El Nio and la

THE WATERs Along the barren coast of Peru are cold and flow northward during most of the year, but around Christmas time, they are warm and flow southward. The latter current was originally given the name El Niño, Spanish for "the boy." Because of its timing, and because it is associated with refreshing rains, the name also refers to Child Jesus.

Every few years the current is exceptionally intense and persistent, bringing very heavy rains that transform parts of the coastal desert into a garden. At such times the fish (such as anchovies) that usually are abundant in the cold water, disappear temporarily. Today, the term El Niño is reserved for these interan-nual events which now are perceived as disasters even though they originally were welcomed as blessings. El Niño was originally regarded as a regional phenomenon, confined to the shores of Peru, but is now recognized as part of the changes in oceanic conditions across the entire tropical Pacific Ocean.

Furthermore, El Niño is not a sporadic departure from "normal" conditions, but is one phase of a continual oscillation with a period of 3 to 5 years; the complementary phase is known as La Niña. (This oscillation is evident in a record of sea surface temperature variations in the eastern equatorial Pacific over the past century. The figures show the spatial structures of El Niño and La Niña.)

The fluctuations in oceanic conditions are in response to fluctuations in the trade winds, which are intense during La Niña, weak during El Niño. Why do the winds change?

Early in the 20th century, Gilbert Walker's attempts to predict failures of the monsoons in India led to his discovery of the Southern Oscillation, which includes oscillations in the intensity of the trade winds over the tropical Pacific. Those oscillations, it turns out, are the ones that induce El Niño and La Niña. From a meteorological perspective, the changes in the wind patterns are a consequence of the changes in the sea surface temperature patterns associated with El Niño and La Niña.

This circular argument—sea surface temperature changes are both the cause and the consequence of changes in the winds—implies that interactions between the ocean and atmosphere are at the heart of the matter. Those interactions are unstable, capable of amplifying small, random disturbances, such as a burst of strong winds, into a major climate fluctuation.

To ask why El Niño (or La Niña) occurs is as meaningless as asking why a pendulum swings. It is more interesting to inquire about the factors that determine the period of the oscillations—the width of the ocean basin is an important one—and to explore the predictability of El Niño.

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