Bibliography - Kirsten L Findell

1. Findell, Kirsten L., and Thomas L Delworth, in press: Impact of common sea surface temperature anomalies on global drought and pluvial frequency. Journal of Climate. 4/09.
   [ Abstract ]

   Climate model simulations run as part of the Clivar Drought Working Group initiative were analyzed to determine the impact of three patterns of sea surface temperature (SST) anomalies on drought and pluvial frequency and intensity around the world. The three SST forcing patterns include a global pattern similar to the background warming trend, a pattern in the Pacific, and a pattern in the Atlantic. Five different global atmospheric models were forced by fixed SSTs to test the impact of these SST anomalies on droughts and pluvials relative to a climatologically-forced control run.
   The five models generally yield similar results in the locations of droughts and pluvials for each given SST pattern. In all of the simulations, areas with an increase (decrease) in the mean drought index values tend to also show an increase in the frequency of pluvial (drought) events. Additionally, areas with more frequent extreme events also tend to show higher intensity extremes. The cold Pacific anomaly increases drought occurrence in the United States and southern South America, and increases pluvials in Central America, and northern and central South America. The cold Atlantic anomaly increases drought occurrence in southern Central America, northern South America, and Central Africa, and increases pluvials in central South America. The warm Pacific and Atlantic anomalies generally lead to reversals of the drought and pluvial increases described with the corresponding cold anomalies. More modest impacts are seen in other parts of the world. The impact of the trend pattern is generally more modest than that of the two other anomaly patterns.

2. Schubert, S D., Thomas L Delworth, and Kirsten L Findell, et al., in press: A US CLIVAR project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results. Journal of Climate. 4/09.
3. Findell, Kirsten L., A J Pitman, M H England, and P J Pegion, in press: Regional and global impacts of land cover change and sea surface temperature anomalies. Journal of Climate. 4/08.
   [ Abstract ]

   The atmospheric and land components of the Geophysical Fluid Dynamics Laboratory’s climate model CM2.1 is used with climatological sea surface temperatures (SSTs) to investigate the relative climatic impacts of historical anthropogenic land cover change (LCC) and realistic SST anomalies. The SST forcing anomalies used are analogous to signals induced by an El Nino-Southern Oscillation (ENSO), a North Atlantic Oscillation (NAO), and the background trend. Coherent areas of LCC are represented throughout much of central and eastern Europe, northern India, southeastern China, and on either side of the ridge of the Appalachian Mountains in North America. Smaller areas of change are present in various tropical regions. The land cover changes in the model are almost exclusively a conversion of forests to grasslands. Model results show that LCC has a negligible impact on the global scale, while the SST anomalies—particularly the ENSO-like signal—have a statistically significant global impact. However, in the regions where the land surface has been altered, the impact of LCC can be equally or more important than the SST forcing patterns in determining the seasonal cycle of the surface water and energy balance. LCC also perturbs the local air temperature and rainfall at a similar level of statistical significance as the SST anomalies. This suggests that proper representation of land cover conditions is essential in the design of climate model experiments, particularly if results are to be used for regional-scale assessments of climate change impacts.

4. Levy II, Hiram, M Daniel Schwarzkopf, Larry Horowitz, V Ramaswamy, and Kirsten L Findell, March 2008: Strong sensitivity of late 21st Century climate to projected changes in short-lived air pollutants. Journal of Geophysical Research, 113, D06102, doi:10.1029/2007JD00917.
   [ Abstract PDF ]

