Observations of ocean-ice shelf interaction at the Totten Glacier

The Antarctic Ice Sheet is the largest reservoir of glacial ice on Earth, containing the ice equivalent to 60 m of global sea level rise. Multiple observations have shown that the Antarctic Ice Sheet is losing mass at an accelerating rate, with the majority of the loss occurring in West Antarctica. This mass loss is triggered by ocean-driven melting of the ice shelves that form where the continental ice extends over the ocean. In contrast to the West Antarctic Ice Sheet, the much larger East Antarctic Ice Sheet has long been considered to be stable. However, recent studies suggest that a large part of the East Antarctic Ice Sheet is grounded well below sea level and therefore exposed to ocean heat flux. Moreover, recent satellite observations have revealed that the Totten Glacier, the largest discharger of ice in East Antarctica, has been losing mass for the past 25 years, suggesting that ocean-driven ice loss may also occur in East Antarctica. In this thesis, the first oceanographic measurements collected near the Totten Glacier on the Sabrina Coast are used to investigate why the glacier is losing mass. Warm Modified Circumpolar Deep Water (MCDW) originating from the Southern Ocean is observed entering the cavity beneath the Totten Ice Shelf near the seafloor through a deep trough. The heat transport is sufficient to drive basal melt at a rate larger than 10 m year-1, the highest rate among the major ice shelves in East Antarctica. These observations suggest that ocean heat flux drives mass loss of the Totten Glacier, as seen in West Antarctica. Additional oceanographic observations show that MCDW is not only found in front of the Totten Glacier, but it fills the bottom layer on most of the continental shelf of the Sabrina Coast. Dense and cold waters typical of the East Antarctic coast are absent, despite the presence of a polynya where strong surface heat loss and salt flux by sea ice formation occur every winter. A simple ocean model driven by observed forcing reveals that freshwater released by ice-shelf basal melt inhibits the formation of dense waters on the Sabrina Coast, inducing a positive feedback of warming and increased ice-shelf melting. Finally, year-round observations of ocean properties collected by icecapable profiling floats show that MCDW intrusions onto the shelf are persistent. Intrusions are warmer and thicker in autumn and early winter. A realistic ocean model shows that interaction between currents on the continental slope and a depression at the shelf break facilitates the MCDW intrusions onto the shelf. The seasonality of these currents explains the warmer and thicker intrusions in autumn and early winter. This thesis provides the first direct evidence that the East Antarctic Ice Sheet is vulnerable to oceanic melting. Considering that the East Antarctic Ice Sheet contains a volume of ice grounded below sea level that is equivalent to 19 m of global sea level rise, the potential for ocean-driven melting to destabilize this large portion of the Antarctic Ice Sheet needs to be accounted for in assessments of future sea level rise.