Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature and even sea surface temperature (SST), via processes such as vertical mixing, vertical advection and transport of water masses. Offshore of the Amazon River, IT have already been detected and studied; however, their impact on temperature, SST and associated processes are not known in this region. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO) which explicitly resolves the internal tides (IT), to assess how they can affect ocean temperature in the studied area. We distinguish the analysis for two contrasted seasons, from April to June (AMJ) and from August to October (ASO), since the seasonal stratification off the Amazon River modulates the IT’s response and their influence in temperature.  The IT are well reproduced by the model, and are in good agreement with observations, for both their generation and their propagation. The simulation with tides is in better agreement with satellite SST data compared to the simulation without tides. During ASO season, stronger meso-scale currents, deeper and weaker pycnocline are observed in contrast to the AMJ season. Results show that the observed coastal upwelling during ASO season is well reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C at sea surface. In the subsurface above the thermocline, the tide simulation is cooler by -1.2 °C, and warmer below the thermocline by +1.2 °C compared to the simulation without the tides. The study further highlights that the IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process explains the cooler temperature at the ocean surface and in the subsurface water above the thermocline and a warming in the deeper layers (below the thermocline). The surface cooling induced in turn an increase of the net heat flux from the atmosphere to the ocean surface, which could induce significant changes in the local and even for the regional tropical Atlantic atmospheric circulation and precipitation. We therefore demonstrate that IT, mainly via vertical diffusivity along their propagation pathways of approximately 700 km offshore, and tides over the continental shelf, play a key role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by IT.  
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.
Very little is known on mesoscale dynamics in the northern Gulf of Guinea, off West Africa. The purpose of our work is to quantify these mesoscale eddies dynamics in this region (0°N-7°N, 10°W-10°E) and their impact on the near-surface ocean and particularly in the coastal upwelling along the northern coast between 2°W and 2°E. We used a regional simulation of the NEMO model at 1/36° resolution of the year 2016 with daily outputs, validated with in situ and satellite data. On average, four cyclonic and four anticyclonic eddies were detected per day with a mean radius of 75 km and 72 km, respectively. Their lifetime is of the order of few days to a month with associated sea level anomaly from 0.5 cm to more than 1cm. The largest eddies with a relatively long life span are located between 2°N and 4°N, east of Cape Palmas (Ivory Coast) and Cape Three Points (Ghana). We then focused on the July-August-September upwelling period, during which we detected a cyclonic eddy east of the Cape of Three Points, from mid-July to mid-August 2016 with an average radius of 75 km. This cyclone is quasi-stationary and is located in the core of coastal upwelling.Using a heat budget, we show that this eddy has an influence on sea surface temperature (SST) with a double effect. It expands offshore the upwelled cold and salty waters from July 14 to 24, then from July 25 until the dissipation of the cyclone, it weakens this upwelling by advection of warm offshore waters towards the coast, which mix with the upwelling cold waters and warm them.A lagrangian study shows that the eddy waters come from the coastal upwelling, then mix with warmer offshore waters and later are transported eastward by the Guinea Current.In conclusion, this study demonstrates the key role of eddies in SST intra-seasonal variability in the northern Gulf of Guinea.Keywords : Gulf of Guinea, Modeling, Eddy, Coastal upwelling, Lagrangian simulation.
Les modèles HYCOM et Wavewatch III seront implémentés dans la chaîne opérationnelle de Vigilance Vague Submersion de Météo-France d'ici 2015 (projet HOMONIM).Dans ce papier sont présentés une évaluation des performances de ces modèles sur des rejeux de tempêtes, et les résultats d'un premier couplage via la prise en compte de la rugosité de surface dans le modèle de surcote et l'effet des courants et variation des niveaux dans le modèle de vagues.
The Congolese upwelling system (CUS), located along the West African coast north of the CongoRiver, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea SurfaceTemperature minimum in the CUS occurs in austral winter, when the winds are weak and notparticularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regionalocean model to identify the key atmospheric and oceanic processes that control the seasonal evolutionof the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in goodagreement with observations on seasonal timescales, and in particular reproduces the signature of thesurface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, andthe signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals acompetition between warming by air-sea fluxes, dominated by the solar flux throughout the year, andcooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. Theseasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurfaceanalysis shows that remotely-forced coastal trapped waves raise the thermocline from April toAugust, which strengthens the vertical temperature gradient at the base of the mixed-layer and leadsto the mixing-induced seasonal cooling in the Congolese upwelling system.
