Changes In The Subantarctic Mode Water Properties And Spiciness In The Southern Indian Ocean Based On Argo Observations

The Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-18. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking at sigma(theta) = 26.7-26.8 kg m(-3) in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-18. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy fluxes. An increase in the mixed layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.Significance Statement Subantarctic Mode Water is a distinct water mass with vertically uniform properties in the Southern Hemisphere's subtropical gyres. Climate change is imprinted in the SAMW through the ventilation at the base of the winter mixed layer. The ocean modulation associated with wind-forced large-scale waves and circulation also plays an essential role in heat, salinity, and water mass redistribution. A net increase in volume-weighted potential temperature and salinity of the SAMW is found during the Argo era since 2004, resulting from a combination of climate change and ocean modulation through opposite heaving and spiciness processes. This study improves our understanding of the dynamics and thermodynamics involved in the SAMW formation during rapid climate change.