Changes in salinity driven stratification and impacts on the deep-water CO2 ventilation in the Southern Ocean

The Southern Ocean considerably influences the global climate by exchanging heat and carbon between the deep ocean and the surface. Historically, it mitigated surface warming by absorbing 70% of excess heat and over 10% of human-induced CO₂ emissions. The future of this role is strongly linked to salinity changes, as salinity controls, through its influence on the density stratification, the vertical exchange of water masses, heat and carbon.  A strong freshening of the Southern Ocean surface waters in the decades before 2016 has resulted in increased surface density stratification all around Antarctica. This enhanced stratification reduces the mixing between deep and surface waters, and in particular the vertical mixing of carbon-rich deep waters into the surface layer. By comparing post-2010 hydrographic sections in the GLODAP database to the climatology, we observe consistent and significant anomalies in the biogeochemical properties of the top 500 m of all the sectors of the Southern Ocean. While the surface layer is freshening, salinity, temperature, dissolved inorganic carbon (DIC) and total alkalinity (TA) increase in the subsurface layer. We find that this increase results from the shallowing of upper circumpolar deep water south of 50°S. We investigate the variability in properties of the surface and subsurface layers over the last decade, as well as the impact of such changes on the potential fugacity of CO₂ to better understand how the change in stratification may impact the air-sea CO₂ flux.