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West African Coastal Ocean - Finescale physics, biogeochemistry and climate change

Periodic Reporting for period 1 - WACO (West African Coastal Ocean - Finescale physics, biogeochemistry and climate change)

Reporting period: 2018-03-01 to 2020-02-29

Eastern boundary upwelling systems (EBUS) are located along the west coasts of the Americas and Africa, where strong equatorward winds promote cold and nutrient rich waters to be brought to the surface - a process called upwelling. The injection of nutrients to the sunlit surface layer enables proliferation of phytoplankton and makes upwelling regions some of the ocean’s most biologically productive regimes. They constitute less than 1% of the total ocean area, but account for nearly 20% of global fish catch. EBUS are hotspots of global climate change. Regional and more localized changes are expected in wind forcing, temperature as well as stratification. Deoxygenation and acidification will be additional sources of stress for EBUS ecosystems. However, fine-scale “chaotic” details of the oceanic circulation, with horizontal sizes between ~ 1 and 100 km arising from the turbulent nature of ocean flows can modulate biogeochemical changes in ways that are poorly understood until today. In addition, the effects of fine-scale ocean turbulence may be different in each EBUS.

West African (WA) societies such as Senegal, Mauritania and Guinea depend heavily on a healthy ocean which provides food and employment. For example in Senegal, small fish mainly caught close to the shores by artisanal fisherman is the most important source of animal protein for the population. In a perspective of adaptive management, it is key to understand how climate change is impacting these resources and ecosystem services. To provide robust and reliable projections of the future states of the WA coastal ocean, the interactions between fine-scale ocean turbulence and biogeochemistry has to be understood and accounted for.

WACO aims to clarify the role of fine-scale physical processes in the present-day functioning and future evolutions of the WA coastal ocean, including its biogeochemical cycles.
To achieve the objectives of the WACO project, two main numerical modelling approaches have been combined: 1) regional realistic modelling of the WA coastal ocean led by Senegalese colleagues at University of Dakar with WACO contribution and 2) WACO idealized modelling in which a simplified prototypical eastern boundary upwelling system is simulated and analyzed to unravel the generic processes at play. The analysis of interdisciplinary observations collected offshore of Mauritania prior to the project have also supported the advancement of the project.

Fundamental progress has been made during WACO on the understanding of the role of ocean turbulent processes of 1 to 100 km size in the functioning of upwelling systems. In the past, it has generally been assumed that the amount of coastal upwelling (i.e. the amount of deep nutrient rich waters being brought to the surface) simply depends on alongshore wind strength. This pillar of upwelling ecosystem studies is being challenged by the work carried out in WACO. We show that the amount of surface ocean warming driven by atmospheric fluxes (i.e. solar radiation) has to be taken into account to quantify mean upwelling rates. Turbulent transfers of ocean properties such as plankton and nutrients have also been clarified in the context of upwelling systems by WACO. These results have implications for the whole biogeochemical cycles. Overall, ocean turbulence in the form of so-called eddies, fronts and filaments tend to counteract the dominant process of wind-driven upwelling. This effect is modest when the ocean is strongly warmed by the atmosphere (i.e. due to strong solar irradiation in summer seasons) as typical occurring in tropical EBUS. However, this counteracting effect can increase significantly under cloudy conditions or during winter seasons when ocean surface warming driven by the atmosphere is absent. These findings are qualitatively applicable to all eastern boundary upwelling systems including the WA system. Guidelines have been provided by WACO on quantitative differences between upwelling systems.

Realistic modelling of the WA under plausible perturbations induced by climate change at the 2100 horizon shows that ocean circulation changes are likely to be of minor importance. This is because anticipated wind changes, which may act as a driver of ocean circulation change, are small with an expected reduction by 10 % (Sylla et al, 2019). And further, the ocean circulation response is minimized by a negative feedback. Until now, the contribution of fine-scale turbulence to this negative feedback is unclear.
Until very recently, the main concern of WA fishery policy makers and stakeholders has been that climate-change would affect the WA wind regime, which in turn would affect ocean currents and ecosystems. A WACO contribution has been on the numerical identification of plausible circulation trends for the WA coastal ocean. Considering only the potential changes related to ocean physics, this work suggests that ocean warming induced by climate change poses a more serious threat to WA ecosystems than wind regime alterations.

During WACO, new ways of international knowledge exchange have been tested using virtual communication tools. Together with other early career researchers from around the globe, WACO researcher S. Thomsen has created an inclusive low-carbon virtual knowledge exchange platform called “EBUS Webinars”. This online platform regularly reaches 30 to 75 researchers from around 12 countries and will have long lasting effects on the knowledge exchange within the upwelling community well beyond the WACO project.