Intermittent african easterly waves in a simple GCM

A dynamical model is constructed of the summertime global circulation maintained by empirically derived diabatic forcing. It is based on the same dynamical code that has recently been used to study African easterly waves as convectively triggered perturbations (Thorncroft et al, 2008, Leroux and Hall, 2009). In this new configuration, the model simulates observed distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to study the two-way interaction between easterly waves and the African easterly jet (AEJ). 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. The magnitude of these simulated easterly waves appears to be limited by nonlinear processes. The waves also display intraseasonal intermittence that is strikingly similar to the observed intermittence of African easterly waves. Further experiments designed to discern the source of this intermittence in the model show that the simulated African easterly waves are triggered by dynamical precursors coming from the North Atlantic storm track. This extratropical triggering of easterly waves is found to be very efficient when compared to simulations of convectively triggered waves using both the perturbation model and the simple GCM.