Dynamics off the Congo River and its impact on the water exchange between the coastal and open ocean at different timescales.

The Congo River represents one of the largest freshwater discharges in the Atlantic Ocean. In this study, we investigate the role of mesoscale and submesoscale dynamics in modulating salinity transport, using a 3 km-resolution realistic numerical simulation. We focus on the year 2016 due to the availability of in situ observations, satellite data, and currents inferred from Automatic Identification System (AIS). These data allow us to confirm the appropriate representation of the Congo River plume, regional circulation, and mesoscale features. For one large anticyclonic eddy in March–April 2016, we demonstrate how mesoscale eddies can trap low-salinity waters and transport them into the open ocean. Motivated by this particular event, we perform a salinity and volume budgets analysis combined with an eddy-mean flow decomposition that confirms the key role of mesoscale activity in the exchange of waters between the coastal and open ocean. Western boundary fluxes dominate the net export of plume waters. Strong salinity changes occur locally due to the eastern and vertical fluxes, which bring fresh and salty waters. Mesoscale dynamics contribute through intense but intermittent events that can dominate freshwater export, particularly during the maximum plume extension. Submesoscale contributions can locally exceed 30% of the salinity transport during short-lived events. However, the season dynamics dominate over the year. While our results highlight the importance of mesoscale variability in episodically exporting freshwater, future work should extend the analysis to multiple years to assess interannual variability and leverage upcoming high-resolution satellite missions to better resolve mesoscale and submesoscale processes in this highly dynamic river–ocean system.