land masses and are thus somewhat sheltered from
ployments to the north Chukchi during the Arctic
the degree of ice movement and consolidation that
Marine Transportation Program, however, con-
occurs in more exposed locations. Ice here is usu-
cluded that year-round transits are technically--
ally less than 30 cm thick during periods of pre-
but probably not economically--feasible using
vailing northerly wind. Zones 5 and 6 are regions
current U.S. icebreaking capabilities. The study
of dynamic ice displacement, resulting in pressure
pointed out that the full 60,000-shaft horsepower
ridges, rubbled floes, and open leads caused by
(shp) operational capacity of the Polar-class ice-
ice drift averaging 0.3 to 0.5 kn. Ice, which is trans-
breakers was often required and that ice piloting
ported primarily by wind, consolidates to a thick-
and operating skills were extremely important.
ness of between 30 and 120 cm. The most severe
Average air temperatures for coastal stations in
ice conditions in the Alaskan Arctic are found just
February, the coldest month, range between 18
and 30C, while extremes of 45C and below
south of the Bering Strait (zone 13). Ice becomes
extremely rubbled and compacted to the wind-
have been recorded. Winds can be severe and pro-
ward side of St. Lawrence Island.
longed, leading to extreme ice pressures and dan-
Year-round marine traffic using current ice-
gerous wind-chill conditions for personnel. The
breaking capabilities is technically feasible
five-year return period for maximum sustained
throughout most of the Bering Sea, but it is usu-
winds for both the Chukchi and Beaufort Seas is
ally suspended from late December to mid-May,
68 kn.
according to the U.S. Coast Pilot manual (USDC/
The disintegration of the seasonal ice cover be-
NOAA 1981). Fast ice begins to form along the
gins, usually around mid-May, at the seaward edge
coast and in sheltered areas sometime in October.
of the fast ice along Alaska's northwest coast in
Pack ice formation in the more open areas begins
response to strong offshore (easterly) winds that
its southward progression in November with the
develop in March and April. However, the pack
seasonal cooling of the water. According to Brower
ice generally remains close in to Pt. Barrow until
et al. (1988), 97% of the ice found in the Bering Sea
late July or early August.
actually forms there with very little southward
influx of ice through the Bering Strait. Winter ice
The Beaufort Sea
coverage in the Bering is highly variable from year
The Beaufort Sea presents the most challenging
to year, and even month to month, due to shifts in
navigation conditions in Alaskan waters. The nar-
prevailing winds that produce leads and polynyas
row region along the northern coastline (zone 8)
of short duration. Thick, multiyear ice is generally
grows fairly stable landfast ice averaging 200 cm
not found in the Bering Sea, since the ice cover
thick. The area contains not only level first-year
melts out completely during the summer.
sea ice, but also has an abundance of pressure
ridges and multiyear ice. At the seaward edge of
The Chukchi Sea (Alaskan)
the fast ice, at approximately the 20-m isobath, is a
The U.S. portion of the Chukchi Sea is divided
shearing zone marking the transition to highly
into three different zones of severity. Zone 7, adja-
dynamic pack ice that constitutes zones 11 and 12.
cent to the Alaskan coastline, is a region of rela-
Shear ridges in this transition zone are larger and
tively stable and level shorefast ice ranging in thick-
more extensive than anywhere else on the Alas-
ness from 155 to 190 cm. Zones 9 and 10 roughly
kan coastline. Both zones have an abundance of
divide the Chukchi at 69N latitude. The ice con-
very large (several km in diameter) multiyear floes
ditions in zone 9 are generally more serious than
and first-year rubble that produces massive pres-
those found anywhere in the Bering (except zone
sure ridges. Zone 12 experiences a greater fre-
13) and include pressure ridges, thick rubble ice,
quency and intensity of pressured ice conditions
and ice pressure. Zone 10 conditions are made
because of its relative position with Alaska's north
more serious still by the greater concentration of
coast and prevailing northeasterly winter winds.
drift ice that enters from the Beaufort Sea. The
February air temperatures along the coast average
28 to 30C. Summertime air temperatures aver-
drifting ice ranges from 150-cm-thick first-year ice
age 2 to 6C. Because ice transit operations there
to 9-m-thick multiyear floes.
Freezeup in the northeast Chukchi begins in
are so demanding, refueling stops for nonnuclear-
mid-September to early October and progresses
fueled icebreakers along the way would be re-
southwestward. The U.S. Coast Pilot manual rec-
quired where none currently exist. The median
ommends that southbound ships be south of the
date for opening the cargo transport season around
Bering Strait by early November. Two winter de-
Barrow to Prudhoe Bay is August 2, but it has
29
Previous Page
