the 30-day forecast experiment conducted at GFDL. The first part is a summary of 8 January case studies, using a finite difference GCM without the anomalous boundary forcings of sea surface temperature (SST). The experiment reveals that the forecast skill of 10-day mean variables is marginal at the end of a month, but that the removal of systematic bias (climate drift) from the original forecasts raises the skill scores appreciably, producing useful one-month prognoses. However, the climate drift is alarmingly large; for example, the forecast error for the 500 mb geopotential height due to the drift is 64% of the total root mean square error. The second part of the paper discusses the forecasts incorporating the observed SST instead of the climatological SST. A series of forecasts was carried out for the most dramatic El Niño event of January 1983. In this study, forecasts were improved for the tropics by using the observed SST, whereas the impact for the extratropics was not beneficial. Four possible causes for the adverse effect of tropical SST were examined, i.e., the cumulus parameterization, the accuracy of SST, the initialization, and the tropical land surface condition. Preliminary investigations suggest that the forecast tropical divergence fields are quite different from those observed, in particular with respect to the components of large scale divergence associated with the 40-50 day oscillation. It is likely that the current initialization of the GFDL forecast system is deficient in treating this distinct tropical oscillation.

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