   This study examines the impact of projected changes (A1B “marker” scenario) in emissions of four short-lived air pollutants (ozone, black carbon, organic carbon, and sulfate) on future climate. Through year 2030, simulated climate is only weakly dependent on the projected levels of short-lived air pollutants, primarily the result of a near cancellation of their global net radiative forcing. However, by year 2100, the projected decrease in sulfate aerosol (driven by a 65% reduction in global sulfur dioxide emissions) and the projected increase in black carbon aerosol (driven by a 100% increase in its global emissions) contribute a significant portion of the simulated A1B surface air warming relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally, 0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40% of the total) in the summertime United States. These projected changes are also responsible for a significant decrease in central United States late summer root zone soil water and precipitation. By year 2100, changes in short-lived air pollutants produce a global average increase in radiative forcing of ∼1 W/m²; over east Asia it exceeds 5 W/m². However, the resulting regional patterns of surface temperature warming do not follow the regional patterns of changes in short-lived species emissions, tropospheric loadings, or radiative forcing (global pattern correlation coefficient of −0.172). Rather, the regional patterns of warming from short-lived species are similar to the patterns for well-mixed greenhouse gases (global pattern correlation coefficient of 0.8) with the strongest warming occurring over the summer continental United States, Mediterranean Sea, and southern Europe and over the winter Arctic.

5. Delworth, Thomas L., and Kirsten L Findell, 2007: Decadal to centennial scale changes in summer continental hydrology In Climate Variability and Change: Past, Present, and Future, John E. Kutzbach Symposium, Gisela Kutzbach, Ed., Madison, WI, Ctr. of Climatic Research, U. Wisconsin-Madison, 49-56.
   [ Abstract ]

   Past studies have suggested that increasing atmospheric CO₂ will lead to a substantial reduction of soil moisture during summer in the extratropics. We revisit this topic using a new climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory. The new model has a horizontal resolution of 2.5° longitude by 2.0° latitude, with 24 vertical levels, and has both a diurnal and seasonal cycle of insolation. The model incorporates substantially updated physics relative to previous versions. Results from earlier studies showed, among other things, an increase in wintertime rainfall over most mid-latitude continental regions when CO₂ is doubled, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer in mid-latitude in doubled-CO₂ experiments. The new model shows similar results, and the processes discussed above are important in this model as well. In addition, we find that changes in atmospheric circulation play an important role in regional hydrologic changes. Additional experiments have been run to probe the causes of the circulation changes. These simulations show that global scale sea surface temperature increases caused by the CO₂ doubling explain the majority of the atmospheric circulation changes, while positive feedbacks from the land surface have a secondary impact. These results highlight the importance of global scale sea surface temperature changes for future regional hydrology changes.

6. 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.

7. 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

8. 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.

9. Findell, Kirsten L., and Thomas L Delworth, 2005: A modeling study of dynamic and thermodynamic mechanisms for summer drying in response to global warming. Geophysical Research Letters, 32, L16702, doi:10.1029/2005GL023414.
   [ Abstract PDF ]

   Past studies have suggested that increasing atmospheric CO2 will lead to a significant reduction of soil moisture during summer in the extratropics. These studies showed an increase in wintertime rainfall over most mid-latitude continental regions when CO2 is doubled, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer. We find that the above processes are important in simulated summer drying in a newly developed climate model. In addition to these thermodynamic processes, we find that changes in atmospheric circulation play an important role in regional hydroclimatic changes. Additional experiments show that the atmospheric circulation changes are forced by the CO2-induced warming of the ocean, particularly the tropical ocean. These results highlight the importance of sea surface temperature changes for regional hydroclimatic changes.

10. 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.

11. Findell, Kirsten L., and E A B Eltahir, 2003: Atmospheric controls on soil moisture-boundary layer interactions. Part I: Framework development. Journal of Hydrometeorology, 4(3), 552-569.
    [ Abstract PDF ]

