Water vapor transport associated with the summertime North American monsoon as depicted by ECMWF analyses

The origins and transport of water vapor into the semi-arid Sonoran Desert region of southwestern North America are examined for the July-August wet season. Vertically integrated fluxes and flux divergences of water vapor are computed for the 8 summers 1985-1992 from ECMWF mandatory level analyses possessing a spectral resolution of triangular 106 (T106). The ECMWF analyses indicate that transports of water vapor by the time-mean flow dominate the transports by the transient eddies. Most of the moisture at upper levels (above 700 mb) over the Sonoran Desert arrives from over the Gulf of Mexico, while most moisture at low levels (below 700 mb) comes from the northern Gulf of California. There is no indication of moisture entering the Sonoran Desert at low levels directly from the southern Gulf of California or the tropical East Pacific. Water vapor from the tropical East Pacific can enter the region at upper levels after upward transport from low levels along the western slopes of the Sierra Madre Occidental of Mexico and subsequent horizontal transport aloft. The T106 ECMWF analyses, when only the mandatory level analyses are used, do not possess sufficient resolution to yield accurate estimates of highly differentiated quantities such as the divergence of the vertically integrated flux of water vapor. Even at a T106 resolution, the northern Gulf of California and the terrain of the Baja California peninsula are not adequately resolved.