Interannual fluctuations of observed summer rainfall across the monsoon region of the southwestern United States are analyzed to ascertain their spatial coherence and to rest the hypothesis that antecedent spring snowpack anomalies may modulate the monsoon and exhibit an inverse correlation with summer rainfall anomalies. To characterize the spatial coherence of seasonal rainfall anomalies, an objective linear analysis of interannual variability is applied to climate divisional data across the Southwest. Three coherent subregions are identified, broadly representing rainfall anomalies across Arizona, eastern New Mexico/western Texas (the Southwest Plains), and most of New Mexico. Interannual fluctuations of summer rainfall in the New Mexico region exhibit a very significant negative correlation with a large-scale index of the antecedent 1 April snowpack over the southern U.S. Rocky Mountains during the 1961-90 climatic averaging period. This strong relationship seems to break down in the years before and after this period, possibly indicating a shift in climate associated with other forcing factors.
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