The genesis and evolution of African easterly waves

The genesis and evolution of easterly waves over the Atlantic Ocean are investigated with a moist multi-level quasi-geostrophic model. The origin of African waves is first re-examined as a linear instability problem of a general basic zonal flow with/without a parameterized latent heating. The method of eigenvalue-eigenfunction analysis is used. The analysis reveals that it is important to use a faithful representation of the structure of the basic flow as observed over the region of northwestern Africa. The kinetic energy of the unstable wave is primarily generated via the barotropic instability process associated with the horizontal shear near the 650 mb level. But the vertical shear of the zonal flow also strongly influences the thermal structure of the unstable wave. It has a thermally indirect secondary circulation centering at about 550 mb and a thermally direct circulation centering at about 850 mb. The effect of the self-induced latent heating strongly depends upon the particular baroclinic structure. The growth rate, the propagation speed and the overall structure of the observed easterly waves (Reed et al., 1977) are well reproduced in the model.;A numerical modeling study is next made for the purpose of determining the physical conditions necessary for the subsequent intensification of an easterly wave to a tropical depression. It is found that for the wave to be able to subsequently intensify to a great intensity, its cold core must be quickly transformed into a warm core within a few days under the influence of a sufficiently large moisture supply as it propagates westward. Such a condition is represented by the heating intensity parameter in the model. The fact that relatively few easterly waves intensify to tropical depressions may be therefore understood as the consequence of a combination of two factors: (1) the easterly waves typically have unfavorable thermal structures, and (2) the moisture supply is usually not sufficiently large. It also follows that if an African wave has a more baroclinically favorable structure, it could intensify to a tropical depression even if the moisture supply over the Atlantic Ocean is only moderately large.