West African Summer Monsoon Onset Variability and Prediction

Forecasting the onset of rainfall is crucial for early warning activities in rainfed agriculture economies like West Africa. This study seeks to examine the utility of satellite-based products in predicting the onset of rainfall in the Sahel region of West Africa to promote early warning in drought-prone areas. The onset is defined as the first day of a wet season when a wet spell of accumulated rainfall in three consecutive days is at least 20 mm, and there is no dry spell of at least 7-days in the next 21-days. For the Sahel region, the search for the onset of June-September (JJAS) rainy season starts on May 15. Maximum dry and wet spell lengths (in days) are determined for specified rainfall thresholds (defaults to 1.0 mm for dry and 5.0 mm for wet days). The Climate Prediction Center morphing method (CMORPH) and Rainfall Estimate version 2 (RFE2) data were used to analyze the historical onset of the rains from 2001-2022. The onset date for 2023 was predicted using the NCEP coupled forecast system model version 2 (CFS2) and Environment and Climate Change Canada (ECCC) global model precipitation hindcast (1993-2016) and forecast (246 days ahead on a daily time scale) data. The forecast was calibrated using linear regression. Results showed that the rainfall season was early in 2005, 2012, and 2017 and late in 2002, 2006, and 2015 over the western Sahel region. The onset dates showed higher variability (about 20-30 days) in the northern Sahel region than in the Sudano Guinean (5-10 days) and Guinean regions (0-5 days). Compared to the observations, the calibrated forecasts showed improvement in most areas in the Sahel region. For example, over Niger and Chad, the onset date was earlier in the north than in the south, which is inconsistent with climatology, and the calibrated forecast corrected this error. Consistent with recent observations, the predicted rainfall onset for JJAS 2023 showed enhanced probability for earlier-than-normal onset over the western Sahel region, especially in southern parts of Mauritania, southwestern Mali, northern Burkina Faso, southern Niger, and northern Nigeria. Overall, the correlation between observed and predicted onset dates is positive for 2001-2022, with the highest skill (>0.7) over east-central Senegal, southwestern Mali, southern Niger, and northern Nigeria.