Review of „Enhanced Southern Ocean CO2 outgassing as a result of stronger and poleward shifted southern hemispheric westerlies” by Menviel et al.

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 1/10^<em>th</em> degree, we explore the changes in total, natural and anthropogenic CO₂ fluxes in the Southern Ocean over the period 1970&ndash;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.