El Niño is a large-scale ocean-atmospheric phenomenon characterised by unusually warm ocean temperatures in the Equatorial Pacific. After the seasons, El Niño and the associated La Niña phenomenon are most significant causes of year-to-year natural climate variability on Earth.
El Niño is the warm phase of a natural oscillation in the Earth's climate system. It involves both the ocean and the atmosphere and occurs when the tropical surface waters of the Pacific Ocean warm, the westward trade winds slacken, and the region of strongest rainfall moves eastwards, from Indonesia out into the Pacific Ocean.
It is not totally predictable but on average occurs about once every 4 years and lasts between 12 and 18 months. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere-ocean phenomenon is collectively known as El Niño-Southern Oscillation, or ENSO.
Global temperatures tend to be slightly higher during El Niño events. Its effect on the tropical atmosphere also sends out a ripple of influences that can have an effect on global weather patterns.
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- El Niño is an example of natural climate variability and a potential source of error in climate projections. However, uncertainty in climate projections that arises from natural variability can be estimated, and this has been done in UKCP09. This is described in the UKCP09 Climate Change Projections report.