INFLUENCE OF THE NORTH-AMERICAN MONSOON SYSTEM ON THE US SUMMER PRECIPITATION REGIME

Key features of the U.S. summer precipitation regime are examined with in the context of the evolving North American monsoon system. The focu s is on the antecedent and subsequent atmospheric conditions over the conterminous United Stares relative to the onset of monsoon precipitat ion over the southwestern United States, which typically begins in ear ly July. The onset of the monsoon in this region is determined using a precipitation index, based on daily observed precipitation for a 31-y r (1963-94) period. Lagged composites of the observed precipitation an d various fields from the NCEP-NCAR reanalysis for the period 1979-94 provide a comprehensive picture of atmospheric conditions during the e volution of the U.S. warm season precipitation regime. The summer prec ipitation regime is characterized by an out-of-phase relationship betw een precipitation over the Southwest and the Great Plains-northern tie r and an in-phase relationship between precipitation over the Southwes t and the East Coast. Changes in the upper-tropospheric wind and diver gence fields (mean vertical motion) are broadly consistent with the ev olution of this precipitation pattern. Enhanced upper-tropospheric div ergence in the vicinity and south of the upper-tropospheric monsoon hi gh coincides with enhanced upper-tropospheric easterlies and Mexican m onsoon rainfall after onset. Over the Great Plains and along the north ern tier, the middle-and upper-tropospheric flow is more convergent an d rainfall diminishes after onset to the north and east of the monsoon high. The frequency of occurrence of the Great Plains low-level jet ( LLJ) and southerly moisture transport change little during the evoluti on. However, LLJ-related precipitation is controlled by changes in the large-scale flow related to the North American monsoon system. There is increased upper-tropospheric divergence and precipitation after ons et in the vicinity of an ''induced'' trough over the eastern United St ates. The pattern of evaporation minus precipitation from the NCEP-NCA R reanalysis shows broad consistency with the divergence of the vertic ally integrated flux of water vapor during the monsoon, although the r esolution in the NCEP-NCAR reanalysis is inadequate to yield quantitat ively accurate regional estimates of these fields. In agreement with e arlier studies, the NCEP-NCAR reanalysis indicates that most of the mo isture below 850 hPa over the desert Southwest comes from the northern Gulf of California, while most of the moisture at and above 850 hPa a rrives from over the Gulf of Mexico.