El Niño is a large scale ocean-atmospheric phenomenon characterised by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which characterised by unusually cold ocean temperatures in the Equatorial Pacific. After the seasons, El Niño and La Niña are the single largest cause of year-to-year natural climate variability on Earth.
El Niño (Spanish for 'the boy') 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.
La Niña (Spanish for 'the girl') is the cold phase of a natural oscillation in the Earth's climate system. It also involves both the ocean and the atmosphere but tends to be weaker than El Niño. It occurs when the tropical surface waters of the Pacific Ocean cool, the westward trade winds strengthen, and the region of strongest rainfall is confined to South-East Asia and the west Pacific. A La Niña event will typically last between 12 and 18 months and often follows an El Niño event.
Both El Niño and La Niña affect the weather globally. Global temperatures tend to be slightly lower during La Niña events and slightly higher during El Niño events. Their effect on the tropical atmosphere also sends out a ripple of influences that can have an effect on global weather patterns.
El Niño and La Niña are examples of natural climate variability, 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 Section 2.2 of the UKCP09 Climate change projections report.