Saharan Dust Transport by African Easterly Waves: Theory, Modeling, and Reanalysis

Theory, modeling, and reanalysis are combined to provide insights into the physical and dynamical processes that control the transport of Saharan mineral dust aerosols by African easterly waves (AEWs). The theory is built around an analytical analysis that provides the basis for interpreting and predicting how the AEWs waves and (zonal-mean) background flow combine to affect the zonal-mean eddy transports of dust. The analytical analysis shows that the eddy dust transports are a consequence of the eddy momentum fluxes acting on the background dust gradients. The analytically derived transport equations predict that the eddy transports of dust are largest where the maximum in the background dust gradients coincide with a critical surface, i.e., where the Doppler-shifted frequency of the wave field vanishes. The modeling of the eddy dust transports are carried out with a mechanistic version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model. The simulations show that the eddy dust transports, which are directed down the background dust gradients, are largest near the critical surface, a location that coincides with the maximum in the meridional gradient. The theoretical predictions and mechanistic modeling results agree with the transport results obtained from calculations based on 37 summers (1980–2016) of data from the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2). The modeling and reanalysis together confirm that the theoretical framework developed in this study can be used as both an interpretive and a predictive tool in the study of Saharan dust transport by AEWs.