Report 18 -- Q1

## Is the Southern Ocean a net source of CO2 to the atmosphere or a net sink? Of what magnitude?

Studies of delta-pCO2 between the atmosphere and oceanic surface waters have previously indicated the Southern Ocean (defined rather broadly in those studies as the zonal band between 40 and 70°S) to be an area of net CO2 flux into the ocean, with an estimated magnitude of 2.7 GT C y-1 (Takahashi et al., 1986). That flux is approximately equal to the global net uptake of CO2 by the oceans (ibid.), indicating that CO2 fluxes are approximately in balance for the rest of the ocean. In other words, the Southern Ocean may dominate the global net uptake of CO2 by the oceans. That view can be disputed, however, as studies of meridional CO2 gradients in the atmosphere suggest little or no net uptake of CO2 by the ocean in the Southern Hemisphere (e.g., Tans et al., 1990). With known areas of net uptake in the vicinity of the Subtropical Convergence (Takahashi et al., 1986), a net Southern Hemisphere flux of approximately zero would imply net outgassing from the waters to the south. The large-scale circulation of the Southern Ocean is characterized by zonal bands of convergence and divergence, suggesting a reasonably complex pattern of alternating sources and sinks, with the result that the overall net balance is difficult to evaluate. The spatial coverage of existing delta-pCO2 data is not particularly good in the Southern Ocean and seasonal information is almost completely lacking, especially in the Pacific sector. Thus the net CO2 flux is difficult to constrain using the available data to no better than -3 to +1 GT C y-1, with negative values denoting uptake.

Implications for field measurements: Field measurements of delta-pCO2 should be given a high priority within the Southern Ocean JGOFS programs of all nations, and should seek to maximize both spatial (especially meridional) resolution and seasonal coverage. The overall purpose of this work should be to constrain the net CO2 exchange within much tighter limits than it is now possible to do.

Implications for modeling: It would be helpful to develop satellite-observable proxies for oceanic pCO2 (based on surface temperature and perhaps ocean color). Once developed, these could be merged with wind data to model CO2 fluxes after 1998.