La Niña is a large-scale ocean-atmospheric phenomenon characterised by unusually cold ocean temperatures in the Equatorial Pacific. After the seasons, La Niña and the associated El Niño phenomenon are the most significant causes of year-to-year natural climate variability on Earth.
La Niña 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.
Global temperatures tend to be slightly lower during La Niña 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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· La Niña is an examples 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 section 2.2 of the UKCP09 Climate Change Projections report.