This article is about Pacific ocean temperature anomalies.

Chart of ocean surface temperature anomaly [°C] during the last strong El Niño in December 1997
Chart of ocean surface temperature anomaly [°C] during the last strong El Niño in December 1997

El Niño and La Niña (Spanish for "boy" and "girl", often written in English as El Nino and La Nina) are major temperature fluctuations in the tropical Pacific Ocean. They are Pacific signatures of the global ENSO phenomenon (El Niño-Southern Oscillation). Their effect on climate in the southern hemisphere is profound. Their role in global warming or cooling is an area of active research, with no clear consensus yet.


El Niño

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El Niño Conditions. Warm water pool approaches South American coast. Absence of cold upwelling increases warming. (NOAA / PMEL / TAO)
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Normal Pacific pattern. Equatorial winds gather warm water pool toward west. Cold water upwells along South American coast.
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La Niña Conditions. Warm water is further west than usual.

El Niño is officially defined as sea surface temperature anomalies greater than +0.5°C across the central tropical Pacific Ocean. The name comes from the Spanish name for the Christ child; the name was given to the phenomenon by fishermen working off the coast of Peru and Ecuador, who noticed it often occurs around Christmas. It occurs at irregular intervals of 2-7 years and usually lasts one or two years.

El Niño's warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water of the Humboldt Current which fish prefer. In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and its economic impact to local fishing can be serious.

Recent El Niños have occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, and 2002-2003. A new El Niño has been in effect since September 2004. The El Niño of 1997-1998 was particularly strong, while the period from 1990-1994 was unusual in that El Niños rarely occur in such rapid succession. They were generally weak, however.

Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean.

In South America, the effects of El Niño are direct and stronger than in North America. An El Niño is associated with warm and very wet summers (December-February) along the coasts of northern Peru and Ecuador causing major flooding whenever the event is strong or extreme. The effects during the months of February, March and April may become critical. Southern Brazil and northern Argentina also experience wetter than normal conditions but mainly during the spring and early summer. Central Chile receives a mild winter with large rainfall, and the Peruvian-Bolivian Altiplano is sometimes exposed to unusual winter snowfall events. Drier and hotter weather occurs in parts the Amazon River Basin, Colombia and Central America.

Direct effects of El Niño resulting in drier conditions occur in Indonesia, the Philippines, and northern Australia. Drier than normal conditions are also generally observed along the eastern half of Australia during June-August.

West of the Antarctic Peninsula, the Ross, Bellingshausen, and Amundsen Sea sectors have more sea ice during El Niño. The latter two and the Weddell Sea also become warmer and have higher atmospheric pressure.

In North America, typically, winters are warmer than normal in the upper midwest states and Canada, while central and southern California, northwest Mexico and the southeastern U.S., are wetter than normal. Summer is wetter in the intermountain regions of the U.S. The Pacific Northwest states, on the other hand, tend to be drier during an El Niño. During a La Niña, by contrast, the midwestern U.S. tends to be drier than normal.

Finally, Africa experiences December-February wetter than normal conditions in the Sahel region, which is equatorial Africa) along the southern edge of the Sahara desert. There also are drier than normal conditions in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique and Botswana.

Non-climate effects

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East Pacific fishing

Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry.

The local fishing industry along the affected coastline can suffer during long-lasting El Niño events. The world's largest fishery collapsed due to overfishing during the 1972 El Niño anchoveta reduction. During the 1982-83 event, jack mackerel and anchoveta populations were reduced, scallops increased in warmer water, but hake followed cooler water down the continental slope, while shrimp and sardines moved southward so some catches decreased while others increased. Horse mackerel have increased in the region during warm events.

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Fishing in warm water for cold-loving fish.

Shifting locations and types of fish due to changing conditions provide challenges for fishing industries. Peruvian sardines have moved during El Niño events to Chilean areas. Other conditions provide further complications, such as the government of Chile in 1991 creating restrictions on the fishing areas for artisanal fishermen and industrial fleets.

