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El Niño-Southern Oscillation

2007 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

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

   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:
    1. Rise in air 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 deserts there
    5. 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.
   Regional impacts of warm ENSO episodes (El Niño).
   Regional impacts of warm ENSO episodes (El Niño).
   Regional impacts of cold ENSO episodes. (La Niña)
   Regional impacts of cold ENSO episodes. (La Niña)

   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.
   East Pacific fishing
   East Pacific fishing

   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).

Related images

   Average equatorial Pacific temperatures.

   El Niño effects upon North American weather and atmospheric
   circulation.

   Map showing Nino3.4 and other index regions

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