Abstract
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.
Highlights
The Eastern Boundary Upwelling Systems (EBUS) (California, Humboldt, Canary, and Benguela upwelling systems) are specific areas connecting the coastal zone to the open ocean with the subtropical gyres
Sensitivity analyses were performed on key parameters in order to better represent the distribution of the well-known biogeochemical fields (O2, NO−3 and Chl a) and the rates of the microbial loop (NH+4 and NO−2 oxidation, NO−3 reduction, anammox)
We developed a biogeochemical model (BioEBUS) taking into account the main processes linked with EBUS and associated Oxygen Minimum Zones (OMZs)
Summary
The Eastern Boundary Upwelling Systems (EBUS) (California, Humboldt, Canary, and Benguela upwelling systems) are specific areas connecting the coastal zone to the open ocean with the subtropical gyres. Other important features contribute to these highly productive zones such as the wind curl (responsible of a second upward vertical advection zone offshore of the continental shelf), the coastal topography, and the poleward undercurrent with water masses enriched in nutrient and depleted in oxygen originating from the equatorial zone These EBUS are crudely represented in the global climate atmosphere–ocean models used within the Coupled Model Inter-comparison Project 5 (CMIP5) due to their coarse resolution. In the Benguela, thanks to several initiatives from different countries (e.g., South Africa, Namibia, Germany, Norway), observations of important variables and fluxes were made as N2O concentrations, nitrification, denitrification, and anammox processes These three processes represent a loss of bioavailable (fixed) nitrogen through the production of gaseous products (N2O and/or N2), with the potential to affect biogeochemical cycles. We discuss the influence of these key parameters on important quantities such as the volume, minimum oxygen concentrations, N losses, N2O concentrations, and emissions before concluding
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