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Keywords:

  • Africa;
  • Easterly waves;
  • Jet instability

Abstract

The nature of unstable easterly waves growing on three different easterly jets has been examined. the ‘shearjet’ with strongest horizontal mean shear on the equatorward side of the jet is characterized by linearly unstable waves with maximum amplitude equatorward of the jet maximum and are dominated by barotropic energy conversions associated with positive horizontal momentum fluxes. the normal modes of the shear-jet have very little amplitude at the surface.

The ‘desert-jet’ with lower mean static stability better represents the atmospheric conditions on the poleward side of the jet, and is characterized by linearly unstable waves with maximum amplitude at the surface. the modes have much larger baroclinic energy conversions than those of the shear-jet, although barotropic energy conversions are still larger. About 25% of the value of the barotropic energy conversions is attributable to the vertical Reynolds stress term associated with vertical momentum fluxes.

The ‘combined-jet’ with both enhanced equatorward horizontal shear and low static stability combined, has also been examined. the linear normal modes associated with this jet are characterized by significant amplitudes at both the jet-level and at the surface. the ratio of barotropic to baroclinic energy conversions is between that associated with the modes growing on the shear- and desert-jets.

The nonlinear behaviour of 3000 km wavelength easterly waves growing on the jets has been examined. All life-cycles are characterized by a transition from initial barotropic energy conversions dominance to baroclinic energy conversions dominance. the nonlinear part of the shear-jet life-cycle is dominated by negative horizontal momentum fluxes on the poleward side of the jet and maximum amplitude at the surface in the region of the temperature gradients. In disagreement with observations only very weak nonlinear positive horizontal momentum fluxes exist on the equatorward side of the jet. the nonlinear desert-jet life-cycle as for the normal mode continues to be characterized by maximum amplitudes at low-levels and becomes dominated by baroclinic energy conversions sooner. the nonlinear combined-jet life-cycle evolves in a similar manner to the desert-jet life-cycle but with more significant jet-level amplitudes.

The synoptic evolution of the relative vorticity in the life-cycles is examined. A single surface relative vorticity maximum, associated with the meridional surface temperature gradient has been identified in all idealized integrations examined, including those with latent heating, in disagreement with observations which indicate two maxima north and south of the jet. At about 850 mb however, two positive vorticity centres do exist; a weak equatorward one associated with the developments at the jet level and a stronger poleward one associated with the low-level temperature gradients.

The shear life-cycle is re-examined with a simple moist parametrization included. Baroclinic energy conversions increase but barotropic energy conversions are almost unaffected by the latent heating and the jet-level structure almost unchanged. the surface vorticity maxima is increased. the relationship between moisture availability and the jet is shown to be important in determining the nature of surface developments and the ascent pattern as well as the magnitude of the baroclinic energy conversions.

It is suggested that differences between observations and the nonlinear equatorward structures identified here, may be reduced with a more accurate representation of diabatic processes.