Influence of Wind and Surface Buoyancy Flux on the Variability of the Oceanic Mixed Layer Depth in the Northern Gulf of Guinea Coastal Upwelling

The coastal upwelling along the northern Gulf of Guinea (GG) plays an important role in the regional climate through the interaction between the air and the mixed layer (ML). It is then very important to understand the mixed layer depth (MLD) variability in this region. Combining the oceanic (ORAP5) and the atmospheric (ERA5) reanalyses to investigate the influence of the wind and the surface buoyancy flux (Bo) on the MLD variability along the northern GG coastal upwelling region, this study showed that shallower MLs and high productive ocean are observed during the period of cooling while deeper MLDs and less productive ocean occur during the warm season. During the upwelling season, the Bo and the Ekman pumping (Wek) contribute to thinning the ML while strong mixing at the ML base deepens it. The Bo and the Wek control the MLD variability during the upwelling season. The MLD decreases from June to August because of an increasing stratification (Bo < 0) and wind-driven divergence (Wek > 0). These effects of Wek and Bo counteract the MLD deepening linked to the shear-mixing at the ML base. Outside the upwelling season, thicker MLD is maintained by the Bo and the Wek when the shear-mixing at the ML base has totally weakened. The minimum MLD during November results from negative Bo that overcomes the deepening due to the Wek in the absence of mixing at the ML base. The high Chlorophyll-a concentration during the upwelling season results from strong mixing and Ekman pumping.