The Relationship of Highly Reflective Clouds to Tropical Climate Anomalies

The interannual variability of tropical convection related to the Southern Oscillation (SO) and regional climate anomalies is studied from satellite-derived estimates of highly reflective clouds (HRC) during 1971–87. The novel HRC data bank provides a particularly useful measure of tropical convection for the purposes of climate diagnostics, because of its length and continuity of record. For the first time, maps are presented of the patterns of correlation between the SO, as well as regional rainfall anomalies, and convection over the global tropics. Throughout the year, the SO (high SO phase defined by anomalously high/low pressure at Tahiti/Darwin) exhibits a highly significant negative correlation with HRC in the equatorial Pacific but a much weaker positive correlation with Indonesia. The SO is correlated positively with HRC in the Amazon basin in boreal winter but negatively with HRC over central Africa throughout most of the year. The three equatorial convection centers tend to vary in unison, in particular those over the Amazon basin and central Africa, while the positive correlations of any of these centers with the SO are much weaker. Copious precipitation during the March-April rainy season of northeast Brazil is associated with a southward displaced low-pressure trough and embedded wind confluence, as well as a southward shift of the convection belt in the sector extending from South America across the Atlantic into equatorial Africa. During abundant Nordeste rainy seasons, as in the high SO phase, convective activity tends to be enhanced over Indonesia but reduced in the equatorial Pacific. Copious rainfall in Subsaharan West Africa (Sahel) tends to be associated with the high SO phase and thus intense convection over Indonesia and reduced convective activity in the equatorial central Pacific. Another new finding is the strong inverse relationship of Sahel rainfall with the convection over central Africa. Abundant Indian summer monsoon rainfall is accompanied by enhanced convective activity over the Indian Ocean and Indonesia and reduced convection in the equatorial central Pacific, characteristics of the high SO phase.