Gaëlle De Coëtlogon

Seasonal influence of the sea surface temperature on the low atmospheric circulation and precipitation in the eastern equatorial Atlantic

The air–sea interaction in the Gulf of Guinea and its role in setting precipitation at the Guinean coast is investigated in the present paper. This study is based on satellite observations and WRF simulations forced by different sea surface temperature (SST) patterns. It shows that the seasonal cold tongue setup in the Gulf of Guinea, along with its very active northern front, tends to strongly constrain the low level atmospheric dynamics between the equator and the Guinean coast. Underlying mechanisms including local SST effect on the marine boundary layer stability and hydrostatically-changed meridional pressure gradient through changes in SST gradient are quantified in WRF regarding observations and CFSR reanalyses. Theses mechanisms strongly impact moisture flux convergence near the coast, leading to the installation of the first rainy season of the West African Monsoon (WAM) system. The current study details the mechanisms by which the Atlantic Equatorial cold tongue plays a major role in the pre-onset of the boreal WAM.

Les avancées d'AMMA sur les interactions océan-atmosphère

The results gathered since the AMMA/EGEE campaigns of 2005-2007 in the field of air-sea interactions are presented. They show the key role of the Atlantic cold tongue (ACT) in the eastern equatorial Atlantic on the West African monsoon. The strengthening of south easterlies, associated with the Saint-Helena anticyclone, promotes the preconditioning of the thermocline along the equator and the development of the ACT.The equatorial front on its northern boundary, modifies the surface heat fluxes that affect the circulation in the lower atmosphere. This circulation helps to push moisture northward, toward the West African sub-continent, and impacts the monsoon jump.