An ocean-atmosphere phenomenon in the tropical Pacific that occurs at irregular intervals between two to seven years.
During El Niño the central and eastern part of the Pacific ocean warm above normal with global consequences.
El Niño is characterized by an unusual warming of the ocean off the coast of Ecuador and Peru during the spring and early summer. The rise in sea temperature culminates at the end of the year, and then usually returns to normal again during the next spring and summer.
Sometimes the eastern Pacific continues cooling below the normal sea temperature after an El Niño, this phenomenon is called La Niña.
During normal non-El Niño conditions the surface water in the western part of the Pacific is significantly warmer than in the eastern. This is mainly because the eastern wind (the trade wind) around the equator pushes the warm surface water west. This simultaneously causes an upwelling of nutritious deep cold water off the west coast of South America, supporting a rich marine ecosystem. Due to the evaporation from the warm water in the western Pacific, the region normally experience more rainfall than the relatively dry eastern Pacific.
El Niño starts by a temporary reduction of the trade wind in the western Pacific that allows a wave of warm water to slowly move east. When the warm water reaches the east Pacific, it reduces the temperature difference that normally exist between the western and eastern Pacific. This causes a range of atmospheric changes. Rainfall increases in the eastern and central Pacific, while it decreases in the western Pacific causing drought in Indonesia and Australia. Furthermore, there is a decrease in the eastern wind that would normally maintain the temperature difference in the Pacific.
The effects of El Niño are global, and for example also affect the climate in North America. Climate models based on data collected during previous El Niño conditions also suggest an influence on the weather in Europe, although probably weak.
Eventually, usually after one year, the wave of warm water from the east bounces off the west coast of South America and returns west. The normal conditions then slowly return, as the Pacific cools again in the east.