Watermass transformations near the tip of the Antarctic Peninsula

The global overturning circulation is a density-driven circulation that controls Earth's climate through its latitudinal transport of heat, freshwater and carbon. In the Southern Ocean, which links all the major oceans of the world, watermasses that are critical to the Meridional Overturning Circulation (MOC) outcrop and transform as they exchange properties that affect their density (e.g heat, salt), acting to close the mass budget of the MOC. Two cells make up the Southern Ocean component of the MOC: a lower cell in which the major portion of global dense waters are formed and the upper cell in which waters subducted elsewhere are returned to the surface via upwelling. The watermasses that transform to drive Southern Ocean overturning have been warming and freshening over recent decades, which has implications for overturning and upwelling strength as well as the drawdown of carbon during dense water formation. The turbulent mixing processes that drive these watermass transformations and the origins of seasonally conditioned dense waters that form the densest watermass are poorly quanti�ed and spatially characterised. This limits our ability to forecast climate change in this region, its global transmission and the performance of climate models. The aim of this thesis is to investigate processes that influence watermass transformations that are critical to the Southern Ocean MOC. Over the continental slope at Elephant Island, we observe the first direct evidence of elevated yet intermittent diapycnal mixing at mid-depths and propose that observed mixing is forced by the locally-generated internal tide. These events occur between overturning cells and are associated with shear instability and turbulent mixing, driving watermass transformation at depth instead of at the surface following upwelling. This has implications for Southern Ocean overturning and upwelling strength. At the same location but two years later, we capture direct heat exchange between a lens (likely an eddy) of Upper Circumpolar Deep Waters (UCDW) and Lower Circumpolar Deep Waters (LCDW). Heat, mass and buoyancy are lost from UCDW to the cooler LCDW beneath as they transform at mid-depth, with instabilities across their shared boundary consistent with symmetric forcing in phase with the baroclinic tide. We propose a mechanism whereby eddy-tide interaction might drive mid-depth heat dispersion from warm UCDW-core eddies, which are the main form of heat transport onto the shelf along the Western Antarctic Peninsula where the recent acceleration of ice-shelf melt is linked to ocean warming. Finally, we link the magnitude of seasonal dense-water pulses at this same location with that of previous-season sea-ice concentration over a wider area dominated by Powell Basin in the west Weddell Sea (the prime site of seasonally-conditioned dense water production), which is separated from waters at Elephant Island by the South Scotia Ridge. We find that the time-lag and the location of three further observations of dense water crossing the South Scotia Ridge via western Hesperides Trough and on the continental slope east of Elephant Island are consistent with tidally-induced currents along connecting isobaths. This pathway is more direct than previously proposed, allowing exported waters to maintain more cohesive dense, cold properties upon arrival at the continental slope where they may interact with warm, saline CDW and convect downslope to replenish and ventilate Antarctic Bottom Waters. This work highlights the contribution of tides to processes that in uence the Southern Ocean MOC. The findings of the thesis implicate the tide in forcing mid-water diapycnal mixing as the baroclinic tide interacts with topography and in driving overturns as the baroclinic tide modifies a submesoscale feature. These processes drive watermass transformations at depth where they can influence upwelling rates and facilitate mid-depth ocean warming where glacier melt is sensitive to heat content along the Western Antarctic Peninsula. Additionally, it is proposed that the tidally-induced flow around bathymetry might play a role in exporting Weddell Sea waters onto the slope at Elephant Island. This work demonstrates that temporally-intermittent and submesoscale features and flows (i.e sub-grid processes) contribute to watermass transformations in the Southern Ocean; depending on location, these can have a signi�cant influence on regional climate. This work makes a contribution towards understanding the spatial and temporal variability of these sub-grid processes and highlights regions where these processes may be studied further.