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

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The fugacity of C0 2 (fCO 2) 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 fCO 2 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 fCO 2 by biological activity. Meanders of the Polar Frontal jet and a mesoscale eddy were reflected in spatial variability of fCO 2 and chlorophyll a. Systematic observations indicated relationships between fCO 2 and chlorophyll a, albeit changing with time. The combination of biological CO 2- uptake with formation of Antarctic Intermediate Water (AAIW) makes the Polar Front a site of combined biological/physical CO 2-drawdown from the atmosphere. In the southern part of the Antarctic Circumpolar Current (sACC) and the Southern Frontal region, fCO 2 increased 7–8 μatm due to surface-water warming of 0.5°C. A sharp rise of surface water fCO 2 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 CO 2. The Weddell Gyre would become a strong CO 2-source after the imminent retreat of the ice. Clearly mechanisms behind changes of fCO 2 in surface waters differ for the hydrographic regions. Interstitial brines of sea-ice had fCO 2 as low as 100 μatm and had been depleted in nutrients. The summation of significant sources and sinks in the different regions indicates an overall minor oceanic CO 2-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 C0 2 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 CO 2 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 C0 2 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.