AN IDEALIZED STUDY OF AFRICAN EASTERLY WAVES .1. A LINEAR VIEW

The linear instability problem of the African easterly jet has been in vestigated using the primitive equations on a sphere. This has include d an examination of the linear growth mechanisms and structure using d iagnostics traditionally employed for mid latitudes, such as Eliassen- Palm (EP) fluxes and potential vorticity. It has been shown that a gro wing normal mode on an African easterly jet is characterized by diverg ent EP fluxes in the region of the jet, implying both barotropic and b aroclinic energy conversions. The linear instability is dominated by t he interaction between positive and negative potential vorticity gradi ents at the level of the jet and, as found in previous studies, the no rmal modes grow mainly through barotropic energy conversions. Many of the synoptic features associated with the normal modes are in good agr eement with those observed, except for the vertical-velocity pattern, which has a 'checkerboard' structure in the vertical and is much weake r. Changing the jet latitude while keeping the baroclinicity constant changed the growth rates very little, and although barotropic energy c onversions remained dominant, the most unstable modes became more baro clinic when the jet was more poleward. The most unstable modes, which grow on a thinner jet, have a larger growth rate, a smaller wavelength and stronger barotropic energy conversions. The 'checkerboard' patter n in the vertical velocity persists with the normal modes which grow o n these jets. The effect of including diabatic effects in the linear p roblem has also been examined. First, a simple boundary-layer scheme w as found to have very little effect on the normal modes. With simply p arametrized latent heat release however, the growth rates of the most unstable modes were increased slightly and the modes became less domin ated by barotropic energy conversions. Also, an asymmetry is found bet ween the ascent and descent regions in the wave, with the length scale of the updraught contracted relative to that of the downdraught. The unrealistic 'checkerboard' pattern in the vertical velocity is almost removed and the amplitude is increased. The structure of the normal mo de, using a CISK-type scheme for the latent heating, has more in commo n with the observed structures over west Africa than the structure of the dry modes. It is suggested that African easterly waves may arise t hrough a mixed barotropic/baroclinic instability mechanism where the r ole of latent heating is important in increasing the baroclinic energy conversions relative to the barotropic energy conversions, and also i n determining the synoptic structure.