Marked warming of the eastern Pacific Ocean that occurs when a warm current of water moves from the western Pacific, temporarily replacing the cold Peru Current along the west coast of South America. This results in a reduction in marine plankton, the main food source in the ocean, and fish numbers decline. The atmospheric circulation in the region is also seriously disturbed, and may result in unusual climatic events, for example floods in Peru, and drought in Australia. El Niño events occur at irregular intervals of between two and seven years. El Niño events have increased in frequency and intensity in recent decades. It is unclear whether global warming is responsible or whether the change is part of natural variability.
El Niño frequently occurs around Christmas, and its name, ‘the boy-child’, refers to Jesus. The corresponding cooling of the eastern Pacific that occurs between El Niño periods is called El Niña, ‘the girl-child’.
El Niño is believed to be caused by the failure of trade winds and, consequently, of the ocean currents normally driven by these winds. Warm surface waters then flow in from the east. The phenomenon can disrupt the climate of the area disastrously, and has played a part in causing famine in Indonesia, drought and bush fires in the Galapagos Islands, rainstorms in California, USA, and in South America, and the destruction of Peru's anchovy harvest and wildlife in 1982–83. El Niño contributed to algal blooms in Australia's drought-stricken rivers and an unprecedented number of typhoons in Japan in 1991. It is also thought to have caused the 1997 drought in Australia and contributed to certain ecological disasters such as bush fires in Indonesia.
El Niño usually lasts for about eighteen months, but the 1990 occurrence lasted until June 1995; US climatologists estimated this duration to be the longest in 2,000 years. The last prolonged El Niño of 1939–41 caused extensive drought and famine in Bengal. In-depth weather data on the El Niño effect were not gathered prior to 1980 and this delayed the development of accurate predictions on the effects of the phenomenon.
El Niño was blamed for disastrous weather in February 2003 in Peru, where abnormally high rainfall caused extensive flooding in the southeast of the country, destroying 6,000 homes and displacing 59,000 people.
In a small way, El Niño affects the rotation of the planet. The wind patterns of the 1998 El Niño slowed the Earth's rotation, adding 0.4 milliseconds to each day, an effect measured on the Very Long Baseline Interferometer (VLBI).
By examining animal fossil remains along the west coast of South America, US researchers estimated in 1996 that El Niño events began 5,000 years ago.
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