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Weather
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Southern Oscillation & El Niño history
Southern Oscillation
The fluctuations in ocean temperatures during El Niño and La Niña are linked with an even larger-scale fluctuations in air pressure between the western and eastern tropical Pacific known as the Southern Oscillation.
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The Southern Oscillation is a change in air pressure measured between the eastern (Tahiti) and western (Darwin) parts of the Pacific. When the pressure is high at Darwin it is low at Tahiti and vice versa.
El Niño, and its sister event "La Niña" represent the opposite extreme phases of the southern oscillation. During El Niño higher than average air pressure covers Indonesia and the western tropical Pacific and below-average air pressure covers the eastern tropical Pacific.
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1. Map of the South Pacific, showing Darwin in Australia and Tahiti, one of the Pacific Islands
Please click the map for a higher resolution! (60 K)
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These pressure departures are reversed during La Niña, which features below-average air pressure over Indonesia and the western tropical Pacific and above-average air pressure over the eastern tropical Pacific.
To sum up, El Niño refers to the oceanic component of the El Niño/Southern Oscillation (ENSO) system, the Souther Oscillation to the atmospheric component, and ENSO to the coupled system, the circulation related to both the atmosphere and the oceans. ENSO has 3 phases: warm (El Niño), cold (La Niña) and neutral (which refers to those periods when neither El Niño nor La Niña is present).
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2. The normal mean sea level pressure (MSPL) at Tahiti is higher than at Darwin. If the pressure at Darwin rises relative to Tahiti the SOI becomes negative and we have an El Niño event.
source: Climate System Lectures, Univ. of Columbia, NY
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Southern Oscillation Index (SOI)
The Southern Oscillation Index (SOI) is designed to measure the strength and phase of the Southern Oscillation. During El Niño episodes the SOI has a large negative value due to lower-than-average air pressure at Tahiti and higher-than-average pressure at Darwin.
During La Niña episodes the SOI has a positive value due to higher-than-average air pressure at Tahiti and lower-than-average pressure at Darwin.
El Niño episodes occur in every 2-7 years.
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3. The southern oscillation index SOI. Strong negative red values stand for El Niño events, strong positive blue values stand for La Niña conditions. Please click the graph for a higher resolution.
Source: Long Paddock website, Gov. of Queensland
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4. Sea Level Pressure 1960-1984
source: U. S. Army Topographic Engineering Center
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Historical review on El Niño events
El Niño is not a new phenomenon, it has been around for thousands of years. Chemical signatures of warmer sea surface temperatures and increased rainfall caused by El Niño appear in coral specimens at least 4000 years old, but some researchers claim to have found coral records that hold evidence of El Niño cycles more than a 100,000 years ago!
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The records of El Niño date back as far as the 1500s. At that time fisherman off the coast of Peru started noticing that periodic warm waters hold down their anchovy catch. But also peruvian farmers noticed that the warm waters lead to increased rainfall, transforming normally barren areas into fertile farmland.
In the period of 1700-1900 European sailors made sporadic attempts at documenting the phenomenon, hence scientists become interested in identifying it's cause.
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5. Fishermen at Peru - the warm water during El Niño makes fishes avoid the Peruvian cost and is very bad for fishery
© downtheroad.org - Peru
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Still, until the middle of the 20th century, little was known about conditions during El Niño years. Interest intensified in the late 1960s and early 1970s, and using new observations including satellite data, climatologists and oceanographers recognized that El Niño meant a whole lot more than just a local feature of climate variability. Only from that time were the major elements of this climate oscillation linked together.
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6. Nowadays the Pacific Ocean is observed by fixed Atlas buoys, measuring several parameters, also in deeper water layers, by coastal tide gauge stations, by satellites from the space (sea surface temperature), by drifting buoys and research ships. From this data an El Niño early warning system helps the people to be ready for the next event.
© clivar.org
In 1923, a British Scientist, Sir Gilbert Walker discovered that when air pressure is high in the Pacific, it is low in the Indian Ocean from Africa to Australia, and vice versa. His find, which he named the Southern Oscillation, is the first indication that weather conditions in distant parts of the tropical pacific are connected.
Fifty years later, in the late 1960s, Jacob Bjerknes Norwegian meteorologist and professor of the university of California at Los Angeles came up with the first detailed description of how El Niño works. He made the connection between Walker’s Southern Oscillation and El Niño which is now officially known as El Niño/Southern Oscillation or ENSO for short.
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8. a) Please click to see an animation of the Walker circulation over the Pacific. The image shows the circulation for neutral conditions.
© original: Bildungszentrum Markdorf
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8. b) Please click on this image in order to see what changes during El Niño conditions.
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9. Comparison of the absolute sea surface temperature for normal conditions and the El Niño event in 1982.
source: Climate science lecture, Univ. of Columbia, NY - Please click to enlarge! (120 K)
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In the period of 1982-83, the strongest El Niño ever recorded made havoc around the world. Related floods,droughts & wildfires killed about 2,000 people worldwide. Damage is estimated at $13 Billion.
In 1997-1998, an El Niño more powerful than the record 1982-83 event developed in the pacific. Warnings were issued in mid 1997, and emergency preparedness conferences were convened. By March 1998, El Niño-related events globally caused US$34 billion direct loss, and killed 24 thousand people. 111 million people were affected in any way, and the related events left 6 million displaced people behind them.
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About this page:
author: Vera Schlanger - Hungarian Meteorological Service
scientific reviewing: Dr. Ildikó Dobi Wantuch / Dr. Elena Kalmár - Hungarian Meteorological Service, Budapest
last updated: 2004-02-23 |
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Further reading:
http://whyfiles.org/
http://ww2010.atmos.uiuc.edu/
http://www.oc.nps.navy.mil/
http://cpc.ncep.noaa.gov/
http://www2.sunysuffolk.edu/
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