The Influence of African Easterly Waves on Tropical Cyclone Activity in a TC-Permitting Global Atmospheric Model

Within the framework of a high-resolution global atmospheric model, we disentangle the relationship between African easterly waves (AEWs) and Atlantic tropical storms (TCs) from the large-scale environmental factors that may obscure their connection. The internal variability of the relationship between AEW and TC count, including the sensitivity to ENSO phase and annual trends, and the interplay between environmental factors, AEW activity, and TC activity are probed using three sets of simulations: 1) climatological simulations, consisting of three ensemble members forced with historical seasonally and annually varying SST; 2) simulations with interannually invariant forcing, including a control simulation with climatological mean SST and a perpetual La Niña simulation with composite SST from strong La Niña years; 3) perturbed simulations, in which the large-scale environment is drastically altered through the manipulation of African albedo. Since variability exists in AEW count that is unexplained by known indicators of large-scale environmental favorability, across all simulations and multiple timescales, it is unlikely that the ubiquitous covariance between AEW and TC count is simply a response to environmental factors in the model. While roughly half of the statistically significant covariance between AEW and TC count interannually seems to be mediated by the large-scale environment, there is evidence that the other half is due to stochastic AEW variability.