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Renu Joseph and Ning Zeng

water vapor of about 3% in both the model and observations during the peak of Pinatubo cooling. In addition, some studies have also examined the relationship of changes in radiative forcing and precipitation. Wild et al. (2008) used observations to connect changes in net reduction in shortwave (SW) radiation to changes in precipitation. Using idealized GCM experiments, Yang et al. (2003) show that the response of precipitation to radiative changes in the atmosphere depends on both the radiative

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Rachel R. McCrary and David A. Randall

indicate that land–atmosphere feedbacks play an important role in the initiation and persistence of long-term droughts over the Great Plains region ( Namias 1991 ; Oglesby and Erickson 1989 ; Schubert et al. 2004a , 2008 ). Although these factors may all contribute to dryness over the Great Plains in some ways, for reasons discussed later it appears that three primary mechanisms cause Great Plains precipitation anomalies to persist for long periods of time. These are 1) variations in tropical

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Philip J. Pegion and Arun Kumar

Geophysical Fluid Dynamics Laboratory (GFDL), the University of Maryland–National Center for Atmospheric Research (UMD–NCAR), the University of Miami–Center for Ocean–Land–Atmosphere Studies (UM–COLA), and the National Centers for Environmental Prediction/Climate Prediction Center (NCEP/CPC). Five of the groups (with the exception of UM–COLA) preformed the experiments with a global AGCM forced with the identical SSTs. UM–COLA preformed the experiments with a coupled model. The atmospheric models employed

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

1. Introduction The central United States is a hydroclimatically and economically sensitive region given its agricultural prominence and significant warm season precipitation variability. The proximity of this region to the Rocky Mountains, Gulf of Mexico, and Atlantic and Pacific Oceans provide a unique combination of potential climate influences, including large-scale atmospheric circulation variations emanating over the adjoining ocean basins and local land–atmosphere interactions. As such

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Alfredo Ruiz-Barradas and Sumant Nigam

( Gutzler and Schubert 2007 ). In this task, a central idea has been for several modeling groups to do identical idealized experiments to look in detail at the physical mechanisms linking SST changes to droughts. In a nutshell, there are five research centers, with their respective state-of-the-art atmospheric GCMs, that participate in the idealized experiments: 1) the National Center for Atmospheric Research’s (NCAR) Community Atmosphere Model, version 3.5 (CAM3.5) ( Neale et al. 2008 ; Oleson et al

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Siegfried Schubert, David Gutzler, Hailan Wang, Aiguo Dai, Tom Delworth, Clara Deser, Kirsten Findell, Rong Fu, Wayne Higgins, Martin Hoerling, Ben Kirtman, Randal Koster, Arun Kumar, David Legler, Dennis Lettenmaier, Bradfield Lyon, Victor Magana, Kingtse Mo, Sumant Nigam, Philip Pegion, Adam Phillips, Roger Pulwarty, David Rind, Alfredo Ruiz-Barradas, Jae Schemm, Richard Seager, Ronald Stewart, Max Suarez, Jozef Syktus, Mingfang Ting, Chunzai Wang, Scott Weaver, and Ning Zeng

5) organize a community workshop to present and discuss the results. This paper provides an overview and some results of task 3 of the working group, involving the design, coordination, implementation, and initial evaluation of a new set of model simulations that address the roles of sea surface temperature forcing and land–atmosphere feedbacks in the development and maintenance of drought. This work extends and builds upon recent modeling studies (e.g., Hoerling and Kumar 2003 ; Schubert et

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Randal D. Koster, Hailan Wang, Siegfried D. Schubert, Max J. Suarez, and Sarith Mahanama

increase in evaporation; at higher levels, the sensitivity disappears and the evaporative fraction remains constant. Hydrologists sometimes refer, somewhat imprecisely, to these two regimes as the “soil moisture controlled” and “energy controlled” evaporation regimes. In the latter, wetter regime, transport of moisture through the soil matrix and vegetation is relatively efficient and no longer acts as a bottleneck to the net transfer of water from the soil to the atmosphere. K09 , recognizing the

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M. Biasutti, A. H. Sobel, and Suzana J. Camargo

21C] by 24 coupled models (one single run per model). The 20C runs are forced by the historical anthropogenic emissions of greenhouse gases and sulfate aerosols and (for a subset of the models) by other anthropogenic and natural forcings. The 21C simulations assume a middle-of-the-road increase of greenhouse gases that stabilizes at 700 ppm and sulfate aerosols emissions increasing up to 2020 and decreasing afterward. We will focus on the difference between the last 25 years of the twenty

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Alfredo Ruiz-Barradas and Sumant Nigam

most difficult aspects of understanding and projecting changes in regional hydroclimate are associated with changes in the circulation of the atmosphere and oceans. This is particularly challenging over the central United States where regional hydroclimate strongly depends on the moisture transport from the Gulf of Mexico via the Great Plains low-level jet (e.g., Ruiz-Barradas and Nigam 2005 , 2006 ; Cook et al. 2008 ; Weaver and Nigam 2008 ). Several empirical and atmosphere

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Yochanan Kushnir, Richard Seager, Mingfang Ting, Naomi Naik, and Jennifer Nakamura

Administration’s (NASA’s) Seasonal-to-Interannual Prediction Project model 1 (NSIPP1)]. They demonstrated the importance of the tropical Pacific forcing by comparing the results of ensembles forced with observed global SSTs and several idealized integrations with SSTs prescribed in different ocean basins. The explanation to the EEP SST impact lies in the response of the atmosphere to ENSO (see Seager et al. 2003 , 2005a ). In particular, warmer (colder)-than-normal EEP SSTs lead to an overall warming

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