Ice Physics and Physical Processes - Sea Ice: Physics and Remote Sensing - page 36

by the history of ice growth. The growth is related to the
meteorological conditions (including oceanic) that pre-
vailed at each growth stage.
There are innumerable examples that can be cited to
confirm that congelation sea ice in the form of columnar
ice is commonly found in relatively narrow channels,
fjords, and bays, and in fast ice along the shores of open
seas in the Arctic: for example, Weeks and Lee [1958] at
Hopedale, Labrador; Weeks and Hamilton [1962] at Point
Barrow, Alaska; Pounder [1965] for Button Bay off
Hudson Bay, Resolute Bay, etc.; Peyton, [1963] at Barrow,
Alaska; Nakawo and Sinha [1981] in Eclipse Sound,
Baffin Island; Weeks and Ackley [1982] in Alaskan coast;
Shokr and Sinha [1994] in Resolute Bay; Nunavut, and
Timco and Frederking [1996] in Canadian Beaufort Sea.
It is also found in undeformed second‐year ice [ Bjerklund
et al. , 1985; Sinha, 1985 b ] as well as in multiyear ice
[ Shokr and Sinha, 1994; Sinha, 1991]. Gow et al. [1987]
collected cores from ice floes in pack ice in Farm Strait
and found that they typically contain 75% congelation
versus 25% frazil ice. Eicken et al. [1995] confirmed this
result in the Eurasian Sea of the Arctic basin and Meese
[1989] confirmed it in the Beaufort Sea. Naturally, conge-
lation ice is not expected in the pressure ridges or rubble
fields containing damaged and crushed material, except
within the fragments or blocks of ice inside the ridges or
rubbles [ Sinha, 1991]. Meese [1989] found 85% frazil ice
in thin ice (0.44 m) in a lead in the Beaufort Sea and
between 30% and 70% in areas near a pressure ridge.
Weeks [2010] mentions that exact percentages of frazil
versus congelation ice separated on a regional basis do
not appear to be available. In general, in the Arctic, the
wind and ocean wave conditions after the initial ice for-
mation usually favor the formation of congealed ice.
There are marked differences in the conditions for
growth of sea ice in the Arctic and the Antarctic.
Practically all the area of sea ice formation in the Antarctic
is a seasonal or temperate, equivalent to sub‐Arctic zone.
Treshnikov [1966] estimated that about 75% of the
Antarctic ice melts during the summer, compared to about
25% in the Arctic. Older sea ice survives essentially in
Weddel Sea and some areas of the Ross and Bellinshausen
Seas. Moreover, the vast perimeter of the annual ice
around the continent of Antarctica is actually within the
South Temperate Zone (STZ), not really in the south
polar region (between 66.6°S and North pole), and can be
considered primarily as the marginal ice zone (MIZ). This
MIZ in the STZ is subjected to strong winds of the
Southern Ocean and the velocity shear across the Antarctic
polar front [ Wadhams, 1986]. The persistent stormy condi-
tions associated with high winds cause the water surface
to be almost always turbulent. Naturally, the Antarctic
sea ice is expected to feature less congelation ice and more
frazil ice. Gow et al. [1987] measured the ratio of frazil
to congealed ice in the Weddell Sea during the austral

(a)

(b)

Figure 2.12 Pancakes with diameters up to 5 m in Davis Strait
in November 1982; note (a) the intrapancake ridge and (b) the
fractured pancake (photo by N. K. Sinha, unpublished).

Fragmented sections of pancakes, such as (b), may also
be pushed against each other and get fused by forming
intrapancake ridges as pointed out by (a) in Figures 2.12.
If conglomeration continues, large thin sheets are formed
laterally.

2.2.2. Vertical Ice Growth (Congelation Ice)

Following the formation of nilas either by completion
of freezing of agglomerate of needles, snow particles, and
small discoids at the surface or by consolidation of pan-
cakes, the ice sheet has no room to grow horizontally. The
ice skin at the surface can only grow vertically down along
the direction of the maximum heat flow from the underly-
ing water to the atmosphere. This process is known in the
ice community as congelation. In metallurgy, this type of
unidirectional growth is known as directional solidifica-
tion (DS). In fact, turbine blades used in the hottest sec-
tions of gas turbine engines (e.g., jet engine) are exclusively
made of nickel‐based superalloys using the DS process
[ Sims et al., 1987] and were shown to behave mechanically
like DS columnar-grained ice [ Sinha, 2009b].
Nilas and pancake ice are usually composed of frazil or
granular ice crystals. Congelation ice starts to grow under
this layer and very similar to what is commonly seen in
lakes and rivers. The top layers become part of the transi-
tion layer to contrast with the congelation layer at the
bottom. Structurally, these two types have distinguishable
fabric. The differences lie in the solidification process
involved. Granular or frazil ice in the transition layer
develops after the interparticle liquid freezes as the heat
continues to flow from the bottom to the top of the ice
sheet. The columnar grains formed during congelation
are significantly larger than the grains of frazil or granu-
lar ice. They are shaped like a pencil with their length
along the vertical direction. The grain shape, orientation,
boundaries, subboundaries, and the geometry of the
entrapped inclusions (liquid brine and gases) are affected