Recorded presentation5A.3
Monsoon breaks over northern Africa: Role of cold air surges
Edward K. Vizy, Cornell University, Ithaca, NY; and K. H. Cook
The existence and recurrence of cold air surges over northern Africa in summer is demonstrated, and their dynamics is diagnosed. Cold air surges occur in conjunction with pronounced mid-tropospheric ridge/trough patterns, typically near topography. Such a pattern is set up over northern Africa and the Mediterranean Sea in summer due to the topography of northwestern Africa in combination with thermal forcing due to land/sea constrasts. Intensification of the upper-tropospheric westerly jet enhances upper-level wind convergence and subsidence over the eastern Mediterranean Sea, generating the downward advection of warmer potential temperatures over the eastern Mediterranean Sea. The warming is balanced primarily by the equatorward advection of cooler air across northern Africa, into the central and eastern Sahel. The cold air surges are associated with intraseasonal variations in rainfall, including monsoon breaks over the Sahel and the formation of mesoscale convective systems (MCSs) and rainfall enhancements farther south. Three cold air surge events are examined in detail. Mesoscale convective activity initially increases over the eastern Sahel as low-level wind convergence is enhanced when the cold air surge frontal boundary pushes equatorward. In the wake of the frontal boundary passage, rainfall rates abruptly diminish over the eastern Sahel as convection shifts southward and is suppressed over the Sahel by the stabilization of the vertical column. Such breaks in rainfall last between 2 - 7 days in the case studies selected. The MCSs associated with the cold air surge convergence front over eastern Africa propagate westward into central and western Africa south of the Sahel, enhancing rainfall in these regions. Recorded presentation
Session 5A, African Climate: V. Eastern and Southern Africa
Tuesday, 22 January 2008, 8:30 AM-9:45 AM, 215-216
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