El Niño During the nineteenth century Peruvian fishermen were aware of a warm water current that flowed southwards along the coast. Because it usually started after Christmas, Los Dias del Niño, the days of the (Christ) Child, they termed it Corriente del Niño or the El Niño current. The tropical marine conditions that El Niño brings are in great contrast to the habitual coolness caused by upwelling of cold water from Antarctica along the western South American coast. The annual El Niño current heralds summer rains over the coastal desert and Andean Mountains, bringing much-needed run-off for the irrigation of crops. The fishing industry also has to adapt from pursuing schooling fishes of the cool, dense, nutrient-rich upwelling water to foraging predator species of the less productive and less saline warmer waters.

At irregular intervals the El Niño current is markedly stronger than normal and carries the warmer, less saline water much further south. Scientists now use the term El Niño to refer to these irregular, stronger events, whose effects can last for more than a year and have important repercussions for weather and climate, and for social and economic well-being, throughout the Pacific basin and even further afield. The first written evidence for an El Niño event (though not named as such) is found in the campaign diaries of Pizarro in 1525–6. El Niño events seem to have occurred at intervals of three to four years, with the strongest type occurring at intervals of more than 20 years. During the twentieth century there were nine strong and 16 moderate El Niño events.

The atmospheric equivalent of El Niño is termed the Southern Oscillation (SO). The Southern Oscillation is a large-scale atmospheric pressure change over the south-eastern Pacific and Indian Ocean. It was first documented in 1932 by Sir Gilbert Walker as Director-General of Observatories in India was studying the causes of variation in the Indian Monsoon. During the ‘high phase’ of the Southern Oscillation, strong tropical convection causes air to ascend over the western Pacific and Indonesia, producing low pressure which draws in strong trade winds over the ocean surface from the south-east. The air rises and returns at higher altitudes before descending and causing high pressure in the south-eastern Pacific. During a ‘low phase’ the pressure pattern is reversed as the zone of low pressure associated with tropical convection moves to the middle and eastern Pacific, resulting in weaker than normal trade winds.

The Southern Oscillation Index (SOI) is a measure of the pressure difference between Darwin in Australia and Tahiti (Fig. 2). Troughs in the index correspond to El Niño events when pressure is low in Tahiti and high in the western Pacific, whereas peaks in the Southern Oscillation Index correspond to an accentuated ‘normal’ phase (now termed La Niña) characterized by strong trade winds. Scientists were unaware of the link between the Southern Oscillation and El Niño until 1957, when a strong correlation was noted between the SOI and sea surface temperatures off Peru.

The Norwegian scientist Jacob Bjerknes was the first to suggest a mechanism for linking these atmospheric and oceanic phenomena. A simplified structure of the tropical ocean can be thought of as comprising a layer of less dense, fresher, warmer water overlying a layer of denser, colder, more saline water, the two separated by a zone of rapidly changing density and temperature known as the thermocline. During ‘normal’ conditions, the west-blowing trade winds move the upper warmer layer towards the western Pacific, where it increases in thickness such that the thermocline can be at depths of 150 to 200 m, leaving only a shallow upper layer and thermocline (30–50 m) in the east. At the same time the west-blowing trade winds and the Earth's rotation combine to produce a surface movement of water away from the Equator, both to north and south. This in turn induces upwelling of cold water from below, and an elongated tongue of cool surface water is produced. The cool tongue does not extend to the western Pacific, because here the trade winds are weaker and the thermocline is deeper. When the trade winds relax at the start of an El Niño event, the upwelling ceases and the upper warm water moves eastwards, bringing with it the zone of low pressure and strong atmospheric convection.

The relaxation of the trade winds initiates a large-scale oceanic wave termed a Kelvin wave, which travels from west to east in the equatorial region at maximum speeds of 2.8 m s⁻¹, crossing the basin in one to two months, lowering the thermocline and raising sea level as it passes. Once a Kelvin wave reaches the eastern side of the Pacific ocean, some of the wave energy may also be translated into coastal Kelvin waves which travel north and south parallel to the coast of the Americas, taking up to three to four months to reach high latitudes. In regions away from the Equator the principal large-scale wave form is a westward-travelling Rossby wave, which can be produced by reflection of Kelvin or coastal Kelvin waves. The speed of a Rossby wave decreases with distance from the Equator, and so El Niño effects that are carried into middle and high latitudes may persist for months and even years. At the latitude of Japan, for instance, a Rossby wave reflected westwards across the Pacific would take several years to cross the basin. Research has indicated that after particularly strong El Niño events, such as in 1982–3, the effects on sea surface temperature and, by implication, weather conditions have been found at higher latitudes up to 11 years later.

