Response of the Benguela upwelling system to four decades of global warming

The Benguela upwelling system (BUS) is one of the most productive marine systems in the world oceans, with about 50 times the productivity per unit area compared to the global ocean average. Such high productivity is attributed to the upwelling process, in which Equatorward alongshore winds combined with the Coriolis effect force surface coastal water offshore, resulting in bringing deep, cold, nutrient-rich waters up to the photic layer, triggering the primary production. The BUS is highly likely to be affected by global warming. Nevertheless, it’s response seems complex because warming might affect the system by two counteracting ways. First, as warming proceeds, upwelling favorable winds might intensify leading to stronger upwelling events. On the other hand, increased stratification potentially counteracts the efficacy of upwelling to deliver nutrients to surface layers. Overall, such possible alterations could have drastic impacts on the physical and biogeochemical characteristics of the BUS and primarily the primary productions rates. Thus, investigating the response of the BUS to recent global warming is crucially important.

To this purpose, a coupled highly resolved 3D physical-biogeochemical model is implemented based on NEMO (Nucleus for European Modelling of the Ocean) and BFM (Biogeochemical flux model). The coupled model is being constructed via an online nesting approach to maintain high resolution for a small-scale process like the upwelling, but at the same time to provide the Benguela domain precise boundary conditions. The grid refinement has been conducted using AGRIF (Adaptive Grid Refinement in Fortran). A two-way online nesting has been applied, where information from the child is allowed to propagate back into the parent domain. With a tripolar ORCA025 grid, the nesting (parent) domain covers the global ocean with a horizontal resolution of 1/4°, while the nested (child) domain for the Benguela domain has a resolution of 1/16° and spatially extend from (7°W to 27°E) and from (15°S to 44°S). Both grids have 75 vertical layers. The coupled model is run over a hindcast simulation encompassing four decades starting from 1980 to 2020.

Regarding the model’s set-up, bottom topography is being derived from GEBCO, while the atmospheric forcing has been retrieved from ERA5, and due to the significance of river freshwater input in such simulation, the coupled model was forced with runoff data from the Global Flood Awareness System (GLOFAS). As for the BFM model configuration, it comprises multiple plankton functional groups, nutrients forms, and O2 dependent processes. BFM initial conditions were retrieved from The Global Ocean Data Analysis Project (GLODAP). Finally, the coupled model is validated using in situ and satellite observational data for physical and biogeochemical state-variables and processes.