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

The direct radiative effects of Saharan mineral dust aerosols on the nonlinear evolution of African easterly waves (AEWs) and their interactions with the African easterly jet (AEJ) are examined using the Weather Research and Forecasting (WRF) model coupled to an online dust model. The model is initialized with zonal-mean distributions of wind, temperature and dust consistent with summertime conditions over North Africa. The dust modifies the lifecycle of the AEWs in the following way: the domain-averaged eddy kinetic energy (EKE) is enhanced during the linear and nonlinear growth phases, reaching a larger peak amplitude that subsequently decays more rapidly, eventually equilibrating at lower amplitude. The increase in EKE during the growth phases is due to local increases in barotropic energy conversions in the dust plume north of the AEJ. The dust-modified, rapidly decaying phase is primarily associated with enhanced barotropic decay that occurs near the top of the plume north of the AEJ. The timing of peak EKE depends on the initial dust concentration. Throughout the evolution of the AEJ-AEW system, the dust increases the maximum zonal-mean wind speeds. The increase is due to the dust-modified mean meridional circulation during the AEW growth phase. The dust-modified maximum wind speeds are also displaced farther southward and upward during the AEW growth phase, which is due to the enhanced wave fluxes decelerating the flow more efficiently north of the AEJ. The spatial-temporal evolution of the dust-modified critical surface is shown to play an important role in the evolution of the AEJ-AEW system.