Geoscience Reference
In-Depth Information
and in both cases they are overlain by large-scale
circulation vortices in the mid-troposphere and
above (see
Figures 7.3
and
7.4
). In many other
respects, the two polar regions differ markedly
owing to geographical factors. The north polar
region comprises the Arctic Ocean, with its
almost year-round sea ice cover (see
Plate 13.5
),
surrounding tundra land areas, the Greenland Ice
Sheet and numerous smaller ice caps in Arctic
Canada, Svalbard and the Siberian Arctic Islands.
In contrast, the south polar region is occupied by
the Antarctic continent, with an ice plateau 3 to
4km high, floating ice shelves in the Ross Sea and
Weddell Sea embayments, and surrounded by a
seasonally ice-covered ocean. Accordingly, the
Arctic and Antarctica are treated separately.
Eurasian sector, moves across the North Pole in
the Transpolar Drift Stream and exits the Arctic
via Fram Strait and the East Greenland Current
(see
Figure 10.35A
). This export largely balances
the annual thermodynamic ice growth in the
Arctic Basin. In late summer, the Eurasian shelf
seas and the coastal section of the Beaufort Sea are
mostly ice-free.
In summer, the Arctic Ocean has mostly
overcast conditions with low stratus and fog.
Snowmelt and extensive meltwater puddles on
the ice keep air temperatures around freezing.
Low pressure systems tend to predominate,
entering the basin from either the North Atlantic
or Eurasia. Precipitation may fall as rain or snow,
with the largest monthly totals in late summer to
early autumn. However, the mean annual net
precipitation minus evaporation over the Arctic,
based on atmospheric moisture transport calcula-
tions, is only about 180mm.
On Arctic land areas, there is a stable snow
cover from mid-September until early June, when
melt occurs within 10-15 days. As a result of the
large decrease in surface albedo, the surface energy
budget undergoes a dramatic change to large
positive values (
Figure 10.38
). The tundra is
generally wet and boggy as a result of the
permafrost table
only 0.5-1.0m below the surface,
which prevents drainage. Thus the net radiation
is expended primarily for evapotranspiration.
Permanently frozen ground is over 500m thick in
parts of Arctic North America and Siberia and
extends under the adjacent Arctic coastal shelf
areas. Much of the Queen Elizabeth Islands,
the Northwest Territories of Canada and the
Siberian Arctic Islands is cold, dry polar desert,
with gravel or rock surfaces, or ice caps and
glaciers. Nevertheless, 10-20km inland from
the Arctic coasts in summer, daytime heating
disperses the stratiform cloud and afternoon
temperatures may rise to 15-20°C.
The Greenland ice sheet, 3km thick and
covering an area of 1.7 million km
2
, contains
enough water to raise global sea level by over 7m
if it were all to melt. However, there is no melting
The Arctic
At 75°N, the sun is below the horizon for about
90 days, from early November until early
February. Winter air temperatures over the Arctic
Ocean average about -32
C, but they are usually
10-12°C higher some 1000m above the surface as
a result of the strong radiative temperature
inversion. The winter season is generally stormy
in the Eurasian sector, where low pressure systems
enter the Arctic Basin from the North Atlantic,
whereas anticyclonic conditions predominate
north of Alaska over the Beaufort and Chukchi
seas. In spring, high pressure prevails, centered
over the Canadian Arctic Archipelago-Beaufort
Sea.
The average 2 to 4m thickness of sea ice in the
Arctic Ocean permits little heat loss to the
atmosphere and largely decouples the ocean and
atmosphere systems in winter and spring. The
winter snow accumulation on the ice averages
0.25-0.30m depth. Only when the ice fractures,
forming a
lead
, or where persistent offshore winds
and/or upwelling warm ocean water form an area
of open water and new ice (called a
polynya
), is the
insulating effect of sea ice disrupted. The ice in the
western Arctic circulates clockwise in a gyre driven
by the mean anticyclonic pressure field. Ice from
the northern margin of this gyre, and ice from the
°
