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these data he expleins that El Nino originates d.ring the transition from one system of
atmoepheric circuletion to another. In the first circulation system, the easterly and
sduth-eagterly winds in the equatorial region of the Pecific becomes stronger, as does the
South Eq.ratorial Cument which flows east-west to the south of the equator (Figure I). The
reautt of these conditions is an accumulation of warm water, the sinking of the thermoeline
and a rise in the level of the western Pacific, while the tenperature of the eastern Pacific,
particularly along the coast of Peru, remaing low. This situetion in the atmosphere and the
ocean develops slowly and may last for several years (Figure 2 A). In the second circulation
system, the eaeterly and south-easterly winds are light, the South Equatorial Current is slow,
the therrnoeline in the eastern Pacific einks and the sea tenperature is high. This situation is
typical of the El Nino periods (Figure 2 B).
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Figure I : The ocean cument system in the Pacific
(adapted from Rand McNally Atlas of the Oceans)
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El Nino originates during a sharp change from the first circulation system to the
second, i.e., urhen the easterly and south-easterly winds suddenly lighten or die after being
abnormally strong. This abrupt change, which occurs mainly in the equatorial central Pacific,
generates a disturbence in the oeean in the form of a wave, known as a Kelvin Equatorial
Wave, which takes approximaLely three months to reach the eas[ coast. When this wave
reaches the South Anprican coast, the thermocline is at a greater depth and warm water
aecumulates off the coast. The presence of this warm water (at least 2oC above normal) off
the coast of Ecuador and Peru for more than f our months is known as the El Nino
phenomenon.