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ENSO Cycle
Introduction
The term ENSO Cycle is used by scientists to label the full cycle of warm events and cold events in the equatorial Pacific Ocean, compared to a long-term average. The El Niño is the oceanic component, and the Southern Oscillation is the atmospheric component, hence the acronym ENSO.
The presentation in this hyperdocument is NOT linear. The pages may be read in any order. In addition to the information on this page, additional information about events and impacts of the ENSO Cycle may be found using in-text links, or the Local Links section. Some in-text links "jump" to specific information about data, instruments, missions, etc.
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El Niño sea surface height anomalies from TOPEX/Poseidon. Deviations shown of sea surface height of warm water (red and white bands in the Central Pacific Ocean) and cool water (blue and purple bands) relative to normal ocean conditions.
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La Niña sea surface height anomalies from TOPEX/Poseidon. |
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The El Niño / Southern Oscillation (ENSO) cycle is irregular, both in duration and in intensity. Each event lasts about a year, but associated climatic anomalies may persist in some parts of the world. The term ENSO is used in various documents in both a wide sense, of the entire cycle of hot, cold, and neutral years, and in a narrow sense, as only the hot event. Here, ENSO is used in the wide sense, spanning several years.
The ENSO events vary widely, with different sequences and distributions of sea-surface warming and wind-pattern changes, for both the warm extremes (El Niño) and the cold extremes (La Niña). Many older sources state that the 1982-83 El Niño was the "biggest event of the century." The 1982-83 El Niño impressed scientists and others that an El Niño was NOT just a local event taking place in a remote part of the world. The extra-large El Niño of 1997-98 was followed by unusually cool and persistent La Niña years: 1998-99, 1999-2000, and 2000-01. The winter of 2001-02 was characterized as a neutral phase (MODIS SST data), while the Northern Hemisphere winter of 2002-03 hasendured the effects of the next El Niño event.
The Southern Oscillation
(NOAA/Climate Diagnostics Center/Multivariate ENSO Index (MEI))
The Southern Oscillation (SO) is a global atmospheric oscillation in temperature and pressure between the Pacific and Indo-Australian areas. Thus, when air pressure is high at Darwin, Australia (western Pacific), it is low at Tahiti (eastern Pacific), and when air pressure is low at Darwin, it is high at Tahiti. Sir Gilbert Thomas Walker (1928) found that when air pressure was very high in the east and low in the west, the monsoon rains in India were heavy. When the pressure difference was small, the rains failed and drought often ensued. These changes in air pressure were noted long before they were connected with variations in the surface of the ocean. The SO is closely related to the recurring warming (El Niño) and cooling (La Niña) of the surface waters of the central and eastern tropical Pacific Ocean.
El Niño - La Niña Events
The El Niño and La Niña events are respectively warm events and cold events in the surface waters of the tropical Pacific Ocean. The irregular El Niño / Southern Oscillation (ENSO) cycle from El Niño to La Niña and back again takes 2 to 7 years. Each event usually lasts no more than 12 months, but a cold event can last 2-3 years.
In a normal year, steady trade winds blow westward and push warm surface water toward the western Pacific Ocean.
In an El Niño year, weakened winds (NSCAT wind data) allow warm water to occupy the entire tropical Pacific (Pathfinder SST data). The warm equatorial waters stand higher and displace the colder waters of the Humbolt Current, cutting off the upwelling process that supports the local fishing industry of Peru and Chile (SeaWiFS chlorophyll data).
ENSO events have been classified into various strengths, depending upon how much the tropical Pacific SST has been elevated. During the period of 1949-2000, there were nine "strong" ENSO events: 1951-52,1957-58, 1965-66, 1968-69, 1972-73, 1977-78, 1982-83, 1991-92, and 1997-98.
In a La Niña year, the trade winds are unusually strong, enhancing the upwelling process that feeds the fish. However, farmers on the Andean foothills frequently experience drought and crop failures.
These warm and cold events affect weather around the globe by heating or cooling the
atmosphere above the surface of the ocean.
Variations from "normal weather" affect man and his crops through:
There is evidence that a climate shift some 5,000 years ago started the ENSO cycle of warm and cold events.
Disclaimer: NASA offers these suggested sites for additional information regarding ENSO events. Web access is required to reach these sites. Link existence and contents are not under the control of the EOSDIS Science Operations Office.
El Niño - La Niña Web Sites