Geology Reference
In-Depth Information
Melting
start
Pond
Pond
Thaw hole
Water
surface
Sea ice
3-4 weeks
Figure 2.56
Decay process in sea ice. It typically takes 3-4 weeks from the onset of surface melt for the appear-
ance of thaw holes in Arctic ice and 2 more weeks until the disappearance of the ice sheet if conditions permit.
Figure 2.57
Two scenes of decaying ice in Allen Bay in the vicinity of Resolute, Canadian central Arctic: FY ice
on June 21, 2002 (left) and MY ice on August 11, 2002 (right) [
Johnston et al
., 2003].
thaw hole is a large melt path through the ice at its thin-
nest point or at the melt pool's deepest point. A single
thaw hole is enough to drain a large area of the ice sur-
face. Eventually melt water drains off into the sea through
existing cracks, over the side of ice floes or through thaw
holes. It should be mentioned that some drainage occurs
in landfast FY ice covers through seal breathing holes
illustrated in Figure 1.9 and reported in details by
Digby
[1984]. Once the disintegration of ice sheets proceeds at
the surface to the point where free water surfaces appear,
the rate of further decay is significantly accelerated.
In an early study,
Billelo
[1977] assembled weekly meas-
urements of ice thickness over a period of 10-15 years
from seven stations in Canada and Alaska to explore the
effect of air temperature and ISR on ice decay. The study
used accumulated thawing degree‐days (ATDDs) as indi-
cators of air temperature. This is defined as the number of
days when the daily average of air temperatures is above
0 °C. It is the counterpart of the accumulated freezing
degree-days (AFDD), defined as the number of days when
average temperature is below freezing. Using air tempera-
ture data from the Resolute Bay weather station, the study
found that the rate of fast ice decay can be defined by two
linear relations (at different slopes) with air temperature.
From the start of the decay until the ice reaches 0.9-1.0 m,
the average ablation rate was approximately 0.09 m per 10
ATDD. After that thickness range, the average ablation
progressed at a much faster rate; approximately 0.17 m/
ATDD. Two anomalies appeared in 2 years (1967 and
1971) and could be attributed to extremes of snow accu-
mulation. Air temperature did not depart appreciably in
these two seasons, but the snow cover in the beginning of
the 1970-1971 winter season was double or four times as
deep as it was in the beginning of 1966-1967 winter and
almost twice as deep at the beginning of the ice deteriora-
tion in May (0.55 m in 1971 and 0.28 m in 1967).
Billelo
[1977] concluded that the extreme variation in the rate of
ice ablation can be attributed to the difference of the
amount of snow on ice. Thick snow delays ice decay and
vice versa. In this case, the relation between ice and the
ADDT does not apply. The ISR has the same effect as air
temperature on decreasing ice thickness during ice decay.
The problem, however, is the difficulty of using it as an
indicator because of the unpredictable cloud cover.
Properties of FY ice during its decay were measured in
several field programs. The most notable example of con-
tinuous monitoring of growth and decays of FY ice, and
regrowth to form SY (second-year) ice during 1981-83,
