African Easterly Waves and Tropical Cyclogenesis

A connection between African easterly waves (AEWs) and tropical cyclones (TCs) has long been understood to exist. A series of papers in the 1990s by Avila and Pasch of the US National Hurricane Center (NHC) are typically cited as the source of this fact. However, these statistics have not been updated in a similar fashion in 20 years. We update the statistics on this relationship between AEWs and TCs in a survey of NHC TC reports. Our survey shows that AEWs were implicated directly or indirectly in the formation of 75% of all TCs in the North Atlantic during 1995-2015.

Further, when examining the efficiency through which AEWs spawn TCs (genesis efficiency) most studies examine counts of AEWs against TCs forming from AEWs. This method does not investigate properties of the AEWs however. In this study, we take a different approach by examining the Eddy Kinetic Energy (EKE) of the AEWs calculated using gridded reanalyses. The correlation between seasonal mean EKE and TC numbers is not highest in the mid-troposphere where the peak EKE of canonical AEWs is located. Instead the correlation is highest in the lower troposphere and corresponds to the southern track of the AEWs. This suggests that mid-level AEW activity is a poor predictor of TC genesis while lower tropospheric diabatic effects due to moist convection exert stronger control. We also show that genesis efficiency is not strongly limited by AEW activity except when there is weak low-level seasonal mean AEW EKE. This has implications for seasonal TC forecasting.