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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

dry conditions (hereafter referred to loosely as “drought-induced warming”) can lead to increased crop stress and increased overall energy demand, exacerbating the dry period’s economic impact. A recent study ( Koster et al. 2009b , hereafter K09 ) examined drought-induced warming in the context of an underlying functional relationship between soil moisture and evaporation. A highly idealized version of this relationship, long discussed in the literature (e.g., Budyko 1974 ; Manabe 1969

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

1. Introduction Long-lasting drought has an enormous impact on the nation’s economy and society. Skillful drought prediction can mitigate devastating economic effects on people and ecosystems. To improve drought forecasts, one needs to understand the causes that trigger and sustain drought. Because drought implies prolonged rainfall and soil moisture deficits, they are often modulated by low-frequency sea surface temperature anomalies (SSTAs). In the Pacific, decadal trends of SSTAs in the

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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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Rachel R. McCrary and David A. Randall

Pacific SSTs, 2) variations in tropical North Atlantic SSTs, and 3) land–atmosphere interactions that involve feedbacks between soil moisture and rainfall. Recently, modeling studies that use atmospheric global circulation models (AGCMs) forced with historic time series of global SSTs have implicated cool, La Niña–like conditions in the tropical Pacific as the primary cause of long-term Great Plains drought ( Schubert et al. 2004a , b ; Seager et al. 2005a , b , 2007 ; Cook et al. 2007 ; Seager et

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Kirsten L. Findell and Thomas L. Delworth

indicator of longer-term drought. Both indices were justified in their original publications as improvements over the Palmer drought severity index (PDSI; Palmer 1965 ). The PDSI was a landmark development in drought research because it was the first water-budget-based index: it incorporated antecedent precipitation, moisture supply, and moisture demand into a hydrologic accounting system ( Heim 2002 ). It was widely adapted to regions far from its original development zone in the Great Basin of the

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

) strengthens the GPLLJ while the opposite weakens it. There are also implications for the timing of the peak magnitude in the seasonal cycle with the PcAw exhibiting a maximum one month earlier than the PwAc scenario. This undoubtedly would have significant implications for the timing of peak moisture availability and dynamic low-level convergence in the central United States. The precipitation response exhibits similar characteristics; however, the degree of sensitivity to the prescribed SST is weaker

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Renu Joseph and Ning Zeng

Agung events from the model. The anomalies of Pinatubo and El Chichón are from the 1981–2000 climatology while that of Agung is from 1961–1980 climatology. ENSO is regressed out in the observations as described in the text. Units are in mm day −1 . The contours in precipitation are for the 5 mm day −1 climatology; and this qualitatively indicates the seasonal movement of precipitation. To further understand the decrease in the monsoonal flow, in Fig. 6 we show the total moisture convergence

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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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Bradfield Lyon

land to be negatively correlated is well known ( Madden and Williams 1978 ; Huang and van den Dool 1993 ; Trenberth and Shea 2005 ; Déry and Wood 2005 , and many others). The physical linkage is via the surface energy budget with below-average precipitation typically associated with reduced soil moisture and increased insolation with both favoring an increase in the surface sensible heat flux and therefore higher surface air temperature. The full picture is somewhat more nuanced, as it is the

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