The Characteristics of African Easterly Waves Coupled to Saharan Mineral Dust Aerosols (Invited Presentation)

African easterly waves (AEWs) are synoptic-scale disturbances that regularly interact with large amounts of Saharan mineral dust (SMD) aerosols emitted from the desert during summer. This study examines the characteristics of AEWs that are coupled to synoptic-scale plumes of SMD. The characteristics are determined using the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). The SMD-coupled AEWs are obtained by using a linear least squares regression technique to project fields from the MERRA-2 reanalysis onto the 2-6 day band-passed filtered aerosol optical depth for a specified location over Africa.

Results from the regression analysis show that the kinematic, thermodynamic and convective structures of the SMD-coupled AEWs have similar characteristics to AEWs observed in previous studies. As for the SMD field, positive anomalies occur in two regions relative to the AEW: one region is ahead of the AEW trough along the baroclinic zone (15-20°N), while the other region is farther north (22-27°N) and behind the AEW trough. The SMD signals are transported in the low-level ascent regions of the vertical motion north of the AEJ over Africa, but as the AEW crosses the Atlantic Ocean, the ascent regions weaken and the SMD propagation speeds change. The result is an elongated SMD plume over the Ocean, which is due to the merging of the signals between two successive AEWs.

The direct radiative effects of dust on the SMD-coupled AEWs are also investigated. The perturbation diabatic heating rate is combined with the perturbation temperature field to infer the structure of the diabatic generation term in the energetics of the SMD-coupled AEWs. The energetics are compared with the SMD-modified energetics obtained in a linear modeling study by Grogan et al. (2016, J. Atmos. Sci.).