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

1. Introduction The nature and cause of North American hydroclimate variability is a subject of heightened concern because of the recent droughts 1 in the American West 2 and in northern Mexico ( Seager 2007 , 2009 ) and because projections of anthropogenic influence on the climate of the twenty-first century indicate a turn toward increasing aridity there ( Seager et al. 2007 ). The latter finding, based on output from climate models that participated in the Intergovernmental Panel on

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Alfredo Ruiz-Barradas and Sumant Nigam

hydroclimatic conditions over North America. It has been shown that SST structures with contrasting signs in the Pacific and Atlantic Oceans [cold (warm) Pacific and a warm (cold) Atlantic] are conducive to the most extreme [droughty (pluvial)] conditions over the central Great Plains (e.g., Hoerling and Kumar 2003 ; Schubert et al. 2009 ). If both basins have the same sign, they are still capable of producing extreme hydroclimatic conditions over the central United States ( Schubert et al. 2009 ). In any

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

). Given the dominance of the ENSO signal in generating global-scale climate anomalies, Atlantic SST influences on North American hydroclimate have only recently begun to gain traction, especially in summer. Emerging evidence suggests a significant role for the Atlantic in generating intraseasonal-to-interannual warm season precipitation anomalies over the continental United States, including atmospheric North Atlantic Oscillation (NAO) variability ( Ruiz-Barradas and Nigam 2005 ; Weaver and Nigam

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Rachel R. McCrary and David A. Randall

al. 2008 ). Similar modeling studies have also shown that three long-term droughts that occurred in the mid-to-late nineteenth century were also forced by variations tropical Pacific SSTs ( Herweijer et al. 2006 ). These studies point to a number of different ways in which tropical Pacific SSTs are linked to changes in North American precipitation. For example, Seager et al. (2005a) found that changes in tropical Pacific SSTs are associated with changes in the subtropical jets, which affect the

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Alfredo Ruiz-Barradas and Sumant Nigam

1. Introduction The U.S. Climate Variability and Predictability (CLIVAR) Drought Working Group (DWG) has been leading the national efforts toward a better understanding of multiyear droughts for the past two and a half years. The main objective of the group has been to facilitate progress on the understanding and prediction of long-term multiyear droughts over North America and other drought-prone regions of the world, including an assessment of the impact of global change on drought processes

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

streamflow in winter. Different phases of the AMO also link to different summer precipitation modes of the North American monsoon ( Hu and Feng 2008 ). With the progress of climate modeling, current climate models in general are able to predict ENSO relatively well ( Saha et al. 2006 ). Responses to SSTAs in the Atlantic simulated by atmospheric GCMs (AGCMs) are more diverse. In the review paper by Kushnir et al. (2002) , the authors stated that the atmosphere indeed responds to the SSTAs in the

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Matías Méndez and Víctor Magaña

ascending motions that copious rains occurs in the northeastern states. Over northwestern Mexico, the North America monsoon ( Higgins et al. 2006 ) is associated with numerous mesoscale convective systems and severe rain events from July through September. Large-scale climatic phenomena, such as El Niño–Southern Oscillation (ENSO), result in climate variability on interannual time scales at locations about the globe ( Diaz and Markgraf 2000 ). In Mexico, El Niño (La Niña) conditions during summer lead

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Kerry H. Cook and Edward K. Vizy

work to further our understanding of the dynamics of the climatological CLLJ, including how the jet’s variations are controlled on seasonal and diurnal time scales. In addition, the relationship between the CLLJ and rainfall is examined, with special attention to any connections between the jet and regional drought. Current literature on the CLLJ is reviewed in the following section, illustrated with figures from the North American Regional Reanalysis (NARR; Mesinger et al. 2006 ), which also

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Antonietta Capotondi and Michael A. Alexander

abandonment of the ancestral Pueblo Indians sites at the beginning of the fifteenth century ( Douglass 1929 , 1935 ; Dillehay 1974 ; Jones et al. 1999 ; Benson et al. 2007 ). This time coincided with the Medieval Warm Period in Europe ( a.d. 800– a.d. 1400), indicating that long-lasting droughts in the western United States may be associated with global-scale precipitation anomalies ( Seager et al. 2007 ). In particular, dry conditions in western North America tend to be associated with dry

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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

1. Introduction In recognition of the profound societal impact of drought in many regions of the world and the emerging capabilities in simulating drought with global climate models, the U.S. Climate Variability and Predictability (CLIVAR) program initiated a drought working group in 2006 to “facilitate progress on the understanding and prediction of long-term (multi-year) drought over North America and other drought-prone regions of the world, including an assessment of the impact of global

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