Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 75

causes the ice to move from the Siberian coast of Russia
across the Arctic basin and exit into the North Atlantic
off the east coast of Greenland usually in 1 or 2 years.
It can also push ice into Baffin Bay and the Canadian
Archipelago. Sea ice typically drifts a few kilometers per
day; but under severe stormy weather, it may drift over
25 km/day [ Pärn and Haapala , 2011]. The large‐scale ice
motion triggered by BSG or TDS causes significant diver-
gence and convergence of Arctic ice cover. Several studies
were conducted to explore the role of atmospheric and
stratospheric forcing on the ice motion in the Beaufort Sea
region. Lukovich et al . [2009] discussed and characterized
the linking between stratospheric and surface events using
the relative vorticity and the square of strain computed at
different pressure levels.
Although large-scale ice deformation occur under com-
pression forces, it is well established that tensile strength
of ice [ Hawkes and Mellor , 1972; Sinha , 1983c; Richter-
Menge and Jones , 1983] is significantly lower than its com-
pressive strength [ Sinha , 1984b, 1985b 1986, 1987a; Sinha
et al ., 1996] at any given rate of deformation. Although
compressive strength at a given temperature depends
strongly on the rate of loading (strain- or stress-rate), ten-
sile strength is significantly less rate-sensitive. However,
bending of ice sheets during compressive interactions pro-
duces tensile stresses at the bottom and results in tensile
failures, thereby producing visibly large cracks. It should
also be noted that in concentrated pack ice a piece of sea
ice responds to wind fields integrated over a large distance.
Therefore, synoptic atmospheric pressure patterns, which
create large‐scale wind fields, can create divergence, con-
vergence, shear, or any combination of these stresses over
very large areas of the pack ice. Using remote sensing
imagery, the National Snow and Ice Data Center (NSIDC)
in Colorado reported a fracture in the Arctic ice, observed
in February 2013, which extended for thousands of kilom-
eters from Ellesmere Island to Barrow (Alaska).
The term “fracture” refers to any opening that exposes
seawater to the atmosphere. It may take the form a crack
or a lead (definitions are provided in “ Manual of
Standard Procedures for Observing and Reporting Ice
Conditions ” [ MANICE , 2005]). A brief description of
each form is presented in the remaining of this section
but more discussions on their formation, characteris-
tics, and methods of identification using remote sensing
observation are presented in section 9.2.
Cracks are the opening in an ice sheet, which are devel-
oped when the sheet diverges or shears in order to
relieve the localized tensile stresses [ Schulson and Hibler ,
1991]. They are usually observed in fast ice, consolidated
ice or a single but large FY ice floe. A crack may arise in
response to wind or tidal action which forces a split of an
ice sheet/floe or breaks fast ice. Figure 2.53 illustrates a
typical crack in FY ice sheet in Resolute Passage. This

crack extended for a few kilometers. Note the zig‐zag path
of the crack and its varying width. That is because cracks
follow weak links along grain and subgrain boundaries of
ice (section 4.1.3).
Figure 2.54 includes a pair of photographs of a ther-
mally induced crack in fast ice that ran across the Frederick
Hyde Fjord (83°1′N, 29°, 50′W), at the northern tip of
Greenland in May 1994. The ice was used as an opera-
tional runway for a Boeing 727 jet [see Figure 1.14]. This
was the most northerly point on floating first-year sea ice
cover that a Boing 727 jet aircraft had ever landed [ Pole ,
1995; Sinha , 1995]. This exercise was also used for valida-
tion of the aircraft landing manual [ Sinha, et al ., 1996]
and illustrated in Figure 1.12. The average ice thickness in
the fjord was 2.32 m, the average width of the crack was
around 150 mm and the freeboard was almost constant at
220 mm. The crack appeared suddenly overnight on May
22, 1994, after a rapid drop in the air temperature. It wid-
ened slowly during the next 3 days. It became a real haz-
ard and accordingly the “touch‐down” point for the
Boeing 727 had to be adjusted. In the background of the
Twin Otter aircraft, several vertically oriented snow cov-
ered grooves on the mountain can be seen. These were
sites of snow avalanches that also brought down crum-
pled rocks at the bottom. Strangely, except for the near-
shore areas, there was no snow on the entire ice cover for
several kilometers. The ice looked green‐turquoise from

Figure 2.53 Crack in 1.2 m thick first‐year ice in the Resolute
Passage, Canadian Arctic in May 1990 (photo by M. Shokr).