The role of the Southern Oscillation (SO) in the interannual variability of convection and rainfall over Mexico is studied by correlation analysis of long‐term precipitation records, a satellite‐derived set of highly reflective clouds (HRC), upper‐air soundings, surface ship observations, and an SO index defined as high for anomalously high low pressure at Tahiti/Darwin. Highly reflective clouds and rainfall have a similar annual cycle and interannual variability, except in certain regions and seasons with prevalence of stratiform cloudiness. During the boreal winter (November‐April) dry season, the low SO phase (or El NiñMo) is, in much of the country, characterized by increased precipitation associated with the enhanced influence of mid‐latitude westerlies. However, in the Isthmus of Tehuantepec, most exposed to Norte invasions, precipitation is larger in the high SO phase. During the boreal summer (May‐October) rainy season, precipitation is more abundant in the high SO phase, when the northward retraction of the North Atlantic High, weaker trades, and a northward displaced near‐equatorial trough are conducive to enhanced ascending motion over the area. Copyright © 1990 John Wiley & Sons, Ltd
