Geoscience Reference
In-Depth Information
convergence zone at about 50°S marked by
westerly submarine jet streams reaching velocities
of 0.5 to 1m s
-1
. South of the West Wind Drift,
the Antarctic Divergence with rising water is
formed between it and the East Wind Drift closer
to Antarctica. In the Northern Hemisphere, a
great deal of the eastward-moving current in
the Atlantic swings northwards, leading to
anomalously very high sea temperatures, and is
compensated for by a southward flow of cold
arctic water at depth. However, more than half of
the water mass comprising the North Atlantic
Current, and almost all of that of the North Pacific
Current, swings south around the east sides of
the subtropical high pressure cells, forming the
Canary and California currents. Their Southern
Hemisphere equivalents are the Benguela,
Humboldt (or Peru), and West Australian
currents (
Figure 7.29
).
Ocean fronts are associated particularly with
the poleward margins of the western boundary
currents. Temperature gradients can be 10
analogous to atmospheric low and high pressure
systems. Ocean mesoscale systems are much
smaller than atmospheric depressions (which
average about 1000km diameter), travel much
slower (a few kilometers per day, compared with
about 1000km per day for a depression) and persist
from one to several months (compared with a
depression life of about a week). Their maximum
rotational velocities occur at a depth of about
150m, but the vortex circulation is observed
throughout the thermocline (
ca.
1000m depth).
Some eddies move parallel to the main flow
direction, but many move irregularly equatorward
or poleward. In the North Atlantic, this produces
a 'synoptic-like' situation in which up to 50 percent
of the area may be occupied by mesoscale eddies
(see
Plate 7.4
). Cold-core cyclonic rings (100-
300km diameter) are about twice as numerous as
warm-core anticyclonic eddies (100km diameter),
and have a maximum rotational velocity of about
1.5m s
-1
. About 10 cold-core rings are formed
annually by the Gulf Stream and may occupy
10 percent of the Sargasso Sea.
C over
50km horizontally at the surface and weak
gradients are distinguishable to several thousand
meters depth. Fronts also form between shelf
water and deeper waters where there is conver-
gence and downwelling.
Another large-scale feature of ocean circula-
tion, analogous to the atmosphere, is the Rossby
wave. These large oscillations have horizontal
wavelengths of 100s-1000s km and periods of
tens of days. They develop in the open ocean
of mid-latitudes in eastward-flowing currents.
In equatorial, westward-flowing currents, there
are faster, very long wavelength Kelvin waves
(analogous to those in the lower stratosphere).
°
2
Deep ocean water interactions
Upwelling
In contrast with the currents on the west sides of
the oceans, equatorward-flowing eastern currents
acquire cyclonic vorticity, which is in opposition
to the anticyclonic wind tendency, leading to
relatively broad flows of low velocity. In addition,
the deflection due to the Ekman effect causes the
surface water to move westward away from the
coasts, leading to replacement by the upwelling
of cold water from depths of 100-300m (
Figure
7.31
). Average rates of upwelling are low
(1-2m/day), being about the same as the offshore
surface current velocities, with which they are
balanced. The rate of upwelling therefore varies
with the surface wind stress. As the latter is
proportional to the square of the wind speed,
small changes in wind velocity can lead to marked
variations in rates of upwelling. Although the
band of upwelling is of limited width (about
Mesoscale
Mesoscale eddies and rings in the upper ocean are
generated by a number of mechanisms, some-
times by atmospheric convergence or divergence,
or by the casting off of vortices by currents such
as the Gulf Stream where it becomes unsteady
around 65
W (
Figure 7.30
). Oceanographic eddies
occur on the scale of 50-400km diameter and are
°
