El Nio-Southern Oscillation diversity and Southern Africa teleconnections during Austral Summer

A wide range of sea surface temperature (SST) expressions have been observed during the El Nio-Southern Oscillation events of 1950-2010, which have occurred simultaneously with different global atmospheric circulations. This study examines the atmospheric circulation and precipitation during December-March 1950-2010 over the African Continent south of 15S, a region hereafter known as Southern Africa, associated with eight tropical Pacific SST expressions characteristic of El Nio and La Nia events. The self-organizing map method along with a statistical distinguishability test was used to isolate the SST expressions of El Nio and La Nia. The seasonal precipitation forcing over Southern Africa associated with the eight SST expressions was investigated in terms of the horizontal winds, moisture budget and vertical motion. El Nio events, with warm SST across the east and central Pacific Ocean and warmer than average SST over the Indian Ocean, are associated with precipitation reductions over Southern Africa. The regional precipitation reductions are forced primarily by large-scale mid-tropospheric subsidence associated with anticyclonic circulation in the upper troposphere. El Nio events with cooler than average SST over the Indian Ocean are associated with precipitation increases over Southern Africa associated with lower tropospheric cyclonic circulation and mid-tropospheric ascent. La Nia events, with cool SST anomalies over the central Pacific and warm SST over the west Pacific and Indian Ocean, are associated with precipitation increases over Southern Africa. The regional precipitation increases are forced primarily by lower tropospheric cyclonic circulation, resulting in mid-tropospheric ascent and an increased flux of moisture into the region.