The Congolese upwelling system (CUS), located along the West African coast north of the CongoRiver, is one of the most productive and least studied systems in the Gulf of Guinea. The Sea SurfaceTemperature minimum in the CUS occurs in austral winter, when the winds are weak and notparticularly favorable to coastal upwelling. Here, for the first time, we use a high-resolution regionalocean model to identify the key atmospheric and oceanic processes that control the seasonal evolutionof the mixed-layer temperature in a 1°-wide coastal band from 6°S to 4°S. The model is in goodagreement with observations on seasonal timescales, and in particular reproduces the signature of thesurface upwelling during the austral winter, the shallow mixed-layer due to salinity stratification, andthe signature of coastal wave propagation. The analysis of the mixed-layer heat budget reveals acompetition between warming by air-sea fluxes, dominated by the solar flux throughout the year, andcooling by vertical mixing at the base of the mixed-layer, as other tendency terms remain weak. Theseasonal cooling is induced by vertical mixing, but is not controlled by the local wind. A subsurfaceanalysis shows that remotely-forced coastal trapped waves raise the thermocline from April toAugust, which strengthens the vertical temperature gradient at the base of the mixed-layer and leadsto the mixing-induced seasonal cooling in the Congolese upwelling system.
Mesoscale dynamics is essential to understanding the physical and biological processes of the coastal ocean regions due to its ability to modulate water properties. However, on the shelf, interactions between eddies, coastal currents, and topography involve complex processes whose observation, understanding, and accurate simulation still pose a major challenge. The purpose of our work is to quantify the mesoscale eddies in the northern Gulf of Guinea, off West Africa (10°W–10°E, 2°N–7°N), and their dynamical interaction with the near-surface ocean particularly in the coastal upwelling that occurs in summer between 2°W and 2°E. We used a regional NEMO model simulation at 1 / 36° resolution over the 2007–2017 period with daily outputs. A total of 38 cyclonic and 35 anticyclonic eddy trajectories were detected over the 2007–2017 period in July–August–September (JAS), with a mean radius along their trajectories of 95 km and 125 km, respectively. The mean lifetime for cyclones and anticyclones is approximately 1 month with an associated sea-level amplitude between 1 and 2 cm. We then focused on the JAS upwelling period of the year 2016 and found a 73 km radius cyclonic eddy east of Cape Three Points (Ghana) with a lifetime of 1 month which interacted with the coastal upwelling. Indeed, the quasi-stationary eddy dwelled within the coastal upwelling region from mid-July to mid-August 2016. A Lagrangian study shows that the eddy waters come from the coastal upwelling, then mix with warmer offshore waters, and later are transported eastward by the Guinea Current. Using a heat budget analysis, we show that this eddy–coastal upwelling interaction has an impact on sea surface temperature (SST) with a double effect: i) the eddy expands offshore the cold and salty waters (23°C and 35.6) of the coastal upwelling from 14 to 26 July; and ii) from 27 July until its dissipation, the eddy weakens this upwelling by an easterly inflow of warm offshore waters. This study highlights how the eddy–upwelling interaction can modulate the coastal upwelling in the northern Gulf of Guinea.
Abstract. The Tropical Atlantic is facing a massive proliferation of Sargassum since 2011, with severe environmental and socioeconomic impacts. The development of Sargassum modelling is essential to clarify the link between Sargassum distribution and environmental conditions, and to lay the groundwork for a seasonal forecast on the scale of the Tropical Atlantic basin. We developed a modelling framework based on the NEMO ocean model, which integrates transport by currents and waves, physiology of Sargassum with varying internal nutrients quota, and considers stranding at the coast. The model is initialized from basin scale satellite observations and performance was assessed over the Sargassum year 2017. Model parameters are calibrated through the analysis of a large ensemble of simulations, and the sensitivity to forcing fields like riverine nutrients inputs, atmospheric deposition, and waves is discussed. Overall, results demonstrate the ability of the model to reproduce and forecast the seasonal cycle and large-scale distribution of Sargassum biomass.
