Variability in boundary layer structure during HAPEX-Sahel wet-dry season transition

The variability of the Sahelian boundary layer has been studied with streamline analyses, rainfall measurements, and upper air soundings during its transition from wet to dry season. The 1992 rainy season ended prematurely because of the early arrival of westerly troughs over West Africa. The change in the circulation is related to global-scale atmospheric circulations as successive westerly troughs over this region can be traced back upstream on a planetary scale. Once the upper level easterlies changed to westerlies, the large-scale circulation brought the surface northeasterly flow southward, which led to the retreat of the Southwest Monsoon in Niger. The boundary layer responded quickly to this transition of synoptic events from wet to dry seasons. During the wet period, the boundary layer was relatively cool and moist because evapotranspiration dominated, keeping the surface cool and preventing significant direct sensible heating of the boundary layer. During the transition period, less extensive showery weather allowed the boundary layer to have more time to recover from rainfall episodes, leading to a warming and drying trend. During the dry period, soil moisture contents dropped rapidly. With more sensible heat flux made available for boundary layer heating and energetics, the boundary layer reached its maximum temperatures and minimum moisture contents during the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) intensive observational period. Budget calculations indicate that the horizontal advection and vertical flux divergence terms were most important during the wet period, whereas during the dry period, the subsidence and vertical flux divergence terms were most important. From wet to dry seasons, the vertical wind shear of the zonal wind was reduced from 23 m s−1 to 16 m s−1, consistent with vertical wind shear differences between wet and dry years as reported in the literature. Similarities and differences with the First ISLSCP Field Experiment (FIFE-89) boundary layer are also examined. It is hypothesized that the retreat of the southwesterly monsoon could be upheld by a sustained secondary circulation if the wet season rainfall pattern imprints an organized south to north soil moisture gradient maintaining a concurrent reverse gradient in surface sensible heating. The boundary layer circulation that would be established in response to the heating gradient would reinforce surface southwesterlies, as well as reinforcing mid-level easterlies of which the African Easterly Jet is a part, and thus help uphold the intrusion of westerlies and the monsoon retreat. Such a mechanism, whose effectiveness would be a function of how distinct the south-north soil moisture gradient develops from the wet season precipitation pattern, could help explain the large interannual variability of rainfall over the Sahel