Changes of carbon dioxide in surface waters during spring in the Southern Ocean

The fugacity of C02 (fCO2) and the content of chlorophyll a in surface-water were determined during consecutive sections between 47° and 60°S along 6°W in austral spring, October- November 1992. In the Polar Frontal region, the fCO2 of surface-water decreased from slightly below the atmospheric value to 50 μatm below it. This was accompanied by the development of diatom blooms. Seasonal warming of 1.2°C and air-sea exchange partly compensated the decrease of fCO2 by biological activity. Meanders of the Polar Frontal jet and a mesoscale eddy were reflected in spatial variability of fCO2 and chlorophyll a. Systematic observations indicated relationships between fCO2 and chlorophyll a, albeit changing with time. The combination of biological CO2- uptake with formation of Antarctic Intermediate Water (AAIW) makes the Polar Front a site of combined biological/physical CO2-drawdown from the atmosphere. southern part of the Antarctic Circumpolar Current (sACC) and the Southern Frontal region, fCO2 increased 7–8 μatm due to surface-water warming of 0.5°C. A sharp rise of surface water fCO2 of 13 μatm occurred south of the southern Frontal jet. As the ice-cover disappeared, the Boundary between the ACC and the Weddell Gyre released significant amounts of CO2. The Weddell Gyre would become a strong CO2-source after the imminent retreat of the ice. Clearly mechanisms behind changes of fCO2 in surface waters differ for the hydrographic regions. Interstitial brines of sea-ice had fCO2 as low as 100 μatm and had been depleted in nutrients. mmation of significant sources and sinks in the different regions indicates an overall minor oceanic CO2-sink of 0.3 mmol m−2 day−1 throughout the cruise, on the basis of the Wanninkhof relationship at in situ wind speed without skin effect. Uptake of C02 increased to 1.0 mmol m−2 day−1, when a uniform cold skin temperature difference of 0.2°C was assumed. The skin temperature difference derived from the physical model by Soloviev and Schl\̈ussel (1994a,b) had an average value of 0.2°C, leading to an uptake of CO2 of 1.2 mmol m−2 day−1. The measured skin temperature difference exceeded the calculated value. These assessments underline the uncertainty in the estimated air-sea exchange of C02 due to the thermal skin effect, the chosen parametrization of the gas transfer velocity, and the selected length of the wind speed interval. Limited understanding of the mechanistics of gas exchange, as well as large seasonal and spatial variability of the air-sea flux, still preclude a reliable estimate of the basin-wide annual flux for the Southern Ocean.