Upper Circumpolar Deep Water Properties: Means and Trends From 2005 Through 2024

Abstract Upper Circumpolar Deep Water (UCDW) is carbon‐rich, oxygen‐poor, nutrient‐rich, and relatively warm and salty compared to waters above and below it. Where it is entrained into the surface mixed layer or impinges on the Antarctic Continental Shelf, it can outgas carbon, promote productivity, and melt sea ice and marine terminating ice sheets. Here, we analyze 20 years (2005–2024) of temperature and salinity profile data from CTD (conductivity‐temperature‐depth) instruments, both mounted on Argo floats and deployed during ship‐based campaigns, to map mean water‐property distributions and temporal trends at the UCDW temperature maximum. This circumpolar analysis of mean pressure, temperature, salinity, and density fields show, consistent with previous studies, the cyclonic subpolar gyres as domes in pressure with warm temperatures swirling in from the north and cold temperature from the coastal regions. Also consistent with previous studies, the westward‐flowing Antarctic Shelf Current is characterized by a relatively cold and deep temperature maximum adjacent to the continental shelf present in all but the Amundsen and Bellingshausen seas. The analysis also reveals that over the past 20 years, UCDW has generally shallowed, warmed, and freshened in the Weddell Sea and off East Antarctica, whereas it has deepened, cooled, and gotten saltier from Drake Passage westward to the eastern edge of the Ross Sea, a striking regional dichotomy in the trends.