Geoscience Reference
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Figure 7.31 Schematic illustration of
mechanisms that cause ocean upwelling.
The large arrows indicate the dominant
wind direction and the small arrows the
currents. (A) The effects of a persistent
offshore wind. (B) Divergent surface
currents. (C) Deep-current shoaling.
(D) Ekman motion with coastal blocking
(northern hemisphere case).
Source : Partly modified after Stowe (1983)
Copyright 1983 © John Wiley & Sons, Inc.
Reproduced by permission.
flow direction, but many move irregularly equatorward
or poleward. In the North Atlantic, this produces a
'synoptic-like' situation in which up to 50 per cent of the
area may be occupied by mesoscale eddies (see Plate
B). Cold-core cyclonic rings (100 to 300-km diameter)
are about twice as numerous as warm-core anticyclonic
eddies (100-km diameter), and have a maximum rota-
tional velocity of about 1.5 m s -1 . About ten cold-core
rings are formed annually by the Gulf Stream and may
occupy 10 per cent of the Sargasso Sea.
width (about 200 km 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 (Plate 16), 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 20-km-wide shelf and offshore
easterly winds. Transport is offshore in the upper 20 m
but onshore at 30 to 80 m depth. This pattern is forced
by the offshore airflow normally associated with the
large-scale convective 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 B, C).
2 Deep ocean water interactions
a 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 to
300 m (Figure 7.31 A, D). Average rates of upwelling
are low (1 to 2 m/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
b 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 temper-
ature differences - a thermohaline circulation. These
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