The summer plateau low pressure system of Mexico

Average July heights and winds over a 10-yr period are computed at 850 mb o ver Mexico using both surface and radiosonde data. During the day central M exico at this level is dominated by low pressure, with some smaller areas o f high pressure. The pressure systems were less pronounced in 1990 and the smaller high pressure areas were notably weaker during 1993, showing that t here is important interannual variability in these patterns. At night, the low pressure weakens and there are generally expanded areas of high pressur e. This area of low pressure is caused by the high plateau, which acts as an e levated heat source for the atmosphere during the day. Thus, the circulatio n is an extension of a similar one previously observed in the United States and is of about the same magnitude. Although computation of 850-mb heights from surface data helps improve horizontal resolution at this level, addit ional data would be needed to assess the vertical structure of the plateau circulation system as well as to provide more details on the diurnal variat ions.