Intermittent African easterly wave activity in a dry atmospheric model: influence of the extratropics

A dynamical model is constructed of the northern summertime global circulation maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (Thorncroft et al, 2008, Leroux and Hall, 2009). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no-convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward propagating 3-5 day disturbances over West Africa. These simulated waves display intraseasonal intermittency that is strikingly similar to the observed intermittency of AEWs. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.