Relationships between rainfall over semi-arid southern Africa, geopotential heights, and sea surface temperatures

An observational study has been conducted of the southern summer rainfall variability over the semi-arid region of southern Africa (SASA) since 1958, as related to the tropical African rainbelt, tropical atmospheric circulation, and sea surface temperature (SST) patterns. A rotated empirical orthogonal function (R-EOF) analysis was applied to the global monthly SST anomalies. On a year-to-year basis, the first R-EOF mode, related to the El Niño/Southern Oscillation (ENSO) phenomenon, was most strongly correlated with the southern African rainfall and two rainbelt indices; the latitude of its center of gravity and the total rainfall that occurs over the entire rainbelt zone. The anomalous warming of the central-eastern equatorial Pacific, identified as the first R-EOF mode, was associated with increased 700 hPa heights throughout the tropics except for the eastern Pacific, especially marked over southern Africa and the eastern Indian Ocean-central Pacific region. Simultaneous intensification of the 700 hPa subtropical high over southern Africa was found to cause a northward displacement in the rainbelt's center of gravity and a decrease in total rainfall. These changes in the rainbelt parameters resulted in decreased rainfall for the SASA region, located south of the main rainbelt. On the other hand, the anomalous cooling of the eastern Pacific was related to a weakening of the southern African sub tropical high. This leads to a simultaneous southward advance and enhanced convection of the rainbelt, which ultimately result in increased SASA rainfall. On the decadal time scale, the third R-EOF mode, characterized by a dominant signal over the South Atlantic, is most strongly correlated with trends in the SASA rainfall and rainbelt parameters. The warming of the South Atlantic and simultaneous increases in the 700 hPa heights over southern Africa result in decreased rainfall; cooling and decreased 700 hPa heights lead to increased rainfall.