Geoscience Reference
In-Depth Information
temperature, but by the vertical profile of salinity - lower salinity means lower den-
sity. Consequently, the halocline is associated with a strong increase in density with
depth, known as a “pycnocline.” As density increases strongly with depth, the upper
Arctic Ocean is very stably stratified, which inhibits vertical mixing. The impor-
tance of the strong “cold Arctic halocline” - so-called because of the associated low
temperatures - cannot be overstressed. As will be examined further in
Chapter 7
,
the fresh surface layer and limited vertical mixing with the warmer waters below is
the key feature of the Arctic Ocean that - along with low winter air temperatures -
allows sea ice to form readily.
Maintenance of the fresh, low-density surface layer (and, in turn, the cold
Arctic halocline) is in large part determined by river input. The importance of this
river inflow is demonstrated by the fact that the Arctic Ocean contains only about
1 percent of the global volume of seawater, yet it receives about 11 percent of the
world's river flow, which peaks in June because of melt of the terrestrial snow-
pack (see
Chapter 6
). The Eurasian drainages of the Ob, Yenisey, and Lena, along
with the Mackenzie River in Canada, supply two-thirds of the total river discharge.
Figure 2.8
gives the boundaries of the Arctic terrestrial drainage and these four
major individual watersheds. The Arctic drainage, which extends well into middle
latitudes, is defined in the figure as all areas draining into the Arctic Ocean as well
as into Hudson Bay, James Bay, the Hudson Strait, the Bering Strait, and the north-
ern Bering Sea. The influence of river inflow is immediately evident in the map of
mean surface salinity for summer (
Figure 2.9
)
. The freshwater discharged by the
major rivers during early summer gives rise to very low salinity values along the
Siberian continental shelves and near the Mackenzie delta; this fresh water in turn
mixes outward to impact much of the central Arctic Ocean. The contrast with the
winter mean surface salinity field when river discharge is small (
Figure 2.10
) is
obvious.
Another contributor to the fresh, low-density surface layer is the influx of
relatively low salinity waters from the Pacific Ocean into the Arctic Ocean
through the Bering Strait. It is the influence of closer proximity to river runoff
and Pacific-derived waters that gives rise to the lower surface salinities in the
Beaufort Sea example in
Figure 2.7
. A further significant contribution to the
fresh surface layer is made by net precipitation (precipitation less evaporation)
over the Arctic Ocean itself. The salinity of the surface layer is also influenced
by the growth and melt of the sea ice cover. As sea ice forms, brine is rejected.
Although this brine rejection explains why the sea ice itself is quite fresh, the
density of the underlying surface water layer increases as does the depth of
vertical mixing. By contrast, as ice melts in summer, the surface layer becomes
fresher, and vertical mixing is inhibited.
Understanding the observed vertical ocean structure also requires us to consider
the inflow of Atlantic-derived waters.
Figure 2.11
is a simple schematic of the mean
surface and deep ocean currents in the Arctic Ocean. First, it is necessary to provide
a brief word on the surface currents, setting the stage for more detailed investiga-
tions in
Chapter 7
. Immediately obvious is the surface/near surface flow entering the
