Maintenance of the Midtropospheric North African Summer Circulation: Saharan High and African Easterly Jet

Abstract

The conspicuous feature of the midtropospheric North African summer circulation is the Saharan high surrounded on its southern rim by the African easterly jet (AEJ). Like a major monsoon circulation, the Saharan high is juxtaposed with the North African divergent center to the east and the eastern Atlantic convergent center to the west. Different from a major monsoon circulation, these pronounced midtropospheric circulation components are overlaid by the western part of the Tibetan high. Because of the unique roles played by the Saharan high and the African easterly jet in the North African summer circulation, an effort is made to explore maintenance mechanisms of these two midtropospheric circulation elements. Major findings of this effort are summarized as follows: In terms of the velocity potential maintenance equation, it is shown that the North African divergent center over the Chad?Sudan region is maintained by the vertical differential heating established by the Saharan thermal-low heating in the lower troposphere and the Saharan radiative cooling in the upper troposphere. The Saharan high is spatially in quadrature with the North African divergent center. It is inferred from the streamfunction budget analysis that the Saharan high is maintained by the east?west circulation, which is formed by the east?west differential heating between the Saharan thermal-low heating and the eastern North Atlantic cooling. This inference is further substantiated by forced barotropic model simulations. The AEJ around the tropical periphery of the Saharan high is almost perpendicular to the equatorward divergent northerlies spilling out of the North African divergent center. The energetic interaction between divergent and rotational flows reveals that this jet is maintained by the Coriolis acceleration associated with these divergent winds. These findings not only reveal the maintenance mechanism of the Saharan high and the associated AEJ, but also facilitate the search for answers to some problems of the North African summer weather/climate system.