Subcritical Destabilization of African Easterly Waves by Saharan Mineral Dust

Studies of African easterly waves (AEWs) have occupied more than four decades of research, evidence of their importance to the meteorology over North Africa and the Eastern Atlantic Ocean. A large body of that research attributes the origin of the AEWs to the combined barotropic-baroclinic instability of the African easterly jet (AEJ). In this study, we show that the interactions between synoptic-scale disturbances in wind, temperature and Saharan mineral dust (SMD) aerosols can yield wave instabilities in zonal-mean background flows that are subcritical with respect to barotropic and baroclinic instability.

In order to expose the radiative-dynamical relationships that govern the SMD-induced instabilities, we develop a theoretical framework that is built on coupled equations for quasigeostrophic potential vorticity, temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in potential voriticty and SMD.

Using an idealized version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model, a linear analysis shows, in agreement with the theoretical prediction, that for a subcritical AEJ and background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their eastward propagation. The SMD-induced growth rates are commensurate with those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic-baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.