A note on long-term variations of the African easterly jet

The African easterly jet (AEJ) is analysed at 600 hPa in August for the period 1948-2003. In most years, the jet reveals strong zonal asymmetries. In many cases a double-core pattern is evident, such that a western (AEJ/w) and eastern (AEJ/e) component of the jet can be distinguished. The inter-annual variations of intensity and latitude of the two components demonstrate markedly different behaviour. Intensity and latitude are negatively correlated for the AEJ/e while uncorrelated for AEJ/w. The latitude of the AEJ/w is also relatively invariant, while from year-to-year the latitude of the AEJ/e varies by some ten degrees. On inter-decadal to multi-decadal timescales, a decrease (increase) in mean intensity of the AEJ/w (AEJ/e) is apparent in the mid-1970s, coincident with onset of the long dry period in the Sahel. The composite analysis of the wettest and driest 5-year periods suggests that in the wet years the dominant core is the western one, while the eastern one is dominant in the dry years. Our explanation for the existence of the two cores and their disparate behaviour involves an inertial instability mechanism that controls the lower and mid-tropospheric winds in the eastern portion of the AEJ (i.e. in the central and eastern Sahel). Copyright. (C) 2010 Royal Meteorological Society