FY 2026

Upper circumpolar deep water properties: Means and trends from 2005–2024

Johnson, G.C.

J. Geophys. Res., 131(1), e2025JC023154, doi: 10.1029/2025JC023154, View open access article at AGU/Wiley (external link) (2026)

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.

Plain Language Summary. Ocean waters are a major source of heat for melting of sea ice and glacial ice around Antarctica. In particular, relatively warm water, called Upper Circumpolar Deep Water, characterized by a subsurface temperature maximum, is drawn toward the surface near Antarctica. It has been shown to promote glacial ice melt off Western Antarctica, especially in the Bellingshausen and Amundsen seas. Over the past two decades, robotic profiling devices called Argo floats have collected thousands of temperature and salinity profiles per year that sample this temperature maximum. We map the means and the trends of its pressure, temperature, salinity, and density. Clockwise gyres in the Weddell and Ross seas are characterized by a shallow temperature maximum with warmer water swirling in from the northeast toward the coast and colder waters swirling in from the southwest toward offshore. Trends in those properties at the temperature maximum are consistent with a strengthening and expansion of the Weddell Gyre over the last two decades. The temperature maximum has warmed and shallowed from the Weddell Gyre eastward to the western edge of the Ross Gyre. In the Ross Gyre, the temperature maximum has deepened and cooled with these trends extending eastward to Drake Passage.