Influence of Sea Surface Temperature Anomalies on the Precipitation Regimes over the Southwest United States

Abstract

Precursory signals in sea surface temperature anomalies (SSTAs) associated with summer precipitation over the Southwest (SW) United States are examined using data from 1900 to 1996. Two patterns of SSTAs are found to influence summer and winter precipitation evolution. One pattern shows positive anomalies extending from the central Pacific to the west coast of the Americas during winter. Warm SSTAs over the equatorial central Pacific in winter are associated with wet conditions in the SW and dry conditions over the Pacific Northwest. The following summer, SSTAs in the central Pacific diminish, but positive SSTAs remain over the tropical eastern Pacific and in the vicinity of Central America. Warm SSTs are conducive to enhanced rising motion and convection. A compensating downward branch is found over the SW favoring dry conditions over that area. Negative SSTAs have the opposite effect. This mode is similar to the SSTA pattern discussed by Higgins et al. When this mode is active, a wet (dry) SW winter is followed by a dry (wet) summer. In addition to SSTAs over the central and eastern Pacific, another controlling factor is the SSTA over the North Pacific. Negative (positive) SSTAs over the North Pacific persist from winter to summer and favor positive (negative) rainfall anomalies over the SW. When this mode is active, a wet (dry) SW persists from winter to summer. The authors conclude that SW rainfall evolution from winter to the following summer is modulated by both SSTAs over the equatorial Pacific and over the North Pacific. SSTAs in the equatorial Pacific alone are not sufficient to explain the observed summer rainfall variability over the southwest United States.