A typical El Niño event seems to be presaged by a period of stronger than normal trade winds. In the onset phase, which usually begins in the early part of the year, there is a westerly (east-blowing) component to the wind in the western and central equatorial Pacific. In April, peak sea-surface temperatures are noted off Peru and Ecuador corresponding to the arrival of the oceanic Kelvin wave. Sea level also rises because of the lower atmospheric pressure and the arrival of warmer surface water from the west. High sea-surface temperatures can last until the following boreal spring of year 2, perhaps with another small peak during the winter before collapsing at the end of the sequence.

The effects of an El Niño event can be global. The eastward movement of the low-pressure convection area results in drought conditions in Australia, Indonesia, Africa, and India. The reverse occurs in the central Pacific, where there is intense rainfall and hurricane initiation. Western South America experiences heavy rainfall, resulting in flooding and erosion, and the effects can be felt in North America, where the change induced in the pressure patterns results in frontal systems bringing high rainfall further south to the western and southern United States.

The 1982–3 El Niño was exceptional in its severity and was undoubtedly the strongest of the twentieth century; it was also one of the best recorded so far. There were record-breaking amounts of rainfall in Peru and Ecuador in early 1983, resulting in record amounts of run-off, flooding, avalanches, and erosion. Translation of the effects took place to higher latitudes with concomitant flooding and costal erosion. Further afield there was a record drought in Australia. New types of disturbance were noted to the marine biota on a global scale. For instance, the deepening of the thermocline and the associated fall in nutrients within the surface waters of the eastern Pacific resulted in the devastation of many coral reefs which had experienced almost uninterrupted growth for several centuries. The loss of the nutrients led to a decrease in the zooplankton, which resulted in fish and squid mortality, reproductive failure in marine birds, and food shortages for penguins, cormorants, seals, and sea lions. In addition, rough seas and high sea levels made feeding very difficult for near-shore feeders such as marine iguanas, and kelp colonies were devastated.

Historical studies over longer timescales using a wide range of approaches suggests that El Niño events can be detected using such proxy evidence as changes in tree ring widths in the south-west of the United States and Mexico; Nile River flood data; tropical and subtropical ice cores; coral growth records; fishery catch records; and from a wide range of evidence derived from marine and lake sediments.

It is now evident that El Niño teleconnections (atmospheric and oceanic responses away from the equatorial Pacific) indicate that the El Niño phenomenon is the largest source of interannual climatic variability on the global scale. Attempts at predicting the onset and severity of El Niño events have not so far been satisfactory. Much more work will have to be done by ocean modellers and palaeoclimatologists before the factors behind the variability are understood.

B. A. Haggart

Bibliography

Bigg, G. R. (1990) El Niño and the Southern Oscillation. Weather 45, 2–8.
Enfield, D. B. (1989) El Niño, past and present. Reviews of Geophysics 27, 159–87.

El Niño

El Niño (pronounced el-NEEN-yo) is the name given to a change in the flow of water currents in the Pacific Ocean near the equator. El Niño—Spanish for "the child" because it often occurs around Christmas—repeats every three to five years. Although El Niño takes place in a small portion of the Pacific, it can affect the weather in large parts of Asia, Africa, Indonesia, and North and South America. Scientists have only recently become aware of the far-reaching effects of this phenomenon.

What is El Niño?

The rotation of Earth and the exchange of heat between the atmosphere and the oceans create wind and ocean currents. At the equator, trade winds blow westward over the Pacific, pushing surface water away from South America toward Australia and Indonesia. These strong trade winds, laden with moisture, bring life-giving monsoons to eastern Asia. As warm surface water moves west, cold, nutrient-rich water from deep in the ocean rises to replace it. Along the coast of Peru, this pattern creates a rich fishing ground.

