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Kirsten L. Findell and Thomas L. Delworth

occurring SST patterns in driving and/or exacerbating drought. The goal of this paper is to document the impact of the three primary SST patterns on drought and pluvial frequency and intensity around the world throughout the annual cycle, as determined by the multimodel mean of these experiments. The dominance of the Pacific forcing stands out in these results, with secondary yet notable impacts forced by the North Atlantic and the global-scale warming trend. In addition, results show that regions with

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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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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

precipitation variations over the United States. GCM experiments have also been used to diagnose the role of tropical SST forcing on central U.S. warm season precipitation. Bates et al. (2001) show that the 1993 pluvial over the Great Plains was related to tropical Pacific SST anomalies; however, no such conclusion was drawn for the 1988 drought. Decadal SST variability has also been implicated in forcing drought and pluvial over the United States ( Schubert et al. 2004 , 2008 ; Seager et al. 2005

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Alfredo Ruiz-Barradas and Sumant Nigam

, the models used, the SST forcing, and an overview of the results. The hierarchy of interactions that give rise to precipitation variability within a model, that is, local land surface–atmosphere versus remote SST–moisture fluxes, plays a crucial role in the simulation of regional summer hydroclimate variability. Regional hydroclimate over the central United States strongly depends on the moisture transport from the Gulf of Mexico via the Great Plains low-level jet, particularly in the summer

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Kingtse C. Mo, Jae-Kyung E. Schemm, and Soo-Hyun Yoo

’s climatology. Therefore, both observations and model experiments are needed to study the extreme precipitation events and atmospheric responses to the low-frequency SSTA forcing. The purpose of this paper is to examine the impact of (i) ENSO, (ii) the AMO, and (iii) the combinations of the different phases of the AMO and ENSO on drought and wet spells over the United States. We draw our conclusions from composites based on observations and model simulations from the U.S. CLIVAR experiments. The

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Bradfield Lyon

) Coupled Model Intercomparison Project Phase 3 (CMIP3) multimodel dataset. The Twentieth-Century Climate in Coupled Model (20C3M) runs, which include forcing from observed aerosols and greenhouse gas concentrations are used for this purpose. The possible future behavior of drought and heat waves, considered separately and jointly, are then evaluated using the same CMIP3 models forced with the Special Report on Emissions Scenarios (SRES) A1B greenhouse gas scenario where CO 2 concentrations reach a

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Kerry H. Cook and Edward K. Vizy

that the CLLJ is basically geostrophic. Here, we extend their analysis to understand what regional force imbalances cause the jet to accelerate and decelerate on diurnal time scales ( Fig. 4 ) and how and why the momentum balances vary with season ( Figs. 2 , 3 ). Ageostrophic components of the flow are of special interest, because these may be convergent or divergent and therefore related more directly to the precipitation field. Of particular interest also is the generation of the northward

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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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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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Antonietta Capotondi and Michael A. Alexander

obtained with climatological SST forcing was not statistically distinguishable from the model climatology, supporting the idea that SST forcing can play a significant role in the occurrence of droughts over the central United States and can increase prediction capabilities. Significant correlations between summer precipitation and Pacific SSTs were also noted by Ting and Wang (1997) at interannual time scales. The SST-forced atmospheric simulations of Schubert et al. (2004a , b) and Seager et al

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