The sea ice cover

The surface of the Arctic Ocean is characterized by its floating cover of sea ice. The processes of sea ice growth, melt, circulation and variability are discussed at length in Chapter 7. As will become clear in the next few chapters, sea ice is intimately coupled with the atmospheric energy budget, the atmospheric circulation, the surface energy

Figure 2.2 Definition of Arctic seas, based on Russian sources. 1. Greenland Sea, 2. Labrador Sea, 3. Baffin Bay, 4. Canadian Arctic Archipelago, 5. Beaufort Sea, 6. Bering Sea, 7. Chukchi Sea, 8. East Siberian Sea, 9. Laptev Sea, 10. Kara Sea, 11. Barents Sea, 12. Greenland Arctic Basin, 13. North American Arctic Basin, 14. Russian Arctic Basin, 15. European Arctic Basin, 16. Kane Basin, 17. Norwegian Sea (from Welsh etal., 1986, by permission of United States Government).

Figure 2.2 Definition of Arctic seas, based on Russian sources. 1. Greenland Sea, 2. Labrador Sea, 3. Baffin Bay, 4. Canadian Arctic Archipelago, 5. Beaufort Sea, 6. Bering Sea, 7. Chukchi Sea, 8. East Siberian Sea, 9. Laptev Sea, 10. Kara Sea, 11. Barents Sea, 12. Greenland Arctic Basin, 13. North American Arctic Basin, 14. Russian Arctic Basin, 15. European Arctic Basin, 16. Kane Basin, 17. Norwegian Sea (from Welsh etal., 1986, by permission of United States Government).

budget and the hydrologic budget. Accordingly, we introduce some basic aspects of sea ice processes and Arctic oceanography here.

On average, the Northern Hemisphere sea ice cover, the bulk of which lies within the Arctic Ocean, ranges in areal extent from about 15 million km2 in March to about 8 million km2 in September. There is large variability from year to year. Mean ice extent for March and September, defined as regions with an ice concentration (i.e., fractional ice cover) of least 15%, is shown in Figure 2.4. During winter, ice extent in the Arctic Ocean is largely constrained by the surrounding landmasses and in the Greenland, Norwegian and Barents seas (hereafter termed the Atlantic sector) by relatively warm ocean waters that discourage ice formation. The sea ice cover is not a featureless slab, but consists of individual ice floes ranging from a few meters to many kilometers across. Floes are separated by roughly linear openings, known as leads, and irregularly shaped

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Figure 2.3 Bathymetry of the Arctic Ocean and major geographical features (adapted from Clark and Grantz, 2002, by permission of AGU).

Figure 2.3 Bathymetry of the Arctic Ocean and major geographical features (adapted from Clark and Grantz, 2002, by permission of AGU).

openings known as polynyas. Typical winter ice concentrations over the central Arctic Ocean are 98-99%, compared to 85-95% during late summer. During winter, leads generally quickly refreeze to form areas of new, thin ice. Ice concentrations decrease near the sea ice-open ocean margins. Figure 2.5 shows a typical summer sea ice field over the central Arctic Ocean based on high-resolution visible-band satellite imagery. Figure 2.6 is another summer scene that captures the transition from sea ice to open ocean waters (termed the marginal ice zone). As is typical in the Arctic, note how the sea ice margin ranges from rather sharp to very diffuse. The image clearly shows numerous large individual floes, as well as polynyas.

Apart from areas of landfast ice along coasts (ice locked to the shoreline), the sea ice cover is in a state of constant motion. This is due to the influence of wind stress on the upper surface of the sea ice cover, water stress (ocean currents) on the lower surface of the ice, floe-to-floe interactions, and ocean tilt (see Chapter 7 for discussion of the sea ice momentum budget). Ice motion varies strongly from day to day, largely in response to variations in the wind field. Leads and polynyas in the pack ice are largely the result of differential ice motion. Divergent ice motion acts to reduce the ice concentration,

Figure 2.4 Mean sea ice extent for March (all shaded areas) and September (dark shading), based on data from the Special Sensor Microwave/Imager (SSM/I) over the period 1979-2001. SSM/I has a nominal resolution of 25 km (courtesy of K. Knowles, NSIDC, Boulder, CO).

Figure 2.4 Mean sea ice extent for March (all shaded areas) and September (dark shading), based on data from the Special Sensor Microwave/Imager (SSM/I) over the period 1979-2001. SSM/I has a nominal resolution of 25 km (courtesy of K. Knowles, NSIDC, Boulder, CO).

while convergent motion increases it. Convergent motion can force one floe to ride over another, a process known as rafting, producing thicker deformed ice characterized by ridges and corresponding underwater keels. Much of the ice cover consists of deformed ice.

Ice growth and melt are determined by both deformation and thermodynamic forcing, the latter at both the top and the bottom of the ice cover. During autumn, the marginal seas and openings in the pack ice refreeze, forming new ice that thickens during the winter to become firstyear ice. New ice thickens at a rate of between 3 and 10 cm per day for SATs between about -10 °C and -30 °C (Maykut, 1986). However, the rate decreases by an order of magnitude as the ice thickens from 10 cm to 60 cm. Nearly half of the ice area is accounted for by deformed ice that persists through the summer, becoming second-year ice and potentially multiyear ice. Ice thicknesses across the Arctic are extremely variable. Multiyear ice is typically 3-5 m in thickness. Undeformed firstyear ice at the end of winter is generally up to 2 m thick. Areas of

Figure 2.5 Sea ice field for the central Arctic Ocean for July 6, 2001, based on visible-band Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The horizontal resolution is about 250 km. The image is about 376 km by 226 km. The "strip" in the middle of the image shows a typical configuration of leads in the sea ice cover. At the top and bottom of the image, cloud cover masks the surface, a pervasive problem in the Arctic with respect to visible- and infrared-band systems. Passive microwave sensors like SSM/I have all-weather capability and can monitor sea ice conditions during polar darkness, but at a relatively coarse spatial resolution (courtesy of T. Haran, NSIDC, Boulder, CO).

Figure 2.5 Sea ice field for the central Arctic Ocean for July 6, 2001, based on visible-band Moderate Resolution Imaging Spectroradiometer (MODIS) imagery. The horizontal resolution is about 250 km. The image is about 376 km by 226 km. The "strip" in the middle of the image shows a typical configuration of leads in the sea ice cover. At the top and bottom of the image, cloud cover masks the surface, a pervasive problem in the Arctic with respect to visible- and infrared-band systems. Passive microwave sensors like SSM/I have all-weather capability and can monitor sea ice conditions during polar darkness, but at a relatively coarse spatial resolution (courtesy of T. Haran, NSIDC, Boulder, CO).

deformed ice can exceed 10 m in thickness. In general, the thickest ice is found off the Canadian Arctic Archipelago and northern Greenland (mean values of 6-7 m). Except when very young, sea ice is essentially fresh water - as ice forms, brine is rejected. For firstyear ice thicker than 1 m, a salinity of 2-6 parts per thousand (usually expressed as equivalent practical salinity units, or psu) could be considered typical. Salinities are considerably lower for multiyear ice.

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