Exploring the Role of Intraseasonal Oscillations on African Easterly Wave Predictability in the ECMWF Ensemble Prediction System

During the boreal summer, African Easterly Waves (AEWs) are the primary synoptic-scale feature that influences North African weather, and the ability to accurately predict these features in operational numerical prediction systems is a necessity for the production of skillful forecasts. However, a comprehensive assessment of AEW predictability remains absent from the literature, which limits our understanding of what environmental conditions result in more or less predictable AEW forecasts.  Recent work has begun to address this issue by surveying the predictability (i.e., the error/spread relationship) of AEW position and intensity forecast during July-August-September 2007—2009, in the ECMWF ensemble prediction system.  Over all cases, the AEW position and intensity standard deviation exhibits a statistically significant relationship with the large-scale water vapor and divergence, with moister and higher divergence environments exhibiting greater standard deviation.  Previous studies have shown that intraseasonal oscillations (ISOs), such as convectively coupled Kelvin waves (CCKWs), explain much of the variability in these two quantities over Africa.  Therefore, this study focuses on the role ISOs have on the predictability of AEW position and intensity forecasts by binning the ECMWF ensemble forecasts as a function of ISO phases.  In particular, the study will test the hypothesis that convectively-favorable phases of ISOs will lead to less predictable forecasts by comparing the ensemble standard deviation for cases in the convectively-active phase with those from the convectively inactive phase.