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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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Bradfield Lyon

with observations. The stations within the boxed region of Fig. 1 were selected for being generally away from coastal locations where local factors relating to topography and oceans affect heat wave occurrence, as discussed further in section 3b . The results ( Fig. 3 ) indicate the strongest relationship between Tx anomalies and drought, in both observations and the models, occurs on the seasonal time scale with a tendency for more frequent occurrences of above-average Tx values during drought

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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

al. 2004a , b ; Wang et al. 2008 , 2009 ) as well as numerous observationally based studies (e.g., Trenberth and Guillemot 1996 ; Mo et al. 1997 ; Ting and Wang 1997 ; Nigam et al. 1999 ; Koster et al. 2003 ; Ruiz-Barradas and Nigam 2004 ; McCabe et al. 2004 ; Wang et al. 2006 ) that have provided substantial insights into the nature of drought and the important role of both the oceans and land–atmosphere interactions. In particular, this work addresses the remaining uncertainties

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Philip J. Pegion and Arun Kumar

Southern Hemisphere. There is also an increase in height around the date line in the North Pacific but lower heights over the Yukon, which suggests a wave train emanating from the tropical western Pacific. This circulation anomaly is more pronounced in boreal winter (not shown) when the precipitation along the South Pacific convergence zone (SPCZ) is suppressed and precipitation in the eastern Indian Ocean enhanced. These results are similar to what has been found in previous works ( Schubert et al

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

dryness over the southern United States and wetness over the Ohio Valley ( Fig. 10a ). The upper-level jet responds to suppressed convection and shifts northward (colored). The jet extends from the North Pacific ( Fig. 10f ) to the Southwest. In the tropics, a couplet of negative anomalies (contoured) straddles the equator over the cold SSTAs in the tropical Pacific. In midlatitudes, there is a Pacific–North American type of wave train with positive height anomalies close to the West Coast and

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

as the cause of a predicted recovery in Sahel rainfall. On the other hand, Held et al. (2005) show a very robust drying in the Sahel even in the presence of a reversal of the Atlantic gradient and attribute it to either a uniform warming or a warming of the Indian ocean ( Held et al. 2005 ; Bader and Latif 2003 ): a warmer ocean would produce a warmer troposphere in the entire tropical band and more stable conditions over Africa, leading to a reduction of rainfall over the Sahel ( Giannini et

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

empirical orthogonal function (REOF), representing a pan-Pacific ENSO-like pattern that includes a weak Indian Ocean component and no Atlantic component]; and (ii) the Pacific warm, Atlantic neutral (PwAn) experiment—this is the reverse of PcAn, with two standard deviations of the pan-Pacific ENSO-like REOF added to the climatological seasonal cycle. The two experiments thus represent opposite extremes in Pacific temperature anomalies, with Atlantic anomalies controlled. If Pacific temperatures do have

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