Southern Ocean Water Masses in a Changing Climate: Structure, Variability, and Pathways to the Antarctic Shelf

Abstract

This thesis examines how the configuration and variability of Southern Ocean water masses govern the delivery of oceanic heat to the Antarctic shelf in the context of contemporary climate change. Emphasis is placed on Circumpolar Deep Water (CDW), the main source of oceanic heat responsible for driving basal melting beneath Antarctic ice shelves. Despite its importance for global climate, significant uncertainties remain regarding how CDW has responded to recent climate forcing, particularly in relation to recent observed changes in Antarctic Bottom Water properties and distribution. In addition, whilst the time-mean regimes that govern CDW access to the Antarctic shelf are well-established from targeted regional studies, two important gaps remain: a) uncertainty surrounding seasonal pathways of CDW to the shelf, and b) a lack of a unified framework that captures intrusions across the broader Antarctic margin. To address these questions, a machine learning framework is developed to expand the scope of water mass classification to a wider range of datasets, including gridded model output and large observational records, such as Argo data, which are limited to basic hydrographic measurements. Applied to the Estimating the Circulation and Climate of the Ocean (ECCO) state estimate, this approach demonstrates how shifts in Atlantic water masses can be reconstructed continuously and suggests a possible poleward displacement of CDW within the Southern Ocean. Building on this, a classification applied to repeat hydrographic sections and Argo data produces a circumpolar, observationally-constrained estimate of CDW redistribution for the period 2004–2024. The results reveal a coherent pattern of CDW layer expansion adjacent to the Antarctic continental slope and contraction further north. Finally, a high-resolution eddy-resolving numerical simulation is used to identify the physical regimes that shape seasonal CDW intrusions onto the Antarctic shelf. Four dominant regimes are characterised, demonstrating how bathymetry, winds, and dense shelf water formation interact to modulate shelf heat transport on seasonal timescales. By synthesising insights from previously disparate regional studies within a single, circumpolar and seasonally resolved framework, this work provides a unified view of the processes that control warm water access to the ice shelf base. Together, the thesis provides a multi-scale perspective on how the large-scale structure and pathways of Southern Ocean water masses respond to external forcing and how these changes shape the stability of the Antarctic Ice Sheet.