Structural Changes in the African Easterly Jet and Its Effects on the Dust-Modified Energetics of African Easterly Waves

Studies have shown that during summer over North Africa, plumes of Saharan mineral dust (SMD) aerosols influence the synoptic-scale variability of the African easterly jet (AEJ)−African easterly wave (AEW) system. Precisely how that variability is connected to the structure of the African easterly jet is unclear. In this study, we numerically examine how structural changes to the zonally-averaged AEJ mediate the effects of SMD on the linear dynamics of AEWs. The numerical simulations are conducted using a linearized version of the Weather Research and Forecasting model coupled to an interactive dust model. For the simulations, the maximum meridional and vertical shear of the basic state AEJ are varied, while the basic state SMD distribution remains fixed.

For each of the observationally consistent AEJ structures that were examined, the dust radiative effects enhance the linear growth of the AEWs across all zonal wavelengths (2500-5000 km). The AEW response to the SMD is strongest for AEJs where the ratio of baroclinic to barotropic energy conversions is largest; this occurs when the maximum meridional shear is reduced and the maximum vertical shear is enhanced. Consistent with previous idealized dust-modified AEW studies (Grogan et al. 2016; J. Atmos. Sci.), a local energetics analysis shows that the dust increases the baroclinic energy conversions and the generation of eddy available potential energy at mid-levels (600-800 hPa) north of the AEJ axis (15-20ºN). But for the AEJs with the larger dust-induced growth rates, the local energetics analysis shows an increase in the barotropic energy conversions north of the AEJ axis. These increases are driven by enhanced momentum fluxes produced by stronger dust-induced AEW amplitudes. The application of these results to the observed AEJ-AEW system is discussed.