Dynamics versus Thermodynamics: The Sea Ice Thickness Distribution

Abstract

Changes of sea ice coverage are commonly taken as an indicator for climate change. Since 30 years, the area of the Arctic and Southern oceans covered by sea ice is routinely monitored by satellite radiometers (Chapter 6). These observations show that the ice coverage of the Arctic Ocean strongly declines during summer, with an average rate of -11.1% per decade. However, in 2007 and 2008, this trend was drastically exceeded when sea ice extent reduced to record lows of only 4.13 and 4.52 km 2 , less than 20% of previous summers, and raising concerns that the Arctic Ocean might become ice-free during summers within the next few decades. However, winter ice coverage of the Arctic Ocean decreases at a much slower pace of only -2.8% per decade. And in contrast to the Arctic, sea ice coverage of the Southern Ocean increases slightly, with 0.6% and 3.4% per decade in the winter and summer, respectively.The sea ice decline in the Arctic is much more rapid than predicted by any of the Intergovernmental Panel for Climate Change (IPCC) climate models . This demonstrates our limited understanding of the processes of sea ice growth and melt, and ice motion and deformation. For a full understanding of the areal changes, additional information on ice thickness is required, but is largely missing up to the date of this writing. This chapter will discuss the importance of ice thickness information, the most frequently used ice thickness measurement techniques and results from observations of long-term, interannual and seasonal thickness variations.In Chapter 2, it was described how sea ice initially forms from open water and subsequently grows into an ice cover, or in other terms, how sea ice grows thermodynamically. One of the basic concepts is that the more the ice grows thicker, the colder the air is due to the establishment of greater temperature gradients in the ice, and higher freezing rates. Vice versa, it would follow that as a consequence of climate warming, the polar sea ice cover would become thinner. However, another process contributes to the sea ice thickness distribution: Due to its relative thinness -some decimetres to a few metres -sea ice fl oating over deep water is subject to winds and currents which steadily move the ice around, i.e. the ice cover drifts. As a result, it breaks up into fl oes interspersed by open water leads. With changing drift directions and speeds, the ice fl oes will be pushed together and collide with each other. If the resulting forces in the ice become too large, it will fi nally break. The resulting

Thickness distribution of Antarctic sea ice

“…Their formulation used a fixed slope angle of 26°, however we use an implied variable slope angle which is dependent on the fractional coverage of ridges and the average sail height. In this way, broader ridges are flatter, which is consistent with the theory that ridges should build laterally once the limiting height is reached [Tucker and Govoni, 1981], although it is well known that the shape of individual keels may vary enormously in both the Arctic and Antarctic [Haas, 2003].…”

Interception of atmospheric fluxes by Arctic sea ice: Evidence from cosmogenic7Be

“…In order to avoid any artifacts related to the presence of the vessel, samples were collected at distances of about 200 m away from the ship. Ice thickness was measured by drilling and also by using an electromagnetic induction system (further details are given by Haas et al [1997] and Haas [2003]) [Schauer, 2008]. For measurement of 7 Be in sea ice, surficial snow thickness was first estimated using a calibrated pole, the snow was removed and approximately 100 L of ice from the upper 10 cm were collected using stainless steel shovels.…”

Holocene long‐ and short‐term climate changes off Adélie Land, East Antarctica

“…Sea ice is one of the fastest reacting, most seasonal geophysical parameter on Earth's surface. Its seasonal cycle is strongly dependent on water temperature, surface air temperatures and wind activity [Ackley, 1980] and to incoming solar energy [Comiso, 2003;Haas, 2003]. It is therefore sensible to investigate past sea ice dynamics to better understand the driving mechanisms of Holocene climate change at high southern latitudes.…”