A Study of the Energetics of African Easterly Waves Using a Regional Climate Model

Jen-Shan Hsieh Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York

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Kerry H. Cook Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York

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Abstract

The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa.

Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet is strong. The eddy available potential energy consumed by the baroclinic overturning is compensated directly by the conversion of zonal to eddy available potential energy and the generation of eddy potential energy by diabatic heating. These direct conversions of latent heat and zonal available potential energy suggest that interactions across space scales, from convective space scales to the large scales, are important for generating African waves. The convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction of the waves with convection, corresponding to the formation of organized precipitation migrating with the waves.

A space–time spectral analysis shows that the dispersion characteristics of African easterly waves with wavelengths between 2650 and 4000 km do not follow the dispersion relation of the shallow water waves, indicating that these waves, similar to other easterly waves in the Tropics, possess significant nonlinearity, and cannot be fully explained by linear wave theory.

Corresponding author address: Jen-Shan Hsieh, 618B, Department of Oceanography, Texas A&M University, College Station, TX 77843. Email: jsh@ocean.tamu.edu

Abstract

The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa.

Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet is strong. The eddy available potential energy consumed by the baroclinic overturning is compensated directly by the conversion of zonal to eddy available potential energy and the generation of eddy potential energy by diabatic heating. These direct conversions of latent heat and zonal available potential energy suggest that interactions across space scales, from convective space scales to the large scales, are important for generating African waves. The convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction of the waves with convection, corresponding to the formation of organized precipitation migrating with the waves.

A space–time spectral analysis shows that the dispersion characteristics of African easterly waves with wavelengths between 2650 and 4000 km do not follow the dispersion relation of the shallow water waves, indicating that these waves, similar to other easterly waves in the Tropics, possess significant nonlinearity, and cannot be fully explained by linear wave theory.

Corresponding author address: Jen-Shan Hsieh, 618B, Department of Oceanography, Texas A&M University, College Station, TX 77843. Email: jsh@ocean.tamu.edu

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