The Observed Circumpolar Decline of Antarctic Bottom Water Volume

Abstract Antarctic Bottom Water (AABW) formation is a key component of the global overturning circulation, ventilating the abyssal ocean and sequestering heat and carbon. Using a new reconstruction from the SatGEM‐2 monthly‐varying gravest empirical mode climatology, a continuous, observation‐driven circumpolar record of AABW variability is presented between 2002 and 2023. Comparisons with independent Deep Argo profiles and repeat hydrographic transects confirm that SatGEM‐2 reproduces observed thermohaline structure to within ∼0.04°C and ∼0.003 g/kg. A decline in AABW volume of % relative to a 2002 baseline is observed, increasing fourfold after 2015 to % . By 2023, this corresponds to a cumulative reduction of % of the 2002 circumpolar volume. The recent increase in AABW loss coincides with a rapid decline in Antarctic sea ice since 2016, and AABW volume anomalies are strongly correlated with sea ice extent variability.