Saharan Dust and the Nonlinear Evolution of the African Easterly Jet–African Easterly Wave System

Abstract

Abstract The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume. The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.