The interactions between ocean, ice shelves and sea ice in the southeastern Amundsen Sea

Ice shelves buttressing the Antarctic Ice Sheet are losing mass rapidly mainly due to oceanic melt. In this thesis, I study the interactions between ocean, ice shelf, and sea ice in the southeastern Amundsen Sea, where the strongest melt has been reported, using multi-platform observations and idealised models, proposing physical mechanisms controlling those interactions. This thesis presents the first winter meltwater distribution near Pine Island Ice Shelf, using hydrographic profiles collected by tagged seals, revealing a highly variable meltwater distribution, with two meltwater-rich layers connected by irregularly-spaced meltwater-rich columns. Year-round observations demonstrate that, due to the reduced vertical stratification, a substantial proportion of the meltwater rises to the surface in winter without undergoing intense mixing, providing near-surface heat to maintain polynyas, and nutrients to boost marine production. Velocity measurements in 2019 reveal, for the first time, a small ocean gyre in a habitually ice-covered region to the west of Thwaites Ice Shelf. This gyre rotates anti-cyclonically, despite the climatologically cyclonic wind stress curl in the Amundsen Sea. This thesis uses an idealised barotropic model to reproduce key features of the observed gyres. It shows that the presence of sea ice and/or ice shelves alters the magnitude of the integrated ocean surface stress curl, and hence regulates ocean gyre direction and strength, potentially allowing the gyre to rotate in the opposite sense to the wind stress curl. The first full-depth well-resolved hydrographic time series in the Amundsen SeaPolynya, sampled in autumn 2014, reveals intense mixed-layer cooling, salinification and deepening, with occasional abrupt cooling events that might be caused by storms, eddies and/or waves. An idealised 1-D mixed-layer model is also used to underpin the driving mechanism of the observed mixed-layer features