Geoscience Reference
In-Depth Information
equilibrium thicknesses of 3.5-4.5 m while surviving ten years or more,
is complicated by thermal history, seen as nonlinear vertical temperature
gradients. Ice melts at both its top and bottom surfaces, with the process
again less straightforward for thick ice. For example, in October when
thin ice is growing quickly in leads, thick ice may still be decreasing in
thickness owing to bottom melt, the reason being that the autumn cooling
has not yet affected the lowest part of the ice. Over the past several
decades, the Arctic has lost much of its thickest and oldest multiyear ice.
The Arctic sea ice cover can be separated into different zones - the
Perennial Ice Zone, the Seasonal Ice Zone, the Marginal Ice Zone, the
Fast Ice Zone, and the Shear Zone. Apart from areas of fast ice locked
to the shoreline, the ice cover is in a state of near constant motion. The
large-scale annual mean drift pattern is characterized by the clockwise
Beaufort Gyre, centered in the Canada Basin, and a mean drift of ice from
the Siberian coast, across the pole and through the Fram Strait, known
as the Transpolar Drift Stream. This pattern is maintained by roughly
equal contributions from winds and ocean currents. On shorter (monthly
to daily) time scales, variations in ice motion are primarily wind-driven.
Ice deformation, characterized by shear and divergence that results from
winds and ocean currents that exert forces on the ice, is associated with
the formation of leads, polynyas, pressure ridges and underlying keels.
For example, the mean Beaufort Gyre circulation tends to push ice up
against the coasts of the Canadian Arctic Archipelago and Greenland. As
a result of ridging, the thickest ice in the Arctic tends to be found in this
region. Ice is thinner along the Siberian shelf seas, where the prevailing
winds and current transport newly formed ice offshore. Thinner ice
then forms along the coast in open water areas. Anomalies in ice extent,
concentration, and thickness represent responses to winds, ocean currents,
and both atmospheric and oceanic thermodynamic forcing, superposed
on downward linear trends in all months, smallest in winter and largest in
September.
Figure 7.1
provides a schematic of sea ice types, processes,
and morphology that are addressed throughout this chapter.
In winter, the North Atlantic, especially the Greenland Sea, is an area
of deepwater production, which is important for driving the ocean's
thermohaline circulation. A key observation bearing on why deepwater
production occurs in the North Atlantic but not in the North Pacific is that
surface salinities are higher in the North Atlantic. Although the processes
are still not completely understood, atmospheric transfers of freshwater
from the Atlantic to the Pacific basin continually sharpen the salinity
difference, and ocean transfers work to destroy the salinity difference.
The Arctic Ocean serves as a “back door” by which relatively fresh water
is transported through the Bering Strait and into the North Atlantic via the
Fram Strait and the Canadian Arctic Archipelago. But the Fram Strait ice
and ocean flux is sensitive to the regional atmospheric circulation and to
