Mechanisms driving the seasonality of air-sea CO2 flux in the ice-free zone of the Southern Ocean and how these might evolve: A 1D vertical biogeochemical model approach.

Abstract:

The biogeochemical SWAMCO-3 model is used to understand mechanisms governing the seasonality of air-sea CO₂ exchanges in the ice-free Southern Ocean. 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 phenomenological observations available for these PFTs. The performance of the SWAMCO-3 model coupled to a vertical one-dimensional physical model is assessed at the location of the time-series station KERFIX (around 51°S-68°E). The model was able to reproduce a mean seasonal cycle based on years where a maximum of chemical and biological observations are available. Ocean fCO₂ in equilibrium with the atmosphere are simulated both in 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 simulated in summer driven by primary production. Model scenarios cancelling biological activity or only coccolithophores allowed, by comparison with the standard simulation, untangling the respective role of physical and biological processes in driving the sign and magnitude of air-sea CO₂ exchanges. First, we show that coccolithophores are repressing the ocean C uptake, but only marginally (5%). Second, the model highlights the role of diatoms on the presence of a CO₂ sink in summer. Altogether, this results in a weak annual air-sea CO₂ flux (-0.9 mol m^-2 y^-1 or -0.1 Pg C y^-1 for the ice-free zone south of 50°S), whose variability seems more related to the thermodynamical processes. We then speculate how global warming might influence the latter mechanisms and alter air-sea CO₂ exchanges in this region.