Geoscience Reference
In-Depth Information
Figure 7.30
Schematic map of the western North Atlantic showing the major types of ocean surface circulation.
Source
: From Tolmazin (1994) Copyright © Chapman and Hall.
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°C over 50 km
horizontally at the surface and weak gradients are dis-
tinguishable to several thousand metres' depth. Fronts
also form between shelf water and deeper waters where
there is convergence and downwelling.
Another large-scale feature of ocean circulation,
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)
(3) Mesoscale
Mesoscale eddies and rings in the upper ocean are
generated by a number of mechanisms, sometimes
by atmospheric convergence or divergence, or by the
casting off of vortices by currents such as the Gulf
Stream where it becomes unsteady at around 65°W
(Figure 7.30). Oceanographic eddies occur on the
scale of 50 to 400 km in diameter and are analogous
to atmospheric low- and high-pressure systems. Ocean
mesoscale systems are much smaller than atmospheric
depressions (which average about 1000 km in diameter),
travel much slower (a few kilometres per day, compared
with about 1000 km per day for a depression) and persist
from one to several months (compared with a depression
life of about a week). Their maximum rotational veloc-
ities occur at a depth of about 150 m, but the vortex
circulation is observed throughout the thermocline (
ca
.
1000 m depth). Some eddies move parallel to the main
