Geoscience Reference
In-Depth Information
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 m. 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 and SSMIS 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).
winter to become first-year 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 to 60 cm. The
reason is that the thermodynamic growth rate depends strongly on the temperature
gradient in the ice - as the ice becomes thicker, the gradient becomes smaller, and it
therefore becomes harder to conduct heat upward through the ice. Nearly half of the
ice area is accounted for by deformed ice that persists through the summer, becom-
ing second-year ice. If it survives another melt season, it becomes third-year ice,
and so on. Any ice two years or older is termed multiyear ice. Ice thicknesses across
the Arctic are extremely variable. Undeformed first-year ice at the end of winter
ranges from a thin veneer to about 2 m thick. Multiyear ice is typically thicker,
averaging perhaps 3.5-4.5 m but sometimes much more; but ridging can also result
in very thick first-year ice. 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
first-year ice thicker than 1 m, a salinity of 2-6 psu (dimensionless practical salinity
units) is typical. Salinities are considerably lower for multiyear ice.
