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Philip J. Pegion and Arun Kumar

is in section 3 . The AGCM simulations are described in section 4 . The results are presented in section 5 , and section 6 contains the conclusions. 2. Identification of the leading modes of SST variability A more in-depth description of the forcing patterns used in this study can be found in Schubert et al. (2009) . The leading patterns of SST variability are identified based on a rotated empirical orthogonal function (REOF) analysis of the annual mean SSTs from version 1 of the Hadley

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

likely, other forcings besides SST are playing a role. Another approach for explaining future trends in the Sahel involves the mechanism responsible for the seasonal development of the “elementary” monsoon ( Webster 1987 ). According to this argument, an amplification of the land–sea thermal contrast (which is expected under global warming; Houghton et al. 1996 ; Sutton et al. 2007 ) will deepen the thermal low over the Sahara, amplify the monsoon circulation, and bring more moisture, creating

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Rachel R. McCrary and David A. Randall

suggest that soil moisture rainfall feedbacks may act as a bridging mechanism between cool-season precipitation anomalies, which result from SST forcing, and warm-season precipitation anomalies ( Seager et al. 2005b ; Cook et al. 2007 ). Koster et al. (2000) and Schubert et al. (2008) argue that, in the Great Plains region, precipitation is particularly sensitive to changes in soil moisture conditions, especially during the warm months. The present study addresses the following questions: 1) How

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Matías Méndez and Víctor Magaña

analysis of prolonged drought in Mexico should focus on Northern Hemisphere (NH) summer rains. During this season, trade winds and easterly waves produce moisture flux from the Americas warm pools into continental Mesoamerica (i.e., the geographical area that extends from central Mexico down through Central America) ( Mestas-Nuñez et al. 2002 ; Wu et al. 2009 ). In the northern part of Mexico subsidence persists most of the year. It is only when easterly waves (EW) or tropical cyclones (TC) force

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

extremes in SST forcing, and it is in the difference between these extremes that we expect to find the strongest signals. Notice the addition in the colorbar of an extra shading contour at 0.5°. The AGCMs show many differences in Δ PcAn−PwAn [ T dry ], with particularly strong values in the Great Plains for GMAO, weaker values for CCM3, CAM3, and GFDL, and an almost complete lack of signal for GFS, despite the fact that it does show lower precipitation for PcAn than for PwAn. [The anomalous behavior

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Caio A. S. Coelho and Lisa Goddard

which eastern equatorial Pacific upwelling reduces ocean surface warming from increased longwave radiative forcing, thus strengthening the zonal temperature gradient between eastern and western Pacific. The more El Niño–like response follows from a weakening of the Pacific Walker circulation as precipitation efficiency increases in a warmer, moister world. Such weakening leads to a reduction in the easterlies in the equatorial Pacific, which results in a reduction in the equatorial Pacific east

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Alfredo Ruiz-Barradas and Sumant Nigam

. Bacmeister , 2004 : Causes of long-term drought in the U.S. Great Plains. J. Climate , 17 , 485 – 503 . Schubert , S. D. Coauthors 2009 : A U.S. CLIVAR project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results. J. Climate , 22 , 5251 – 5272 . Seager , R. , Y. Kushnir , C. Herweijer , N. Naik , and J. Velez , 2005 : Modeling of tropical forcing of persistent droughts and pluvials over western North America

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