2B.3
On the generation mechanisms of African easterly waves (Invited)
Jen-Shan Hsieh, Department of Oceanography, Texas A & M University, College Station, TX; and K. H. Cook
African easterly waves have long been associated with the barotropic and baroclinic instability of the African easterly jet through meridional potential vorticity gradient reversals, which also may be achieved by concentrated convective heating of the ITCZ. A primary objective of this study is to explore whether African waves are initiated by the Charney-Stern instability in the region of reversed potential vorticity (PV) gradients, or whether convectively induced instability generates African waves, and the waves in turn enhance and organize the convection. To better understand the generation mechanisms of these waves, a regional climate model is used to investigate the related energetics of African easterly waves and instability growing in regions with reversed potential vorticity gradients over Africa.
A detailed energetics study of these waves over Africa indicates 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 barotroic conversions and baroclinic overturning, which are usually associated with significant rainfall over Africa. It is also found that the convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction (phase-locking) between the waves and convection, corresponding to the formation of organized precipitation migrating with the waves. The barotropic and the baroclinic instabilities associated with the horizontal and the vertical wind shears respectively are further studied along with the formation of PV anomaly near the African easterly jet. A PV budget analysis indicates that this PV anomaly is mainly a result of the convective generation of PV due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere, which suggests that the African easterly jet may become unstable due to convection.
Results indicate that the barotropic instability of the jet initiates primarily outside of the region of strengthened reversed PV gradients, suggesting that the instability is a result of convectively induced eddies extracting energy from the zonal flow rather than the release of zonal kinetic energy to the waves in the unstable region. In contrast, the residual barotropic instability occurs inside the region of reversed PV gradients during the waves' decaying stage when ITCZ convection weakens. This suggests that the decaying waves are maintained through the Charney-Stern instability in the unstable region at the expense of the zonal mean kinetic energy. The baroclinic instability associated with the vertical zonal wind shear in the unstable region becomes distinguishable from that due to surface temperature gradients when the surface heat flux is weak, a condition under which the African easterly jet better acts as an internal jet. Thus, this analysis indicates that African waves are not initiated by the instabilities implied by the Charney-Stern theory. Instead, they appear to result from convective instability, i.e., those induced by ITCZ moist convection or shallow convection beneath the jet.
An analysis of the seasonal mean Eliassen-Palm flux (EP flux) is performed to explore the propagation of the generated waves on the meridional cross section and its effect on the zonal mean flow. This diagnosis revels that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north of the jet. All of this wave energy propagates downward and dissipates in the lower troposphere. The fact that the sign of quasi-geostrophic potential vorticity (q) gradients opposite to the divergence of EP flux for African waves indicates a down gradient flux of quasi-geostrophic eddy potential vorticity of these waves, which decreases the zonal mean gradient of potential vorticity, and thus the meridional shear of the jet. Thus, the 3-5-day waves stabilize the easterly zonal mean flow in the seasonal mean.
Recorded presentationSession 2B, African Climate: II. African Easterly Waves and Dust
Monday, 21 January 2008, 10:45 AM-11:45 AM, 217-218
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