It has been speculated that westward propagating disturbances, known as African waves. originate as perturbations on the low level easterly jet which is present over northern Africa during the season of intense wave activity. A linearized, pseudo-spectral, primitive equation model is used to simulate the response of the observed jet to perturbations on the scale of African waves. Results show that the jet is unstable due to both its horizontal and vertical shears. The sub adiabatic lapse rate, however, acts to stabilize the waves. The most unstable wave supported by the jet has a wavelength of 3000 km and a period of 2.0 -2.5 days. It attains its maximum intensity at the 700 mb level, near l4°N. This compares favorably with the characteristics of the observed waves. The kinetic energy of the waves grows at the expense of the kinetic energy of the mean jet. Energy is transferred at approximately equal rates by the horizontal and vertical Reynolds stresses. Energy conversions involving available potential energy are nearly an order of magnitude smaller. reflecting the fact that the kinetic energy of the wave accounts for about ninety per cent of the total wave energy. The characteristics of the most unstable wave are virtually unchanged when a crude parameterization of latent heat release is included in the model. The modification of the jet due to the growth of the waves is studied by allowing nonlinear interactions to take place between the wave and the zonally averaged flow. The results indicate that, for the simplified system studied, the interaction is too small to affect the stability of the jet on the time scale of a single wave.
westward propagating disturbances, African waves, primitive equation model, kinetic energy of waves
University of Illinois - Urbana-Champaign
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