Case Studies of Cold Surges along the African Highlands

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Wednesday, 7 January 2015
Nicholas D. Metz, Hobart and William Smith Colleges, Geneva, NY; and C. Crossett

North–south-oriented regions of high terrain can serve as a focus for equatorward moving cold surges. Although the east coast of Africa features high terrain, few studies have focused on cold surges that occur to the lee of the African Highlands. A companion climatological study of these cold surges identified almost 150 surge events during the 2008–2012 period along the African Highlands. The purpose of this presentation is to identify representative case studies of these African Highlands cold surges to determine common synoptic-scale patterns and identify how these patterns are similar to and different from those associated with cold surges along other regions of high terrain.

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.