An idealized study of African easterly waves. I: A linear view



The linear instability problem of the African easterly jet has been investigated using the primitive equations on a sphere. This has included an examination of the linear growth mechanisms and structure using diagnostics traditionally employed for mid latitudes, such as Eliassen-Palm (EP) fluxes and potential vorticity. It has been shown that a growing normal mode on an African easterly jet is characterized by divergent EP fluxes in the region of the jet, implying both barotropic and baroclinic energy conversions. The linear instability is dominated by the interaction between positive and negative potential vorticity gradients at the level of the jet and, as found in previous studies, the normal modes grow mainly through barotropic energy conversions. Many of the synoptic features associated with the normal modes are in good agreement with those observed, except for the vertical-velocity pattern, which has a ‘checkerboard’ structure in the vertical and is much weaker.

Changing the jet latitude while keeping the baroclinicity constant changed the growth rates very little, and although barotropic energy conversions remained dominant, the most unstable modes became more baroclinic 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’ pattern in the vertical velocity persists with the normal modes which grow on these jets.

The effect of including diabatic effects in the linear problem has also been examined. First, a simple boundary-layer scheme was found to have very little effect on the normal modes. With simply parametrized latent heat release however, the growth rates of the most unstable modes were increased slightly and the modes became less dominated by barotropic energy conversions. Also, an asymmetry is found between 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 mode, using a CISK-type scheme for the latent heating, has more in common with the observed structures over west Africa than the structure of the dry modes.

It is suggested that African easterly waves may arise through a mixed barotropic/baroclinic instability mechanism where the role of latent heating is important in increasing the baroclinic energy conversions relative to the barotropic energy conversions, and also in determining the synoptic structure.