What drives the seasonality of air-sea CO2 fluxes in the ice-free zone of the Southern Ocean: A 1D coupled physical-biogeochemical model approach

The complex biogeochemical SWAMCO-3 model has been used to assess the response of the ice-free Southern Ocean to the physical and biological mechanisms governing air-sea CO₂ exchanges. For this application, the model 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 and the coccolithophores whose growth regulation by light, temperature and nutrients has been obtained from a literature review of phenomenological observations available for these PFTs. The performance of the SWAMCO-3 model coupled to a vertical one-dimensional physical model was first assessed at the location of the JGOFS time-series station KERFIX. The model was able to reproduce a mean seasonal cycle based on years where a maximum of chemical and biological observations are available at this location (1993-1994, 1994-1995, 1998-1999 and 2000-2001). Ocean fCO₂ in equilibrium with the atmosphere are simulated both in Austral winter associated with surface layer replenishment in DIC due to deep vertical mixing and in late summer as a consequence of the warming effect on the carbonate system. A clear under-saturation is found in spring/summer. Analysis of the modelled seasonal biogeochemical and physical features shows that thermodynamical conditions are driving the 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).