Abstract
The southwestern United States receives more than half of its annual precipitation from winter storms. Winter precipitation is an important source of reservoir recharge (Sheppard et al. 2002) and plays a significant role in determining wildfire conditions in the following summer (Westerling et al. 2006). Generalized drought prediction in the Southwest is complicated by the bimodal distribution of precipitation. By focusing on winter precipitation, we can quantify the relationship between jet position and a significant portion of the annual precipitation. We will present the results from time series analyses comparing the winter Northern Annular Mode (NAM) index to instrumental and tree ring records of winter precipitation in northern Arizona over the twentieth century. Over the past two decades, the NAM has shifted into a predominantly high index pattern. One result has been a northward migration of the jet stream over the Pacific Ocean, and changes in the number of storms reaching the southwest are possible (Thompson and Wallace 1999). Tree ring records have been used to reconstruct winter precipitation records in a number of studies throughout the Southwest (e.g., Salzer and Kipfmueller 2005, Woodhouse 1997). Better constraint of the role of jet position in winter precipitation amount in the Southwest will allow better prediction of reservoir and soil moisture recharge and therefore, better planning of water and fire fighting resources.
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