The contribution of African easterly waves to monsoon precipitation in the CMIP3 ensemble

摘要

African easterly waves (AEWs) are a major source of synoptic-scale rainfall variability throughout West Africa. Given the persistent uncertainty in the response of precipitation over West Africa to enhanced greenhouse forcing, we analyze the contribution of AEWs to the simulation of precipitation over West Africa in the Coupled Model Intercomparison Project Phase 3 (CMIP3) ensemble of general circulation models. Nearly all models that simulate weaker-than-observed AEW activity exhibit a lack of coupling between AEWs and precipitation. Only those models that employ a convective parameterization with a moisture convergence-based closure or trigger, or a prognostic closure with a convective suppression scheme, demonstrate a consistent connection between AEWs and precipitation. The strength of simulated AEW activity is largely dependent on the simulated magnitude and positioning of the African easterly jet and low-level westerlies. We find a strong positive relationship between the magnitude of simulated AEW activity and seasonaland synoptic-scale precipitation. Models with weaker-than-observed AEW activity exhibit a disproportionately low percentage of precipitation variability at AEW timescales. Conversely, in those models that simulate stronger-than-observed AEW activity, dynamical forcing associated with the wave tends to produce overly intense individual precipitation events. Our results suggest that the simulation of AEWs contributes substantially to the spread in mean seasonal precipitation across the CMIP3 ensemble during the 20th century, and that improved understanding of the effect of enhanced greenhouse forcing on the relationship between AEWs and precipitation could help to narrow the uncertainty in the response of West African rainfall to continued global warming.