The Congolese upwelling system (CUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea Surface Temperature minimum in the CUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular reproduces the signature of the surface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals a competition between warming by air-sea fluxes, dominated by the solar flux throughout the year, and cooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurface analysis shows that remotely-forced coastal trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed-layer and leads to the mixing-induced seasonal cooling in the Congolese upwelling system.

Potential vorticity (PV) is a key parameter to analyze the generation and dynamics of oceanic mesoscale eddies. Adiabatic and diabatic processes can be involved in the generation of localized PV anomalies and vortices. However, PV is difficult to evaluate at mesoscale. In this study we argue that eddies created by diapycnal mixing or isopycnal advection of water-masses are associated with PV anomalies and significant isopycnal temperature/salinity anomalies (Ɵ’/S’). In contrast, eddies created by friction are associated with PV anomalies but with non-significant isopycnal Ɵ’/S’. Based on 18 years of satellite altimetry data and vertical Ɵ/S profiles from Argo floats, we analyze the isopycnal Ɵ’/S’ within new-born eddies in the tropical Atlantic Ocean (TAO) and discuss the possible mechanisms involved in their generation. Our results show that on density-coordinates system, both anticyclonic (AEs) and cyclonic (CEs) eddies can exhibit positive, negative or non-significant isopycnal Ɵ’/S’. Almost half of the sampled eddies do not have significant Ɵ’/S’ at their generation site, suggesting that frictional effects play a significant role in the generation of their PV anomalies. The other half of eddies, likely generated by diapycnal mixing or isopycnal advection, exhibits significant positive or negative anomalies with typical Ɵ’ of ±0.5°C. More than 70% of these significant eddies are subsurface-intensified, having their cores below the seasonal pycnocline. Refined analyses of the vertical structure of new-born eddies in three selected subregions of the TAO, show the dominance of cold (warm) subsurface AEs (CEs) likely due to isopycnal advection of large scale PV and temperature.

Internal tides (ITs) in the Indonesian seas were largely investigated and hotspots of intensified mixing identified in the straits in regional models and observations. Both of them indicate strong mixing up to 10⁻⁴cm/s even close to the surface and show that tides at spring-neap cycle cool by 0.2°C the surface water at ITs’ generation sites.These findings supported the idea of strong and surfaced mixing capable of providing cold and nutrient-rich water favorable for the whole ecosystem. However, it has never been assessed through an ad-hoc study. Our aim is to provide a quantification of ITs impact on chlorophyll-a through a coupled model, whose physical part was validated against the INDOMIX data in precedent studies and the biogeochemical part is compared to in-situ samples and satellite products. In particular, explicit tides’ inclusion within the model improves the representation of chlorophyll and of the analyzed nutrients. Results from harmonic analysis of chlorophyll-a demonstrate that tidal forcing modify spring/neap tides’ variability on the regions of maximum concentration in correspondence to ITs’ génération areas and to plateau sites where barotropic tides produce large friction reaching the surface. The adoption of measured vertical diffusivities explains the biogéochemical tracers’ transformation within the Halmahera Sea and used to estimate the nutrients’ turbulent flux, with an associated increase in new production of ~25% of the total and a growth in mean chlorophyll of ~30%. Hence, we confirm the key role of ITs in shaping vertical distribution and variability of chlorophyll as well as nutrients in the maritime continent.