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Matías Méndez and Víctor Magaña

-Castillo et al. 2002 ). In general, during winter, climate across Mexico is influenced by the state of the tropical Pacific Ocean with minor influence from the Atlantic Ocean. Some studies suggest that historical droughts in southern Mexico coincide with the negative phase of the AMO ( Mendoza et al. 2007 ) but may also be affected by the positive phase of the PDO or El Niño conditions ( Seager et al. 2009 ); recurrent droughts are expected over central/southern Mexico during this phase of PDO. The

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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

next? What is the role of the different ocean basins, including the impact of El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), the Atlantic multidecadal oscillation (AMO), and warming trends in the global oceans? What is the role of the land? To what extent can droughts develop independently of oceanic variability resulting from year-to-year memory that may be inherent to the land? To address these questions, the working group proposed that a number of mechanistic

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Antonietta Capotondi and Michael A. Alexander

conditions across the midlatitude North Atlantic Ocean, the Mediterranean, including parts of Europe and the Middle East, and central Asia ( Hoerling and Kumar 2003 ). Sparse paleoclimate evidence also indicates that, when North America is dry, it is wet in northern South America; dry in southern South America, southern Europe and the Mediterranean, and equatorial East Africa; and wet in the Sahel, South Africa, and India ( Seager et al. 2007 ). Numerical simulations performed with atmospheric models

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Caio A. S. Coelho and Lisa Goddard

future climate have clear negative societal implications for many of the already vulnerable tropical land areas. Numerous uncertainties surround El Niño and climate change in coupled general circulation models (CGCMs). First, although the latest models developed for the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4)—the so-called phase 3 of the Coupled Model Intercomparison Project (CMIP3) models—have improved their representation of El Niño–Southern Oscillation

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Kirsten L. Findell and Thomas L. Delworth

increases. The cold Pacific experiment showed modest drought increases in southern Europe extending into central Asia, for example. The warm Pacific experiment shows pluvial increases over a much broader portion of Europe but few grid points with values above 3 months yr −1 in central Asia. There are additional differences around the Indian Ocean: the cold Pacific experiment leads to increased pluvial in the Arabian Peninsula and India, while the warm Pacific experiment leads to increased droughts

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Philip J. Pegion and Arun Kumar

variance. The third mode, which represents only 5.8% of the global variance, is a North Atlantic pattern, similar to the Atlantic multidecadal oscillation. The spatial pattern of the leading REOF shows a fairly uniform loading throughout the world’s oceans ( Fig. 1 ). The highest loading occurs in the southern Indian Ocean, and there is a noticeable minimum in the central tropical Pacific Ocean. The principal component associated with this leading pattern shows a nonsecular increase, with values

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Bradfield Lyon

the comparatively cool southeast South Atlantic Ocean into the western and southern parts of South Africa is consistent with relatively small probabilities of heat waves during drought (including El Niño–related drought). In marked contrast, Fig. 6c shows a composite of 850-hPa reanalysis daily wind anomalies during 19 heat wave events that occurred at St. Lucia (location given by gray dot in figure) between 1961 and 2000. Northwesterly, downslope flow is implied at that location. Similarly

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Alfredo Ruiz-Barradas and Sumant Nigam

American monsoon region; however, the largest regional variability is along the eastern seaboard of Mexico. Summer precipitation variability by the models is quasi-realistic. All models show low (high) variability in the western (eastern) half of United States, as well as large variability over western and southern Mexico. While all models have a maximum over the central United States that resembles the maximum in observations, the apparent connection with the Gulf coast states present in the

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Renu Joseph and Ning Zeng

, the observations show a decrease in temperature over land and ocean in the tropics, with our model results supporting this decrease. We do notice some residual warming in observations over the east Pacific and in the southern Indian Ocean where our ENSO removal technique did not completely remove all the ENSO SST warming during the Agung volcano event as discussed also in the appendix . There is a substantial cooling in the subtropics particularly over the dry arid desert over land both in the

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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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