Eigenvector Analysis of Seasonal Temperature, Precipitation and Synoptic-Scale System Frequency over the Contiguous United States. Part I: Winter

Abstract

Using eigenvector methods, the principal anomaly patterns of winter temperature, precipitation and cyclone/anticyclone frequency over the contiguous United States are described. The period of record used is 1894/95–1977/78; the data consist of state averages for temperature and precipitation and 10° × 10° grid counts for the cyclone/anticyclone analysis. The first three temperature eigenvectors (out of a possible 48) account for 86% of the total seasonal variance; the principal seasonal precipitation eigenvectors account for proportionally smaller variance, with 67% being contained in the first five eigenvectors. An even smaller variance reduction is effected with the variation of cyclone and anticyclone centers over the United States. Nevertheless, the first three components (out of a possible 12) account for 46 and 48%, respectively, of the variance in the series of cyclone and anticyclone frequency counts. The principal modes of variation of mean winter temperature and precipitation appear to be associated with the season-to-season positioning (both in terms of the amplitude and phase) of the major long-wave trough in eastern North America. Principally over the eastern two-thirds of the United States, this results in a predominance of cold/dry and warm/wet winter types. For the western United States the tendency is for colder and wetter versus warmer and drier regimes. Variations in synoptic-scale system frequencies appear to be associated with latitudinal shifts in the mean position of the jet stream from season to season, with land-sea contrasts along the Atlantic Coast, and with changes that take place over the eastern Pacific.