Geoscience Reference
In-Depth Information
A
C
Normal January
December ENSO event
B
D
September pre-ENSO
Post ENSO
Jet stream
Warm surface
water
Wet or under
flood
Surface winds
Very warm
Surface water
Dry or in
drought
Tropical cyclone
Relative pressure
Idealized representation of evolution and effects of an El Niño-Southern Oscillation event.
Fig. 2.7
the west Pacific warm pool in the Philippine Sea. The
supra-elevation of sea level in the west Pacific Ocean
is approximately 1 cm for each 1°C difference in
temperature between the west Pacific and the
South American coast. Normally this supra-elevation
amounts to 13-20 cm.
This atmospheric system is very stable, existing
beyond the annual climatic cycle. The easterly trade
wind flow is termed
Walker circulation
, after the
Indian meteorologist, Gilbert Walker, who described
the phenomenon in detail in the 1920s-1930s. For
some inexplicable reason, this quasi-stationary heating
process weakens in intensity, or breaks down com-
pletely, every 3-5 years - a frequency very suggestive
of a chaotic system. High pressure can become estab-
lished over the Indonesian-Australian area, while low
pressure develops over warm water off the South
American coast. In the tropics, the easterly winds abate
and are replaced by westerly winds. The rainfall belt
shifts to the central Pacific and drought replaces
normal or heavy rainfall in Australia. Such conditions
persisted in the Great World Drought of 1982-1983,
which affected most of Australia, Indonesia, India, and
South Africa.
Because it tends to fluctuate, this phenomenon is
called the
Southern Oscillation
(SO). Simply subtract-
ing the barometric pressure for Darwin from that of
Tahiti produces an index of the intensity and fluctua-
tions of the SO. This value is then
normalized
to a
mean of 0.0 hPa and a standard deviation of 10. This
normalized index, from 1851 to the present, is pre-
sented in Figure 2.8.
Warm water usually appears (albeit in reduced
amounts) along the Peruvian coast around Christmas
each year. This annual warming is termed the
El Niño
,
which is Spanish for 'Christ Child'. However, when
Walker circulation collapses, this annual warming
becomes exaggerated, with sea surface temperatures
increasing 4-6°C above normal and remaining that
way for several months. This localized, above average,
