Abstract
In this study, we diagnose the onset and demise of the rainy season from the daily rainfall over West Africa. We then produce a probabilistic seasonal outlook of the rainy season over this region based on the observed variations of the onset date of the season, which verifies well with observations. We generated 101 ensemble members at every grid point by randomly perturbing the observed series of daily rainfall data obtained from the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) mission, version 6, rainfall analysis. The generated ensemble of time series of daily rainfall accounts for uncertainties at meso- to synoptic scales that could arise in the generation of the observed rainfall analysis. The ensemble members provide a robust estimate of the onset and demise dates of the rainy season. We find that the interannual variations in the onset and demise dates of the rainy season in West Africa significantly influence the corresponding anomalies of the seasonal length and rainfall. Additionally, the interannual variability of the onset dates dominates over the demise dates of the rainy season across West Africa. In contrast, their association with remote, large-scale forcing is not found to be as significant. In addition, we found that the African easterly jet (AEJ) is displaced southward or northward in early or late onset seasons, respectively. This study highlights the effectiveness of utilizing the intrinsic relationships between onset date, seasonal length, and rainfall anomaly to produce useful seasonal outlooks of the rainy season solely by monitoring the onset date of the rainy season.
Significance Statement
The West African region comprising of Liberia, Sierra Leone, Guinea, and Guinea-Bissau is critically dependent on the seasonal evolution of the rainfall for their agricultural activities amid significant seasonal-to-interannual variations of their rainy season. In this study, we offer a simple but reliable methodology to provide a seasonal outlook of the rainy season over West Africa by simply monitoring the evolution of the onset date of the rainy season. Given the availability of gridded rainfall analysis at 10-km grid resolution from the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (IMERG) mission, version 6, at 12-h latency, it is now possible to adapt the proposed methodology for real-time monitoring and seasonal outlook of the rainy season. The veracity of the seasonal outlook of the rainy season from the proposed methodology is analyzed to show that it has much higher skill than a random forecast and could serve as a complementary approach to current practices of seasonal forecast for the region.
© 2025 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).
