Abstract
Several studies on coupled physical–biogeochemical models have shown that major deficiencies in the biogeochemical fields arise from the deficiencies in the physical flow fields. This paper examines the improvement of the physics through data assimilation, and the subsequent impact on the ecosystem response in a coupled model of the North Atlantic. Sea surface temperature and sea surface height data are assimilated with a sequential method based on the SEEK filter adapted to the coupling needs. The model domain covers the Atlantic from 20°S to 70°N at eddy-permitting resolution. The biogeochemical model is a NPZD-DOM model based on the P3ZD formulation. The results of an annual assimilated simulation are compared with an annual free simulation. With assimilation, the representation of the mixed layer depth is significantly improved in mid latitudes, even though the mixed layer depth is generally overestimated compared to the observations. The representation of the mean and variance of the currents is also significantly improved. The nutrient input in the euphotic zone is used to assess the data assimilation impact on the ecosystem. Data assimilation results in a 50% reduction of the input due to vertical mixing in mid-latitudes, and in a four- to six-fold increase of the advective fluxes in mid-latitudes and subtropics. Averaged zonally, the net impact is a threefold increase for the subtropical gyre, and a moderate (20–30%) decrease at mid and high latitudes. Surface chlorophyll concentration increases along the subtropical gyre borders, but little changes are detected at mid and high latitudes. An increase of the primary production appears along the Gulf Stream path, but it represents only 12% on average for mid and high latitudes. In the subtropical gyre centre, primary production is augmented but stays underestimated (20% of observations). These experiments show the benefits of physical data assimilation in coupled physical–biogeochemical applications.
Highlights
Marine biogeochemical cycles are at the heart of environmental issues such as coastal eutrophication, the role of the ocean in the global carbon cycle and in the climate change
The assimilation method is based on the CET03 configuration of the SEEK filter (Pham et al, 1998), modified in order to respect the hydrostatic stability, essential for the coupling
The results have been compared to a free simulation
Summary
Marine biogeochemical cycles are at the heart of environmental issues such as coastal eutrophication, the role of the ocean in the global carbon cycle and in the climate change. The need to understand, monitor and forecast these cycles has motivated the development of coupled biogeochemistry-circulation models, which simulate the complex interactions between ocean physics, chemistry and biology. Convection, vertical velocities, mesoscale and submesoscale currents as well as diapycnal mixing are fundamental processes for the coupling between physics and biochemistry They act directly on the nutrient supply, the availability of light, and on the dissolved organic carbon export (Oschlies and Garçon, 1999; Doney et al, 2004). Oschlies and Garçon (1999) examined the impact of vertical mixing parameterisation and advection numerics on the primary production in the North Atlantic.
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