Enhanced Southern Ocean CO₂ outgassing as a result of stronger and poleward shifted southern hemispheric westerlies

Abstract. While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the total SO CO2 uptake exhibited large (~0.3 GtC/yr) decadal-scale variability over the last 30 years, with a similar SO CO2 uptake in 2016 than in the early 1990s. Here, using an eddy-rich ocean, sea-ice, carbon cycle model, with a nominal resolution of 1/10th degree, we explore the changes in total, natural and anthropogenic CO2 fluxes in the Southern Ocean over the period 1970–2021 and the processes leading to the CO2 flux variability. Over that period, the simulated total CO2 uptake increases by 0.5 GtC/yr, half of which occurs between 1970 and 1982. The simulated total CO2 flux exhibits decadal-scale variability with an amplitude of ~0.1 GtC/yr in phase with observations and with variability in the Southern Annular Mode (SAM). Notably, a stagnation of the total CO2 uptake is simulated between 1982 and 2000, while a re-invigoration is simulated between 2000 and 2012. This decadal-scale variability results from enhanced outgassing of natural CO2 south of the sub-Antarctic front due to the strengthening and poleward shift of the southern hemispheric (SH) westerlies. These wind changes also lead to enhanced anthropogenic CO2 uptake south of the polar front, even though the correlation is low and the amplitude 75 % smaller than for natural CO2 changes. The total SO CO2 uptake capability thus reduced since 1970 in response to a shift towards positive phases of the SAM. Both the multi-decadal and annual changes in SO fluxes can be mostly explained by variations in surface dissolved inorganic carbon (DIC) brought about by a combination of Ekman-driven vertical advection and DIC diffusion at the base of the mixed layer, thus indicating that even in an eddy-rich ocean model a strengthening and/or poleward shift of the southern hemispheric westerlies enhance CO2 outgassing. The projected poleward strengthening of the SH westerlies over the coming century will thus reduce the capability of the SO to mitigate the increase in atmospheric CO2.