Principal components-based regionalization of precipitation regimes across the southwest United States and Northern Mexico, with an application to monsoon precipitation variability

Precipitation is the critical climate variable in the arid borderlands of the United States and Mexico. Although climate is of course contiguous across the political boundary, historically, broad-scale analyses of precipitation across this border region have most often treated the U.S. or Mexican sides separately, partly for reasons of data availability. The region is physically and climatically complex, making simple comparisons of precipitation awkward. Yet, there are several wellrecognized climatic controls on precipitation across the region that reflect its sub-tropical location. The subtropical anticyclones and associated upper-level ridges centered over the region and adjacent Atlantic and Pacific oceans exert a dominant influence year-round, modulated by the role of the mid-latitude westerly circulation in winter and the North American monsoon in