Impact of Mesoscale Eddies on CO₂ Fluxes in the Southern Ocean

The Southern Ocean plays a crucial role in the global carbon cycle, uptaking about 43% of the ocean’s CO₂ uptake. However, significant uncertainties remain regarding the processes governing CO₂ fluxes in this region. Mesoscale eddies have been identified as a potential source of these uncertainties. In this study, we analyze 30 years of daily, high-resolution (10 km) global simulations using the ICON-O ocean model coupled with the HAMOCC biogeochemistry model. The Okubo-Weiss parameter and vorticity is used to classify four flow regimes—anticyclonic eddy cores, cyclonic eddy cores, eddy core peripheries, and quiescent background—allowing us to generate composites for each. Our results show that CO₂ flux is directed into the ocean across all four regimes, with the magnitude of CO₂ uptake varying by regime. Anticyclones have a greater capacity for CO₂ uptake compared to cyclones. In certain regions (i.e. Agulhas retroflection and the Brazil-Malvinas confluence) anticyclones exhibit the highest CO₂ uptake capacity among the four regimes, a pattern linked to the greater eddy intensity of these areas. An analysis of the CO₂ flux terms shows that wind speed and ∆pCO₂ are the primary contributors to flux magnitud and variability. As atmospheric pCO₂ is prescribed, the main changes in ∆pCO₂ are related to changes in oceanic pCO₂. These variations are primarily driven by dissolved inorganic carbon (DIC) and sea surface temperature, which tend to compensate for each other, with DIC having a stronger influence. To explore DIC changes, we analyzed DIC budgets in the first 300 m. In the surface layer, the total DIC tendency is predominantly driven by vertical diffusion, which causes a net loss of DIC to deeper layers and induces atmospheric CO₂ flux into the ocean. Vertical diffusion is particularly stronger in anticyclonic eddies, explaining their enhanced ability to absorb CO₂. In deeper layers, the total DIC tendency is primarily controlled by the divergence of advective fluxes, while changes in DIC from sources and sinks (i.e. biogeochemical processes) are almost entirely balanced by vertical diffusion. These findings highlight the dominant role of mesoscale eddies in oceanic carbon uptake and underscore the need for more refined models to accurately represent their impact on the global carbon cycle.