Downscaling biogeochemistry in the Benguela eastern boundary current |
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| Institution: | 1. Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, Beijing, 100081, China;2. Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY, 13699, USA;1. Leibniz Institute for Baltic Sea Research Warnemünde, Seestraße 15, D-18119 Rostock, Germany;2. Institute of Oceanography, University of Hamburg, Bundesstraße 53, D-20146 Hamburg, Germany;1. South China Sea Marine Prediction Center, State Oceanic Administration, Guangzhou, China;2. First Institute of Oceanography, State Oceanic Administration, Qingdao, China;3. College of Oceanography, Hohai University, Nanjing, China;4. National Marine Environmental Forecasting Center, Beijing, China;1. Development of Skilled Manpower in Earth System Science (DESK), Indian Institute of Tropical Meteorology, Pune, India;2. Department of Meteorology and Oceanography, College of Science and Technology, Andhra University, India;3. Centre for Atmospheric Sciences, Indian Institute of Technology, Delhi, India;4. ESSIC, University of Maryland, USA |
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| Abstract: | Dynamical downscaling is developed to better predict the regional impact of global changes in the framework of scenarios. As an intermediary step towards this objective we used the Regional Ocean Modeling System (ROMS) to downscale a low resolution coupled atmosphere–ocean global circulation model (AOGCM; IPSL-CM4) for simulating the recent-past dynamics and biogeochemistry of the Benguela eastern boundary current. Both physical and biogeochemical improvements are discussed over the present climate scenario (1980–1999) under the light of downscaling.Despite biases introduced through boundary conditions (atmospheric and oceanic), the physical and biogeochemical processes in the Benguela Upwelling System (BUS) have been improved by the ROMS model, relative to the IPSL-CM4 simulation. Nevertheless, using coarse-resolution AOGCM daily atmospheric forcing interpolated on ROMS grids resulted in a shifted SST seasonality in the southern BUS, a deterioration of the northern Benguela region and a very shallow mixed layer depth over the whole regional domain. We then investigated the effect of wind downscaling on ROMS solution. Together with a finer resolution of dynamical processes and of bathymetric features (continental shelf and Walvis Ridge), wind downscaling allowed correction of the seasonality, the mixed layer depth, and provided a better circulation over the domain and substantial modifications of subsurface biogeochemical properties. It has also changed the structure of the lower trophic levels by shifting large offshore areas from autotrophic to heterotrophic regimes with potential important consequences on ecosystem functioning. The regional downscaling also improved the phytoplankton distribution and the southward extension of low oxygen waters in the Northern Benguela. It allowed simulating low oxygen events in the northern BUS and highlighted a potential upscaling effect related to the nitrogen irrigation from the productive BUS towards the tropical/subtropical South Atlantic basin. This study shows that forcing a downscaled ocean model with higher resolution winds than those issued from an AOGCM, results in improved representation of physical and biogeochemical processes. |
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| Keywords: | Ocean modeling Downscaling Upscaling Upwelling Biogeochemistry Benguela ROMS |
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