El Niño

[American Spanish, from Spanish, the Christ child (from its onset being around Christmastide) : el, the (from Latin ille) + niño, child (from Old Spanish ninno , from Vulgar Latin *nīnnus).]

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In general, an invasion of warm water into the central and eastern equatorial Pacific Ocean off the coast of Peru and Ecuador, with a return period of 4–7 years. El Niño events come in various strengths: weak, moderate, strong, very strong, and extraordinary. The size of an El Niño event can be determined using various criteria: the amount of warming of sea surface temperatures in the central and eastern Pacific from their average condition; the areal extent of that warm water anomaly; and the length of time that the warm water lingers before being replaced by colder-than-average sea surface temperatures in this tropical Pacific region.

Under normal conditions the winds blow up the west coast of South America and then near the Equator turn westward to Asia. The surface water is piled up in the western Pacific, and the sea level there is several tens of centimeters above average while the sea level in the eastern Pacific is below average. As the water is pushed toward the west, cold water from the deeper part of the ocean along the Peruvian coast wells up to the surface to replace it. This cold water is rich with nutrients, making the coastal upwelling region along western South America among the most productive fisheries in the world. See also Upwelling.

Every 4–7 years those winds tend to die down and sometimes reverse, allowing the warm surface waters that piled up in the west to move back toward the eastern part of the Pacific Basin. With reduced westward winds the surface water also heats up. Sea level drops in the western Pacific and increases in the eastern part of the basin. El Niño condition can last for 12–18 months, sometimes longer, before the westward flowing winds start to pick up again. Occasionally, the opposite also occurs: the eastern Pacific becomes cooler than normal, rainfall decreases still more, atmospheric surface pressure increases, and the westward winds become stronger. This irregular cyclic swing of warm and cold phases in the tropical Pacific is referred to as ENSO (El Niño Southern Oscillation).

El Niño is considered to be the second biggest climate-related influence on human activities, after the natural flow of the seasons. Although the phenomenon is at least thousands of years old, its impacts on global climate have only recently been recognized. Due to improved scientific understanding and forecasting of El Niño's interannual process, societies can prepare for and reduce its impacts considerably. See also Climatology; Maritime meteorology; Tropical meteorology.

Numerical models that couple the atmosphere to the ocean have been used to successfully predict the sea surface temperature of the tropical Pacific a year or so in advance. The basic reason that the cycle is predictable is that ENSO evolves slowly and regularly. If the initial state of the atmosphere-ocean system can be characterized accurately, the classification of this state in the ENSO sequence is made (even if it is not completely recognizable in each system separately), and the future evolution of the cycle can be predicted. See also Climate modeling.

A disruption along the equatorial Pacific of the east-west Walker circulatory cell. The gradient between the low pressure of the warm, western tropical Pacific and the high pressure of the cold, eastern tropical Pacific decreases, so that the easterly trade winds weaken, allowing the warm surface water to move eastwards. The rising branch of the circulation cell, and therefore the precipitation associated with it, moves with the water. An El Niño is only one element of a dual-phase oscillating ocean-atmosphere system. When the system reverts to its ‘normal’ phase, Pacific waters cool off the coast of Ecuador and Peru, and become warm again in the western Pacific, in the region of Indonesia and northern Australia. Sometimes the eastern Pacific becomes unusually cool; this is a La Niña event. This entire ocean-atmosphere system: El Niño, the ‘normal’ phase, sometimes developing into La Niña, is the El Niño-Southern Oscillation. Dawson and O'Hare (Geography 85) give an exceptionally clear explanation.

These changes in marine and atmospheric circulation caused by an El Niño have been related to abnormally heavy rain in the southern USA and western South America, a decrease in tropical cyclones in the Atlantic, and to droughts in Indonesia and Australia, all of which were witnessed in the 1997-8 El Niño event. This effect occurs about fourteen times per century, but its exact cause is not understood.

El Niño is the unusual warming of the surface waters of large parts of the tropical Pacific Ocean. Occurring around Christmastime, it is named after the Christ child. El Niño occurs erratically every three to seven years. It brings heavy rains and flooding to Peru, Ecuador, and southern California, and a milder winter with less snow to the northeastern United States. Studies reveal that El Niño is not an isolated occurrence, but is instead part of a pattern of change in the global circulation of the oceans and atmosphere. The 1982-83 El Niño was one of the most severe climate events of the twentieth century in both its geographical extent as well as in the degree of warming (14°F or 8°C).

A warming of the surface water of the eastern and central Pacific Ocean, occurring every four to twelve years when cold water does not rise to the surface, causing unusual weather patterns. The warmer water kills fish and plankton, brings heavy rains to western South America, and causes drought in eastern Australia and Indonesia.