Ice Conditions and Ice Edges

General

Both fast ice (see explanation below) and pack ice (see explanation below) can have areas of new ice (less than one year old), second year ice, multi-year ice,and old ice (of unknown age). In winter, multi-year ice forms the majority of ice cover in the northern channels of the central Arctic, such as Melville Sound and M'Clintock Channel. Multi-year ice floes are usually thicker and stronger than first-year ice floes. The roughness of the ice coverage will change with the weather patterns of a certain area. The maximum ice coverage in the Arctic is usually reached in April , and the minimum usually occurs in late August or early September. Areas of water which are ice-free all year-round are called polynyas.

• Fast Ice

Fast ice is usually reformed yearly, although it may contain icebergs and floes of older pack ice. These icebergs and floes can become trapped within the fast ice as it forms in the fall. Although icebergs and ridges within the fast ice can have high relief, the fast ice is generally fairly flat. Ice thickness is usually no more than 2.2 m. Some areas with intense ice movement, such as Parry Channel, can have extensive fields of rough rubble and ridges, where ice thickness is greater.

Fast ice can extend for only a few meters from a shore, ice front, shoal, or grounded iceberg, or may extend for several hundred kilometers from such attachment points. The depth of the sea bottom is the main determining factor in the extent of fast ice coverage. Fast ice usually ends where the water reaches a depth of about 25 m. Beyond this depth, ice ridges and other pieces of ice protruding from the bottom of the ice cover rarely remain aground. Where icebergs become grounded, they may become trapped within fast ice and that ice can extend beyond the usual water depth.

Sections of fast ice sometimes become dislodged from the shore by intense offshore winds, after which they normally become part of the pack ice. In winter, most of the channels between the islands in the central Arctic are covered with fast ice.

• Pack Ice

Pack ice consists of ice originally formed as fast ice, but now dislodged from shore, and ice formed at sea. It can be flat over extensive areas, but is usually covered with areas of rough relief caused by movement of floes against each other. The resulting pressure ridges can be many metres thick. Pack ice is not always continuous, and at times can be very broken, with leads between floes of ice. As leads open and later refreeze, newly formed ice is added to the ice cover throughout the winter.

Pack ice persists in the Arctic Ocean all year. In spring, the extent of the pack ice within the Arctic Archipelago increases as sections of fast ice melt and break up. As a result of melting, in most of the Arctic Archipelago and areas further south, there is an ice-free period of about two months (generally longer further to the south, and sometimes shorter further north and west).

The Shear Ice Zone

Large pressure ridges and rubble fields occur between the moving pack ice and the stationary fast ice. When winds drive pack ice into fast ice, or grind it up against the fast ice laterally along the edge, pressure ridges are formed. These ridges will reach depths of 25 m and act as sea anchors for the adjacent fast ice. The shear ice zone also contains many leads. When offshore winds carry loose ice away from consolidated ice, there is a large lead that forms between the edge of the fast ice and the shear ice. This phenomenon is common in the Beaufort Sea.

Seasonal Ice Edge Location

Whereas polynyas occur in the same general areas every year, there is a great variation in the seasonal location of fast ice edges from one year to the next. This variability is due to vagaries in wind strength and directions, air temperatures, solar radiation and ocean currents in both previous years and in the current year. The variability in the location of the ice edge in the Barrow Strait and Lancaster Sound is exemplified in Table below which shows spring location of fast ice / ice edge for a twenty-six year period from 1964 to 1989.

TABLE