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Kerry H. Cook and Edward K. Vizy

1. Introduction The Caribbean low-level jet (CLLJ) is an easterly jet located over the Caribbean Sea between the northern coast of South America (Venezuela and Columbia) and the Greater Antilles (Cuba, Haiti, Dominican Republic, and Puerto Rico). It is present throughout the year and transports large amounts of moisture from the tropical Atlantic into the Caribbean Sea, into the Gulf of Mexico, across Central America, and into the Pacific basin. In this paper, we build on results from previous

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Matías Méndez and Víctor Magaña

–NCAR) reanalysis ( Kalnay et al. 1996 ) for the period 1948–2002. Specifically, EW activity is analyzed in the IAS region through the estimate of high-frequency (3–9 days) variance of the meridional wind at 700 hPa ( Diedhiou et al. 1999 ) from June through September in the central Caribbean Sea (17.5°N and 70°W). Finally, the Caribbean low-level jet (CLLJ) intensity is defined by averaging the 925-hPa zonal wind anomalies multiplied by −1 over the region 12.5°–17.5°N and 80°–70°W ( Wang 2007 ). 3. Persistent

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

2008 ; Weaver et al. 2009 ) and mean SST in the Atlantic warm pool region ( Wang et al. 2007 ). A vitally important mechanism for warm season central U.S. precipitation variability is the Great Plains low-level jet (GPLLJ). Precipitation variations are extremely sensitive to this dynamic low-level circulation feature (see Fig. 4 herein) ( Helfand and Schubert 1995 ; Higgins et al. 1997b ; Schubert et al. 1998 ). As such, fluctuations in the strength, placement, and timing of the GPLLJ exert

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Antonietta Capotondi and Michael A. Alexander

heights are raised, consistent with tropical warming, while heights are lower in the midlatitudes. The mechanism by which tropical Pacific SST anomalies influence precipitation in the American west involves changes in strength and position of the subtropical jets in both hemispheres and changes in transient eddies and eddy-induced meridional flow ( Seager et al. 2005 ). When the tropical Pacific is cold, the equator-to-pole tropospheric temperature gradient decreases, and the subtropical jet streams

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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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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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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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Kirsten L. Findell and Thomas L. Delworth

these studies shows that drought in much of the United States in particular and throughout the midlatitudes in general tends to occur when the eastern tropical Pacific is cooler than normal (La Niña conditions). Seager et al. (2003) explain that this impact is driven by changes in the locations of the subtropical jets and the effects these jets have on the eddy-driven mean meridional circulation. Hoerling and Kumar (2003) , Schubert et al. (2004) , and others extend this result to show that a

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

this case from both greenhouse gases and aerosols, possibly modified by local land–atmosphere feedbacks; second, as suggested by a preliminary analysis of interannual variability, changes in the tropical SST and in the midlatitude jet. Previous studies have focused on the role of SST trends in forcing rainfall trends in the Sahel. We have shown that the Sahara low is a skillful predictor for Sahel rainfall changes and that land–sea thermal contrast alone does not control those characteristics of

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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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