Sea Ice

Sea ice is ICE formed by the freezing of seawater and floats on the surface of the polar oceans. Its coverage varies with the seasons; in the Northern Hemisphere sea ice ranges from a minimum of about 9 million km² in September to a maximum of about 16 million km² in March. In the Southern Hemisphere the range is from 3 million to 19 million km², with the minimum and maximum coverage occurring in February and September respectively. The thickness of sea ice can vary from a few centimetres for newly formed ice in protected locations to 20 m or more in ridges; however, typical thicknesses are about 3 m in the Arctic and about 1 m in the Antarctic.

Drift Ice
Away from land, sea ice is in nearly constant motion, being driven by WIND and OCEAN CURRENTS. This mobile ice is often called "drift ice." Its motion constantly deforms the ice cover, creating open water "leads" (long cracks a few metres to a kilometre or more wide) in places where the ice is pulled apart, and thick ridges where it is forced together. Ridges are sinuous accumulations of broken ice; the upper part, termed the "sail," may be up to 5 m high, while the submerged portion, termed the "keel," has a depth 4 to 5 times the sail height. When ice is consistently driven away from a coastline, an open water area called a polynya may form. Leads and polynyas provide access through the ice cover and so are an important part of the habitat for marine mammals like whales and seals.

Fast Ice
In COASTAL WATERS near shore, ice motion is restricted by attachment to the coastline and by grounding of ridges in the shallow water. This nearly motionless ice is termed "fast ice" and it predominates in most of the Canadian ARCTIC ARCHIPELAGO. Because it remains relatively undeformed, fast ice is more uniform than drift ice and may reach a thickness of 2.5 m off the northernmost islands of Canada.

When seawater freezes to form sea ice, most of the dissolved SALT is rejected, so that the sea ice is much fresher than seawater. The salt that remains is trapped in small brine pockets. These brine pockets give sea ice an opaque appearance and make it weaker than freshwater ice. In the central Arctic, thick sea ice does not melt completely during the summer, and the melting of snow accumulated on the surface flushes out much of the remaining brine to leave the ice fresher and stronger. Because of the resulting differences in structure and appearance, sea ice is often distinguished by its age. During its first winter it is termed first-year ice, after surviving one summer it is termed second-year ice, and after surviving 2 or more summers it is termed multiyear ice.

Environmental Factors
Sea ice is an important environmental factor in offshore transportation and in oil and gas exploration and development industries. Thick ridges and multiyear ice are serious hazards to navigation, and the necessity of designing for impact of drift ice adds substantially to the cost of offshore structures. In support of these activities, the CANADIAN COAST GUARD operates a fleet of ice-breaking ships, while ice information and forecasting services are provided by the Atmospheric Environment Service of Environment Canada.

Sea ice is also an important part of the CLIMATE system. It insulates the ocean from the cold atmosphere in winter, and in summer it increases the surface albedo (reflectivity) over that of open ocean. An increase in the amount of ice therefore decreases the amount of solar radiation absorbed, reducing surface temperature and allowing enhanced ice growth. The reverse is true for a decrease in ice cover. Since the effect is to reinforce the initial perturbation, it is a positive feedback effect - the so-called ice-albedo feedback. In addition, the transport of drift ice out of the Arctic provides a source of fresh water to the North Atlantic, which affects ocean circulation.

See also ARCTIC OCEANOGRAPHY; CLIMATE CHANGE; CLIMATOLOGY; ICEBERG.

Author GREGORY M. FLATO

Links to Other Sites
Ice Tracker
How modern science is aiding the search for Sir John Franklin's long-lost ships in the Arctic. From the "InnovationCanada.ca" website.

Sea Ice Climatic Atlas for the Northern Canadian Waters
A basic overview of factors affecting ice in the sea. Click on right side menu for related maps and charts. From Environment Canada.

Centre for Earth Observation Science
Click on the interactive map to check out current CEOS scientific research projects. Many photographs and extensive technical data. A University of Manitoba website.

Glossary: Icebergs and Sea Ice
Bilingual glossaries of terms commonly used in the study of icebergs and sea ice. Click on the menu items on the left side of the page to access each glossary. From Environment Canada.

Cryosphere
A definition of the term "cryosphere" from the "Climate and Cryosphere" website, a project of the World Climate Research Program. Browse other sections of this site for summaries of scientific research programs concerning cryosphere regions and their impact on global climate.

Isobel Moira Dunbar
Scroll down this lengthy document (to Page 100)for an extensive obituary for the accomplished Canadian geographer Isobel Moira Dunbar. Describes her contributions to sea ice reconnaissance and her strong support for the equal participation of women in Arctic research projects. From The Royal Society Of Canada. A PDF file.

Sea Ice Studies: Photo and Poster Gallery
A fascinating collection of photos and posters about sea ice and related oceanographic research. Some large pdf documents. From Fisheries and Oceans Canada.

Journal of Glaciology
This site offers free access to selected articles from the "Journal of Glaciology." From the International Glaciological Society.

EducaPoles
In this website you will find several animations designed and produced by the International Polar Foundation on different topics linked to the polar regions, the way our planet's climate functions, climate change and energy.

Glossary: Arctic Climatology and Meteorology
A glossary of meteorological terms prepared for the Arctic Climatology Project Arctic Meteorology and Climate Atlas.

The Oldest Arctic Ice
An article about a small remnant of the former Ellesmere Ice Shelf that has been described as the oldest non-glacial ice in the northern hemisphere. From science20.com.

Sea Ice Cycle
A very readable description of environmental factors that impact on the formation, structure, and disintegration of sea ice. With many illustrations. From Environment Canada's "Educational Resources."