Interannual Variability of the U.S. Summer Precipitation Regime with Emphasis on the Southwestern Monsoon

Relationships between the interannual variability of the U.S. summer precipitation regime and the intensification, weakening, or changes in position of the climatological-mean circulation features that organize this regime are examined. The focus is on the atmospheric conditions over the conterminous United States relative to wet and dry monsoons over the southwestern United States. The onset of the monsoon in this region, which typically begins in early July, is determined using an index based on daily observed precipitation for a 32-yr (1963–94) period. Composites of observed precipitation and various fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research Reanalysis for wet and dry monsoons are used to show that the interannual variability of the summer precipitation regime closely mimics the seasonal changes associated with the development of the North American monsoon system. The warm season precipitation regime is characterized by a continental-scale precipitation pattern consisting of an out-of-phase relationship between the Southwest and the Great Plains/Northern Tier and an in-phase relationship between the Southwest and the East Coast. This pattern is preserved for both wet and dry monsoons, but the Southwest is relatively wetter and the Great Plains are relatively drier during wet monsoons. Wet (dry) monsoons are also associated with a stronger (weaker) upper-tropospheric monsoon anticyclone over the western United States, consistent with changes in the upper-tropospheric divergence, midtropospheric vertical motion, and precipitation patterns. The intensity of the monsoon anticyclone over the western United States appears to be one of the most fundamental controls on summertime precipitation downstream over the Great Plains. Evidence is presented that the interannual variability of the U.S. warm season precipitation regime is linked to the season-to-reason ‘‘memory’’ of the coupled atmosphere–ocean system over the eastern tropical Pacific. In particular, it is shown that SST anomalies in the eastern Pacific cold tongue and precipitation anomalies in the intertropical convergence zone, present during the winter and spring preceding the monsoon, are linked via an anomalous local Hadley circulation to the warm season precipitation regime over the United States and Mexico. Wet (dry) summer monsoons tend to follow winters characterized by dry (wet) conditions in the Southwest and wet (dry) conditions in the Pacific Northwest. This association is attributed, in part, to the memory imparted to the atmosphere by the accompanying Pacific SST anomalies.