Cold Surges along the African Highlands

Equatorward moving cold surges are ubiquitous features along the lee of high terrain, especially during the cold season. These cold surges have been studied along many mountain ranges including, the Andes, Appalachians, Rockies, and Himalayas. However, even though the east coast of Africa features high terrain, a dearth of research exists on cold surges along the African Highlands despite the fact that the surges could have potentially large agricultural effects. The purpose of this presentation is to examine these African Highlands cold surges from both a climatological and case study perspective.

A five-year climatology of African Highlands cold surges was created spanning the 2008 to 2012 period. This climatology revealed that African Highlands cold surges had a climatological maximum in September, and the strongest events were featured throughout the Southern Hemisphere winter. These cold surges feature temperature drops of between 2°C and 11°C, as 925-hPa meridional flow averaging 35 knots advected Antarctic air equatorward. Cold surges along the African Highlands last from one, to fifteen days, with the highest frequency of events spanning a three-day period. A representative case study reveals that during a cold surge event, a surface anticyclone forms near the southern coast of Africa in a favorable region of subsidence, associated with quasi-geostrophic forcing for descent. As the anticyclone progresses eastward, 925-hPa winds become southerly and ageostrophic as they advect cold air equatorward along the lee of the African Highlands.