Temporal variations of mixed-layer oceanic CO<SUB>2</SUB> at JGOFS-KERFIX time-series station: Physical versus biogeochemical processes

Abstract

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 CO 2 fugacity (fCO 2 ) 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/m 2 /yr, the biological pump appears to play a significant role on seasonal and interannual variations in air-sea CO 2 exchanges. Its contribution varies from 10 to 45% of the total sea surface fCO 2 variations depending on the period. This area has been a sink for atmospheric CO 2 with annual mean values of -0.8 to -3.0 mol/m 2 /yr during the whole period investigated. Annually the CO 2 sink is due to the balance between biological activity and mixing processes on fCO 2 inducing thermodynamically mediated variations. The sink's 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 CO 2 increase on oceanic fCO 2 is also investigated. The rate of increase of oceanic fCO 2 (0.6 μatm/yr) was half that of atmospheric fCO 2 (1.2 μatm/yr). The increase of the air-sea CO 2 gradient lead to an increase of the CO 2 sink of about 0.07 mol/m 2 /yr (0.02 GtC/yr) over the five years investigated.