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The goal of this research is to identify key features of atmospheric circulation that influence winter climate variability in the Sonoran Desert region, and to determine how much variability in climate can be explained by these features. The relationship between winter climate and atmospheric circulation is investigated through the use of circulation indices, which describe the principal features of circulation patterns. A set of six circulation indices was used. Existing indices used include the Southern Oscillation Index (SOI) and a sea-surface temperature index (SST) from the equatorial Pacific. Derived indices include a modified Pacific North American (PNA) index, a cyclone frequency index, a south-western trough index, and a Pacific-high south-western low index. Winter climate is described in terms of numbers of rainy days and average maximum temperatures at a network of 40 to 50 climate stations across southern California, Arizona, and western New Mexico.

A rotated principal components analysis on the six indices yielded a component containing SOI, SST, and PNA indices and a component containing the cyclone frequency, south-western trough, and Pacific-high-south-western low indices. These two components appear to be defining two types of circulation mechanisms influential to climate; an El Nino-Southern Oscillation and PNA mechanism, and a South-western Low mechanism, although it is likely that the two are not independent. The two components explained up to 63 per cent of the variation in numbers of rainy days and up to 82 per cent of the variation in maximum winter temperatures. These results suggest that there are two main circulation features that influence climate variation in this area, and have important implications for how changes in global- to hemispheric-scale circulation features may influence climate in the Sonoran Desert region. (C) 1997 by the Royal Meteorological Society.