INTERANNUAL VARIABILITY OF THE US SUMMER PRECIPITATION REGIME WITH EMPHASIS ON THE SOUTHWESTERN MONSOON

Relationships between the interannual variability of the U.S. summer p recipitation 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 o ver the southwestern United States. The onset of the monsoon in this r egion, which typically begins in early July, is determined using an in dex based on daily observed precipitation for a 32-yr (1963-94) period . Composites of observed precipitation and various fields from the Nat ional Centers for Environmental Prediction-National Center for Atmosph eric Research Reanalysis for wet and dry monsoons are used to show tha t the interannual variability of the summer precipitation regime close ly 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 p lains/Northern Tier and an in-phase relationship between the Southwest and the East Coast. This pattern is preserved for both wet and dry mo nsoons, but the Southwest is relatively wetter and the Great Plains ar e relatively drier during wet monsoons. Wet (dry) monsoons are also as sociated with a stronger (weaker) upper-tropospheric monsoon anticyclo ne over the western United States, consistent with changes in the uppe r-tropospheric divergence, midtropospheric Vertical motion, and precip itation patterns. The intensity of the monsoon anticyclone over the we stern United States appears to be one of the most fundamental controls on summertime precipitation downstream over the Great Plains. Evidenc e is presented that the interannual variability of the U.S. warm seaso n 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 convergen ce zone, present during the winter and spring preceding the monsoon, a re linked via an anomalous local Hadley circulation to the warm season precipitation regime over the United States and Mexico. Wet (dry) sum mer monsoons tend to follow winters characterized by dry (wet) conditi ons in the Southwest and wet (dry) conditions in the Pacific Northwest . This association is attributed, in part, to the memory imparted to t he atmosphere by the accompanying Pacific SST anomalies.