Ocean uptake of anthropogenic CO2 in a general circulation. Biogeochemical model including isopycnal diffusion.

The invasion of anthropogenic carbon dioxide (CO2) in the ocean is simulated with an ocean biogeochemical circulation model. The model includes an isopycnal diffusion scheme for realistic tracer distributions in the ocean interior. Anthropogenic CO2 is added to a one-box atmosphere in the course of a simulated time integration from the preindustrial state to the present day. The atmospheric CO2 concentration in the model increases and reaches 354 μatm in 1990. The ocean uptake of anthropogenic CO2 is 2.1 GtC year-1 of the anthropogenic CO2 emission of 6 GtC year-1. The subpolar and polar regions in the basins, the equatorial Pacific and the Southern Ocean are strong sinks. In these regions deep waters, which are not equilibrated with atmospheric CO2, are supplied to the surface primarily by wind-driven upwelling. Much anthropogenic CO2 is accumulated in the subtropical gyres by Ekman convergence of surface waters and is transported to the depths in the North Atlantic through the deep western boundary current, respectively. These results are consistent with previous observational and model studies. The isopycnal diffusion in the model plays an important role in the uptake of anthropogenic CO2 at subpolar and polar latitudes and in its transport to the depths. The time integration into the future indicates that advection by the North Atlantic Deep Water is most effective in the century-scale transport of anthropogenic CO2 into the ocean interior. Central regions of subtropical gyres become filled with anthropogenic CO2 and are less effective for the uptake of anthropogenic CO2.