Theoretical Framework for the Destabilization of African Easterly Waves by Saharan Mineral Dust Aerosols

Among the prominent features of the summertime circulation over North Africa are the African easterly jet (AEJ), African easterly waves (AEWs), and the episodic formation of vast plumes of Saharan mineral dust (SMD) aerosols. In this study, we present a theoretical framework that exposes the causal relationships that govern the dust-induced growth of AEWs. The framework is built on a quasi-geostrophic system governed by coupled equations for potential vorticity, temperature, and dust continuity. The direct dust radiative heating rate includes the effects of both shortwave and longwave radiation. A perturbation analysis yields analytical expressions for the propagation and growth characteristics of the model's AEWs. The expressions are functions of vertically and meridionally averaged wave activity, which depends on wave spatial structure, direct dust-radiative heating, and the background distributions of wind, temperature, and dust mass mixing ratio. More specifically, the propagation and growth rate of the AEWs depend on the background dust concentration, and its meridional and vertical gradients, which are modulated by the Doppler-shifted frequency. The predictions by the theoretical framework are tested against linear stability calculations carried out using an idealized (linear) version of the Weather Research and Forecasting (WRF) model coupled to an online dust model. For a realistic, subcritical background flow, the WRF model calculations show, consistent with the theoretical framework, that the dust may be destabilizing, depending on the locations of the dust plume relative to the jet and the signs and relative magnitudes of the meridional and vertical gradients in the background dust field. The dust-induced amplification of the AEWs is maximized north of the AEJ axis, where the spatial gradients in dust are maximized and the Doppler-shifted frequency vanishes. The clarity of the expressions connecting SMD aerosols to the linear properties of AEWs provides an important interpretive tool for analyzing results obtained from comprehensive model simulations of AEWs, such as those produced by the WRF model.