African Easterly Waves and Convection: A Potential Vorticity Perspective

African Easterly Waves (AEWs) and convection have been associated with each other since the seminal studies on AEWs by Frank, Burpee, and Reed, among others, during the 1960s and 1970s. More recently, studies such as those by Berry and Thorncroft (2005, 2012) have shown that convection is integral to maintaining an AEW during its passage across West Africa. However, a detailed understanding of the exact mechanism(s) through which they interact has eluded the community. In this presentation, we examine the findings of a synoptic-scale approach to understanding this interaction.

This approach primarily examines the evolution of potential vorticity (PV) in AEWs, focusing on its sources and sinks. PV is examined in both a convection-allowing Weather Research and Forecasting model (WRF) simulation of a single AEW and composite average AEWs derived from the ERA-Interim Reanalysis. This approach finds that diabatic sources of PV are maximized at and below the peak of AEW PV, both enhancing and deepening the AEW circulation. Meanwhile, adiabatic sources of PV are important above the peak in AEW PV. Through sensitivity studies, it is shown that the average propagation of AEWs is dominated by advection of the PV anomaly by the mean flow, namely the African Easterly Jet, and convection does not change AEW propagation significantly. Further, we find that while the wave is sustained without diabatic forcing, it is overall weaker, and it’s PV anomaly is limited to levels above the African Easterly Jet.