Impact of Desert Environment on the Genesis of African Wave Disturbances

Abstract

During summer the meteorological conditions in the lower troposphere between the anticyclonic-shear (north) side and cyclonic-shear side of the midtropospheric easterly jet over North Africa are rather different. A significant dynamical feature on the cyclonic-shear side of the jet core is the reversed potential vorticity gradient, which satisfies the Charney-Stern necessary condition for combined barotropic-baroclinic instability. There is no indication of such combined instability on the anticyclonic-shear side. The desert air to the north is characterized by near-neutral static stability and a marked horizontal temperature gradient as a result of strong surface heating. The role of the unique desert environment in the dynamics and energetics of African wave disturbances is examined based on linearized perturbation analysis. A ?dry? (no moist processes) system of primitive equations is used in the analysis. Results reveal that the mean thermal conditions over the east-central African deserts in summer are most favorable for the growth of perturbations having a zonal wavelength in the range 1500 to 3000 km. These amplifying perturbations have many basic characteristics in common with observed African waves. Energy analysis indicates that the baroclinic instability is responsible for the generation of the perturbation kinetic energy on the anticyclonic-shear side of the jet.