Three-Dimensional Transport of Saharan Dust Aerosols by African Easterly Waves: Theory and Reanalysis

Theory and reanalysis are combined to expose the physical and dynamical processes that control the three-dimensional (3D) transports of Saharan mineral dust aerosols by African easterly waves (AEWs). The theory is derived by applying the WKB (Wentzel-Kramers-Brillouin) method to a linear conservation equation for dust-coupled AEWs in time-mean African easterly jets (AEJs). The 3D theory proves to be an important predictive and interpretive tool for the dust transports by AEWs, which are calculated from the 3D meteorological fields obtained from 37 summers of the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2).

The theory accurately predicts where the transports are largest over both land and ocean, which occurs where the eddy momentum fluxes acting on the mean dust gradients are large near a critical surface, i.e., where the 3D Doppler-shifted frequency is small. The reanalysis results show a clear transition in the structure of the eddy dust transports from land to ocean. The zonal transports depend on their position relative to the AEJ axis, whereas the meridional and vertical transports depend on their position relative to the maximum in dust aerosol optical depth. Whether over land or ocean, the eddy dust transport in each direction is dominated by the meridional advection of the time-mean dust by the eddy field. This transport term typically contributes more than 80% to the transports in regions where the transports are largest.