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JMA Index Showing El Niño and La Niña Years

El Niño is a climate phenomenon that results from an intimate connection between ocean and atmosphere. During an El Niño event, slackening trade winds result in unusually warm sea surface temperatures in the eastern and central Pacific. These warm temperatures trigger droughts in Australia, a shift in storm patterns in North America, and a host of other climatic anomalies around the world. El Niño is also know as the warm phase of the El Niño/Southern Oscillation, or ENSO. La Niña is the cold phase of ENSO.

When El Niño is on the way, air pressures rise over northern Australia and fall over the central Pacific—changes indicating the weakening of the westward trade winds. Westward flowing currents that carry warm surface waters in the Pacific slow down. This leads to a rise in surface sea temperatures in the Pacific all along the equator from the west coast of South America to the international date line.

The ENSO Index gives the changing sea surface temperatures from year to year. As you can see, the shift between neutral temperatures, unusually warm temperatures (El Niño years, shown in red), and unusually cold temperatures (La Niña years, shown in blue) is periodic but not predictably so. One goal of ENSO data collection is to find an underlying pattern that can predict El Niño events—and the weather changes they can bring—far in advance.

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JMA Index Showing El Niño and La Niña Years - Shown here are sea surface temperature (SST) measurements taken over the last few decades in a zone of the equatorial Pacific Ocean. The vertical scale on the right shows the temperature anomaly the difference between the measured temperature and the expected temperature. During El Niño events, the water temperatures in this area are warmer than usual, shown here in red as “warm phase.” Source: Center for Ocean-Atmospheric Prediction Studies, Florida State University(COAPS)

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The many factors that affect sea surface temperature make interpretation of ENSO Index data difficult. Beyond daily weather, variable currents and irregular oceanic changes create “noise” that make it hard to identify and explain larger trends. Over geological time scales, even changes in the earth’s orbit may affect ENSO patterns. The seemingly endless influences behind sea surface temperature have made it difficult to develop fully realistic computer models of the ENSO phenomenon.

 related sites  

Frequently Asked Questions About La Niña and El Niño - A series of questions and answers dealing with La Niña and El Niño created by the National Oceanic and Atmospheric Administration (NOAA).

El Niño-Southern Oscillation (ENSO) - A relatively technical discussion of ENSO as part of a larger site dealing with the science of climate change.

Coral Paleoclimatology: What can corals tell us about climate? - An article explaining how old coral can help us understand global climate trends, part of a larger paleoclimatology site by the National Oceanic and Atmospheric Administration.

Exploratorium Webcast: La Niña Summit - Audio archives from the 1998 summit.

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