Sources and Sinks of Warm Circumpolar Deep Water on the Antarctic Continental Shelf

Author: Moorman, Ruth

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Thompson, Andrew F.

Committee Members: Callies, Joern; Stewart, Andrew L.; Minchew, Brent; Thompson, Andrew F.

Option: Environmental Science and Engineering

Abstract

Circumpolar Deep Water (CDW) is the heat source driving ice loss from the Antarctic Ice Sheet. These warm ocean waters can be found everywhere north of the continental shelf break around Antarctica. Along most of the Antarctic coastline, the Antarctic Ice Sheet is protected from this heat reservoir by a buffer of surface cooled continental shelf waters. But where this buffer fails, and warm CDW encroaches on the Antarctic coast without venting its heat to the atmosphere, these warm waters rapidly melt floating ice shelves from below and accelerate the flow of upstream glaciers into the ocean. For decades this has been observed along the Pacific coast of Antarctica, leading to dramatic ice loss from the West Antarctic Ice Sheet. This thesis aims to improve our understanding of the dynamics controlling the supply of warm CDW to the Antarctic continental shelf and determining the fate of warm CDW after it crosses the continental shelf break. An atypical focus on the latter suite of processes, i.e. the "sinks" of CDW on the continental shelf, yields novel insights into the Antarctic coastal environment and the drivers of ice shelf melt variability.

The depth of the thermocline partitioning warm CDW from colder overlying Winter Waters (WW), effectively the thickness of the CDW layer, correlates well with West Antarctic ice shelf melt rates. Using a simple conceptual model, we show that variations in coastal sea ice formation rates can generate large amplitude, decadal-scale thermocline depth variations, even when the supply of CDW from the shelf-break is steady. The modeled variability is sustained by feedbacks between ice shelf melt rates, vertical mixing of CDW across the thermocline, and thermocline stratification strength, here a function of WW density. In a later chapter, relationships between thermocline and WW properties are investigated throughout the West Antarctic continental shelf using observations. This assessment reveals consistent relationships between thermocline depth and thermocline stratification strength throughout the region, but suggests sea ice processes exert differing degrees of influence on CDW thickness in the Amundsen and Bellingshausen sectors. These studies frame the West Antarctic thermocline as a potential site of CDW modification, modulating the diversion of CDW heat away from ice shelves towards the atmosphere.

When warm CDW manages to access Antarctic ice shelves and melt glacial ice, it loses some of its heat to that process. Though this statement may appear banal, the majority of ocean and climate models fail to account for the heat exchanges needed to melt Antarctic glacial ice. Using simulations that represent ice shelf and iceberg melt, we present a detailed heat budget of Antarctica's coastal oceans, demonstrating that this omission neglects the largest ocean heat sink on the Antarctic continental shelf. Experiments where we suppress this heat sink to evaluate the impact of its omission reveal that the supply CDW heat to the continental shelf is dynamically coupled to the consumption of CDW heat on the continental shelf.

Ultimately, this thesis serves as a reminder to consider the processes keeping Antarctica’s coastal oceans cold when investigating the drivers ice shelf melt variability.