Multidecadal CO2 uptake variability of the North Atlantic

The multidecadal variability of air-sea CO(2)fluxes in the North Atlantic under preindustrial atmospheric CO2 conditions is simulated, using a coupled biogeochemical/circulation model driven by long-term surface forcing reconstructed from the leading modes of sea level pressure observations from 1850 to 2000. Heat fluxes are of great importance for the multidecadal CO2 fluctuations, about equal in magnitude to wind stress, in contrast to their less prominent role for CO2 flux variability on interannual timescales. Another difference, compared to higher frequencies, is the dominance of the North Atlantic Oscillation in driving the variability of the air-sea CO2 fluxes. Two spatially distinct regimes lead to large anomalies in the CO2 fluxes but compensate to a large degree. The first regime is advective and has its clear signature southeast of Greenland while the second one, in the vicinity of the Labrador Sea and off Newfoundland, is convective. In both regimes, the multidecadal CO2 fluctuations are driven mainly by variations in temperature, salinity, and DIC content at the sea surface while the role of the biological pump is of minor importance in this particular model. The magnitude of the simulated multidecadal CO2 uptake changes is on the order of 0.02 Pg C/yr and amounts to 10-15% of the estimated annual anthropogenic CO2 uptake of the North Atlantic.