A three-dimensional primitive equation model Océan Parallélisé (OPA) was coupled to the biogeochemical model Pelagic Interaction Scheme for Carbon and Ecosystem Studies to simulate the ocean circulation and the marine biological productivity through the biogeochemical cycles of carbon and the main nutrients (P, N, Si, Fe). We focus on surface phytoplankton dynamics in the Indian Ocean extending from 30°S to 30°N and from 30°E to 120°E. The seasonal cycle of phytoplankton over the Indian Ocean is generally characterized by two blooms, one during the summer monsoon, the other one during the winter monsoon. Based on the method proposed by Lévy et al. (2007), different biogeochemical provinces can be defined during the summer and winter monsoons. The model performed relatively well by simulating the main features of the cumulated increase in chlorophyll, and the time of the bloom onsets is consistent with data. It also reproduced quite well the main biogeochemical provinces in good agreement with data in most of the Arabian Sea (except in the central part), the Bay of Bengal, and in the convergence zone south of the equator. The analysis of the modeled biogeochemical processes has shown that during the blooms onset periods, the most limiting nutrient was nitrogen except some areas around India and the eastern part of the Bay of Bengal where the ecosystem tends toward silicate limitation. These limitations change during the blooms development. The model also highlighted a variety of the critical physical processes (horizontal and vertical advection, turbulent diffusion, mixed layer depth) involved in each biogeochemical province bloom dynamics.

[1] A three-dimensional primitive equation model, the Regional Ocean Modeling Systems (ROMS), coupled to two biogeochemical configurations (NPZD and N 2 P 2 Z 2 D 2 ) was used to study the dynamics of the first trophic levels of the pelagic food web in the southern Benguela upwelling system. The domain extends from the Agulhas Bank bordered by the Agulhas Current to 27°S on the west coast of South Africa. The circulation is driven by monthly climatologies of atmospheric forcing fields. The NPZD ecosystem model consists of four state variables: nutrient (nitrate), phytoplankton, zooplankton and detritus. In the N 2 P 2 Z 2 D 2 model, ammonium has been added and the three other variables have been divided into small and large organisms or detritus. Both models are able to reproduce the spatio-temporal phytoplankton distribution. Along the west coast, chlorophyll concentrations maxima are associated to surface waters. Westward dominating winds generate the lowest chlorophyll concentrations encountered in winter. The small phytoplankton organisms simulated by the N 2 P 2 Z 2 D 2 model are responsible for a weaker chlorophyll inshore/offshore gradient, in closer agreement with observations. Transitions from a regime dominated by new production (high f ratio) to one dominated by regenerated production (low f ratio) happen to be abrupt, underlying the constant competition between small and large organisms with regard to upwelling induced nutrient inputs. On the Agulhas Bank, the summer enrichment is associated with subsurface maxima, while in winter, mixing by storms results in a homogeneous phytoplankton distribution in the water column. Regenerated production plays an important role in maintaining the total phytoplankton growth. Zooplankton biomass reflects the overall patterns of chlorophyll a concentrations with differences between the west coast and the Agulhas Bank, consistent with data, and its distribution exhibits a clear seasonal contrast. The seasonality of small and large zooplankton in the N 2 P 2 Z 2 D 2 model is quite distinct, which allows, from the Agulhas Bank to St. Helena Bay, a food continuum for fish larvae. This was not achieved with the simpler NPZD model, emphasizing the importance of representing the appropriate level of complexity to characterize food availability for higher trophic levels.Citation: Koné, V., E. Machu, P. Penven, V. Andersen, V. Garçon, P. Fréon, and H. Demarcq (2005), Modeling the primary and secondary productions of the southern Benguela upwelling system: A comparative study through two biogeochemical models, Global

Cyclonic eddies generated downstream of Cape Palmas and Cape Three points have been suggested to contribute to the coastal upwelling along the northern coast of the Gulf of Guinea. A numerical analysis using a high-resolution model is used to investigate the mesoscale activity and the coastal upwelling generation processes. An eddy detection and tracking tool is applied to altimeter and model data, showing good agreement between these data sets. About two cyclonic eddies per year with an average radius of 60 km were identified downstream of both capes. These cyclonic eddies have an average lifetime of about 60 days during the major coastal upwelling period (boreal summer) and an eastward propagation. These cyclonic eddies are shallow, energetic (their relative vorticity can reach 3 times the earth's rotation), and dimensionless parameters show that they are in an eddy shedding regime. Mean flow interactions and barotropic instabilities associated with capes are their main generation processes. An idealized experiment is conducted in order to analyze the effect of capes on eddy generation and on coastal upwelling. It reveals that these cyclonic eddies generated downstream of capes are not the process responsible for this coastal upwelling. This experiment also suggests that the cyclonic eddies are the cause of the westward and coastal Guinea Counter Current that is associated with a transfer of energy from eddy kinetic to the mean flow.

The northern Gulf of Guinea is a part of the eastern tropical Atlantic where oceanic conditions due to the presence of coastal upwelling may influence the regional climate and fisheries. The dynamics of this coastal upwelling is still poorly understood. A sensitivity experiment based on the Regional Oceanic Modeling System (ROMS) is carried out to assess the role of the detachment of the Guinea Current as a potential mechanism for coastal upwelling. This idealized experiment is performed by canceling the inertia terms responsible for the advection of momentum in the equations and comparing with a realistic experiment. The results exhibit two major differences. First, the Guinea Current is found to be highly sensitive to inertia, as it is no longer detached from the coast in the idealized experiment. The Guinea Current adjusts on an inertial boundary layer, the inertial terms defining its lateral extension. Second, the upwelling east of Cape Palmas disappears in absence of the Guinea Current detachment. This is in contrast with the upwelling east of Cape Three Points, which is still present. The results suggest that two different generation processes of the coastal upwelling need to be considered: the upwelling east of Cape Palmas (which is due to inertia, topographic variations, and advective terms effects resulting in important vertical pumping) and the upwelling east of Cape Three Points (which is principally induced by local winds). In addition to recent work ruling out the role of eddies, this study clarifies the processes responsible for this coastal upwelling.

The aim of this study is to characterize the coastal upwelling variability at seasonal and inter-annual time scales in the northern Gulf of Guinea (NGoG) using Sea Surface Temperature (SST) collected with autonomous "ONSET" thermometers. Results show that the ONSET SST data are suitable for numerical model evaluation, and provide relevant information in addition to satellite and reanalysis data at seasonal cycle. The minor and major coastal upwellings are present in all the products. The inter-annual SST variability is more pronounced in the western part of the region (Côte d'Ivoire and Ghana) than in the eastern part (Benin and Nigeria). The pattern differences between the west and the east of the region highlight a large spatial variability of the SST in the NGoG. Indeed, the signal of the minor upwelling season is visible only in the west of the basin, namely between Cape Palmas and Cape Three Points. We also observe a well-established thermal gradient between the western and eastern parts of the basin. This gradient is increasing from west to east during the major upwelling season, and decreasing from east to west during the rest of the year. The coastal ONSET data allow to evidence higher SST anomalies than those deduced from satellite and reanalysis products. Although the cold or warm events observed in 2008, 2010 and 2012 are well detected by all products, only the ONSET data set reveal the strong negative SST anomaly observed in 2009 along the coast of Ghana and Côte d'Ivoire.