Geology Reference
In-Depth Information
summer of 1980 and found that the average values were
57% and 43%, respectively. However, they reported signifi-
cant variations in these ratios from floe to floe. For exam-
ple, they found floes containing between 6% and 90%
congelation ice and between 3% and 100% frazil ice.
Lange and Eicken [1991] reported that sea ice in floes of
all ages in the Weddell Sea in the Antarctic is dominated
by granular ice of frazil origin. However, in a field study
in the western area of the Ross Sea, Jefferies and Adolph
[1997] found that thermodynamic thickening of the ice in
the inner pack ice was dominated by congelation ice
growth. They observed that 65% of the ice in the inner
pack (up to 400 km from the coast) was congelation ice
with a mean thickness of 200 mm, while 22% of ice in the
outer pack (>800 km from the coast) was frazil ice with a
mean thickness of 120 mm. They attributed the prepon-
derance of congelation ice in the inner pack ice due to a
less stormy environment, which is necessary for signifi-
cant congelation ice growth. Worby et al. [1998] suggested
that at least 39% of the ice volume in the East Antarctic
is columnar, 47% frazil, 13% snow ice and 1% other
types. As part of the Japanese Antarctic Climate Research
(ACR) program, a 2‐year study of atmosphere/sea ice/
ocean interaction processes off Queen Maud Land and
Enderby Land, Kawamura et al. [1995] performed exten-
sive investigations of sea ice from 1990 to 1992. They
examined the structure and texture of sea ice, along with
measurements on vertical distribution of oxygen isotope
concentration, δ ( 18 O) ( presented earlier in section 2.1.1.),
at 16 stations in Lützow‐Holm Bay. The ice thickness
varied in the range of about 2-3.5 m. Significant variation
in the amount of granular and mixed columnar/granular
ice was noticed. The thickness of this layer varied a great
deal, between 0.5 and 2.5 m, but all the cores exhibited
columnar structure at the bottom. For more and up-to-
date information on Antarctic sea ice, the reader must
consult the National Institute of Polar Research in Tokyo
and the Low Temperature Science Laboratory of
Hokkaido University in Supporro, Japan.
atmosphere. From the viewpoint of the crystallographic
classification of natural ice, superimposed ice is usually
categorized as snow or granular ice. Incorporation of
snow into the sea ice reduces the δ ( 18 O). Therefore, it is
possible to differentiate between snow ice and other
structurally different ice types (of seawater but not snow
mixture origin) using a threshold on the δ ( 18 O) value.
Superimposed ice is not common in the Arctic, though
it is observed in subarctic areas such as the Labrador
Sea, but the ice covers are significantly thinner than those
in other areas. However, this type of ice is frequently
observed in the Antarctic. In the southern hemisphere, the
annual sea ice develops primarily in the South Temperate
Zone (STZ), as mentioned earlier, around the coastal
areas surrounding the continent of Antarctica, except
for the areas covered by Ross Sea, Amundsen Sea,
Bellingshausen Sea and Weddel Sea, all within the Western
Antarctic (and remarkably just west of the International
Date Line and/or longitude of 0°). Since the coastal line
of this continent follows essentially the Antarctic Circle
with a latitude of about 66.6°S, the sea ice regime is con-
fined within a narrow belt between about 60°S and 66.6°S,
except, of course, the western Antarctic. The area, north
of the Antarctic Circle but beyond the coastline of the
continent in the western Antarctic, is occupied largely not
by sea ice but by a collar of freshwater ice in the form of
shelf ice. There are a number of huge ice shelves, for
example, Ross Ice Shelf, Ronne Ice Shelf, and Thwaites
Ice Toungue. Thus, strictly speaking, the Antarctic sea ice
regime is not within the  south polar region. Since it is
found in STZ, this should be named as STZ ice, 'not polar
ice'. The STZ ice is naturally expected to be thinner due
to the warmer atmospheric temperature of the STZ.
Additionally, heavier snowfall in the STZ (primarily
drifted from the land to the ocean surfaces) compared to
that in the Arctic also dampens the growth of sea ice. No
wonder, the Antarctic sea ice is thinner than its counter-
part in the Arctic. Here, of course, we mean the annual
sea ice as the “undeformed” first‐year (FY) ice grown in
oceans without the adverse effects of storms and severe
wind. This speculation is confirmed by Weeks [2010] who
concluded that the thickness of undeformed ice in the
Antarctic rarely exceeds 1 m, which is remarkably thinner
than 1.5-2.5 m typically measured in the Arctic.
Only a few field studies have been conducted in the past
to identify the superimposed ice in the Antarctic. Worby
et al. [1998] observed that more than 50% of thin ice sur-
faces in the eastern Antarctic were flooded. The surface
flooding is caused by a number of processes that include
surface deformation and ice breakup, wave penetration in
ice, snow loading, or upward rejection of brine from
the ice subsurface layer [ Perovich and Richter‐Menge,
1994]. In the western Antarctic, namely in the Ross and
Amundsen Seas, Jefferies et  al. [1994] found that super-
imposed snow ice varied from 13% to 43%, whereas frazil
2.2.3. Superimposed Ice
Superimposed ice is formed on existing ice surfaces as a
result of one of the following three processes: freezing of
rain on existing ice surface, refreezing of ice surface or
snow melt, and freezing of water‐logged snow. The second
scenario occurs when the ice or snow surface melts due
to  warm atmospheric temperature during winter, then
refreezes when cold atmospheric temperature resumes.
The third scenario occurs when the load of snow cover
is sufficient to depress the growing ice sheet below its
freeboard so that the water floods the surface and later
refreezes. In both cases the freezing may proceed either
from top down or from bottom up depending on the
temperature difference between the ice surface and the
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