Mesoscale dynamics off the Congo River in the Gulf of Guinea and its impact on the exchange of waters between coastal and open ocean.

The aim of this study is to characterize and quantify the mesoscale dynamics off the Congo River in the Gulf of Guinea and evaluate its impact on the exchange of fresh and salty waters between the coastal and open ocean in this area. The study area, centered at the mouth of the Congo River (2°S-10°S and 3°E-13.5°E), is characterized by an intense seasonal freshwater cycle related to rainfall-driven fluvial input. We used a 1/36° resolution NEMO model configuration for the Gulf of Guinea, optimised to improve the realism the Congo River plume. Results from this configuration were validated with observations and we analyzed the year 2016. The Congo River combined with the wind forcing strongly influences the ocean circulation in the area. The river plume is associated with positive sea level height at the river mouth and strong horizontal density gradients. Moreover, the river plume stratifies the surface waters leading to a very shallow mixed layer (<10 m) enhancing the wind forcing on the surface waters. Our analysis of the mesoscale dynamics for the year 2016 reveals several events, especially a dipole, with a lifetime of 40 days for the anticyclone and 70 days for the cyclone. This dipole appears between March and April 2016, when the river discharge is high, winds are weak, and the river plume is located south of the mouth. The anticyclonic structure carries low-salinity water (S≈32.5) from the southward extension of the river plume. Lagrangian analyses confirm that the waters trapped in the mesoscale dipole originate partly from the Congo river plume. To investigate the processes driving the offshore water transport, we analyze the salinity variations in a box encompassing the river plume. The horizontal/vertical advection through its boundaries increase salinity whereas vertical diffusion decreases it. At the boundaries, the role of the mesoscale dynamics accounts for up to 53 % of the total fresh/salty transport, showing the key role of mesoscale dynamics, especially towards the open ocean.