A summer-time sink for atmospheric carbon dioxide in the Southern Ocean between 88°W and 80°E

Measurements of surface water dissolved carbon dioxide are presented from two cruises in the Southern Ocean from 88°W (Bellingshausen Sea) to 80°E (Princess Elizabeth Trough) with a number of observations close to the ice edge. The data, collected from early to late Austral summer 1992/1993, indicate that the Southern Ocean in these regions was acting as a sink for atmospheric carbon dioxide at this time. No correlation between carbon dioxide levels and the standing stock of phytoplankton or sea-surface temperature was observed, except in isolated regions of high chlorophyll concentration and across small source regions associated with fronts, respectively. Assuming that the observations can be generalised to the region encircled by the cruise tracks (approximately 20% of the Southern Ocean south of 50°S by area), we calculate C02 uptake for this region during the four months that the cruises took place. Using transfer velocities based on observed winds, we find an uptake of 0.07 Gt C using the Liss-Merlivat (The role of air-sea exchange in geochemical cycling; Reidal Publ., The Netherlands, 1986) parameterisation of transfer velocity, or 0.10 Gt C using that due to Wanninkhof (Journal of Geophysical Research, 97, 7373–7382, 1992) or Tans et al. (Science, 247, 1431–1438, 1990), including an additional flux from the skin effect. No winter-time data are available to assess the sign of the flux annually (sink or source); however, the size of the sink observed during the summer suggests that, if representative of the whole of the Southern Ocean, there is a drawdown of between 0.35 and 0.50 Gt C over 4 months. The observation of a sink is consistent with the most recent estimates of the regional budgets derived from atmospheric isotope data. Most data sets from earlier years show the region as neutral with respect to atmospheric CO2, so it appears possible that the widespread sink we observed, at the very least between 88°W and 80°E, is a recent phenomenon not present in some previous surveys.