Geoscience Reference
In-Depth Information
Building on previous discussions, leads (fractures) in the sea ice cover are typi-
cally on the order of 10-1000 m wide, but can exceed 10 km near coasts and in the
shear zone. The orientations of major fractures are broadly correlated with large-
scale wind fields, and they have similar synoptic space and time scales (Miles and
Barry,
1989
). Polynyas (irregularly shaped openings) may be found throughout the
pack ice (see
Figure 2.6
) but recur along coasts with offshore winds and are com-
mon in the shear zone. One of the most prominent recurrent polynyas is the North
Water of northern Baffin Bay-Smith Sound, forced partly by strong northerly winds
advecting ice southward (Mysak and Huang,
1992
) and partly by upwelling (Steffen,
1985
). As reviewed in
Chapter 5
, leads and polynyas comprise only 1-2 percent of
the total winter ice area, but the heat flux they provide to the atmosphere is 1-2
orders of magnitude larger than that from surrounding thicker ice (Maykut,
1978
,
1986
).
7.2.4
Ice Thickness
Information on the spatial-temporal distribution of ice thickness is provided by
under-ice submarine transects and moored under-ice buoys, which measure ice draft
with upward-looking sonar, as well as satellite altimetry, such as from the ICESat
and CryoSat-2. Draft is the part of the ice (roughly 90 percent) that projects below
the water surface. Mean ice thickness is typically considered from the thickness of
ice that is present averaged along with open water (zero ice thickness).
Figure 7.9
provides an estimate of the mean winter ice thickness based on submarine sonar
data. Immediately obvious is that the thickest ice is along the northern coasts of
Canada and Greenland, where it ranges from 6-8 m. As discussed, this is a result
of the mean Beaufort Gyre ice circulation having a component of on-shore motion,
resulting in ridging. Thinner ice in the Eurasian shelf seas arises from the continual
transport of ice away from this region toward the Fram Strait, forming open water
areas where new, thin ice can grow in autumn and winter. It hence follows that the
Eurasian shelf seas are the primary areas of new ice production in the Arctic. This
region is sometimes referred to as the “ice factory” of the Arctic Ocean.
Given that mean thickness is somewhat of a statistical abstraction, it is better
to characterize the thickness in terms of a probability density function (PDF). The
PDF for spring, based on submarine sonar data in the 1970s and 1980s, primarily
in the PIZ, is characterized by peak probabilities at around 3 m, and a long tail with
increasingly lower probabilities of thicknesses of 6 m or more, much greater than
the 3.5-4.5 m representing the typical maximum thickness of undeformed MYI.
This thick ice represents ice that has been deformed by rafting and ridging, which
will be addressed in more detail in
Section 7.3
. As another way of looking at thick-
ness probabilities, R. Bourke and A. McLaren (
1992
) found that pressure ridges
with keels 9 m deep tend to recur once every 50-200 m of submarine.
Spatial fields of estimated ice thickness and PDFs for the ice cover as a whole
from ICESat appear in
Figure 7.10
for three periods: February-March of 2006,
March-April of 2007, and February-March of 2008. The PDFs are separated into
