Coupled physical/biogeochemical modeling including O<sub>2</sub>-dependent processes in the Eastern Boundary Upwelling Systems: application in the Benguela

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.

Seasonal variability of the oxygen minimum zone off Peru in a high-resolution regional coupled model

“…We use a high-resolution simulation of the southeastern Pacific, based on the hydrodynamic Regional Ocean Modeling System (ROMS) circulation model (see McWilliams, 2005, 2009, for a complete description of the model) coupled with a nitrogen-based biogeochemical model developed for the eastern boundary upwelling systems (BioEBUS; Gutknecht et al, 2013a, b), hereby referred as CR BIO.…”

“…BioEBUS uses two compartments of phytoplankton and zooplankton, small (flagellates and ciliates, respectively) and large (diatoms and copepods, respectively), detritus, dissolved organic nitrogen and the inorganic nitrogen forms nitrate, nitrite and ammonium, as well as nitrous oxide (see Gutknecht et al, 2013a, b, for a description of the model). The open-boundary conditions for the biogeochemical model are provided by the climatological CARS data set (nitrate and oxygen concentrations) and by SeaWiFS archive (chlorophyll a concentration).…”

“…Low-resolution CMIP class coupled models still have severe biases of physical and biogeochemical origins, particularly in eastern boundary current systems (Richter, 2015), which has eluded the interpretation of long-term trends in OMZ (Stramma et al, , 2012Cabré et al, 2015). Regional coupled biogeochemical modeling nonetheless has provided a complementary approach to gain insight in the dynamics of OMZ and its relationship with climate (Resplandy et al, 2012;Gutknecht et al, 2013a). One recent modeling effort to understand the dynamics behind the OMZ in the eastern tropical Pacific comes from Montes et al (2014).…”

“…In the present study, we also take advantage of the regional modeling approach in order to investigate the mechanisms associated with the seasonal cycle of DO within the OMZ. The motivation for focusing on seasonal variability is threefold: (1) a better knowledge of the processes acting on the OMZ at seasonal timescale is viewed as a prerequisite for interpreting longer timescales of variability (ENSO, decadal); (2) the scarcity of quality longterm subsurface biogeochemical data in the SEP is a limitation for tackling the investigation of OMZ variability at low frequency; (3) to the authors' knowledge, this issue has not been addressed in the literature for the eastern tropical Pacific, although it has been a concern for other tropical oceans (Resplandy et al, 2012;Gutknecht et al, 2013a;Duteil et al, 2014).…”

Reconstruction of super-resolution ocean <i>p</i>CO<sub>2</sub> and air–sea fluxes of CO<sub>2</sub> from satellite imagery in the southeastern Atlantic

“…Surface temperature is also related to gas solubility in the ocean, and areas with high temperatures are more suitable for releasing CO 2 to the atmosphere. We have studied the relationship of SST and Chl a variables with pCO 2 using the outputs of a coupled Regional Ocean Modeling System (ROMS) with the BIOgeochemical model of the Eastern Boundary Upwelling System (BIOE-BUS) (Gutknecht et al, 2013). The ROMS includes several levels of nesting and composed grids, which makes it an ideal model for the basis of our methodology in working in two spatial resolutions.…”