Geoscience Reference
In-Depth Information
80°W
70°W
60°W
50°W
40°W
Anticyclonic eddies
40°N
40°N
Cyclonic ring
Extension rings
35°N
35°N
30°N
30°N
Small
eddies
Mesoscale eddies
25°N
25°N
20°N
20°N
T
80°W
70°W
60°W
50°W
40°W
Figure 7.30
Schematic map of the western North Atlantic showing the major types of ocean surface
circulation.
Source: From Tolmazin (1985). Copyright © Chapman & Hall.
200km for the Benguela Current), the Ekman
effect spreads this cold water westward. On the
poleward margins of these cold-water coasts, the
meridional swing of the wind belts imparts a
strong seasonality to the upwelling; the California
Current upwelling, for example, is particularly
well marked during the period March to July.
A major region of deep-water upwelling is
along the West Coast of South America (
Figure
11.52
) where there is a narrow 20km-wide shelf
and offshore easterly winds. Transport is offshore
in the upper 20m but onshore at 30-80m depth.
This pattern is forced by the offshore airflow
normally associated with the large-scale convec-
tive Walker cell (see Chapters 7C.1 and 11G)
linking Southeast Asia-Indonesia with the eastern
South Pacific. Every two to ten years or so this
pressure difference is reversed, producing an El
Niño event with weakening Trade Winds and a
pulse of warm surface water spreading eastward
over the South Pacific, raising local sea surface
temperatures by several degrees.
Coastal upwelling is also caused by less
important mechanisms such as surface current
divergence or the effect of the ocean bottom
configuration (see
Figure 7.31
).
Deep ocean circulation
Above the permanent thermocline the ocean
circulation is mainly wind driven, while in
the deep ocean it is driven by density gradients
due to salinity and temperature differences - a
thermohaline
circulation. These differences are
mostly produced by surface processes, which feed
cold, saline water to the deep ocean basins in
compensation for the deep water delivered to the
