Variations in sea ice thickness in the polar regions

An overview is presented on the evidence for variations in sea ice thickness in the polar regions. Most ice thickness data in the Arctic come from upward-looking sonar profiling by submarines. The available dataset is large, but the sampling has been necessarily unsystematic, so that only very few cases exist of directly comparable profiles from different years or seasons in the same location. Comparisons made so far are reviewed. They show that large fluctuations of mean ice thickness can occur over significant areas (15% over 300,000 km2) but that these are associated strongly with variability in the field of ice motion in regions upstream of land boundaries, where ice deformation is usually an important contributor to the mean draft. Comparisons made in the Trans Polar Drift Stream far from land boundaries show a remarkable consistency in mean draft between seasons and years. In the Antarctic ice thickness data are much sparser, and most have been obtained by direct drilling. Most of the available winter data come from only two cruises, the 1986 and 1989 Weddell Sea cruises of F.S. Polarstern. They show that first-year ice, which comprises most of the ice in the Antarctic, is remarkably thin, with a mean thickness of about 60 cm when undeformed Second-year ice, found in the western Weddell Sea, is much thicker (1.17 m mean thickness when undeformed). Pressure ridging adds about 42% to the mean drafts due to undeformed ice alone, but most ridges are very shallow (less than 5 m deep). There is no evidence of temporal variations between the two cruises, although winter ice extent in the Atlantic sector was much greater in the second year