The influence of an Antarctic ice shelf on land-fast sea ice detected by in-situ and satellite measurements. (2020)
Type of ContentTheses / Dissertations
Thesis DisciplineAntarctic Studies
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
In proximity to ice shelves, the growth of land-fast sea ice can be influenced by meltwater in the upper surface ocean originating from basal melting. In the western Ross Sea, strong southerly winds advect sea ice offshore and form regions of open water called coastal polynyas. Sea ice is produced intensively within polynyas and the rejected brine forms highly dense and saline High Salinity Shelf Water which can sink down into a nearby ice shelf cavity and melt or dissolve the ice shelf base at depth. The meltwater has a potential temperature below the surface freezing point and forms buoyant plumes of Ice Shelf Water which rise along the basal slope becoming in-situ supercooled as the pressure decreases. Supercooling promotes the formation of frazil ice which can grow into larger platelet ice crystals. The outflow of supercooled Ice Shelf Water from the McMurdo-Ross Ice Shelf cavity drives accelerated land-fast sea ice formation, increased sea ice thicknesses, and the formation of an unconsolidated sub-ice platelet layer in McMurdo Sound. The volume of supercooled Ice Shelf Water outflowing into McMurdo Sound is reflected in the thickness of ice shelf-influenced land-fast sea ice and the sub-ice platelet layer beneath. In this thesis, spatio-temporal variability in the distribution of land-fast sea ice and the sub-ice platelet layer was assessed in McMurdo Sound using a combination of drill hole and electromagnetic induction surveying and satellite altimetry techniques. A consistent pattern of thicker ice shelf- influenced land-fast sea ice with a substantial sub-ice platelet layer beneath was observed in the central-western region of the sound. On interannual timescales, thicker and variable sub-ice platelet layers were observed in late spring following winters subject to a higher occurrence of strong southerly wind events and resultant polynya activity. It was hypothesized from this study that a higher frequency of polynya activity over winter and resultant High Salinity Shelf Water production accelerates circulation and increases melting in the proximal shallow McMurdo Ice Shelf and the deeper Ross Ice Shelf regions of the conjoined cavity. The outflow of supercooled Ice Shelf Water and sub-ice platelet layer formation in McMurdo Sound are consequently promoted. The temporal evolution of the sub-ice platelet layer was assessed with stationary electromagnetic induction time-series surveys near the McMurdo Ice Shelf over winter and over a spring-neap tidal cycle in the following late spring. Diurnal variability in the thickness of the sub-ice platelet layer correlated with the oscillation of the tides, and accelerated growth in the sub-ice platelet layer co-occurred with strong southerly wind events and polynya activity in the region in early spring. The sub-ice platelet layer was substantially thicker beneath multi-year ice near the ice shelf. A combination of the tides, wind-driven polynya activity and the presence of multi-year ice influences the circulation of Ice Shelf Water and consequently the evolution of the sub-ice platelet layer over a range of timescales. The capability of the CryoSat-2 satellite radar altimeter to detect the influence of Ice Shelf Water on land-fast sea ice by identifying anomalously higher ice freeboard driven by thicker ice-shelf influenced sea ice and the buoyant forcing of the sub-ice platelet layer in McMurdo Sound was investigated. The spatial distribution of anomalously higher CryoSat-2 derived ice freeboard correlated with the distribution of thicker ice shelf-influenced sea ice, the sub-ice platelet layer, and the supercooled Ice Shelf Water plume in late spring every year. This comprehensive study has bridged the scales between ground-based geophysical observations and satellite-scale assessments, and achieved the objective of enhancing the understanding of interconnections between wind-driven polynya activity, ocean circulation, Ice Shelf Water outflow and Antarctic land-fast sea ice formation.
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