El Niño-Southern Oscillation is a periodic change in the atmosphere and ocean of the tropical Pacific region. It is defined in the atmosphere by the sign of the pressure difference between Tahiti and Darwin, Australia, and in the ocean by warming or cooling of surface waters of the tropical central and eastern Pacific Ocean. El Niño is the warm phase of the oscillation and La Niña is the cold phase. The oscillation does not have a specific period, but occurs every three to eight years. Mechanisms that cause the oscillation remain a matter of research.

The El Niño-Southern Oscillation is often abbreviated as ENSO and in popular usage is commonly called simply El Niño. El Niño is Spanish for "the boy" and refers to the Christ child, because periodic warming in the Pacific near South America is usually noticed around Christmas. "La Niña" is Spanish for "the girl."

Effects on weather vary with each event, but ENSO is associated with floods, droughts and other weather disturbances in many regions of the world. In the Atlantic Ocean, effects lag behind those in the Pacific by 12 to 18 months. Developing countries dependent upon agriculture and fishing, particularly bordering the Pacific Ocean, are especially affected.

Definition

El Niño is defined by sustained differences in Pacific-Ocean surface temperatures when compared with the average value. The accepted definition is a warming or cooling of at least 0.5°C (0.9°F) averaged over the east-central tropical Pacific Ocean. When this happens for less than five months, it is classified as El Niño or La Niña conditions; if the anomaly persists for five months or longer, it is called an El Niño or La Niña "episode." Typically, this happens at irregular intervals of 2–7 years and lasts nine months to two years.

The first signs of an El Niño are:

1. Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
3. Trade winds in the south Pacific weaken or head east
4. Warm air rises near Peru, causing rain in the northern Peruvian deserts
5. Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.

El Niño's warm current of nutrient-poor tropical water, heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming occurs and its economic impact to local fishing for an international market can be serious.

Early stages and characteristics of El Niño

Although its causes are still being investigated, El Niño events begin when trade winds, part of the Walker circulation, falter for many months. A series of Kelvin waves—relatively warm subsurface waves of water a few centimeters high and hundreds of kilometers wide—cross the Pacific along the equator and create a pool of warm water near South America, where ocean temperatures are normally cold due to upwelling. The Pacific Ocean is a heat reservoir that drives global wind patterns, and the resulting change in its temperature alters weather on a global scale. Rainfall shifts from the western Pacific toward the Americas, while Indonesia and India become drier.

Jacob Bjerknes in 1969 helped toward an understanding of ENSO, by suggesting that an anomalously warm spot in the eastern Pacific can weaken the east-west temperature difference, disrupting trade winds that push warm water to the west. The result is increasingly warm water toward the east. Several mechanisms have been proposed through which warmth builds up in equatorial Pacific surface waters, and is then dispersed to lower depths by an El Niño event. The resulting cooler area then has to "recharge" warmth for several years before another event can take place.

While not a direct cause of El Niño, the Madden-Julian Oscillation, or MJO, propagates rainfall eastwardaround the global tropics in a cycle of 30–60 days, and may influence the speed of development and intensity of El Niño and La Niña in several ways. For example, westerly flows between MJO-induced areas of low pressure may cause cyclonic circulations north and south of the equator. When the circulations intensify, the westerly winds within the equatorial Pacific can further increase and shift eastward, playing a role in El Niño development. Madden-Julian activity can also produce eastward-propagating oceanic Kelvin waves, which may in turn be influenced by a developing El Niño, leading to a positive feedback loop.

Southern Oscillation

The Southern Oscillation is the atmospheric counterpart of El Niño. It is an oscillation in air pressure between the tropical eastern and the western Pacific Ocean waters. The strength of the Southern Oscillation is measured by the Southern Oscillation Index (SOI). The SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia. El Niño episodes are associated with negative values of the SOI, meaning that the pressure at Tahiti is relatively low compared to Darwin.

Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water, in part because deep convection over the warm water acts to transport air. El Niño episodes are defined as sustained warming of the central and eastern tropical Pacific Ocean. This results in a decrease in the strength of the Pacific trade winds, and a reduction in rainfall over eastern and northern Australia. Conversely, La Niña episodes are associated with positive values of the SOI and are accompanied by stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time.

Walker circulation

During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks. 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 higher in the western Pacific as the result of this motion.

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 and El Niño condition alternate over a several year cycle.

Effects of ENSO's warm phase (El Niño)

South America

Because El Niño's warm pool feeds thunderstorms above, it creates increased rainfall across the east-central and eastern Pacific Ocean. The effects of El Niño in South America 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. Along the west coast of South America, El Niño reduces the upwelling of cold, nutrient-rich water that sustains large fish populations, which in turn sustain abundant sea birds, whose droppings support the fertilizer industry. This leads to fish kills offshore Peru.

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 Peruvian 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. 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 self-employed 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. There is some evidence that El Niño activity is correlated with incidence of red tides off the Pacific coast of California.

