El Niño

El Niño, oceanic and atmospheric phenomenon in the Pacific Ocean, during which unusually warm ocean conditions appear along the western coast of Ecuador and Peru, causing climatic disturbances of varying severity. The term originally was used to describe the warm southward current that appears in the region every December, but it is now reserved for occurrences that are exceptionally intense and persistent. These occur every three to seven years and can affect climates around the world for more than a year. The name El Niño, Spanish for “the child,” refers to the infant Jesus Christ and is applied because the current usually begins during the Christmas season. Because a fluctuation in air pressure and wind patterns in the southern Pacific accompanies El Niño, the phenomenon is known as the El Niño Southern Oscillation, or ENSO.

The climate disturbances caused by El Niño occur when sea surface temperatures in the southeastern tropical Pacific are unusually high. Normally, the warm waters are confined to the western tropical Pacific, with temperatures more than 10 Celsius degrees (18 Fahrenheit degrees) higher than the eastern waters of coastal Peru and Ecuador. The air pressure is quite low over the warmer waters. Moist air rises in the region, causing the clouds and heavy rainfall characteristic of southeastern Asia, New Guinea, and northern Australia. In the eastern Pacific, the water is cold and air pressure is high, creating the typically arid conditions along coastal South America. The trade winds blow from east to west, pushing sun-warmed surface waters westward and exposing cold water to the surface in the east.

During El Niño, however, the easterly trade winds collapse or even reverse. As the slight weakening of the winds causes a modest change in sea surface temperatures, the change in wind and pressure increases. The warm water of the western Pacific flows back eastward, and sea surface temperatures increase significantly off the western coast of South America. As this happens, the wet weather conditions normally present in the western Pacific move to the east, and the arid conditions common in the east appear in the west. This brings heavy rains to South America and can cause droughts in southeastern Asia, India, and southern Africa. It can also bring unusual weather to large parts of the United States.

Economic effects of El Niño are felt particularly in coastal Peru and Ecuador. These cold-water zones normally support large populations of fish, especially anchovies. The fish are caught commercially and also provide food for seabirds, whose guano is an important component of the regional fertilizer industry. However, during El Niño a layer of warmer, nutrient-depleted water from the west covers the nutrient-rich eastern coastal waters. The fish and birds die or leave the area in search of food, thus upsetting the economy of the region.

The El Niño events that began in 1982 and in 1997 were the most severe of the 20th century. Other recent occurrences began in 1972, 1976, 1987, 1991, and 1994.

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