Distinguishing north and south African Easterly Waves with a spectral method: Implication for tropical cyclogenesis from mergers in the North Atlantic

Abstract

African Easterly Waves (AEWs) are mixed barotropic–baroclinic instabilities, propagating both north (AEW‐N) and south (AEW‐S) of the African Easterly Jet. They are an important component of the North Atlantic region meteorology, influencing precipitation and convection, and are primary seeds for tropical cyclones (TCs) over the North Atlantic. In this study, a novel tracking protocol is introduced based on filtering of the 700‐hPa vorticity over two frequency ranges, suited for each wave track: 2.95–4.55 days (quicker propagating mode) and 4.55–6.35 days (slower propagating mode). It is applied to a 31‐year reanalysis dataset, the European Centre for Medium‐range Weather Forecasts (ECMWF) Fifth Reanalysis. This method effectively isolates north‐track activity even amidst dominant south‐track signals, enabling better identification and tracking of northward vortices. An analysis of the climatological structure of the two modes is conducted. The slower mode propagates around and transitions from 850 hPa above the continent to 700 hPa above the ocean, consistently with AEW‐N characteristics. The quicker mode propagates around and is located at the African Easterly Jet level, between 700 and 600 hPa above the continent and the ocean, consistently with AEW‐S characteristics. The new method allows the detection of AEW‐N even when AEW‐S activity dominates the 2–10 day signal, thereby enabling the detection of more AEW‐S/AEW‐N mergers than in previous studies. When comparing the usual 2–10 day filtering with this new method, the proportion of TCs related to AEW mergers rises from 11% to 25%. 23.4% of mergers are found to be related to TC genesis, making mergers the most efficient precursors for TC genesis in the Atlantic. TC genesis from mergers occurs more frequently on the eastern side of the basin than TC genesis from other precursors.