    This paper investigates the influence of soil moisture on the development and triggering of convection in different early-morning atmospheric conditions. A one-dimensional model of the atmospheric boundary layer (BL) is initialized with atmospheric sounding data from Illinois and with the soil moisture set to either extremely wet (saturated) or extremely dry (20% of saturation) conditions. Two measures are developed to assess the low-level temperature and humidity structure of the early-morning atmosphere. These two measures are used to distinguish between four types of soundings, based on the likely outcome of the model: 1) those soundings favoring deep convection over dry soils, 2) those favoring deep convection over wet soils, 3) those unlikely to convect over any land surface, and 4) those likely to convect over any land surface. Examples of the first two cases are presented in detail. The early-morning atmosphere is characterized in this work by the newly developed convective triggering potential (CTP) and a low-level humidity index, HI_low . The CTP measures the departure from a moist adiabatic temperature lapse rate in the region between 100 and 300 mb (about 1-3 km) above the ground surface (AGS). This region is the critical interface between the near-surface region, which is almost always incorporated into the growing BL, and free atmospheric air, which is almost never incorporated into the BL. Together, these two measures form the CTP-HI_low framework for analyzing atmospheric controls on soil moisture-boundary layer interactions. Results show that in Illinois deep convection is trigged in the model 22% of the time over wet soils and only 13% of the time over dry soils. Additional testing varying the radiative conditions in Illinois and also using the 1D model with soundings from four additional stations confirm that the CTP-HI_low framework is valid for regions far removed from Illinois.

12. Findell, Kirsten L., and E A B Eltahir, 2003: Atmospheric controls on soil moisture-boundary layer interactions. Part II: Feedbacks within the continental United States. Journal of Hydrometeorology, 4(3), 570-583.
    [ Abstract PDF ]

    The CTP-HI_low framework for describing atmospheric controls on soil moisture-boundary layer interactions is described in a companion paper, Part I . In this paper, the framework is applied to the continental United States to investigate how differing atmospheric regimes influence local feedbacks between the land surface and the atmosphere. The framework was developed with a one-dimensional boundary layer model and is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HI_low. These two measures are used to distinguish between three types of early-morning atmospheric conditions: those favoring moist convection over dry soils, those favoring moist convection over wet soils, and those that will allow or prevent deep convective activity, independent of the surface flux partitioning.

13. Findell, Kirsten L., and E A B Eltahir, 2003: Atmospheric controls on soil moisture-boundary layer interactions: Three-dimensional wind effects. Journal of Geophysical Research, 108(D8), 8385, doi:10.1029/2001JD001515.
    [ Abstract PDF ]

    This paper expands the one-dimensionally based CTP-HI_low framework for describing atmospheric controls on soil moisture-boundary layer interactions [Findell and Eltahir, 2003] to three dimensions by including low-level wind effects in the analysis. The framework is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HI_low. These two measures are used to distinguish between three types of early morning soundings: those favoring rainfall over dry soils, those favoring rainfall over wet soils, and those whose convective potential is unaffected by the partitioning of fluxes at the surface. The focus of this paper is the additional information gained by incorporating information about low-level winds into the CTP-HI_lowframework. Three-dimensional simulations using MM5 and an analysis of observations from the FIFE experiment within this framework highlight the importance of the winds in determining the sensitivity of convection to fluxes from the land surface. A very important impact of the 3D winds is the potential for low-level backing or unidirectional winds with great shear to suppress convective potential. Because of this suppression of convection in certain wind conditions, far fewer simulations produced rain than would be anticipated based solely on the 1D framework of understanding. However, when the winds allowed, convection occurred in a manner consistent with the 1D-based expectations. Generally speaking, in the regime where dry soils were expected to have an advantage, convection was triggered over dry soils more often than over wet; in the regime where wet soils were expected to have an advantage, convection was more frequently triggered over wet soils than over dry. Additionally, when rainfall occurred in both simulations with wet soils and simulations with dry soils for a given day, rainfall depths were typically greater in the simulations with wet soils. Similarly, the FIFE data showed numerous days with convective potential but no rainfall: each of these days had low-level backing or strongly shearing winds. Four days with high humidity deficits and veering winds in the lowest 300 mbar did have rain, highlighting the enhanced buoyancy effects of low-level veering winds.

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