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 of the Amazon River Basin, Colombia and Central America.

North America

In North America, El Niño creates warmer-than-average winters in the upper Midwest states and the Northeast. Meanwhile, central and southern California, northwest Mexico and the southwestern U.S. become significantly wetter while the northern Gulf of Mexico states and northeast Mexico are wetter and cooler than average during the El Niño phase of the oscillation. Summer is wetter in the intermountain regions of the U.S. The Pacific Northwest states, on the other hand, tend to experience dry, mild but foggy winters and warm, sunny and early springs.

In Canada, both warmer and drier winters (due to forcing of the Polar Jet further north) occur with warmer and less stormy summers, although relatively little variation is seen in the Maritime Provinces. However, it is believed that the ice-storm in January 1998, which devastated parts of Southern Ontario and Southern Quebec, may have been caused or at least accentuated by El Nino's warming effects.

El Niño is also associated with increased wave-caused coastal erosion along the United States Pacific Coast.

Tropical cyclones

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving into the main belt of the Westerlies. When the subtropical ridge position shifts due to El Nino, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September-November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E which would favor the Japanese archipelago. During El Niño years, Guam's chance of a tropical cyclone impact is one-third of the long term average. The tropical Atlantic ocean experiences depressed activity due to increased vertical wind shear across the region during El Niño years.

Elsewhere

In Africa, East Africa, including Kenya, Tanzania and the White Nile basin experiences, in the long rains from March to May, wetter than normal conditions. There also are drier than normal conditions from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique and Botswana.Direct effects of El Niño resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires and worsening haze and decreasing air quality dramatically. Drier than normal conditions are also generally observed in Queensland, inland Victoria, inland New South Wales and eastern Tasmania from June to 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.

El Niño's effects on Europe are not entirely clear, but certainly it is not nearly as affected as at least large parts of other continents. There is some evidence that an El Niño may cause a wetter, cloudier winter in Northern Europe and a milder, drier winter in the Mediterranean Sea region. The El Niño winter of 2006/2007 was unusually mild in the UK and Western Europe, and the Alps recorded very little snow coverage that season.

Effects of ENSO's cool phase (La Niña)

La Niña is the name for the cold phase of ENSO, during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning "the little girl", analogous to El Niño meaning "the little boy". It has also in the past been called anti-El Niño, and El Viejo (meaning "the old man").

North America

La Niña causes mostly the opposite effects of El Niño. Atlantic tropical cyclone activity is generally enhanced during La Niña. La Niña causes increased rainfall across the United States' Midwest. Other potential impacts include above average precipitation in the Northern Rockies, Northern California, and in southern and eastern regions of the Pacific Northwest. Below-average precipitation is expected across the southern tier, particularly in the southwestern and southeastern states."

In Canada, La Nina will generally cause a cooler, snowier winter, such as the near record-breaking amounts of snow recorded in the La Nina winter of 2007/2008 in Eastern Canada.

Asia

During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China. In March 2008, La Niña caused a drop in sea surface temperatures over Southeast Asia by an amount of 2°C. It also caused heavy rains over Malaysia, Philippines and Indonesia.

Recent occurrences

There was a strong La Niña episode during 1988-1989. La Niña also formed in 1995, from 1998-2000, and a minor one from 2000-2001. The most recent occurrence of El Niño started in September 2006 and lasted until early 2007. From June 2007 on, data indicated a moderate La Niña event, which strengthened in early 2008 and weakened by early 2009; the 2007-2008 La Niña event was the strongest since the 1988-1989 event. According to NOAA, El Niño conditions have been in place in the equatorial Pacific Ocean since June 2009. Therefore the 2009/2010 season is expected to be an El Niño year.

Remote influence on tropical Atlantic Ocean

A study of climate records has shown that El Niño events in the equatorial Pacific are generally associated with a warm tropical North Atlantic in the following spring and summer. About half of El Niño events persist sufficiently into the spring months for the Western Hemisphere Warm Pool (WHWP) to become unusually large in summer. Occasionally, El Niño's effect on the Atlantic Walker circulation over South America strengthens the easterly trade winds in the western equatorial Atlantic region. As a result, an unusual cooling may occur in the eastern equatorial Atlantic in spring and summer following El Niño peaks in winter. Cases of El Niño-type events in both oceans simultaneously have been linked to severe famines related to the extended failure of monsoon rains.

ENSO and global warming

A few years ago, attribution of recent changes (if any) in ENSO or predictions of future changes were very weak. More recent results tend to suggest that the projected tropical warming may follow a somewhat El Niño-like spatial pattern, without necessarily altering the variability about this pattern, while the ENSO cycle may be minimally shortened.

