Case Studies of Cold Surges along the African Highlands
One of these cases occurred from 13–20 August 2011 and featured a surface anticyclone that formed near the southern coast of Africa in a favorable region of subsidence associated with quasi-geostrophic forcing for descent. As the anticyclone progressed eastward, 925-hPa winds became southerly and ageostrophic as they advected cold air equatorward along the lee of the African Highlands. This particular case was associated with 925-hPa ageostrophic winds between 20 and 30 knots, and 925-hPa temperatures that were up to 10°C below normal as far equatorward as 15°S. As the cold surge evolved a well-defined upper-level ridge folded over anticyclonically. Cold air advection remained anchored to the high terrain even as the dynamical forcing associated with the upper-level ridge and surface anticyclone weakened and progressed eastward.