The ENSO variability may contribute to the great success of small fast-growing species along the Peruvian coast, as periods of low population removes predators in the area. Similar effects benefit migratory birds which travel each spring from predator-rich tropical areas to distant winter-stressed nesting areas.

Causes of El Niño

The mechanisms which might cause an El Niño event are still being investigated. It is difficult to find patterns which may show causes or allow forecasts.

Major theories:

  • Bjerknes in 1969 suggested that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, causing weakening in the Walker and trade wind flows which push warm water to the west. The result is increasingly warm water toward the east.
  • Wyrtki in 1975 proposed that increased trade winds could build up the western bulge of warm water, and any sudden weakening in the winds would allow that warm water to surge eastward. However, there was no such buildup preceding the 1982-83 event.
  • Recharge oscillator: Several mechanisms have been proposed where warmth builds up in the equatorial area, then is dispersed to higher latitudes by an El Niño event. The cooler area then has to "recharge" warmth for several years before another event can take place.
  • Western Pacific oscillator: In the western Pacific, several weather conditions can cause easterly wind anomalies. For example, a cyclone to the north and anticyclone to the south force easterly winds between. Such patterns may counteract the westward flows across the Pacific and create a tendency toward continuing the eastward motion. A weakening in the westward currents at such a time may be the final trigger.
  • Equatorial Pacific Ocean may tend to be near El Niño conditions, with several random variations affecting behavior. Weather patterns from outside the area or volcanic events may be some such factors.
  • The Madden-Julian Oscillation (MJO) is an important source of variability that can contribute to a more rapid evolution toward El Nino through related fluctuations in low-level winds and precipitation over the western and central equatorial Pacific. Eastward-propagating oceanic Kelvin waves can be produced by MJO activity.

La Niña

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A schematic diagram of the quasi-equilibrium and La Niña phase of the southern oscillation. The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the eastern Pacific as the result of this motion. The water and air are returned to the east. Both are now much cooler, and the air is much drier. An El Niño episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific.

In the Pacific, La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the same area. The La Niña condition often follows the El Niño, especially when the latter is strong. Strong La Niñas occurred in 1988-1989 and 1998-2001, and weakly in 1995-1996.


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SOI index, 1876-2004

The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.

Sustained negative values of the SOI often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade winds, and a reduction in rainfall over eastern and northern Australia. The most recent strong El Niño was in 1997/98.

Positive values of the SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia, popularly known as a La Niña episode. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal.

The most recent strong La Niña was in 1988/89; a moderate La Niña event occurred in 1998/99, which weakened back to neutral conditions before reforming for a shorter period in 1999/2000. This last event finished in Autumn 2000.


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Regional impacts of warm ENSO episodes (El Niño).
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Regional impacts of cold ENSO episodes. (La Niña)

ENSO (El Niño, Southern Oscillation) is a set of interacting parts of a single global system of climate fluctuations that come about as a consequence of atmospheric circulation. ENSO is the most prominent known source of interannual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. Global ENSO has signatures in the Pacific, Atlantic and Indian Oceans. In the Pacific, during major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SOI intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag those in the Pacific by 12-to-18 months. Many of the countries most affected by ENSO events are developing countries within main continents (South America, Africa...), with economies that are largely dependent upon their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events in the three oceans can have global socio-economical impacts. While ENSO is a global and natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern. Low-frequency variability has been evidenced. Interdecadal modulation of ENSO might exist.

Western Hemisphere Warm Pool

Study of climate records has found that about half of the summers after an El Niño have unusual warming in the Western Hemisphere Warm Pool (WHWP). This affects weather in the area and seems to be related to the North Atlantic Oscillation.

Atlantic effect

An effect similar to El Niño sometimes takes place in the Atlantic Ocean, where water along equatorial Africa's Gulf of Guinea becomes warmer and eastern Brazil becomes cooler and drier. This may be related to El Niño Walker circulation changes over South America.

Related images

External links

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