Temporal variations of mixed-layer oceanic CO2 at JGOFS-KERFIX time-series station: Physical versus biogeochemical processes

The seasonal and interannual variations in mixed-layer carbon dioxide in the Southern Ocean are analyzed from January 1990 to March 1995 at KERFIX time-series station (50°40S-69°25E). The temperature, salinity and chlorophyll time series are used as constraints on a simple box model to extrapolate total dissolved inorganic carbon (DIC), total alkalinity (TA) and oceanic CO2 fugacity (fCO2) over the five years of the monitoring. Results of the simulation are compared to all available observations. Both measured and simulated DIC and TA give seasonal signals of 25 µmol/kg and 8 µeq/kg, respectively. In spite of a weak primary production about 70 gC/m2/yr, the biological pump appears to play a significant role on seasonal and interannual variations in air-sea CO2 exchanges. Its contribution varies from 10 to 45% of the total sea surface fCO2 variations depending on the period. This area has been a sink for atmospheric CO2 with annual mean values of-0.8 to-3.0 mol/m2/yr during the whole period investigated. Annually the CO2 sink is due to the balance between biological activity and mixing processes on fCO2 inducing thermodynamically mediated variations. The sink\u27s interannual variations appear to be mainly due to the high variability of the wind speeds and hence, of the mixed-layer depth. The impact of the anthropogenic atmospheric CO2 increase on oceanic fCO2 is also investigated. The rate of increase of oceanic fCO2 (0.6 µatm/yr) was half that of atmospheric fCO2 (1.2 µatm/yr). The increase of the air-sea CO2 gradient lead to an increase of the CO2 sink of about 0.07 mol/m2/yr (0.02 GtC/yr) over the five years investigated