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Alberto M. Mestas-Nuñez, David B. Enfield, and Chidong Zhang

European Centre for Medium-Range Weather Forecasts (ECMWF) 15-yr global reanalysis (ERA15, Gibson et al. 1997 ). They examine uncertainties arising from choices of area boundaries, calculation algorithms, spatial and temporal resolutions, as well as a combination of these effects. Their main conclusion is that these uncertainties are smaller than the large annual and interannual variabilities in moisture flux divergence estimated from the NCEP–NCAR reanalysis. The implication is that the NCEP

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Katrina Grantz, Balaji Rajagopalan, Martyn Clark, and Edith Zagona

; Douglas et al. 1993 ; Higgins et al. 1997 ; Mitchell et al. 2002 ; Sheppard et al. 2002 ). The variability of this important moisture source is of particular concern for watershed managers, ranchers, and planners in southwestern North America. Too little summer rainfall has negative agricultural and environmental impacts, while heavy summer thunderstorms present the danger of flash floods. Predicting the variability in the strength, location, and timing of monsoonal precipitation is understandably

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Chunmei Zhu, Tereza Cavazos, and Dennis P. Lettenmaier

). Nonetheless, it has tremendous local socioeconomic impact especially considering the rapid population increase in the last few decades in the affected region ( Castro et al. 2001 ). Therefore, improved seasonal predictability of the NAM precipitation would have not only important scientific implications, but important practical ones as well. Improved predictability of NAMS precipitation is also motivated by the fact that NAMS has not only local affects, but is also linked to warm season precipitation over

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Andrea J. Ray, Gregg M. Garfin, Margaret Wilder, Marcela Vásquez-León, Melanie Lenart, and Andrew C. Comrie

northern Mexico, contributing more than 50% of the annual precipitation in some areas ( Sheppard et al. 2002 ). The monsoon typically begins in southern Mexico in early June and progresses northward to the southwestern United States by early July ( Adams and Comrie 1997 ; Higgins et al. 1999 ). The region’s climate is highly variable: in northern Sonora over the past decade, climate variability has included 7–8 yr of drought, intense rains in 1994–95, and freezing temperatures in 1996 (Browning

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Michelle Hallack-Alegria and David W. Watkins Jr.

temperatures (SSTs), with El Niño events resulting in wet winters and La Niña events in dry winters ( Kiladis and Diaz 1989 ). Warm season teleconnections do not appear to be as strong, but Higgins and Shi (2000) suggest that interannual variability in the NAM in the southwestern United States is modulated by long-term fluctuations in Pacific SSTs, and furthermore it is known that the Pacific decadal oscillation has the potential of amplifying ENSO effects. McPhee et al. (2004) further suggest that

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X. Gao, J. Li, and S. Sorooshian

viewpoint of model improvement, it is important to examine how the diagnostic features and their variations can be captured by a numerical model in its case-by-case predictions. This study intends to evaluate the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model’s (MM5) capabilities and limitations in reproducing intraseasonal variabilities in the NAM precipitation in the context of the 2004 monsoon season. Studies for the past two decades have

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Wanqiu Wang and Pingping Xie

. (2005) showed that the simulated NAM rainfall from the PSU–NCAR MM5 was sensitive to the specified SSTs. In addition, accurate estimate of SSTs are also found to be important in the monitoring and forecast of hurricane activities in the Gulf of Mexico and the western Atlantic ( Bender and Ginis 2000 ), which have strong potential impacts over North America. Results from these studies indicate that an accurate SST analysis is essential for understanding the mechanisms of the NAM variability and for

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J. Craig Collier and Guang J. Zhang

intensification of an anticyclonic gyre is a major forcing mechanism for the low-level monsoon moisture-laden circulation over western Mexico. And the monsoon is more than just a hydrologic forcing on the climate system. Barlow et al. (1998) show that in the monsoon’s mature phase diabatic heating over northwestern Mexico is as large as 1 K day −1 , which influences continental-scale divergence and vorticity dynamics. Accurately predicting the interannual variability of the monsoon system would aid water

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