RELATIONSHIPS BETWEEN RAINFALL OVER SEMIARID SOUTHERN AFRICA, GEOPOTENTIAL HEIGHTS, AND SEA-SURFACE TEMPERATURES

An observational study has been conducted of the southern summer rainf all variability over the semiarid region of southern Africa (SASA) sin ce 1958, as related to the tropical African rainbelt, tropical atmosph eric circulation, and sea surface temperature (SST) patterns. A rotate d empirical orthogonal function (R-EOF) analysis was applied to the gl obal monthly SST anomalies. On a year-to-year basis, the first R-EOF m ode, related to the El Nino/Southern Oscillation (ENSO) phenomenon, wa s most strongly correlated with the southern African rainfall and two rainbelt indices the latitude of its center of gravity and the total r ainfall that occurs over the entire rainbelt zone. The anomalous warmi ng of the central-eastern equatorial Pacific, identified as the first R-EOF mode, was associated with increased 700 hPa heights throughout t he tropics except for the eastern Pacific, especially marked over sout hern Africa and the eastern Indian Ocean-central Pacific region. Simul taneous intensification of the 700 hPa subtropical high over southern Africa was found to cause a northward displacement in the rainbelt's c enter of gravity and a decrease in total rainfall. These changes in th e rainbelt parameters resulted in decreased rainfall for the SASA regi on, located south of the main rainbelt. On the other hand? the anomalo us cooling of the eastern Pacific was related to a weakening of the so uthern African sub tropical high. This leads to a simultaneous southwa rd advance and enhanced convection of the rainbelt, which ultimately r esult in increased SASA rainfall. On the decadal time scale, the third R-EOF mode, characterized by a dominant signal over the South Atlanti c, is most strongly correlated with trends in the SASA rainfall and ra inbelt parameters. The warming of the South Atlantic and simultaneous increases in the 700 hPa heights over southern Africa result in decrea sed rainfall; cooling and decreased 700 hPa heights lead to increased rainfall.