West African Monsoon (WAM) and Atlantic Cold Tongue (ACT) onsets using Regional Climate Model

This study focused on the occurrences and relationships of the Atlantic Cold Tongue (ACT) and the West African Monsoon (WAM) onsets in the Sudano-Sahelian region during 1983-2000. Two simulations (WAFSSTERA and WAFSSTREY) were used to investigate the sensitivity of the Regional Atmospheric Model (MAR) for two different datasets of sea surface temperature (ERA-40 and Reynolds SST datasets). The MAR reproduced the rainfall high values in the Guinean zone. A band of weak precipitation (<1 mm/day) was observed in the two simulations. WAFSSTERA gave more annual daily average rainfall (~1.6 mm/day) than WAFSSTREY. The simulated onset date index (ODI) of WAM showed more northward penetration of monsoon flow, particularly during flood years in the Gulf of Guinea. These results also indicated that the monsoon onset did not depend on the SST dataset used to force the model. Finally, the negative difference between ODIs and ACT onset date could be due to warm SST. The yearly evolution of the southern heat transport was also in agreement with this difference. It showed yearly succession of negative and positive anomalies around 7.5°N (mean rainy equilibrium position over West Africa). Such study could be useful for the forecast of WAM onset and ACT scenario relationships. Key words: Atlantic cold tongue, onset date index, regional climate model, sea surface temperature, West African Monsoon.