Diagnostic studies of a mid-tropospheric African easterly jet in the southern hemisphere and implications for the understanding of African climate

An analysis of mean zonal winds over West Africa shows the presence of two mid-tropospheric easterly jets near 650 mb. One of these is the well-known AEJ of the northern hemisphere. The second is a southern-hemisphere jet that is evident as a wind maximum in earlier analysis but which has not been described in detail. Here we use the terms AEJ-N and AEJ-S to distinguish them. The AEJ-S is described for two sectors of the continent (roughly 10W to 20E and 10E to 30E).

In the western sector, the AEJ-S is clearly defined during August through December. Mean core speeds reach 8 m s-1 in October and November, but only 4 to 6 m s-1 in the other months. Hence, it is weaker than the AEJ-N. Some semblance of an easterly jet is discernible in the mean for January, February and July, but it disappears entirely from March through June. Here the location of the AEJ-S ranges from roughly 4 o S in August to 12 o S in December. Its altitude ranges from roughly 700 mb in August to 600 mb in December.

In the eastern sector the annual cycle is similar. The AEJ-S is best defined in August through November. Its position ranges from about 5 o S in August to 8 o S in November. Core speeds range from about 6 m s-1 in August to 10 m s-1 in October. Some semblance of a mid-tropospheric jet is apparent in December through March, but in April through July it is replaced by a lower level easterly maximum near 800 mb.

A comparison with the structure and interannual variability of the rainbelt suggests that the AEJ-S plays a significant role in the development of the rainy season over equatorial and subtropical Africa. The tropical rainbelt is substantial more intense during the months when both the AEJ-N and AEJ-S are present (i.e., August to November). The belt of intense rains is roughly bounded by the core of these two jets during this time period. The cores also roughly define the zones where mesoscale convective complexes (MCCs) occur over equatorial Africa. The MCCs predominantly occur on the equatorward side of the two jets, with a minimum frequency occurring near the equator. The AEJ-S is noticeably stronger during wet years over North Africa.