Reduced Southern Ocean CO2 uptake due to the positive SAM trend

While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the SO total CO2 (tCO2) uptake exhibited large (~0.3 GtC/yr) decadal-scale variability over the last 30 years, with a similar SO tCO2 uptake in 2016 as in the early 1990s. Here, using an eddy-rich ocean, sea-ice, carbon cycle model, with a nominal resolution of 0.1, we explore the changes in total, natural and anthropogenic SO CO2 fluxes over the period 1980-2021 and the processes leading to the CO2 flux variability. The simulated tCO2 flux exhibits decadal-scale variability with an amplitude of ~0.1 GtC/yr globally in phase with observations. Notably, two stagnation in tCO2 uptake are simulated between 1982 and 2000 as well as since 2012, while a re-invigoration is simulated between 2000 and 2012. This decadal-scale variability is primarily due to changes in natural CO2 (nCO2) fluxes south of the polar front associated with variability in the Southern Annular Mode (SAM). Positive phases of the SAM lead to enhanced SO nCO2 outgassing due to higher surface natural dissolved inorganic carbon (DIC) brought about by a combination of Ekman-driven vertical advection and DIC diffusion at the base of the mixed layer. The pattern of the CO2flux anomalies indicate a dominant control of the interaction between the mean flow south of the polar front and the main topographic features. While positive phases of the SAM also lead to enhanced anthropogenic CO2(aCO2) uptake south of the polar front, the amplitude of the changes in aCO2 fluxes is only 25% of the changes in nCO2 fluxes. Due to the larger nCO2 outgassing compared to aCO2uptake as the SH westerlies strengthen and shift poleward, the SO tCO2uptake capability thus reduced since 1980 in response to the shift towards positive phases of the SAM.