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:
- Rise in surface 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 northern Peruvian deserts
- 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 eastward around 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 60 centimetres (24 in) 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 f
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