Dynamics of the Low-Level Westerly Jet Over West Africa

As identified earlier, a low level westerly jet forms over West Africa from June until mid October. This jet plays a major role in transporting moisture into the Sahel during the summer. Also, the strong horizontal and vertical wind shears associated with the jet over coastal West Africa are very important to the summer circulation in the area. Since only a few studies have been done on this low level jet, further understanding is needed. In our paper, the structure, seasonality, diurnal cycles, and dynamics of the low-level westerly jet over West Africa are investigated in the high resolution ERA40 reanalysis, with the lower resolution NCEP/DOE AMIP-II reanalysis added for reference. Five stages of jet development are identified in the daily ERA40 climatology. At 925 hPa, westerly winds start over the ocean in early June at 8°-11°N, between 13°W and the coast of West Africa. Westerly winds expand westward to 22°W during late June into July. From late July to early September, the jet reaches a maximum in the region of 13°-30°W and 9°-11°N with wind speeds about 3-4 m/s in the 26-year NCEP2 climatology and 5-6 m/s in the 44-year ERA 40 climatology. The jet decreases and retreats eastward to 25°W during mid September. From late September to mid October, the westerly winds over the ocean strongly decrease to 1-2 m/s and the jet disappears. The formation of the jet accompanies the development of a westward extension of the continental thermal low over the ocean. A local low-level low is formed when the jet is at its maximum. Study of the vertical structure of the westerly jet shows that in the ERA reanalysis, the jet peaks at 925 hPa over the ocean, which is lower than the zonal wind center associated with the West African monsoon at the same latitudes. Above 700 hPa, the winds change to easterly. At low levels, distinguished from the monsoon flow, the jet does not have a strong meridional wind component. Also, being located within the ITCZ, the jet region exhibits strong vertical convection and low-level convergence. In the 6-hourly ERA40 climatology, the westerly jet shows a weak diurnal cycle with strongest winds at 17 hr local time and weakest at local 11 hr. The amplitude of the diurnal cycle is about 1-2 m/s. Momentum budget analyses are applied to the regions of the jet, the West Africa monsoon, and the transition between them. The results show that in the jet region the zonal winds are nearly geostrophic. As the Coriolis force is a little larger than the pressure gradient force, the resultant ageostrophic winds are positive and the jet is supergeostropic. In both the transition and monsoon regions, the meridional pressure gradient is about as twice large as in jet region and cannot be balanced by the Coriolis force, so the actual wind speeds are subgeostrophic. In the jet region during July to September, weak vertical wind shears tend to increase the westerly wind speeds by vertical advection while ascending vertical winds tend to weaken the jet. Mechanisms that maintain the thermal low extension are also discussed.