Dynamics of the West African Westerly Jet

Abstract

The West African westerly jet (WAWJ) is a low-level westerly jet located at 8°–11°N over the eastern Atlantic and the West African coast. It is clearly distinguished from the monsoon westerly flow by its structure and dynamics, and plays an important role in transporting moisture from the tropical eastern Atlantic to Sahelian West Africa during boreal summer. The WAWJ develops in early June, sustains maximum wind speeds of 5–6 m s−1 from late July to early September, and weakens and dissipates by mid-October. In its mature stage, the WAWJ is located within the Atlantic ITCZ. It extends from the surface to 700 hPa, with maximum speed at 925 hPa. The jet has a weak semidiurnal cycle, with maxima at 0500 and 1700 local time. A momentum budget analysis reveals that the WAWJ forms when a region of strong westerly acceleration is generated by the superposition of the Atlantic ITCZ and the westward extension of the continental thermal low. The WAWJ is supergeostrophic at its maximum, with zonal pressure gradient and Coriolis accelerations both pointing eastward. While much of the WAWJ’s seasonal variation can be explained by the geostrophic wind, the ageostrophic wind contributes more than 40% of the wind speed during the jet’s formation and demise. The westward extension of the thermal low is associated with the formation of an offshore low, which is related to seasonal warming of the ocean between 6° and 18°N along the coast. The coastal SSTs vary in response to a net surface heating pattern with warming to the north and cooling to the south, which is mainly controlled by solar radiative and latent heat fluxes.