Variations and trends of CO2 in the surface seawater in the Southern Ocean south of Australia between 1969 and 2002

Measurements of the partial pressure of CO2 in surface seawater (pCOsw2) were made in the Southern Ocean south of Australia during four cruises in January to February 1969, December 1983 to January 1984, December 1994 to January 1995 and January 2002. The spatial distribution of pCOsw2 for the four cruises showed the same pattern north of the Sub-Antarctic Front (SAF), while year-to-year changes were noted south of the SAF. We evaluated the long-term trend of the pCOsw2 representative of the zone between oceanographic fronts by taking into account changes in the seasonal variation in pCOsw2 and the long-term increase of the sea-surface temperature (SST) of the Southern Hemisphere. The observed growth rate of pCOsw2 was 0.7 ± 0.1 μatm yr−1 at its minimum, which was observed at the SST of 15 °C north of the Subtropical Front (STF), 1.0 ± 0.5 μatm yr−1 in the Sub-Antarctic Zone (SAZ) between STF and SAF, 1.5 ± 0.4 μatm yr−1 in the Polar Frontal Zone (PFZ) between SAF and the Polar Front (PF) and 1.8 ± 0.2 μatm yr−1 in the Polar Zone (PZ) between PF and 62°S, determined as the northern edge of the Seasonal Sea Ice Zone (SSIZ) on the basis of surface salinity and satellite images. These increases were caused by the uptake of anthropogenic CO2 as well as variations in the thermodynamic temperature effect, ocean transport and biological activity. In the SSIZ between 62 and 66.5°S, we could not clearly evaluate the long-term trend of pCOsw2 due to the remarkable CO2 drawdown due to biological activity in January 2002. The relatively low growth rates of pCOsw2 close to the STF and in the SAZ are probably associated with the formation of Subtropical Mode Water and Sub-Antarctic Mode Water in their respective zones. Between the north of the STF and the PZ, the growth rate of total dissolved inorganic carbon was calculated to be about 0.5–0.8 μmol kg−1 yr−1 via the buffer factor.