Geoscience Reference
In-Depth Information
Figure 7.2.
Temperature versus density
plot for fresh water and ice at standard
atmospheric pressure (from Maykut,
1985
,
by permission ofApplied Physics Laboratory,
University of Washington, Seattle, WA).
Imagine that winter is approaching, and that the temperature in a fresh water col-
umn (e.g., in a lake) is initially higher than the maximum density temperature T
m
.
The water column is cooling from the top, which sets up a vertical temperature gradi-
ent in the water. However, as density increases as temperature decreases, the cooling
destabilizes the column. This generates vertical convection, mixing the cooler surface
water downward and the warmer water at depth upward. This process continues until
the entire water column reaches the temperature of maximum density. From this point
on, any further surface cooling is associated with a decrease in density. This allows
the water column to become stably stratified, meaning that the lower density water is
at the top. Once the column is stably stratified, conduction, rather than convection,
dominates the heat transport. As compared to convection, conduction is a compara-
tively inefficient mechanism for heat transport. Because it is difficult to bring up warm
water from lower depths, surface cooling can proceed quickly until the freezing point
(T
f
) is reached. Further cooling results in a slight supercooling and ice formation pro-
ceeds. Because of the stable stratification, ice can readily form at the surface, even
though the bulk of the water column is significantly above the freezing point.
If salt is added, the situation changes radically. With salinities below 24.7 psu,
the temperature of maximum density is higher than the water's freezing point. At
salinities above this value, the temperature of maximum density equals the freez-
ing point (
Figure 7.3
)
. Furthermore, the presence of salt in solution depresses the
freezing point of water. For water with a typical ocean salinity of about 35 psu, the
freezing point is −1.8°C. Imagine that it is autumn in the Arctic Ocean. As in the
case of a freshwater body, cooling of the water surface initially results in a density
increase and vertical mixing. The plot in
Figure 7.2
could lead one to believe that
the entire water column would have to be cooled to the salinity adjusted freezing
point before ice could form. Because much of the Arctic Ocean is 2-3 km deep, ice
formation would seem to be rather difficult. The reason it can readily form is that
there is strong, preexisting stability in the upper ocean, which limits the depth of
water that needs to be cooled.
The basis of this salinity structure was examined in
Chapter 2
. To reiterate, the
low-salinity surface layer (
Figure 2.7
) is maintained by discharge from the rivers
draining into the Arctic Ocean, the inflow of comparatively low-salinity waters
through the Bering Strait, and positive net precipitation over the Arctic Ocean.
Seasonally, summer melt of the ice is important (see following discussion). Beneath