Every three to five years, however, the trade winds slacken, or even reverse direction, allowing winds from the west to push warm surface water eastward toward South America. This change is called the Southern Oscillation (oscillation means swinging or swaying), and it is brought about by a shifting pattern of air pressure between the eastern and western ends of the Pacific Ocean. The warm water, lacking nutrients, kills marine life and upsets the ocean food chain. The warm, moist air that slams into the South American coast brings heavy rains and storms. At the same time, countries at the western end of the Pacific—Australia, Indonesia, and the Philippines—have unusually dry weather that sometimes causes drought and wildfires.

Another type of unusual weather that often follows an El Niño is called La Niña, which is Spanish for "the girl." El Niño and La Niña are opposite phases in the Southern Oscillation, or the back and forth cycle in the Pacific Ocean. Whereas El Niño is a warming trend, raising the water temperature as much as 10°F (5.6°C) above normal, La Niña is a cooling of the waters in the tropical Pacific, dropping the temperature of the water as much as 15°F (8°C) below normal.

Global effects of El Niño

Meteorologists believe the altered pattern of winds and ocean temperatures during an El Niño changes the high level winds, called the jet streams, that steer storms over North and South America. El Niños have been linked with milder winters in western Canada and the northern United States, as more severe storms are steered northward to Alaska. The jet streams altered by El Niño can also contribute to storm development over the Gulf of Mexico, which brings heavy rains to the southeastern United States. Similar rains may soak countries of South America, such as Peru and Ecuador, while droughts may affect Bolivia and parts of Central America.

El Niño also appears to affect monsoons, which are annual shifts in the prevailing winds that bring on rainy seasons. The rains of the monsoons are critical for agriculture in India, Southeast Asia, and portions of Africa. When the monsoons fail, millions of people are at risk of starvation. It appears that wind patterns associated with El Niños carry away moist air that would produce monsoon rains.

La Niña can bring cold winters to the Pacific Northwest, northern Plains states, Great Lakes states, and Canada, and warmer-than-usual winters to the southeastern states. In addition, it can bring drier-than-usual conditions to California, the Southwest, the Gulf of Mexico, and Florida, as well as drought for the South America coast and flooding for the western Pacific region.

Not all El Niños and La Niñas have equally strong effects on global climate; every El Niño and La Niña event is different, both in strength and length.

Words to Know

Jet streams: High velocity winds that blow at upper levels in the atmosphere and help to steer major storm systems.

Monsoon: An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.

Worst El Niños of the century

According the National Oceanic and Atmospheric Administration (NOAA), 23 El Niños and 15 La Niñas took place in the twentieth century. Out of those, the four strongest occurred after 1980. Scientists are unsure if this is an indication that human activity is adversely affecting the weather or if it is simply a meaningless random clustering.

The El Niño event of 1982–83 was one of the most destructive of the twentieth century. It caused catastrophic weather patterns around the world. Devastating droughts hit Africa and Australia while torrential rains plagued Peru and Ecuador. In the United States, record snow fell in parts of the Rocky Mountains; drenching rains flooded Florida and the Gulf of Mexico's coast; and intense storms brought about floods and

mud slides in southern California. French Polynesia in the South Pacific was struck by its first typhoon in 75 years. It is estimated this particular El Niño killed 2,000 people and caused $13 billion worth of property damage.

Less than 15 years later, another destructive El Niño pattern developed. This one, however, was much more devastating than the 1982-83 event. In fact, it was the worst in recorded history. Beginning in late 1997, heavy rain and flooding overwhelmed the Pacific coast of South America, California, and areas along the Gulf Coast. Eastern Europe and East Africa were affected, as well. Australia, Central America, Mexico, northeastern Brazil, Southeast Asia, and the southern United States were all hit hard by drought and wildfires. In the United States, mudslides and flash floods covered communities from California to Mississippi. A series of hurricanes swept through the eastern and western Pacific. Southeast Asia suffered through its worst drought in fifty years. As a result, the jungle fires used to clear lands for farming raged out of control, producing smoke that created the worst pollution crisis in world history. At least 1,000 people died from breathing problems. By the time this El Niño period ended some eight months later in 1998, the unusual weather patterns it had created had killed approximately 2,100 people and caused at least $33 billion in property damage.

