Geoscience Reference
In-Depth Information
Figure 7.3.
The effect of salinity on the
temperature of maximum density (dashed
curve) and the freezing point temperature
(solid curve) (from Maykut,
1985
, by
permission of Applied Physics Laboratory,
University of Washington, Seattle, WA).
the surface layer is the Atlantic layer. The Atlantic layer is comparatively warm, with
temperatures above 0°C. It represents a potential source of ocean heat to the surface,
which would inhibit ice formation. But at the low temperatures on the Arctic Ocean,
the vertical density structure is determined by salinity, rather than temperature. The
halocline between the surface and warm Atlantic water hence acts as a strong den-
sity gradient (pycnocline) that suppresses vertical mixing.
In the Arctic Ocean, the depth to which the water must be cooled before freezing
can commence (Z
c
) is typically 10-40 m, although values in excess of 70 m have
been observed. Where Z
c
is 50 m, there can be a delay of up to two months in the
date of initial ice formation compared to where Z
c
is 10 m. The surface mixed layer
varies seasonally. As new ice forms in autumn, brine is rejected from the ice, which
mixes downward, increasing the density of the surface layer and weakening the
pycnocline. In summer, ice melt freshens the surface layer. This enhances the den-
sity stratification in the uppermost 30-50 m, strengthening the pycnocline, further
decoupling heat exchange with Atlantic water (Carmack,
1990
).
7.1.2
Primary Sea Ice Types
Sea ice is a complex material, comprising a solid component of ice crystals, a liq-
uid component of brine solution in pockets, and a gaseous component of air pock-
ets. Although there are many different types of sea ice (World Meteorological
Organization,
1989
), some of which will be introduced in the next section, there
are several basic categories. First-year ice (FYI), which historically has comprised
about 40 percent of the Arctic sea ice cover (Rothrock and Thomas,
1990
; Romanov,
1991
), represents the ice growth of a single season. In the broadest definition, new,
or young ice, falls into the FYI category. The growth and melt of first-year ice in
the marginal seas is primarily responsible for the large seasonal variations in total
ice extent shown in
Figure 2.4
. Away from coastal regions, about 60 percent of the
ice cover has historically been represented by second-year ice which has survived
one summer melt season and ice that has survived more than one melt season. For
operational applications and remote-sensing mapping, all ice that has survived at
least one melt season is classified as multiyear ice (MYI). MYI may typically attain
a thickness of 3.5-4.5 m thick, whereas FYI typically gets up to about 2 m, limiting
