Over the years, a complex terminology has evolved concerning sea ice, its development and various forms. The World Meteorological Organization (WMO) published standardized definitions for many of these terms more than 30 years ago. The purpose of this week’s Supplemental Information is to define some terms that you may encounter while exploring sea ice.
Sea ice refers to any form of ice floating in ocean waters that originated from the freezing of seawater. As noted in Chapter 3 of your Oceanography text, the salt dissolved in seawater depresses slightly the freezing point of water. For example, seawater having a salinity of 34 psu (psu = practical salinity units, which is nearly identical to 3.4% salt) begins to freeze when its temperature drops to about –1.89 °C (28.6 °F) whereas the freezing point of fresh water is 0 °C (32 °F). Note that icebergs are not sea ice; they are massive chunks of ice that break away from a glacier that accumulated on land (mainly Antarctica and Greenland) or an ice shelf and consist of frozen fresh water. Icebergs may be floating or aground.
Sea ice is broadly classified based on its age (and thickness) as new ice (less than 10 cm thick), young ice (10-30 cm thick), first-year ice (more than 30 cm and perhaps as much as 120 cm thick), or old ice (second-year or multi-year ice). First-year ice represents no more than one winter’s growth of sea ice. Second-year ice has survived one melt season (summer) whereas multi-year ice has persisted through at least two summer’s melt. The latter is typically around 3 m in thickness and is more common in the Arctic Ocean than the Southern Ocean. Sea ice near Antarctica is very seasonal. While much variations occur, summer ice cover usually ranges from zero to 150 km in width, while winter ice can vary from as little as 450 km to more than 1700 km. Maximum coverage tends to occur in the Weddell Sea, but tends to melt back each summer.
The initial stage of sea ice formation consists of the formation of frazil ice in the top few centimeters of ocean waters. Frazil ice consists of millimeter-sized crystals that give the sea surface an oily appearance. Wave action can stir frazil ice to a depth of several meters. With continued freezing, frazil ice crystals coagulate to form a soupy mix that has a matte surface, sometimes referred to as grease ice. Frazil ice and grease ice are forms of new ice but do not form distinctive ice floes. (An ice floe is any contiguous piece of ice that can vary greatly in size from meters to more than 10 km across.) Other types of new ice include frozen slush (an accumulation of water-saturated snow on an ice surface or floating in water), nilas (a thin crust of elastic gray-colored ice on a calm sea that readily bends into interlocking fingers), and pancake ice.
Grease ice aggregates into small chunks of ice, which then become pancake-shaped ice floes. Pancake ice consists of circular pieces of ice from 0.30 to 3.0 m in diameter and having a thickness of up to 10 cm. Collisions with other ice floes are responsible for the upturned rim that is characteristic of pancake ice. Eventually, these pancake-shaped ice floes freeze together into larger floes or form an extensive ice cover. With falling temperature, the ice cover thickens. Winds, waves, and currents cause ice floes to override one another (known as rafting) further thickening the ice cover generally to 0.4 to 0.6 m. However, in some fierce winter storms in the Bering Sea, scientists have reported 3-m thick ice floes rafting onto other ice floes and building the ice to a thickness of 6 m, of which 90% or 5.4 m is below sea level. Rafting also produces a rough surface on the ice floe.
Fast ice is sea ice that forms along the coast and is attached to the shore or shallow sea bottoms (e.g., typical off the coast of northern Alaska) so that it cannot move laterally. It may extend from a few meters to several hundred kilometers offshore and may be more than one year old. Pack ice is any area of sea ice that is not anchored to land and moves with the wind and ocean currents. Pack ice is described as very open (1/10 to 3/10 ice cover), open (4/10 to 6/10 ice cover), close (7/10 to 8/10 ice cover), very close (9/10 to less than 10/10), and compact (10/10 ice cover with no water visible). If ice floes are frozen together, the compact pack ice is described as consolidated. The extent of sea ice is defined as the area in which ice covers at least 15% of the ocean surface.
A significant portion of Antarctic sea ice thickens by snow accumulation on top of the ice being flooded by seawater with the slush then freezing. Snow that accumulates on the surface of an ice floe can be flooded via several processes. For one, a sufficiently massive snow cover will suppress the ice/snow interface below sea level allowing seawater to intrude and turn the snow to slush. In addition, water may migrate upward through brine channels within the sea ice and enter the snow. At low temperatures, the slush freezes to so-called snow ice.
Stresses cause fractures in sea ice and vary from a few meters to many kilometers in length. Where fast ice is attached to the shore, tide-induced vertical motions of the ice may produce tide cracks. These fractures allow penguins and seals access to the ocean. A fracture that is wide enough to be navigable by surface vessels is known as a lead and may occur between the shore and the pack ice or between fast ice and pack ice or simply between large ice floes or in the ice pack at sea. At sea, leads provide breathing holes for whales. Leads also permit exchange of heat and moisture between the ocean and atmosphere. In addition, converging ice floes may be forced upward into a wall of fractured ice, known as a pressure ridge. Pressure ridges can extend several meters above the sea ice and ocean surface. The fractured ice forced downward meters below the ridge is known as an ice keel.
A non-linear shaped kilometer-scale opening in the sea ice cover that persists or reoccurs regularly (often annually), is called a polynya. These holes in the ice would not normally exist because of freezing temperatures were it not for mechanisms to keep them open. Persistent gravity-driven cold winds (katabatic winds) blow ice away from land (e.g., Antarctica), islands, and grounded ice leaving the surface ice-free. In a second mechanism, warm water welling up from below supplies sufficient heat to melt the ice cover. Also, polynyas are common at the mouths of large rivers. Major polynyas in the Northern Hemisphere include the St. Lawrence polynya in the Bering Sea and polynyas off the northeast and northwest coasts of Greenland. These are called North East Water and North Water respectively.
For more information including photographs of the various forms of sea ice, see http://oceans-www.jpl.nasa.gov/polar/SeaIce.html or http://www.antcrc.utas.edu.au/aspect/seaiceglossary.html .
You can monitor the variations of sea ice in the Arctic Ocean, the Bering Sea and the Southern Oceans around Antarctica through the "Global Ice Extent" link on the DataStreme Ocean home page. The Marine Modeling and Analysis Branch of the Environmental Modeling Center, a part of the National Environmental Prediction Centers (NCEP), routinely analyzes the sea ice based upon data received from passive microwave sensors onboard polar orbiting satellites.
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URL: DSOcean/supl.html
Prepared by H.J. Niebauer, Ph.D. and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2004, The American Meteorological Society.