A model perspective on the drivers of the shallow oxygen minimum zone off the northwestern African coast

The Oxygen Minimum Zone (OMZ) off the coast of Mauritania and Senegal is characterized by a shallow and a deep oxygen minimum, each with potentially different formation mechanisms. Although the shallow OMZ has been linked to the en-route degradation of organic matter within highly productive, coastal-generated eddies, less attention has been paid to hypoxic Dissolved Oxygen (DO) concentrations observed along the coastal region, where low-oxygen eddies are formed. This study aims to clarify the spatio-temporal dynamics and underlying mechanisms that lead to the formation of the shallow OMZ along the northwestern African coast.

To achieve this, a Eulerian-Lagrangian numerical framework was employed by combining a coupled physical-biogeochemical model with a Lagrangian particle-tracking simulation. The model domain covers the entire Tropical Atlantic with an horizontal resolution of 3 km, achieving a good representation of the horizontal and vertical structure of the North Atlantic OMZ. To assess the pathways and evolution of the water masses that form the shallow OMZ, lagrangian particles were released in grid cells with DO < 40 μmol.l^-1 and traced backwards in time.

Our results reveal distinct seasonal and latitudinal variations of DO concentrations along the coast, with DO concentrations significantly decreasing in the transition from the upwelling to the relaxation season (from May to July). Associated with the transport of more oxygenated South Atlantic Central Waters (SACW), the influence of the Poleward Undercurrent (PUC) on the ventilation of the coastal region is evident, especially when the current loses intensity and becomes a surface-intensified feature in summer. When the PUC reaches its maximum intensity in autumn, its core deepens below the mixed layer and replaces the older, oxygen-poor waters with ventilated waters of southern origin.

The impact of eddies on coastal dynamics was also explored. A quasi-permanent Anticyclonic Modewater Eddy (ACME) formed during the upwelling season by the interaction of the PUC with the Cap-Vert headland is the main mechanism behind the import of offshore waters to the coastal region. Lagrangian particle trajectories suggest that this eddy prevents the direct northward transport of SACW by the PUC. Whilst some of the particles are trapped and subsequently transported offshore inside the eddy, other particles are stirred with an older, less oxygenated SACW variety in the offshore region and re-circulate to the coastal region. Similar particle re-circulation patterns are also observed further north, coinciding with cyclonic and ACME formation hotspots.

Our findings suggest that in addition to their role in the formation and advection of oxygen-depleted waters to offshore, coastal-generated eddies play a crucial role in modulating DO levels along the northwestern African coast.