Subcritical Destabilization of African Easterly Waves by Saharan Mineral Dust

• Published in: Journal of the atmospheric sciences Vol. 74; no. 4; pp. 1039 - 1055
• Main Authors: Nathan, Terrence R., Grogan, Dustin F. P., Chen, Shu-Hua
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.04.2017
• Subjects: Aerosols; African easterly waves; Atmosphere; Atmospheric particulates; Atmospheric sciences; Baroclinic instability; Barotropic mode; Climatology; Destabilization; Doppler sonar; Dust; Dust storms; Easterly waves; Exact solutions; Formulas (mathematics); Frameworks; General circulation models; Gradients; Growth rate; Instability; Linear analysis; Mathematical analysis; Mathematical models; Mixing ratio; Perturbation methods; Phase velocity; Potential vorticity; Predictions; Propagation; Science programs; Temperature effects; Vorticity; Weather forecasting; United States > US; North Africa; California
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-16-0247.1

Abstract A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD. Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.