El Niño-Southern Oscillation
2008/9 Schools Wikipedia Selection. Related subjects: Climate and the Weather
El Niño-Southern Oscillation (ENSO) is a global coupled ocean-atmosphere phenomenon. The Pacific ocean signatures, El Niño and La Niña (also written in English as El Nino and La Nina) are important temperature fluctuations in surface waters of the tropical Eastern Pacific Ocean. The name El Niño, from the Spanish for "the child", refers to the Christ child, because the phenomenon is usually noticed around Christmas time in the Pacific Ocean off the west coast of South America. La Niña means "the little girl". Their effect on climate in the southern hemisphere is profound. These effects were first described in 1923 by Sir Gilbert Thomas Walker from whom the Walker circulation, an important aspect of the Pacific ENSO phenomenon, takes its name. The atmospheric signature, the Southern Oscillation (SO) reflects the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. The most recent occurrence of El Niño started in September 2006 and lasted until early 2007.
ENSO is a set of interacting parts of a single global system of coupled ocean-atmosphere climate fluctuations that come about as a consequence of oceanic and atmospheric circulation. ENSO is the most prominent known source of inter-annual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. 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 SO intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag behind 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-economic 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 the theory of global warming is an important concern. Low-frequency variability has been evidenced: the quasi-decadal oscillation (QDO). Inter-decadal (ID) modulation of ENSO (from PDO or IPO) might exist. This could explain the so-called protracted ENSO of the early 90s.
El Niño and La Niña
El Niño and La Niña are officially defined as sustained sea surface temperature anomalies of magnitude greater than 0.5°C across the central tropical Pacific Ocean. When the condition is met for a period of 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 classified as an El Niño or La Niña episode. Historically, it has occurred at irregular intervals of 2-7 years and has usually lasted one or two years.
The first signs of an El Niño are:
- Rise in air pressure over the Indian Ocean, Indonesia, and Australia
- Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
- Trade winds in the south Pacific weaken or head east
- Warm air rises near Peru, causing rain in the deserts there
- Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing rainfall in normally dry areas and extensive drought in eastern areas.
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, also known as the Peru Current, which support great populations of food fish. 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 for an international market can be serious.
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 60 cm 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. Atlantic tropical cyclone activity is generally enhanced during La Niña. The La Niña condition often follows the El Niño, especially when the latter is strong.
El Niño/Southern Oscillation (ENSO) - A shift in the normal relationship between the atmosphere and ocean in the tropical Pacific Ocean. Normally, strong winds (called trade winds because they aided sailing ships transporting goods) blow to the west in the Pacific, moving warmer surface water away from North and South America. Simultaneously, cold water from the ocean depths rises to the surface off the west coast of South America. This upwelling brings nutrients to the surface, supporting fisheries and ecosystems in the area. In an El Niño event, these trade winds die down, causing warmer surface water to accumulate off western North and South America. This leads to increased rainfall, storm activity, and flooding in the Americas (especially the southwestern United States and Peru) and drought conditions in Australia and other areas in the western Pacific and the Indian Ocean. Fisheries on the west coasts of North and South America are also seriously affected.
Wider effects of El Niño conditions
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. 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.
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.
In North America, typically, winters are warmer than normal in the upper Midwest states, the Northeast, and Canada, while central and southern California, northwest Mexico and the southwestern U.S., are wetter and cooler than normal. Summer is wetter in the intermountain regions of the U.S. The Pacific Northwest states, on the other hand, tend to experience dry but foggy winters and warm, sunny and precocious springs during an El Niño. During a La Niña, by contrast, the Midwestern U.S. tends to be drier than normal. El Niño is associated with decreased hurricane activity in the Atlantic, especially south of 25º N; this reduction is largely due to stronger wind sheartropics.
Finally, 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.
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.
Cases of double El Niño events have been linked to severe famines related to the extended failure of monsoon rains, as in the book Late Victorian Holocausts.
Non-climate effects
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.
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 of the Pacific coast of California.
It has been postulated that a strong El Niño led to the demise of the Moche and other pre-Columbian Peruvian cultures.
A recent study of El Niño patterns suggests that the French Revolution was caused in part by the poor crop yields of 1788-89 in Europe, resulting from an unusually strong El-Niño effect between 1789-93.
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.
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:
- Jacob 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 circulation 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 westerly wind anomalies. For example, cyclones north and south of the equator force west-to-east winds between. Such wind may counteract the typical easterly 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 needed to shift into an El Nino.
- Equatorial Pacific Ocean may tend to be near El Niño conditions, with several random variations affecting behaviour. 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 Niño conditions 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. Eisenman, Yu, and Tziperman (2005 ) have suggested that the MJO may in turn be influenced by a developing El Nino, leading to a positive feedback.
- Adams, Mann and Ammann showed in 2003, using statistical analysis of paleoclimatic records, that a volcanic event in the tropics tends to trigger a three year El Niño followed by three years of La Niña.
History of the theory
The first mention of the term "El Niño" to refer to climate occurs in 1892, when Captain Camilo Carrilo told the Geographical society congress in Lima that Peruvian sailors named the warm northerly current "El Niño" because it was most noticeable around Christmas. However even before then the phenomenon was of interest because of its effects on biological productivity, with its effects on the guano industry.
Normal conditions along the west Peruvian coast are a cold southerly current (the Peru current) with upwelling water; the upwelling nutrients lead to great oceanic productivity; the cold currents leads to very dry conditions on land. Similar conditions exist elsewhere (California current; Benguela current off south Africa). Thus the replacement of this with warmer northerly water leads to lower biological productivity in the ocean, and more rainfall - often flooding - on land; the connection with flooding was reported in 1895 by Pezet and Eguiguren.
Towards the end of the nineteenth century there was much interest in forecasting climate anomalies (for food production) in India and Australia. 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. In 1924 Gilbert Walker (for whom the Walker circulation is named) first coined the term "Southern Oscillation".
For most of the twentieth century, El Niño was thought of as a largely local phenomenon.
The major 1982-3 El Niño lead to an upsurge of interest from the scientific community.
The 1998 El Nino event caused an estimated 16% of the world’s reef systems to die. Since then, mass coral bleaching has become common worldwide, with all regions having suffered ‘severe bleaching’.
History of the phenomenon
ENSO conditions seem to have occurred at every two to seven years for at least the past 300 years, but most of them have been weak.
Major ENSO events have occurred in the years 1790-93, 1828, 1876-78, 1891, 1925-26, 1982-83, and 1997-98.
Recent El Niños have occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, 2002-2003, and 2006-2007.
The El Niño of 1997 - 1998 was particularly strong and brought the phenomenon to worldwide attention, while the period from 1990-1994 was unusual in that El Niños have rarely occurred in such rapid succession (but were generally weak). There is some debate as to whether global warming increases the intensity and/or frequency of El Niño episodes. (see also the ENSO and Global Warming section above).