Anthropogenic CO.sub.2, air-sea CO.sub.2 fluxes, and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX

The temporal variation of the carbonate system, air-sea CO.sub.2 fluxes, and pH is analyzed in the southern Indian Ocean, south of the polar front, based on in situ data obtained from 1985 to 2021 at a fixed station (50°40.sup.' S-68°25.sup.' E) and results from a neural network model that reconstructs the fugacity of CO.sub.2 (fCO.sub.2) and fluxes at monthly scale. Anthropogenic CO.sub.2 (C.sub.ant) is estimated in the water column and is detected down to the bottom (1600 m) in 1985, resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of C.sub.ant . At the subsurface, the trend of C.sub.ant is estimated at +0.53±0.01 µmol kg.sup.-1 yr.sup.-1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic fCO.sub.2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO.sub.2 and dissolved inorganic carbon (C.sub.T) trends depending on the decade and emphasizing the role of biological drivers on air-sea CO.sub.2 fluxes and pH inter-annual variability. The regional air-sea CO.sub.2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m.sup.-2 yr.sup.-1 in 1985 to a sink of -0.5 molC m.sup.-2 yr.sup.-1 in 2020. Over 1985-2020, the annual pH trend in surface waters of -0.0165±0.0040 per decade was mainly controlled by the accumulation of anthropogenic CO.sub.2, but the summer pH trends were modulated by natural processes that reduced the acidification rate in the last decade. Using historical data from November 1962, we estimated the long-term trend for fCO.sub.2, C.sub.T, and pH, confirming that the progressive acidification was driven by the atmospheric CO.sub.2 increase. In 59 years this led to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO.sub.2 is expected to increase in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future C.sub.T concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to -0.86 lower than pre-industrial pH and -0.71 lower than pH observed in 2020. The aragonite undersaturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton speci