EVOLUTION OF THE NORTH-AMERICAN MONSOON SYSTEM

A dynamically oriented description of the North American summer. monso on system, which encompasses the Mexican monsoon and the associated la rge-scale circulation over the:continental United States, is provided, by developing an evolution climatology of the precipitation, troposphe ric circulation, moisture fluxes, diabetic heating, convective environ ment, and the adjoining basin SSTs. A distinguishing aspect of this st udy is the amount of independent data analyzed, such as the newly avai lable European Centre for Medium-Range Weather Forecasts (ECMWF) reana lyses, the National Centers for Environmental Prediction (NCEP) reanal yses, both satellite-derived and station data-based precipitation esti mates, and the heating diagnosed from both reanalyses. This also provi des a preliminary evaluation and comparison of the newly available NCE P and ECMWF reanalyses at the regional level, including the model-gene rated precipitation and heating distributions. The principal findings are the following. The accompaniment of the Mexican monsoon onset by d ecreased precipitation to the east is shown to be a robust climatologi cal feature. This striking linkage is also evident in the associated t ropospheric circulation and, notably, in the upper-level heating field s. The climatological phasing of the precipitation between the two are as is coherent even at the pentad timescale; While the Mexican monsoon onset is closely associated with thermodynamic favorability, the link age to the central United States, as reflected in the vertical velocit y and the low-level height fields, appears to be consistent with sever al possible forcings: the monsoon deep heating, the elevated heating o f the North American cordillera and plateau, and orographic forcing as sociated with the seasonal movement of the easterlies encroaching on t he North American cordillera. Although both reanalyses yield a tropica l-type deep tropospheric heating distribution in the Mexican monsoon r egion and, therefore, a potentially prominent role for the monsoon in the regional circulation, the considerable differences in the diagnose d heating vertical structure, thermodynamic balance, and the overall h eating magnitude between the two reanalyses, and even between the NCEP reanalysis-consistent heating and the NCEP model-produced heating, su ggest potentially significant differences in the implied dynamics of t he North American monsoon system.