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 ...