| We develop a zonally averaged model of tracer transport in the Antarctic Circumpolar Current (ACC), based on residual mean theory, and used it to consider the uptake and transport of anthropogenic CO2. Scaling analysis and explicit numerical integrations provide a clear framework for interpreting spatial patterns and amplitudes of the modeled ocean uptake and its sensitivity to physical forcing parameters. Eddy transfer plays a fundamental role in the dynamical and tracer balances in the ACC through its control on both the residual mean flow and isopycnic stirring. Scaling analysis predicts that the regional cumulative uptake is sensitive to the surface residual flow, the entrainment of deep waters, and the isopycnic stirring. The theory suggests an enhanced uptake of fossil fuel CO2 to the south of the polar front due to the large scale upwelling of deep waters. To the north of the polar front, another region of enhanced uptake is expected due to the intense isopycnal stirring and large gas transfer velocity, both associated with the maximum in the surface westerly wind. The explicit, zonally averaged model is used to simulate the uptake of fossil fuel CO2 and to test the predictions from the scaling analysis. Sensitivity experiments reveal the relationships between physical forcing and the uptake of fossil fuel CO2, and the simulated fluxes are consistent with the theoretical predictions. Baroclinic eddies play a central role in controlling both the magnitude and the spatial patterns of the uptake through their influence on the residual circulation and isopycnal stirring. The surface buoyancy flux ultimately control the spatial pattern of the oceanic uptake through its control on the residual flow. In contrast, the integrated cumulative uptake is more sensitive to surface wind stress and gas transfer velocity which controls the isopycnic stirring and the saturation state of the surface waters. This may explain the similarities and differences seen in comparisons of three-dimensional model simulations of anthropogenic CO2 uptake. |
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