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 De sert region of southwestern North America are examined for the July-Au gust wet season. Vertically integrated fluxes and flux divergences of water vapor are computed for the 8 summers 1985-1992 from ECMWF mandat ory level analyses possessing a spectral resolution of triangular 106 (T106). The ECMWF analyses indicate that transports of water vapor by the rime-mean flow dominate the transports by the transient eddies. Mo st of the moisture at upper levels (above 700 mb) over the Sonoran Des ert arrives from over the Gulf of Mexico, while most moisture at low l evels (below 700 mb) comes from the northern Gulf of California. There is no indication of moisture entering the Sonoran Desert at low level s directly from the southern Gulf of California or the tropical East P acific. Water vapor from the tropical East Pacific can enter the regio n at upper levels after upward transport from low levels along the wes tern slopes of the Sierra Madre Occidental of Mexico and subsequent ho rizontal transport aloft. The T106 ECMWF analyses, when only the manda tory level analyses are used, do not possess sufficient resolution to yield accurate estimates of highly differentiated quantities such as t he divergence of the vertically integrated flux of water vapor. Even a t a T106 resolution, the northern Gulf of California and the terrain o f the Baja California peninsula are not adequately resolved.