Environmental Engineering Reference
In-Depth Information
Figure 2.9
Sea surface temperature (SST) anomalies in the Pacific Ocean: December 1982
Source:
After Rasmusson and Hall (1983)
(see Figure 2.8). It has a periodicity of one to
five years, and in its wake it brings changes in
wind fields, sea surface temperatures and ocean
circulation patterns. When pressure is high over
Tahiti and low over Darwin, the general wind
and surface water flow is from east to west, and
there is a tendency for relatively warm water to
pond up at the western end of the Pacific Ocean.
The removal of the warm surface water from
the eastern Pacific allows the upwelling of
relatively cold water from below in that area.
These conditions are reversed following the
Oscillation. Easterly winds are replaced by the
westerlies, as the atmospheric pressure changes,
and warm water begins to flow east again to
replace the cold (see Figure 2.9). It is this
phenomenon which has come to be called El
Niño. When it is strongly developed, it may
keep areas in the eastern Pacific warmer than
normal for periods of up to a year (Rasmusson
and Hall 1983), but its influence extends far
beyond the immediate area (see Chapter 3).
During some years—when the difference
between the high pressure over Tahiti and low
pressure over Darwin is particularly well-marked,
for example—the equatorial easterly winds are
stronger than normal, and push the cold water
upwelling off the South American coast far across
the Pacific. This creates a cold current, flowing
east to west, which has been given the name La
Niña. Like El Niño, it appears to have an effect
outside the region, but as a recently identified
phenomenon (1986), its full climatological
significance is as yet uncertain (Hidore and Oliver
1993).
In addition to its direct role in energy
transfer, the oceanic circulation also contributes
to the earth's climate through its participation
in the global carbon cycle. The oceans contain
close to 80 per cent of the earth's total carbon at
any one time. It is held in active storage, and is
transferred between the oceanic and
atmospheric reservoirs in the form of carbon
dioxide. Horizontal and vertical mixing within
the oceans helps to control the rate of that
exchange, which impacts on the greenhouse
effect and therefore on the earth's energy budget
(Taylor 1992).
The circulation of the atmosphere
George Hadley developed the classic model of
the general circulation of the atmosphere some
250 years ago. It was a simple convective system,