El Niño "Modoki"

One recent study suggests that El Niños have been changing. This study says there are two similar forms of warming in the Pacific Ocean. One is the eastern Pacific Warming, which is the same as a typical El Niño. The central Pacific Warming has temperature anomalies near the dateline rather than near South America. There is some reason to believe that this warming in the central Pacific reduces El Niño's usual suppression of Atlantic hurricane activity. This new type of El Niño has been dubbed "El Niño Modoki" (Modoki is Japanese for "similar, but different".) The change from El Niño to El Niño Modoki may be due to a weakening of the southeasterly trade winds. It is not known whether this is due to natural variablity, the effects of climate change, or some other cause. Research is ongoing to determine whether a similar thing is happening to La Niña.

Cultural History and Pre-historic Information

ENSO conditions have occurred at two- to seven year intervals for at least the past 300 years, but most of them have been weak. There is also evidence for strong El Niño events during the early Holocene epoch 10,000 years ago.

El Niño affected pre-Columbian Incas and may have led to the demise of the Moche and other pre-Columbian Peruvian cultures. A recent study suggests that a strong El-Niño effect between 1789-93 caused poor crop yields in Europe, which in turn helped touch off the French Revolution.

An early recorded mention of the term "El Niño" to refer to climate occurs in 1892, when Captain Camilo Carrillo told the Geographical society congress in Lima that Peruvian sailors named the warm northerly current "El Niño" because it was most noticeable around Christmas. The phenomenon had long been of interest because of its effects on the guano industry and other enterprises that depend on biological productivity of the sea.

Charles Todd, in 1893, suggested that droughts in India and Australia tended to occur at the same time; Norman Lockyer noted the same in 1904.An El Niño connection with flooding was reported in 1895 by Pezet and Eguiguren. In 1924 Gilbert Walker (for whom the Walker circulation is named) coined the term "Southern Oscillation".

The major 1982-83 El Niño lead to an upsurge of interest from the scientific community. The period from 1990-1994 was unusual in that El Niños have rarely occurred in such rapid succession. An especially intense El Niño event in 1998 caused an estimated 16% of the world’s reef systems to die. The event temporarily warmed air temperature by 1.5°C, compared to the usual increase of 0.25°C associated with El Niño events. Since then, mass coral bleaching has become common worldwide, with all regions having suffered ‘severe bleaching’.

Major ENSO events were recorded in the years 1790-93, 1828, 1876-78, 1891, 1925-26, 1972-73, 1982-83, and 1997-98. Recent El Niños have occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, 2002-2003, 2004-2005 and 2006-2007.

See also

• Indian Ocean Dipole
• Pacific Decadal Oscillation

References

1. Australian Bureau of Meteorology
2. Climate Prediction Center. Average October-December (3-month) Temperature Rankings During ENSO Events. Retrieved on 2008-04-16.
3. Climate Prediction Center. Average December-February (3-month) Temperature Rankings During ENSO Events. Retrieved on 2008-04-16.
4. http://www.davidsuzuki.org/Climate_Change/Impacts/Extreme_Weather/El_Nino.asp
5. Joint Typhoon Warning Center (2006). 3.3 JTWC Forecasting Philosophies. United States Navy. Retrieved on 2007-02-11.
6. Pacific ENSO Applications Climate Center. Pacific ENSO Update: 4th Quarter, 2006. Vol. 12 No. 4. Retrieved on 2008-03-19.
7. http://news.bbc.co.uk/2/hi/europe/6185345.stm
8. http://www.ec.gc.ca/doc/smc-msc/2008/s3_eng.html
9. M. C. Wu, W. L. Chang, and W. M. Leung (2003). Impacts of El Nino-Southern Oscillation Events on Tropical Cyclone Landfalling Activity in the Western North Pacific. Journal of Climate: pp. 1419–1428. Retrieved on 2007-02-11.
10. http://www.cpc.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
11. El Niño Modoki - new type could mean more hurricanes make landfall, Science Codex, 2009-07-02, accessed 2009-09-27
12. Modoki: The Mimetic Tradition in Japan (article by Sakabe Magumi), p251- in Modern Japanese Aesthetics - A Reader, ed Michelle Marra, 1999, University of Hawaii Press
13. http://news.bbc.co.uk/2/hi/science/nature/25433.stm

Further reading

External links

• National Academy of Sciences El Nino/La Nina article
• NOAA FAQ "What is ENSO?"
• Lastest El Nino/La Nina Data from NASA
• Economic Costs of El Niño / La Niña and Economic Benefits from Improved Forecasting from "NOAA Socioeconomics" website initiative
• El Niño and La Niña from the 1999 International Red Cross World Disasters Report by Eric J. Lyman.
• ENSO (El Niño-Southern Oscillation)
• La Niña episodes in the Tropical Pacific
• NOAA announces 2004 El Niño
• NOAA El Niño Page
• Ocean Motion: El Niño
• SOI (Southern Oscillation Index)
• The Climate of Peru
• What is El Niño?
• What is La Niña?