Modified circumpolar deep water intrusions causing enhanced ice shelf melting in East Antarctica

Glaciological evidence shows that the Antarctic Ice Sheet is losing mass, due to the thinning of Antarctic ice shelves caused by warm modified Circumpolar Deep Water (mCDW) intrusions. While mCDW intrusions are well-documented in West Antarctica, their temporal and spatial variability in East Antarctica remain uncertain. Hence, this thesis utilizes a combination of observational data, along with a high-resolution ocean-sea ice model, to investigate mCDW intrusions in two locations in East Antarctica, Vincennes Bay and the Shackleton Ice Shelf (SIS), focusing on the spatial and temporal variability of intrusions, their interaction with local ice shelves, and how they get transported onto the continental shelf. There are three key results. Firstly, this study describes the first observations of warm mCDW intrusions into Vincennes Bay, and attributes these to enhanced melting of the Vanderford Glacier. The consequent input of fresh meltwater increases the stratification of the water column, ultimately making it harder for polynyas - areas of intense, wind-driven sea-ice formation - to form Dense Shelf Water (DSW), and by extension, Antarctic Bottom Water (AABW). Observations show that top-to-bottom convection, which is required to form DSW, began very late in the season (after June) and that the DSW was fresher than in other AABW production regions (average S = 34.49). Secondly, this study examines the temporal variability of mCDW intrusions on the continental shelf west of the Shackleton Ice Shelf (SIS). Examining historical data prior to 1996, the intrusions observed near the SIS were relatively cool (mCDW’s θ < -1.6 ◦C). After 2010, significantly warmer mCDW Glaciological evidence shows that the Antarctic Ice Sheet is losing mass, due to the thinning of Antarctic ice shelves caused by warm modified Circumpolar Deep Water (mCDW) intrusions. While mCDW intrusions are well-documented in West Antarctica, their temporal and spatial variability in East Antarctica remain uncertain. Hence, this thesis utilizes a combination of observational data, along with a high-resolution ocean-sea ice model, to investigate mCDW intrusions in two locations in East Antarctica, Vincennes Bay and the Shackleton Ice Shelf (SIS), focusing on the spatial and temporal variability of intrusions, their interaction with local ice shelves, and how they get transported onto the continental shelf. There are three key results. Firstly, this study describes the first observations of warm mCDW intrusions into Vincennes Bay, and attributes these to enhanced melting of the Vanderford Glacier. The consequent input of fresh meltwater increases the stratification of the water column, ultimately making it harder for polynyas - areas of intense, wind-driven sea-ice formation - to form Dense Shelf Water (DSW), and by extension, Antarctic Bottom Water (AABW). Observations show that top-to-bottom convection, which is required to form DSW, began very late in the season (after June) and that the DSW was fresher than in other AABW production regions (average S = 34.49). Secondly, this study examines the temporal variability of mCDW intrusions on the continental shelf west of the Shackleton Ice Shelf (SIS). Examining historical data prior to 1996, the intrusions observed near the SIS were relatively cool (mCDW’s θ < -1.6 ◦C). After 2010, significantly warmer mCDW Finally, experiments utilizing the ACCESS-OM2-01 high-resolution numerical model show that Ekman-driven upwelling over the slope, which drives a shoaling of isopycnals, promotes the flow of mCDW onto the continental shelf in Vincennes Bay. Variations in the easterly winds (weak/strong) drive variability in the Ekman upwelling/downwelling, causing the shelf warming/cooling responses that follow. As the westerly winds over the Southern Ocean move poleward under anthropogenic forcing (Thompson & Solomon, 2002), a reduction in the meridional extent and strength of the polar easterlies is predicted (Neme et al., 2022). Therefore, the results imply that warm intrusions into the region may become more frequent in the future. The findings of this thesis highlight the increased vulnerability of East Antarctic Ice Shelves, in relation to increasing mCDW intrusions onto its continental shelves and their potential to cause higher basal melt rates. Considering the large sea level equivalent stored within East Antarctica, and its capacity to alter ocean circulation, ongoing research into mCDW intrusions in East Antarctica is paramount to improving future sea-level rise and climate projections.