Evolution of the North American Monsoon System

A dynamically oriented description of the North American summer monsoon system, which encompasses the Mexican monsoon and the associated large-scale circulation over the continental United States, is provided by developing an evolution climatology of the precipitation, tropospheric circulation, moisture fluxes, diabatic heating, convective environment, and the adjoining basin SSTs. A distinguishing aspect of this study is the amount of independent data analyzed, such as the newly available European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses, the National Centers for Environmental Prediction (NCEP) reanalyses, both satellite-derived and station data–based precipitation estimates, and the heating diagnosed from both reanalyses. This also provides a preliminary evaluation and comparison of the newly available NCEP and ECMWF reanalyses at the regional level, including the model-generated precipitation and heating distributions. The principal findings are the following. The accompaniment of the Mexican monsoon onset by decreased precipitation to the east is shown to be a robust climatological feature. This striking linkage is also evident in the associated tropospheric circulation and, notably, in the upper-level heating fields. The climatological phasing of the precipitation between the two areas is coherent even at the pentad timescale. While the Mexican monsoon onset is closely associated with thermodynamic favorability, the linkage to the central United States, as reflected in the vertical velocity and the low-level height fields, appears to be consistent with several possible forcings: the monsoon deep heating, the elevated heating of the North American cordillera and plateau, and orographic forcing associated with the seasonal movement of the easterlies encroaching on the North American cordillera. Although both reanalyses yield a tropical-type deep tropospheric heating distribution in the Mexican monsoon region and, therefore, a potentially prominent role for the monsoon in the regional circulation, the considerable differences in the diagnosed heating vertical structure, thermodynamic balance, and the overall heating magnitude between the two reanalyses, and even between the NCEP reanalysis-consistent heating and the NCEP model-produced heating, suggest potentially significant differences in the implied dynamics of the North American monsoon system.