Geoscience Reference
In-Depth Information
the amount of ice that can form in one winter season. Historically, some MYI floes
have been observed to be ten years or more in age. As sea ice forms, salt in the form
of brine is rejected. Typical mean salinities for FYI thicker than 1 m range from 2 to
6 ppt. Owing to gravity-induced brine drainage, the salinity of MYI is considerably
lower than that of FYI (Weeks and Ackley,
1986
), with salinity varying with depth,
being lowest in the top part of the floe. Old MYI is essentially fresh water. The small
amount of remaining salt is trapped in brine pockets.
Ice age can be tracked via remote sensing techniques.
Figure 7.4
shows the spa-
tial pattern of ice age for September 2012 and 2007 (2007 having the second lowest
extent in the satellite record) along with the time history of September ice age clas-
ses for the Arctic as a whole back to 1983. The time history shows an overall tran-
sition toward both less ice and less coverage by the older ice age classes; especially
notable is the decline in the coverage of MYI that is four years or older.
7.1.3
Sea Ice Growth and Melt
Reviews of sea ice growth and melt processes are given by Maykut (
1985
,
1986
),
W. Weeks and S. Ackley (
1986
), C. Parkinson et al. (
1987
), Barry et al. (
1993
),
P. Wadhams (
2000
), and Barry and Gan (
2011
). Initial ice formation occurs at the
surface as small platelets an needles termed frazil ice. Frazil crystals are generally
less than 3-4 mm in size. Continued cooling forms a slurry of unconsolidated frazil
crystals, termed grease ice. Under calm conditions, frazil crystals freeze together
forming a solid, continuous ice cover of 1-10 cm thickness. However, in the more
typical situation, a solid ice cover is inhibited by wind-induced turbulence in the
water. Winds and waves advect frazil ice downwind where accumulations of up to 1
m thick can form in front of obstacles (e.g., existing ice floes).
Once the fraction of ice-covered area exceeds 30-40 percent, there is sufficient
bonding between individual ice crystals to drastically reduce their mobility, ini-
tiating the transition to a solid ice cover. In the ocean, this is usually seen in the
transition to pancake ice, rounded masses of semi-consolidated slush of 0.3 to 3.0
m in diameter formed by wind and wave action. Initially, the ice remains near the
salinity-adjusted water freezing point, with large turbulent heat and longwave radia-
tion losses from the surface resulting in rapid ice formation. As the ice thickens, the
ice surface temperature declines. Additional growth then occurs mainly by bottom
accretion. As outlined in
Chapter 3
, the latent heat release from ice growth repre-
sents a significant source of heat to the Arctic atmosphere during winter.
Field investigations have shown that the growth rate of young sea ice (less than
100 cm) is closely related to the cumulative number of freezing degree days. A
number of empirical formulae have been developed to express this relationship (see
Maykut,
1986
), for example:
H = 1.33θ
0.58
(Lebedev,
1938
)
(7.1)
H
2
+ 5.1H = 6.7θ (Anderson,
1961
)
(7.2)
H
2
+ 50H = 8θɪ (Zubov,
1945
)
(7.3)
