Abstract
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.
Highlights
In the last twenty years, many studies have been conducted to understand how the surface conditions could influence the West African Monsoon (WAM) precipitations
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
These biases calculations have been performed from WAFSSTERA (Figure 2a) and WAFSSTREY (Figure 2b) and allow characterization of the difference between the simulation results compared to Climatic Research Unit (CRU) dataset
Summary
In the last twenty years, many studies have been conducted to understand how the surface conditions could influence the West African Monsoon (WAM) precipitations These works, which focused on Sahel region, identified the global Sea Surface Temperature (SST) (Lamb, 1978; Folland et al, 1986; Janicot et al, 1996), the continental surface conditions (Nicholson, 1989; Cook, 1994) and the large-scale circulation as potential factors (Nicholson, 2013). Vizy and Cook (2001) found that the impact of SST anomalies on West African rainfall was more important in the Gulf of Guinea than in the north of the tropical Atlantic. They investigated the sensitivity of some Regional Climate Models (RCM) to regional SST anomalies and found that RCMs simulated better the main processes that influence the inter-annual variability of the monsoon during the June and July periods
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