Résumé : The complex biogeochemical SWAMCO-3 model has been used to assess the response of the ice-free SouthernOcean to the physical and biological mechanisms governing air–sea CO2 exchanges. For this application, themodel explicitly details the dynamics of three Phytoplankton Functional Types (PFTs) of importance for C, N, P,Si, Fe cycling and air–sea CO2 exchange in this area. These are the diatoms, the pico–nanophytoplankton andthe coccolithophoreswhose growth regulation by light, temperature and nutrients has been obtained from a literaturereview of phenomenological observations available for these PFTs. The performance of the SWAMCO-3model coupled to a vertical one-dimensional physical model was first assessed at the location of the JGOFStime-series station KERFIX. The modelwas able to reproduce a mean seasonal cycle based on yearswhere amaximumofchemical and biological observations are available at this location (1993–1994, 1994–1995, 1998–1999and 2000–2001). Ocean fCO2 in equilibriumwith the atmosphere are simulated both in Australwinter associatedwith surface layer replenishment in DIC due to deep verticalmixing and in late summer as a consequence of thewarming effect on the carbonate system. A clear under-saturation is found in spring/summer. Analysis of themodelled seasonal biogeochemical and physical features shows that thermodynamical conditions are drivingthe air–sea exchange of CO2 in the region, while the biological activity under the control of light and iron availability,is responsible for the predicted relatively modest annual carbon sink (−0.9 mol C m−2 y−1).