Observational evidence for a poleward migration of warm Circumpolar Deep Water towards Antarctica

The formation and downwelling of cold Antarctic Bottom Water (AABW) and the concurrent upwelling of warm Circumpolar Deep Water (CDW) around the Antarctic margin are key processes in the global climate system, driving the renewal of cold dense water to the deep ocean, while transporting heat, nutrients, and carbon upwards and poleward towards Antarctic ice shelves. Observational and modelling studies have documented that AABW has warmed, freshened, and contracted in recent decades. However, the response of CDW to anthropogenic forcing is less clear, with some model-based studies projecting a poleward migration of its southern boundary. In this study, we utilise the physical and chemical seawater properties measured from relatively sparse ship-based observations to robustly classify Southern Ocean water masses. We then train a random forest machine learning model to accurately apply the water mass classification to the abundant Argo temperature and salinity data spanning the Southern Ocean over the past two decades, allowing for local assessment of variability on monthly-to-decadal timescales. Our analysis reveals an increase (decrease) in CDW thickness near (away from) the continent, indicating a poleward migration of CDW upwelling. This trend is accompanied by a contraction in AABW layer thickness along the Antarctic margin. This spatial redistribution suggests an enhanced heat flux toward the Antarctic continental shelf, with potential implications for basal ice shelf melting and sea-level projections.