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

The Southern Oceanconsiderablyinfluences 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 CO2emissions. The future of this roleis strongly linked tosalinity changes, as salinitycontrols,throughits influence on the densitystratification,the vertical exchange of water masses, heat andcarbon. A strongfreshening of the Southern Ocean surface watersin the decades before 2016has resulted in increased surface densitystratificationall around Antarctica. Thisenhancedstratification 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-2010hydrographicsections in the GLODAP database to the climatology, we observeconsistent andsignificant anomalies in thebiogeochemical properties of thetop 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)increasein the subsurface layer.We find that this increase resultsfromthe shallowing ofupper circumpolar deep water south of 50S. We investigate thevariabilityin properties of thesurface and subsurface layers over the last decade, as well as the impact of such changes on the potential fugacity of CO2to better understand how the change in stratification may impact the air-sea CO2flux.