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Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels.

Abstract. The Eastern Boundary Upwelling Systems (EBUS) contribute to one fifth of the global catches in the ocean. Often associated with Oxygen Minimum Zones (OMZs), EBUS represent key regions for the oceanic nitrogen (N) cycle. Important bioavailable N loss due to denitrification and anammox processes as well as greenhouse gas emissions (e.g, N2O) occur also in these EBUS. However, their dynamics are currently crudely represented in global models. In the climate change context, improving our capability to properly represent these areas is crucial due to anticipated changes in the winds, productivity, and oxygen content. We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated OMZs. We implemented this model in a 3-D realistic coupled physical/biogeochemical configuration in the Namibian upwelling system (northern Benguela) using the high-resolution hydrodynamic ROMS model. We present here a validation using in situ and satellite data as well as diagnostic metrics and sensitivity analyses of key parameters and N2O parameterizations. The impact of parameter values on the OMZ off Namibia, on N loss, and on N2O concentrations and emissions is detailed. The model realistically reproduces the vertical distribution and seasonal cycle of observed oxygen, nitrate, and chlorophyll a concentrations, and the rates of microbial processes (e.g, NH4+ and NO2− oxidation, NO3− reduction, and anammox) as well. Based on our sensitivity analyses, biogeochemical parameter values associated with organic matter decomposition, vertical sinking, and nitrification play a key role for the low-oxygen water content, N loss, and N2O concentrations in the OMZ. Moreover, the explicit parameterization of both steps of nitrification, ammonium oxidation to nitrate with nitrite as an explicit intermediate, is necessary to improve the representation of microbial activity linked with the OMZ. The simulated minimum oxygen concentrations are driven by the poleward meridional advection of oxygen-depleted waters offshore of a 300 m isobath and by the biogeochemical activity inshore of this isobath, highlighting a spatial shift of dominant processes maintaining the minimum oxygen concentrations off Namibia. In the OMZ off Namibia, the magnitude of N2O outgassing and of N loss is comparable. Anammox contributes to about 20% of total N loss, an estimate lower than currently assumed (up to 50%) for the global ocean.

SUMMARYIt is common in meteorological applications of variational assimilation to specify the error covariances of the model background state implicitly via a transformation from model space where variables are highly correlated to a control space where variables can be considered to be approximately uncorrelated. An important part of this transformation is a balance operator which effectively establishes the multivariate component of the error covariances. The use of this technique in ocean data assimilation is less common. This paper describes a balance operator that can be used in a variable transformation for oceanographic applications of three-and fourdimensional variational assimilation. The proposed balance operator has been implemented in an incremental variational data assimilation system for a global ocean general-circulation model. Evidence that the balance operator can explain a significant percentage of background-error variance is presented. The multivariate analysis structures implied by the balance operator are illustrated using single-observation experiments.

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sources of mixing play a major role in the overall system dynamics through their impact upon the general shelf thermohaline structure, in particular in the vicinity of the upwelling zone. Systematic alongshore variability in thermohaline properties highlight important limitations of the 2D idealization framework that is frequently used in coastal upwelling studies.

[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

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