Geoscience Reference
In-Depth Information
TPDS progressively increase toward the Fram Strait, where mean drift speeds
are 5-10 cm s
−l
. Even larger ice velocities are observed in the Greenland Sea.
Approximately 20 percent of the total ice area of the Arctic Basin annually exits
through the Fram Strait (Thorndike,
1986
), of which 80 percent consists of MYI
(Gow and Tucker,
1987
).
7.2.2
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 extent
shown in
Figure 2.4
(the PIZ has, however, shrunk over the past several decades).
During winter, the PIZ typically consists of 10-15 percent FYI formed in leads, and
the remainder is MYI. In summer, much of the FYI melts. The Arctic's perennial ice
zone contains about two thirds of all the MYI in the world's oceans. 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 extent (
Figure 2.4
).
Undeformed ice in the SIZ is typically less than 2 m in thickness - the limit of
annual thermodynamic ice growth - but large amounts of ridged ice are also present
(ice deformation will be addressed in detail in
Section 7.3
).
The fast ice zone refers to areas where ice is anchored to the bottom of the shore-
line, usually to 2 m depth, but up to 18 m in the southern Beaufort Sea by grounded
pressure ridges (Kovaks and Mellor,
1974
; Mahoney et al.,
2007
). 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 (
Figure 7.6
).
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 has his-
torically been quite thick (greater than 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, Barry, and Weaver,
1975
) and for the Beaufort Sea coast
(Barry, Mortiz, and Rogers,
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 7.7
for July 21, 2001 shows a section of the
shear zone along the coast of the Canadian Arctic Archipelago. North is approxi-
mately 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
