Antarctic Winter Water: its role in Southern Ocean dynamics and sea ice variability

Abstract

The Southern Ocean is central to the global climate system, connecting the world's major ocean basins and regulating intense exchange of carbon and heat between the atmosphere and deep ocean. Antarctic sea ice is a critical component to this system, influencing planetary albedo, modulating air--sea exchanges, and regulating the vertical structure of the upper ocean.

In recent years, Antarctic sea ice has undergone an unprecedented and rapid change, shifting from a multi-decadal state of gradual sea ice expansion to a regime of low and variable coverage---proposed as a new sea ice state. The processes driving this transition, particularly the vertical ocean--ice interactions, remain poorly understood and models fail to reproduce the sea ice conditions.

This thesis examines the role of Antarctic Winter Water---the remnant cold wintertime mixed layer sandwiched between the warm and fresh summertime mixed layer and the warm and salty subsurface ocean interior. I investigate Winter Water's role in the overturning circulation system, how it modulates vertical fluxes, and its contribution in shaping Antarctic sea ice variability, hypothesising that it acts as a stratification barrier.

Using a circumpolar dataset of quality-controlled multi-platform \textit{in-situ} observations (including Argo, SOCCOM, MEOP seal-borne sensors, gliders and ship-based CTDs) spanning from 2004 to 2022, combined with satellite records of sea ice concentration and re-analysis output of various other parameters, I characterise Winter Water properties, quantify their role in recent sea ice changes, and assess regional variations in ocean--ice coupling.

Four main studies underpin this thesis. First, I describe the seasonal cycle, spatial distribution, and export pathways of Winter Water, highlighting its role within the overturning circulation. Second, I show that from 2005–2015, Winter Water shoaled across much of the Southern Ocean, preconditioning the ocean for enhanced vertical ocean heat fluxes that contributed to the 2015 transition to a new sea ice regime. Third, I reveal that this transition was not circumpolarly uniform: contrasting hydrographic structures across sectors led to opposing regional sea ice responses. Finally, I demonstrate that giant iceberg melt in the Weddell Sea modifies Winter Water properties, increasing stratification and altering upper-ocean heat content.

The findings identify Winter Water as a critical component of the Southern Ocean system. By regulating vertical heat exchange, Winter Water regionally influences sea ice distribution and ocean density structure. Improving the representation of Winter Water in climate models is essential for better projections of Antarctic sea ice and the Southern Ocean under continued climate change.