Observed Trends and Variability in the Water Masses of the Southern Ocean

Abstract We present SatGEM‐2, a new version of the satellite gravest empirical mode (SatGEM) Atlas for the circumpolar Southern Ocean, incorporating all available CTD and Argo profile data through December 2022. SatGEM‐2 uses in situ hydrographic observations to construct static gravest empirical mode (GEM) fields in pressure–sea surface height (sea surface height [SSH]) space, linking each SSH value to a full‐depth temperature, salinity, and density profile. This framework enables reconstruction of a four‐dimensional (latitude–longitude–depth–time) view of the Southern Ocean from maps of SSH. SatGEM‐2 provides daily, full‐depth fields of temperature, salinity, and density from 1993 to 2023 on a grid with 10 dbar vertical resolution. Using SatGEM‐2, we identify robust changes in Southern Ocean water‐mass volumes driven by southward‐shifting SSH isolines: Subantarctic Mode Water, Antarctic Intermediate Water, and Upper Circumpolar Deep Water have increased by , , and , while Lower Circumpolar Deep Water has declined by . Together, these trends correspond to an approximate 1.4 Sv increase in upper‐ocean water‐mass volume over 30 years. Southern Ocean heat content has also risen sharply, with trends during 2014–2023 more than doubling the long‐term mean . These results demonstrate the power of combining satellite altimetry with the GEM framework to monitor long‐term Southern Ocean water‐mass change and provide a tool for assessing future oceanic variability in a warming climate.