[See also Atmospheric pressure; Ocean; Weather; Wind ]

El Niño-Southern Oscillation (ENSO) , large-scale climatic fluctuation of the tropical Pacific Ocean. The El Niño [Span.,=the child] itself is a warm surface current that usually appears around Christmas in the Pacific off Ecuador and Peru and disappears by the end of March, but every two to seven years it persists for up to 18 months or more as part of an ENSO. While the ENSO results from the dynamic and thermodynamic interactions among the atmosphere, oceans, and land surfaces, exactly what initiates an ENSO is unclear. It seems certain that pressure changes and wind currents play a vital role. Some researchers have implicated the greenhouse effect (see global warming ), while others have attributed it to activity occurring on the ocean floor, such as underwater earthquakes.

In a typical ENSO, the strong easterly winds of the equatorial Pacific weaken, which allows warm eastward-flowing subsurface waters to rise, increasing surface temperatures 1-2°C (2-3.5°F), and sometimes as much as 4-6°C (7-11°F), in the central and E Pacific. Along the W coast of South America, El Niño's warm waters persist and deepen, and cold, upwelling, nutrient-rich waters fail to reach surface waters; the resulting warm, nutrient-poor waters devastate coastal fisheries. Heavy rain falls along the South American coast, and heavy rainfall also moves from the western to central Pacific, causing drier than normal conditions in Indonesia and nearby areas. An ENSO also affects the climate of the northern latitudes, particularly North America, which experiences warmer temperatures along the Pacific coast, increased rainfall in the Gulf states, and weaker Atlantic hurricanes.

Severe ENSO events can be economically disruptive worldwide. Of the 29 ENSOs that occurred between 1700 and 1999, the 1982-83 El Niño was the strongest and most devastating. It caused droughts in Africa, Australia, India, Indonesia, and the Philippines, flooding in Peru and Ecuador, and devastating coastal storms in California. The ENSO was blamed for 1,300-2,000 deaths and more than $13 billion in damage to property and livelihoods.

The effects of El Niño were documented in Peru as early as the Spanish conquest in 1525. By the end of the 19th cent. the phenomenon was being studied by Peruvian oceanographers, although the effects were thought to be limited to the W coast of South America. It was not until the systematic studies of the International Geophysical Year of 1957-58 that the extent of the meteorological impact of El Niño was recognized.

La Niña, a similar climatic fluctuation, involves the abnormal cooling of the waters off Ecuador and Peru. Penetrating westward, the cold current is believed to affect weather in areas in the middle latitudes in the western Pacific Ocean and to cause extremely hot summers in Japan.

Bibliography: See M. H. Glantz, Currents of Change: El Niño's Impact on Climate and Society (1996); B. Fagan, Floods, Famines, and Emperors: El Niño and the Fate of Civilizations (1999).

El Niño is a phenomenon that occurs every three to seven years in the Pacific Ocean. During a normal year sea temperatures warm up briefly during the Christmas season off the coasts of Peru and Ecuador (El Niño is Spanish for the infant Jesus). However, during an El Niño these warm temperatures persist for many months, coinciding with aberrant changes in air pressure and wind patterns throughout the southern Pacific, known as El Niño Southern Oscillations (ENSOs). Periodically, the Intertropical Convergence Zone (ITCZ) in the atmosphere, which normally sits over the Indonesian region, migrates eastwards over the central Pacific. The ITCZ is a zone of high atmospheric pressure where the rising air triggers heavy rainfall and high air temperatures. This warms the ocean surface and triggers a planetary wave that travels eastwards. This wave is slow moving and takes about a year to cross the Pacific along the equator. Its effect is to deepen the thermocline so that the upwelling that occurs along the equator no longer brings cool nutrient-rich water from below the thermocline up to the surface. Normally the cool waters that occur off the coasts of Peru and Ecuador are not only highly productive, but they also keep the coastal climate arid.

During an El Niño event the major fishery for anchovetta (Centegraulis mysticetus), normally one of the biggest in the world, collapses and there are mass mortalities of seabirds, such as pelicans and guanay cormorants, whose droppings were the main source of nitrates used to manufacture the gunpowder used in 19th-century European wars. The warm seas also trigger heavy rainfall over the normally arid coastal regions, resulting in disastrous flooding. When the planetary wave encounters the continental margin of America it is diverted both north and south, resulting in similarly anomalous warm seas and heavy rainfall along the normally arid coasts of North and South America. The repercussions are felt in other regions such as South Africa, India, and South-East Asia, where seasonal rains fail resulting in disastrous droughts and human misery.

