Mesoscale dynamics and its influence on coastal upwelling in the northern Gulf of Guinea

Very little is known on mesoscale dynamics in the northern Gulf of Guinea, off West Africa. The purpose of our work is to quantify these mesoscale eddies dynamics in this region (0°N-7°N, 10°W-10°E) and their impact on the near-surface ocean and particularly in the coastal upwelling along the northern coast between 2°W and 2°E. We used a regional simulation of the NEMO model at 1/36° resolution of the year 2016 with daily outputs, validated with in situ and satellite data. On average, four cyclonic and four anticyclonic eddies were detected per day with a mean radius of 75 km and 72 km, respectively. Their lifetime is of the order of few days to a month with associated sea level anomaly from 0.5 cm to more than 1cm. The largest eddies with a relatively long life span are located between 2°N and 4°N, east of Cape Palmas (Ivory Coast) and Cape Three Points (Ghana). We then focused on the July-August-September upwelling period, during which we detected a cyclonic eddy east of the Cape of Three Points, from mid-July to mid-August 2016 with an average radius of 75 km. This cyclone is quasi-stationary and is located in the core of coastal upwelling.

Using a heat budget, we show that this eddy has an influence on sea surface temperature (SST) with a double effect. It expands offshore the upwelled cold and salty waters from July 14 to 24, then from July 25 until the dissipation of the cyclone, it weakens this upwelling by advection of warm offshore waters towards the coast, which mix with the upwelling cold waters and warm them.
A lagrangian study shows that the eddy waters come from the coastal upwelling, then mix with warmer offshore waters and later are transported eastward by the Guinea Current.
In conclusion, this study demonstrates the key role of eddies in SST intra-seasonal variability in the northern Gulf of Guinea.
Keywords : Gulf of Guinea, Modeling, Eddy, Coastal upwelling, Lagrangian simulation.