THE WEST AFRICAN MONSOON: From a better understanding of the intraseasonal and synoptic variability toward forecast
Abstract:
The intraseasonal variability (ISV) is critical in West Africa where resources are highly rainfall dependent. Chauvin et al. (2010) identified a robust propagative mode of variability of the SaharPoan et al. (2013) assessed the Sahelian ISV from a moist perspective based on precipitable water (PW). While the eastern Sahel PW variability is dominated by time scales longer than 10 days, synoptic scales dominate in the western Sahel, due to the influence of African easterly waves (AEWs). A composite study of AEWs provides a documentation of their structure and of the role of diabatic processes along their propagation and growth that the dry dynamics alone cannot explain (Poan et al. 2014). Corresponding dry and wet PW anomalies significantly modulate convective activity and can be tracked back to the eastern Sahel over a long period.
To explore the potential forecast skill brought about by the advanced understanding of the ISV, the MISVA collaborative project between Senegal and France aims at monitoring and forecasting the ISV over Africa in real-time. This project provides a series of diagnostics and indices during the WAM season since 2011. This project has showed that dry and moist events can be detected several days in advance, even if Numerical Weather Prediction models have low skill for precipitation. In general such events are the result of the combination of several synoptic-to-intraseasonal modes. The physical understanding of the WAM discussed above helped to improve forecasting methods over West Africa and led to the writing of the "Forecaster's Handbook for West Africa", in close collaboration between forecasters and scientific international communities, under the WMO umbrella.