The El Niño events of 1982 and 1997 were the most extreme on record. What determines the irregular occurrences of ENSO events is not understood, but many scientists believe that the recent increase in their frequency and severity is one expression of global climate change. If this is indeed so then these events may become the norm with severe implications for human societies.

See also environmental issues.

www.elnino.noaa.gov/

M. V. Angel

El Niño (Sp. (Christ) ‘child’) Warm surface current that flows in the equatorial Pacific Ocean towards the South American coast. It occurs around Christmas time. An easing or reversing of the trade winds over the s Pacific Ocean, causes warm surface waters that have ‘piled-up’ in the w Pacific to flow back and warm the coastal waters of South America by 2 to 3°C. It has a dramatic effect on climate patterns in Australia and Southeast Asia, and may affect rainfall as far away as Africa. In normal years, trade winds blow e to w along the Equator, dragging warm surface waters into a pool off n Australia and monsoon rains to Indonesia. In the w Pacific, the Humboldt Current pushes the surface waters away from the coast of Peru, bringing cold water to the surface. This upwelled, nutrient-rich water stimulates phytoplankton production and swells the population of anchovies, a mainstay of the Peruvian fishing industry. In an El Niño year, the upwelling ceases and the biological productivity of the area collapses. In 1982–83, the anchovy catch fell by 600%. In addition, mean sea-level along the coast of Latin America may increase by as much as 50cm (20in), causing widespread flooding. Some scientists believe that the frequency (presently every 2–10 years) and effects of El Niño may be increasing.

http://www.elnino.noaa.gov

El Niño A weakening of the Equatorial Current, allowing warm water to accumulate off the S. American Pacific coast; it is associated with a change in the atmospheric circulation known as a southern oscillation, the two together comprising an El Niño—Southern Oscillation (ENSO) event. Its climatic effects are felt throughout the Pacific region. (A similar phenomenon may occur in the Atlantic.) About once every 7 years, during the Christmas season (midsummer in the southern hemisphere), prevailing trade winds weaken, the Equatorial counter-current strengthens, and warm surface waters that are normally driven westwards by the wind to form a deep layer off Indonesia flow eastwards to overlie the cold waters of the northward-flowing Peru current. In exceptional years (e.g. 1953, 1972–3, 1982–3, and 1997–8) the severity with which the upwelling of nutrient-rich cold water is inhibited causes the death of a large proportion of the plankton population and a consequent decline in the numbers of fish.

El Niño A warm-water current which periodically flows southwards along the coast of Ecuador. It is associated with the Southern Oscillation (these effects are collectively known as an El Niño—Southern Oscillation, or ENSO, event) and with climatic effects throughout the Pacific region. A similar phenomenon may also occur in the Atlantic. Approximately once every seven years, during the Christmas season (the name refers to the Christ child), prevailing trade winds weaken and the Equatorial countercurrent strengthens. Warm surface waters, normally driven westward by the wind to form a deep layer off Indonesia, flow eastwards to overlie the cold waters of the Peru current. In exceptional years, e.g. 1891, 1925, 1953, 1972–3, 1982–3, 1986–7, 1994–5, and 1997–8 the extent to which the upwelling of the nutrient-rich cold waters is inhibited causes the death of a large proportion of the plankton population and a decline in the numbers of surface fish.

El Niño A weakening of the Equatorial Current, allowing warm water to accumulate off the S. American Pacific coast; it is associated with the southern oscillation (these two effects are known collectively as an El Niño–Southern Oscillation or ENSO event) and with climatic effects throughout the Pacific region. A similar phenomenon may also occur in the Atlantic. Approximately once every seven years, during the Christmas season (the name refers to the Christ child), prevailing trade winds weaken and the Equatorial Countercurrent strengthens. Warm surface waters, normally driven westward by the wind to form a deep layer off Indonesia, flow eastwards to overlie the cold waters of the Peru Current. In exceptional years (e.g. 1953, 1972–3, 1982–3, and 1997–8) the extent to which the upwelling of the nutrient-rich cold waters is inhibited causes the death of a large proportion of the plankton population and a consequent decline in the numbers of surface fish.

Ni·ño, El • see El Niño.

El Niño •bagnio •dal segno, jalapeño •cursillo, Trujillo •caudillo • El Niño • yo-yo