Mediation of air-sea CO2 fluxes by mesoscale eddies in the Southern Ocean

The eddy-rich Southern Ocean (SO) has a huge seasonal variability in mixed layer depth (MLD) ranging from O(10 m) in summer to a few hundred meters in winter. The modulation of MLD by mesoscale eddies and implications for biogeochemistry are expected to be substantial, yet not fully understood partially due to the challenge of separating mesoscale effects from those of the larger-scale circulation.

In this study, we report the significant impact of mesoscale eddies on air-sea CO₂ fluxes using offline simulations of a biogeochemical model, which provides a clear and definitive way to separate out the effect of mesoscale processes. Two simulations of carbon cycle are compared. The first one is forced by an eddy-rich circulation, the second by a smoothed circulation in which mesoscale kinetic energy is suppressed by half but the mean circulation is kept the same. Strikingly, smoothing out the mesoscale leads to significantly less outgassing of CO₂: by 0.3 Pg C yr^-1 which is roughly 1/3 of the seasonal variability in SO CO₂ flux.

Reduction in outgassing rates is a consequence of reduced vertical mixing and associated MLD shallowing, by O(100 m), in the smoothed solution, lowering the supply of carbon-rich deep water into the surface layer. Our results clearly demonstrate the importance of parameterizing the interaction between vertical mixing and mesoscale circulations in order to realistically represent ocean biogeochemical processes in climate models.