Poleward migration of warm Circumpolar Deep Water towards Antarctica

The upwelling of warm Circumpolar Deep Water is a key process in the global climate system, transporting heat, nutrients, and carbon poleward towards Antarctic ice shelves. Here we use physical and chemical seawater properties from repeat ship-based observations to classify Southern Ocean water masses and show changes in warm water abundance south of the Antarctic Circumpolar Current over the past two decades. We then train a random forest model ensemble to extend this classification to a monthly gridded Argo climatology beginning in 2004, enabling further decomposition of the spatial and temporal variability of the signal. Both analyses reveal an increase in upper-2000~m warm water thickness near the continent, consistent with a circumpolar-mean poleward redistribution of the upper Circumpolar Deep Water core of 1.26~km\,yr⁻¹ (95% CI: $0.53$--$1.98$). Together, these shifts suggest enhanced heat flux towards the Antarctic shelf, with implications for basal ice shelf melting and sea-level rise.