Sea ice zones

The sea ice cover can be divided into different ice zones. Again following Maykut (1985), the perennial ice zone (PIZ) is where ice is present throughout the year. The PIZ can be broadly considered as the area north of the mean September ice margin shown in Figure 2.4. During winter, the PIZ typically consists of 10-15% FYI formed in leads, and the remainder MYI. In summer, most of the FYI melts. The Arctic's perennial ice zone contains about two-thirds of all the MYI in the world's oceans.

Recent satellite analyses indicate that MYI covers about 60% of the Arctic Ocean in January (Kwok, 2004). The seasonal ice zone (SIZ) is where ice is present only on a seasonal basis. It can be considered as the area between the seasonal maximum and minimum ice margins (Figure 2.4). Undeformed ice in the SIZ is typically less than 2 m in thickness, but large amounts of ridged ice are also present.

The fast ice zone refers to areas where ice is anchored to the shoreline, and grounded to about 2 m depth, but up to 20 m in the southern Beaufort Sea by grounded pressure ridges (stamukhi) (Kovaks and Mellor, 1974). Fast ice forms early in the winter in shallow water where the water column can cool rapidly to the freezing point. Fast ice development is most extensive in areas where the shelf slope is gentle, such as the Mackenzie Delta and Laptev Sea. Fast ice is typically undeformed over large areas but can contain pressure ridges and keels. Near the shore it is typically highly deformed and is very difficult to navigate. Ice that fills bays and channels in regions like the Canadian Arctic Archipelago is also considered to be fast ice. Fast ice in the Canadian Arctic Archipelago is typically quite thick (>5 m). Fast ice breakup can be estimated from cumulative thawing degree days. Studies based on this approach have been carried out in Baffin Bay (Jacobs et al., 1975) and for the Beaufort Sea coast (Barry et al., 1979).

The shear zone is a region of highly deformed ice along the coasts of Alaska, the Canadian Arctic Archipelago and northern Greenland. The shear zone can be considered as a boundary between the fast ice and the central (perennial) pack ice which is in near constant motion. The shear zone is readily observed in satellite imagery. The MODIS image in Figure 7.4 for July 21, 2001 shows a section of the shear zone along the coast of the Canadian Arctic Archipelago. North is approximately to the right. Borden Island is at the top of the figure and Axel Heiburg Island is at the bottom. The shear zone runs from the top middle to bottom right. Fast ice is to the left of the zone while the moving pack ice is to the right. The pack ice has a mean component of motion toward the top of the image associated with the Beaufort Gyre circulation. While there is strong shear, there is also a component of motion into the shore. This results in convergence and the formation of thick, ridged ice (seen as the light colored streaks). There is also locally strong divergence, producing leads. The very obvious lead north of (to the right of) Borden Island is roughly 1-2 km wide. Ice deformation will be reviewed more formally in Section 7.3. On the basis of submarine sonar data, Wadhams (1980) suggests that the shear zone extends about 400 km off the coast of Ellesmere Island where ice motion is normal to the coast and about 160 km off Alaska where the motion is more parallel to the coast.

Recall from Chapter 2 that the marginal ice zone (MIZ) represents the boundary between the sea ice and open ocean. The boundary is generally (but not always) gradual, perhaps 100-200 km in width. Figure 2.6 shows an example of conditions along the MIZ. Penetration of surface waves into the ice pack breaks ice into small floes with an average size increasing rapidly with distance from the ice edge. During winter, the MIZ is associated with strong horizontal temperature gradients. Enhanced baroclinicity along the MIZ may influence cyclone development. Horizontal variations

Marginal Zone Ice Winter Satelite

Figure 7.4 MODIS satellite image showing the shear zone along the coast of the Canadian Arctic Archipelago. North is roughly to the right. Axel Heiburg Island is at the bottom. The image covers an area of approximately 526 km by 376 km, with a resolution of 250 m (courtesy of T. Haran, NSIDC, Boulder, CO).

Figure 7.4 MODIS satellite image showing the shear zone along the coast of the Canadian Arctic Archipelago. North is roughly to the right. Axel Heiburg Island is at the bottom. The image covers an area of approximately 526 km by 376 km, with a resolution of 250 m (courtesy of T. Haran, NSIDC, Boulder, CO).

Plate 4 Ice concentration for the Arctic and peripheral seas (see color bar) for the period January 5-11, 1994, based on the analysis of the National Ice Center (courtesy of K. Knowles, NSIDC, Boulder, CO): See color plates section.

in the density structure along the MIZ can also produce mesoscale phenomena in the ocean, such as eddies, jets and upwelling. Biologic activity is high along the MIZ.

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