The Southwest Branch of the North American Monsoon during Summer 1979

Abstract

The physical characteristics of the southwest branch of the North American monsoon system during the summer of 1979 are studied with the FGGE dataset. The combined features such as the low pressure trough over northwestern Mexico, the penetration of the easterly flow from the Atlantic Ocean, and the intensification. of the anticyclonic gyre of the South Pacific, are responsible, during the summer, for a well established cross-equatorial surface air flow along the eastern tropical Pacific Ocean and western Mexico. Mean monthly maps of vertically integrated water-vapor flux show the development of a low-level jet favoring the penetration, into western Mexico, of a cross-equatorial moist flow which originates over the southern Pacific. It is shown that the South Pacific anticyclone gyre is an important feature which brings moist air along western Mexico. The mean monthly evolution of the net water-vapor flux divergence suggests a strong association with the precipitation pattern observed over Mexico.