What is El Niño?

El Niño is a weather event involving the eastward migration of a mass of warm water normally found in the western equatorial Pacific Ocean. Periodically (usually every three to seven years), the easterly trade winds in the Pacific weaken and allow the pool of warm water to drift from Australia to the western coast of South America, often triggering heavy rains there. This vast pool of warm water is thought to set off a chain reaction that can affect jet stream and weather patterns around the world, especially in the winter months in the northern hemisphere. El Niño is sometimes referred to as ENSO, for El Niño-Southern Oscillation. The Southern Oscillation is a seesaw of air pressures on the eastern and western halves of the Pacific.

What does "El Niño" mean?

Peruvian fishermen first noticed the effects of a new El Niño at Christmas time, when storminess off the coast reduced the supply of fish. "El Niño" is Spanish for "little boy," and is used to refer to the Baby Jesus.

How does El Niño usually affect the winter weather in the United States?

Because even the most dedicated scientists do not thoroughly understand El Niño (we do not know, for instance, why the trade winds suddenly die down and allow the warm water pool to move eastward), we can only describe certain tendencies in the weather during a strong El Niño year. In the past, El Niño has often brought heavy rains to southern California and to a portion of the South from Atlanta to Cape Hatteras; it can bring relatively mild winter temperatures to the northern third of the country. However, these effects are not consistent in every El Niño event on record.

To learn more about El Niño, a good place to start is the Center for Ocean-Atmospheric Prediction Studies Library index of El Niño-related Web sites.

The National Oceanic and Atmospheric Administration (NOAA) has very detailed information about El Niño:

• NOAA's El Niño Basics
  
• NOAA's El Niño Impacts
  
• NOAA's El Niño Forecasts

For a scientific discussion of El Niño-Southern Oscillation (ENSO) try the ENSO Colloquium Web site which features atmospheric, oceanic, societal, environmental, and policy perspectives on the phenomenon.