Warm Ocean Water Migrates Towards Antarctica, With Potential Implications for Ice Melt

The Southern Ocean is a key component of the global climate system, where cold Antarctic Bottom Water (AABW) forms and downwells, and warm Circumpolar Deep Water (CDW) upwells. While recent decades have documented changes in AABW, the response of CDW to anthropogenic forcing has remained less clear. Some model-based studies projected a poleward migration of CDW's southern boundary; however, robust observational clarity was needed regarding whether CDW, a key transporter of heat, nutrients, and carbon, is indeed migrating poleward towards the Antarctic ice shelves.

To address this uncertainty, researchers employed an innovative approach combining traditional oceanographic measurements with advanced machine learning. They first utilized physical and chemical seawater properties measured from relatively sparse ship-based observations to robustly classify Southern Ocean water masses. This classification then served as training data for a random forest machine learning model, which was subsequently applied to the abundant Argo temperature and salinity data spanning the Southern Ocean over the past two decades. This methodology allowed for local assessment of water mass variability on monthly-to-decadal timescales.

The analysis revealed an increase in CDW thickness near the continent and a decrease away from it. This spatial redistribution indicates a poleward migration of CDW upwelling. Concurrently, the study observed a contraction in the AABW layer thickness along the Antarctic margin. These findings provide observational support for CDW's movement towards the continent, clarifying its response to anthropogenic forcing.

This poleward shift of warm CDW carries significant implications. As lead author Lanham notes in the paper, "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."