17:15 〜 19:15
[MIS15-P04] Seasonal variation of landfast ice porosity and its implications for ice thermodynamic and mechanical properties in Prydz Bay, East Antarctica
キーワード：ice porosity, landfast ice, Antarctic
Sea ice has an intricate and highly variable internal structure, detailed descriptions of which can be used to estimate the ice thermodynamic and mechanical parameters. The relatively stationary and stable properties, combined with the associated logistical support from shore-based research stations, make landfast ice an ideal platform for conducting the observations of sea ice physics.
Supporting by the program of Chinese National Antarctic Research Expedition (CHINARE), the ice-core based salinity and density data were obtained for two ice seasons from November 2004 to December 2006 on first-year landfast ice off Zhongshan Station in Prydz Bay, East Antarctica. In addition, the thermodynamic growth and temperature profile of landfast ice have been observed for 8 ice seasons (2005-2006, 2009-2010, 2013-2015 and 2021) during these two years and beyond using ice mass balance buoys (IMBs). Based on the 2-year-long core-based measurements, a simple parameterization scheme was developed to characterize the ice basic physical parameters of salinity and density, then the vertical distribution and seasonal evolution of the ice porosity was derived combined with the ice temperature profiles observed by the IMBs.
There are significant difference in the vertical distribution of landfast ice porosity among the ice growth, transition and melting season. The estimated air volume fraction has an obvious seasonal characteristic, reaching a minimum close to zero in mid-August and a maximum with an average of about 9% in late January. However, its interannual variation is almost negligible. The seasonal variation of the bulk brine volume fraction was relatively trivial, less than 5% through most of the year. While the year-to-year fluctuation of the bulk brine volume fraction is comparable to or even larger than that of its seasonal variation, which is closely related to the synoptic-scale warming events. Analysis on these warming events during the ice growth season shows that the warming magnitude, duration, and snow accumulation accompanied with low pressure systems can largely regulate the brine volume fraction. At the top and middle ice layers, the greater the near-surface air temperature rise, the longer the warming event lasts, and the thinner the snow, the greater the increase in the brine volume fraction. While the bottom brine volume fraction is generally smaller at the end of the warming event than at the beginning, likely related to the downward movement of the ice-water interface as ice grows, which promotes the discharge of brine and offsets the effect of warming. The sensitivity of brine inclusions to ice temperature varies in different temperature ranges, with two jumps at -5℃ and close to the freezing point. The potential of estimating the thermal and mechanical parameters of sea ice based on the parameterized ice salinity, density, and porosity is discussed. The brine convection and gas transport within the ice depends on the permeability, which is the function of the bine volume fraction. The air volume fraction would greatly affect its thermal conductivity, particularly for the ice with lower densities. The ice mechanical strength is largely dependent on its total ice porosity rather ran the brine volume fraction.
The results given by this study can provide important parameterizations supporting for the numerical simulations of thermodynamics and dynamics of landfast ice, as well as the study of interactions of icebreaker-sea ice, and ecosystems involved in landfast ice. Although the results obtained in this study are based on observations of landfast ice off Zhongshan Station, but are also applicable, to a certain extent, to the level ice floes formed through thermodynamic growth in the Southern Ocean.
Supporting by the program of Chinese National Antarctic Research Expedition (CHINARE), the ice-core based salinity and density data were obtained for two ice seasons from November 2004 to December 2006 on first-year landfast ice off Zhongshan Station in Prydz Bay, East Antarctica. In addition, the thermodynamic growth and temperature profile of landfast ice have been observed for 8 ice seasons (2005-2006, 2009-2010, 2013-2015 and 2021) during these two years and beyond using ice mass balance buoys (IMBs). Based on the 2-year-long core-based measurements, a simple parameterization scheme was developed to characterize the ice basic physical parameters of salinity and density, then the vertical distribution and seasonal evolution of the ice porosity was derived combined with the ice temperature profiles observed by the IMBs.
There are significant difference in the vertical distribution of landfast ice porosity among the ice growth, transition and melting season. The estimated air volume fraction has an obvious seasonal characteristic, reaching a minimum close to zero in mid-August and a maximum with an average of about 9% in late January. However, its interannual variation is almost negligible. The seasonal variation of the bulk brine volume fraction was relatively trivial, less than 5% through most of the year. While the year-to-year fluctuation of the bulk brine volume fraction is comparable to or even larger than that of its seasonal variation, which is closely related to the synoptic-scale warming events. Analysis on these warming events during the ice growth season shows that the warming magnitude, duration, and snow accumulation accompanied with low pressure systems can largely regulate the brine volume fraction. At the top and middle ice layers, the greater the near-surface air temperature rise, the longer the warming event lasts, and the thinner the snow, the greater the increase in the brine volume fraction. While the bottom brine volume fraction is generally smaller at the end of the warming event than at the beginning, likely related to the downward movement of the ice-water interface as ice grows, which promotes the discharge of brine and offsets the effect of warming. The sensitivity of brine inclusions to ice temperature varies in different temperature ranges, with two jumps at -5℃ and close to the freezing point. The potential of estimating the thermal and mechanical parameters of sea ice based on the parameterized ice salinity, density, and porosity is discussed. The brine convection and gas transport within the ice depends on the permeability, which is the function of the bine volume fraction. The air volume fraction would greatly affect its thermal conductivity, particularly for the ice with lower densities. The ice mechanical strength is largely dependent on its total ice porosity rather ran the brine volume fraction.
The results given by this study can provide important parameterizations supporting for the numerical simulations of thermodynamics and dynamics of landfast ice, as well as the study of interactions of icebreaker-sea ice, and ecosystems involved in landfast ice. Although the results obtained in this study are based on observations of landfast ice off Zhongshan Station, but are also applicable, to a certain extent, to the level ice floes formed through thermodynamic growth in the Southern Ocean.