Winter Sea Ice Growth in McMurdo Sound, Antarctica

• Main Author: Purdie, Craig Rolfe
• Other Authors: Langhorne, Pat, Vennell, Ross
• Format: Thesis
• Language: English
• Published: University of Otago 2012
• Subjects: sea ice; Antarctica; ice growth; ice salinity; platelet ice; adcp; acoustic detection of ice; ocean currents; Antarctic; McMurdo Sound; The Antarctic; Antarc*; Ice Shelf; Sea ice
• Online Access: http://hdl.handle.net/10523/2387

Winter sea ice growth determines not only the ice thickness, but also important properties such as salinity, and changes in structure such as platelet ice growth. However, there are few direct measurements of ice growth over winter, and none previously that simultaneously measured the oceanographic properties that are believed to affect ice growth near an ice shelf. This thesis provides a long-needed parameterisation of the growth and properties of landfast sea ice in McMurdo Sound, through simultaneous measurement of ice properties and oceanographic conditions during the Antarctic winter. In situ ice temperature profile measurements made over winter produced a continuous record of ice growth, relating depths of platelet ice identified in cores to oceanographic conditions when the ice formed. An ice growth model using measured temperature profiles to determine heat conduction, showed residual ice growth due to times of negative oceanic heat flux. This is mostly associated with times of platelet ice growth, accounting for about 35% of the platelet ice or 15% of the total ice thickness. A lack of correlation between the oceanic heat flux and supercooling measured in the water suggests that platelet ice growth is not entirely due to turbulent heat exchange between the ice and the ocean. Additional platelet ice growth is identified with the addition of ice crystals suspended in the water, which were detected in backscatter measurements from an ADCP. Backscatter measurements from an Acoustic Doppler Current Profiler (ADCP) are calibrated and analysed through a comprehensive review of scatterers in the water of McMurdo Sound. Episodic peaks of over 30 dB increase in measured backscatter are attributed to ice crystals suspended in the ocean during times of measured supercooling and observations of platelet ice. A model of acoustic backscatter from ice platelets is presented to determine mass concentrations of suspended ice crystals in the water from ADCP backscatter measurements and vertical velocities. Ice crystal c-axes in columnar ice are demonstrated to align with current direction measured at the time of growth. This is the first time that currents have been measured under natural sea ice during the entire growth history. The rate of c-axis alignment in columnar ice is found to be slightly slower than expected from the measured current speed according to an existing parameterisation. This is thought to be due to the tidal variability of the ocean currents compared with the constant currents used in the tank based parameterisation. Sea ice salinity evolution is parameterised using only measured ice temperature profiles, and the empirical desalination relations from Cox and Weeks (1988b). Salinity profiles from the parameterisation match measured salinity cores but give a greater temporal resolution which is useful for parameterising bulk salinity evolution and salt flux from the ice to the ocean. The resolution of the temperature profile measurements is also sufficient to capture some individual brine drainage events.