A Reexamination of the Genesis of African Waves

Abstract

The problem of genesis of easterly African waves is reexamined as a linear eigenvalue instability problem of a supposedly realistic basic zonal shear flow with and without a cumulus heating. Evidence will be presented to show how the growth rate, the propagation speed, and the overall composite structure of the observed tropical easterly waves can he well reproduced in the model in conjunction with a CISK representation of the latent heating. The degree of resemblance between the structure of the model unstable wave and the observed African waves is found to be significantly dependent upon the secondary features of the basic shear flow. This finding is based on a detailed comparison of the results using different structures for the basic flow including the one used by Mass. Much of the kinetic energy of the unstable wave is generated via the barotropic instability process associated with the horizontal shear of the Sahara jet centered at the 650 mb level. But the vertical shear of the zonal flow also strongly influences the thermal and vertical velocity fields of the unstable wave. Slightly ahead of the trough, there is a thermally indirect secondary circulation centered at about 550 mb and a direct circulation centered at about 850 mb. From the energetics point of view, the latent heating tends to stabilize the motion because of such a structure at the middle level (400 mb?600 mb). The opposite occurs at the lower level (below 600 mb). The unique structure of this mutual cancellation plays a crucial role in the evolution of the wave disturbance. Its significant implication on the subsequent evolution of the African wave over the Atlantic Ocean is examined in a later paper.