A Case Study of the Effects of Saharan Mineral Dust Aerosols on the Structure of the African Easterly Jet–African Easterly Wave System

Understanding the interaction between Saharan mineral dust (SMD) aerosols and the African Easterly jet–African easterly wave (AEJ-AEW) system is central to understanding the meteorology over North Africa and the eastern Atlantic Ocean. In this study, we use the Weather Research and Forecasting (WRF) model coupled to an on-line dust model to carry out a case study of the effects of SMD on the structure of the AEJ-AEW system. The case study is for a single summer. Only direct dust-radiative feedbacks are included in the preliminary experiments, of which there were two: one without dust-radiative feedbacks and the other with dust-radiative feedbacks. For both experiments, the WRF-dust model was initialized using ECMWF reanalysis data. The model was then integrated forward in time for three months—July, August and September, 2006. The data was then averaged for the region containing the AEWs, i.e., West Africa to the Eastern Atlantic. Two aspects of the dust-modified AEJ-AEW system were examined: The location and orientation of the AEJ axis, and the north and south tracks of the AEWs. The AEJ axis was identified using a linear regression of the maximum wind speeds in the jet core. To determine the AEW tracks, we filtered the model data in time and space in order to isolate the AEWs. The filtered maximum relative vorticity variance was then calculated at 900mb (north track) and 700mb (south track). The tracks were then determined based on a “best fit” of the maximum vorticity variance. The preliminary results show that the dust shifts the AEJ northward and upward, and rotates its axis clockwise. The northward and upward shift in the AEJ axis is consistent with previous studies, which show that the SMD affects the thermal field, which, via thermal wind balance, changes the AEJ. The dust-radiative feedbacks also shift the AEW tracks northward, though by a lesser amount than the AEJ. The following questions are among those currently being addressed: To what extent does the SMD affect the power spectra and area-averaged strength of the AEWs? What is the relative importance of direct versus indirect dust effects on the AEJ-AEW system? And how does the model results compare with observations of the AEJ-AEW system and spatial distribution of aerosol optical depth?