Thermodynamics and dynamics of polar ice sheets, glaciers and ice shelves under changing climate

Abstract

Polar ice sheets, glaciers, and ice shelves, referred to as land ice in this document, are under transition in the changing climate. Observations show that glaciers have retreated and melt water discharge from land ice has increased together with the warming climate. Decreasing volume of land ice is reflected to the whole Earth system via changes in surface radiation balance, sea level rise, and water balance in inland watersheds. Warming and melting land ice creates positive feedback loops that further increase melting. The study of polar land ice in changing climate is challenging due to lack of observations from the remote areas. Large interannual variability of climate, rapid changes in temperature and ice conditions, and short observational timeseries further complicate the research.

This Ph.D. thesis concentrates on the aspects of melting land ice in the polar regions. The focus is on the interannual variability of surface melt and weather conditions on two of the Antarctic Peninsula Ice Shelves (Larsen C and Wilkins) and the Greenland Ice Sheet. Surface melt is addressed through the surface energy balance and other direct measures of melt. Furthermore, the changes in surface accumulation and ablation patterns have been modelled on a small Arctic valley glacier, Midtre Lovénbreen. This modelling study presents for the first time a method that allows to produce high resolution maps of accumulation and ablation, using only a glacier flow model and the digital elevation models of the glacier surface.

Surface energy balance controls the melt of snow and ice. The effect of atmospheric moisture or clouds on the surface energy balance was important in the Antarctica Peninsula region and on the Greenland Ice Sheet. Cloud cover fraction was related to the wintertime surface net heat flux on Larsen C Ice Shelf. A multi-regression model, including the cloud cover fraction as one of the explanatory variables, explained up to 80% of the interannual variation of the surface net heat flux in June-August. On the Greenland Ice Sheet the vertically integrated total column water was positively correlated with the summertime surface melt.

Local near surface winds (at 10 m height above ground level) were important in explaining the surface net heat flux on Larsen C Ice Shelf in summer, autumn, and winter. In Greenland, the wind components correlated locally with the number of melt days. The positive correlation was likely related to the adiabatic heating of descending air on the lee side of the ice sheet and, in other locations, to the advection of warm air from lower latitudes. The number of melt days on the Greenland Ice Sheet were also correlated with positive North Atlantic Oscillation Index and Greenland Blocking Index, indicating that these large scale patterns contribute in creating conditions that favour melt. The large scale atmospheric conditions that increase humidity or advect warm air to the polar regions are likely to increase surface melt in Greenland or Antarctic Peninsula region.

Nevertheless, explaining the high resolution melt patterns requires understanding of the local conditions and topographic features. This Ph.D. thesis contributes in understanding local surface melt as part of the large scale climate system.