On the instability of the African easterly jet and the generation of African waves: Reversals of the potential vorticity gradient

The relationship between the generation of African easterly waves and instability growing in regions with reversed potential vorticity gradients is studied using a regional climate model. Results indicate that the convective generation of potential vorticity (PV) due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere causes a vertically elongated PV anomaly on the southern flank of the African easterly jet. This PV maximum at 9 N in the midtroposphere, together with a PV minimum near 15 N at lower levels because of dry convection over the Sahara, reverses the meridional PV gradient between 9 and 15 N, which suggests that the zonal flow may be unstable in this region. Analysis of the seasonal mean Eliassen-Palm flux for African waves indicates that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north of the jet. The barotropic conversion of the jet initiates primarily outside of the region of strengthened reversed potential vorticity (q) gradients, suggesting that this barotropic conversion is a result of convectively induced eddies extracting energy from the zonal flow rather than the release of zonal kinetic energy to the waves in the unstable region. In contrast, the residual barotropic conversion occurs inside the region of reversed q gradients during the waves' decaying stage when ITCZ convection weakens. The baroclinic instability in the unstable region becomes distinguishable from that due to surface temperature gradients when the surface heat flux is weak, a condition under which the African easterly jet better acts as an internal jet. Thus, this analysis indicates that the shear instability of the jet occurs to sustain the waves at the decaying stage rather than to initiate the waves, since it does not appear strong enough to reenergize the waves.