Summer Circulation Climate of the American Southwest, 1945-1984

The dominant atmospheric circulation controls of summer climate in the U.S. Southwest are shown from classification of daily 500mb charts for the period 1945-84. I calculate seasonal synoptic circulation indices and test their relationships with Arizona summer rainfall variations using correlation analysis. Summer rainfall is related to anticyclonic activity aloft; on average, “wet” anticyclonic patterns dominate between about mid- to late July; “dry” anticyclonic patterns feature strongly in early July, early August, and early September; and cyclonic types increase in frequency from about mid-August onward. A statistically significant southward shift in the mean latitude of the subtropical ridge took place between the anomalously wet summers of the 1950s and the considerably drier summers of the 1970s. This shift was accompanied by important changes in the persistence of wet and dry anticyclonic patterns and the synoptic types that characteristically succeed such spells. There is, however, no evidence of a significant long-term trend in the position of the subtropical ridge for the entire 1945-84 period. No statistically significant relationship is found between the atmospheric circulation over the Southwest U.S. and the coupled tropical sea surface temperature/atmospheric index representative of the Southern Oscillation (SOI) for the full 40-year record. Statistically significant lag correlations are found, however, between the summer circulation and the SOI for two subperiods (1945-63, 1964-83). These results confirm that low-latitude teleconnections to the extratropics are not stable over time.