What Is an El Niño?
The waters off the west coast of South America are normally very cold for their latitudes. At Lima, Peru (12°S), ocean temperatures vary from about 16°C in the winter to about 20°C in the summer. This cold current of water extends nearly to the equator before turning to the west. Normally, for a period of just a few weeks around Christmas each year, this cold water is replaced by a warm current. This event is called El Niño, Spanish for "the child." Every 2 to 7 years, however, this warm water event lasts much longer and is much more pronounced. Then it is called a major El Niño event.
For years, most scientists viewed any El Niño strictly as a regional event, but during the last half century they have realized that major El Niños are correlated with weather events in distant regions of the world. The El Niño of 1982-83 provided a tremendous amount of data concerning this phenomenon since this El Niño was the most pronounced of the century.
During a major El Niño, the normally cold water off the west coast of South America becomes much warmer, exceeding the normal temperatures by several degrees, while the waters in the western Pacific cool. These changes are accompanied by a change in the surface atmospheric pressure patterns. The relatively high pressure in the eastern Pacific and the relatively low pressure in the west are reversed. This periodic reversal in the pressure pattern has been recognized for many years and is called the Southern Oscillation. Fairly recently, the connection between these two events has been noted. As a result, the term ENSO, for El Niño Southern Oscillation, was coined.
Frequently, a major El Niño event is referred to as the warm phase of an ENSO. It is accompanied by a decrease in speed and sometimes a reversal of the easterly tradewinds. A study of the 1982-83 El Niño and historical records showed that the effects of an El Niño had gone far beyond the Pacific and had probably influenced events worldwide. Teleconnections is the term used to describe the link between atmospheric/oceanic events in one region of the world to events in another region.
The figures show water temperatures during El Niño and non-El Niño years. Note how the cold water reaches the surface in the eastern Pacific in an El Niño year, but falls below the surface in a non-El Niño year. Graphics: Courtesy of NOAA.
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