Water Mass Changes and Carbon Uptake by Subantarctic Pacific Waters

The Southern Ocean (SO) is a critical component of the global carbon cycle, acting as a significant sink for atmospheric carbon dioxide (CO₂). Understanding the intricate processes governing CO₂ uptake in the SO is paramount for comprehending the global carbon budget and predicting future climate scenarios. Recent observations suggest that changes in SO water masses, driven by climate-induced alterations in temperature and circulation patterns, can significantly impact CO₂ uptake. Understanding these feedbacks is crucial for predicting the SO's future role as a carbon sink and its broader implications for climate mitigation efforts. In this work, we determine changes in the water mass composition and their characteristics, including their CO₂ content, along the CUSTARD transect (54ºS-59ºS 90ºW) in Subantarctic Pacific waters. The CUSTARD transect crosses a region of formation of mode and intermediate waters. We use an extended Optimum Multiparameter (eOMP) analysis and data from three repeats of the CUSTARD transect in 1993 (expocode 316N19930222; data from GLODAPv2.2023), 2005-2006 (316N20050821 and 316N20060130; from GLODAPv2.2023), and 2019-2020 (74EQ20191202; the CUSTARD cruise). We observe isopycnal heaving in the southern part of the transect from 1993 to 2020. In the upper ocean (neutral density (γⁿ) < 27.2 kg m^-3), isopycnal heaving is linked to a temperature decrease of up to -2ºC and a salinity decrease of up to -0.15 between 1993 and 2005, extending to γⁿ < 27.5 kg m^-3 in 2019-2020. The physicochemical changes in the upper ocean are linked to changes in the water mass composition, including an increase in the volume of Antarctic Surface Water and Antarctic Intermediate Water and a decrease in the volume of SubAntarctic Mode Water over the 18-year study period. These water mass changes are accompanied by decreases in concentrations of oxygen, dissolved nutrients, and total alkalinity, along with an increase in total dissolved inorganic carbon of up to 40 µmol kg^-3  for γⁿ < 27.5 kg m^-3 from 1993 to 2019-2020. For 27.5 kg m^-3 < γⁿ <28.2 kg m^-3, salinity increased by 0.05 from 1993 to 2005 and by 0.15 over the 18-year studied period in the southern part of the transect. This salinity increase extends northward in 2019-2020. These changes in salinity are linked to an increase in Circumpolar Deep Water volume. In the deep layer (γⁿ > 28.2 kg m^-3), Ross Sea Bottom Water replaces Adélie Bottom Water from 1993 to 2019-2020. The changes in water mass composition observed along the CUSTARD transect indicate circulation variations linked to the Southern Annular Mode (SAM), with a prevalent positive phase since 1995. Positive SAM pahses increase upwelling south of the Antarctic Polar Front and downwelling in the Subantarctic Zone. Due to these circulation changes, the SO’s uptake of atmospheric CO₂ decreases during positive SAM phases, which are predicted to intensify with climate change.