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

• Published in: Biogeosciences
• Main Authors: Menviel, Laurie C., Spence, Paul, Kiss, Andrew E., Chamberlain, Matthew A., Hayashida, Hakase, England, Matthew H., Waugh, Darryn
• Format: Text
• Language: English
• Published: 2023
• Subjects: Southern Ocean; Sea ice
• Online Access: https://doi.org/10.5194/bg-20-4413-2023; https://bg.copernicus.org/articles/20/4413/2023/

While the Southern Ocean (SO) provides the largest oceanic sink of carbon, some observational studies have suggested that the SO total CO 2 (tCO 2 ) uptake exhibited large ( ∼ 0.3 GtC yr −1 ) decadal-scale variability over the last 30 years, with a similar SO tCO 2 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 CO 2 fluxes over the period 1980–2021 and the processes leading to the CO 2 flux variability. The simulated tCO 2 flux exhibits decadal-scale variability with an amplitude of ∼ 0.1 GtC yr −1 globally in phase with observations. Notably, two stagnations in tCO 2 uptake are simulated: between 1982 and 2000, and between 2003 and 2011, while re-invigorations are simulated between 2000 and 2003, as well as since 2012. This decadal-scale variability is primarily due to changes in natural CO 2 (nCO 2 ) fluxes south of the polar front associated with variability in the Southern Annular Mode (SAM). Positive phases of the SAM, i.e. stronger and poleward shifted southern hemispheric (SH) westerlies, lead to enhanced SO nCO 2 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 CO 2 flux 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 CO 2 (aCO 2 ) uptake south of the polar front, the amplitude of the changes in aCO 2 fluxes is only 25 % of the changes in nCO 2 fluxes. Due to the larger nCO 2 outgassing compared to aCO 2 uptake as the SH westerlies strengthen and shift poleward, the SO tCO 2 uptake capability thus reduced since 1980 in response to the shift towards positive phases of the SAM. Our results indicate that, even in an eddy-rich ocean model, a ...