A numerical study of ocean circulation and ice-ocean interaction, beneath the Amery Ice Shelf, Antarctica

Simulations of the ocean dynamics in the cavity under the Amery Ice Shelf, East Antarctica, were carried out using a three-dimensional numerical ocean model. The aims were as follows: first, to describe the ocean circulation, the temperature and salinity distributions, and the interaction of the ocean with the base of the ice shelf; second, to investigate the strength of the influence of the external circulation in Prydz Bay on the circulation in the ocean cavity; third, to investigate the impact of any change in climate on the dynamics of the ocean cavity and the basal mass balance of the Amery Ice Shelf. The simulations show that the circulation in the ocean cavity is predominantly horizontal and approximately follows the contours of water column thickness. It is driven by the density gradient in the cavity, which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice-ocean interface and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front. This horizontal exchange is strongly influenced by the velocity field at the ice front. Estimates of the basal component of the mass loss of the Amery Ice Shelf were found from model runs with different ice front boundary conditions. The computed loss varied between 10.6 Gta\\(^{-1}\\) and 20.2 Gta\\(^{-1}\\), depending on which boundary condition was applied at the ice front. The bulk of the melting occurred near the southern grounding line of the ice shelf, although substantial melting also occurred in areas where heat transport by the horizontal circulation was large. Accretion was restricted to areas where water, from upstream melting, was supercooled as it ascended the ice shelf base. In further studies the model domain was expanded to the north to encompass the adjacent ocean in Prydz Bay. The aim was to allow flow across the ice front to evolve freely. To ensure realistic circulation and water mass distribution in Prydz Bay, temperature and salinity observations were assimilated into the Prydz Bay portion of the domain. This eliminated the need for complicated sea ice models and the arbitrary specification of water mass characteristics. This was only partially successful. The model circulation in Prydz Bay was consistent with observations, but the temperature and salinity were not. This inconsistency raises doubts about the corresponding temperature and salinity distribution in the ocean cavity, and about the fluxes between the ocean and the ice shelf. The impact of possible ocean climate change to the north of the Amery Ice Shelf was investigated using the smaller domain model. First, two cooler ocean scenarios appropriate to glacial climates or reduced sea ice growth were applied. Second, temperature increases in the range of 0.1°C to 3.0°C were applied at the ice front boundary. For some of these scenarios changes in salinity were considered. The circulation in the ocean cavity, in particular the strength and structure of some gyres close to the ice front, was affected by the changes. In addition, heat flux from warmer water increased the mean melt rate and the net rate of mass loss from the ice shelf. These rates increased approximately linearly with temperature (at ~0.45 ma\\(^{-1°}\\)C\\(^{-1}\\) and ~22.0 Gta\\(^{-1°}\\)C\\(^{-1}\\)), suggesting substantial modification of the ice shelf would occur in a warmer climate.