Influence of the North American Monsoon System on the U.S. Summer Precipitation Regime

Key features of the U.S. summer precipitation regime are examined within the context of the evolving North American monsoon system. The focus is on the antecedent and subsequent atmospheric conditions over the conterminous United States relative to the onset of monsoon precipitation over the southwestern United States, which typically begins in early July. The onset of the monsoon in this region is determined using a precipitation index, based on daily observed precipitation for a 31-yr (1963–94) period. Lagged composites of the observed precipitation and various fields from the NCEP–NCAR reanalysis for the period 1979–94 provide a comprehensive picture of atmospheric conditions during the evolution of the U.S. warm season precipitation regime. The summer precipitation regime is characterized by an out-of-phase relationship between precipitation over the Southwest and the Great Plains–northern tier and an in-phase relationship between precipitation over the Southwest and the East Coast. Changes in the upper-tropospheric wind and divergence fields (mean vertical motion) are broadly consistent with the evolution of this precipitation pattern. Enhanced upper-tropospheric divergence in the vicinity and south of the upper-tropospheric monsoon high coincides with enhanced upper-tropospheric easterlies and Mexican monsoon rainfall after onset. Over the Great Plains and along the northern tier, the middle- and upper-tropospheric flow is more convergent and 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 evolution. 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 onset in the vicinity of an “induced” trough over the eastern United States. The pattern of evaporation minus precipitation from the NCEP–NCAR reanalysis shows broad consistency with the divergence of the vertically integrated flux of water vapor during the monsoon, although the resolution in the NCEP–NCAR reanalysis is inadequate to yield quantitatively accurate regional estimates of these fields. In agreement with earlier studies, the NCEP–NCAR reanalysis indicates that most of the moisture below 850 hPa over the desert Southwest comes from the northern Gulf of California, while most of the moisture at and above 850 hPa arrives from over the Gulf of Mexico.