El Niño

El Niño, the Spanish name for "Christ Child", is the name given to the occasional development of warm surface waters in the Pacific Ocean along the coast of equatorial South America. El Niño occurs roughly every 2 to 7 years, usually around Christmas, and lasts usually for a few weeks or months. Sometimes an extremely warm event can develop that lasts much longer. In the early 1990s, a strong El Niño developed in 1991 that remained until 1995. It is possible that global warming could bring more frequent and intense El Niño weather patterns.

The formation of El Niño is linked with the cycling of a Pacific Ocean circulation pattern known as the El Niño Southern Oscillation or ENSO. In a normal year, low atmospheric pressure develops over northern Australia and Indonesia, with high pressure over the Pacific. Consequently, winds over the Pacific move from east to west. The easterly flow of the trade winds carries warm surface waters westward bringing rainstorms to Indonesia and northern Australia. Along the coast of Peru and Ecuador, cold deep water wells up to the surface to replace the warm water that is pulled to the west.

In an El Niño year, low pressure over northern Australia is replaced by high pressure, whilst air pressure falls over large areas of the central Pacific and along the coast of South America. This change in pressure pattern causes the normal easterly winds to be reduced and sometimes reversed. This allows warm equatorial water to flow or "slop" back eastward across the Pacific, and accumulate along the coastlines of Peru and Ecuador. The warm water off the equatorial South American coast cuts off the upwelling of cold nutrient-rich deep ocean water, drastically reducing fish populations in this part of the Pacific Ocean. The warm moist air on the eastern side of the Pacific leads to drastically increased rates of rainfall and flooding. In contrast, the high pressure and cooler waters around Australasia reduce the formation of storm clouds, leading to drought and sometimes extensive bush fires as the vegetation dries up.

After an El Niño event, weather conditions usually return back to normal. However, in some years the easterly winds can become extremely strong and an abnormal accumulation of cold water can occur in the central and eastern Pacific. This event is called a La Niña. The cold La Niña events sometimes (but not always) follow El Niño events.

El Niño has the capacity to influences atmospheric wind patterns worldwide. As well as flooding in Peru and Ecuador, and drought in Indonesia and Australia, possible impacts include a shifting of the jet stream, storm tracks and monsoons, producing unseasonable weather over many regions of the globe. During the El Niño event of 1982-1983, some of the abnormal weather patterns which were observed included drought in Southern Africa, Southern India, Sri Lanka, Philippines, Indonesia and Australia; heavy rain and flooding in Bolivia, Ecuador, Northern Peru and Cuba; and hurricanes in Tahiti and Hawaii. Because El Niño may influence the mid-latitude Northern Hemisphere jet stream, even the weather in Europe and North America can be influenced. The most recent El Niño episode in 1997 and 1998 brought record high winter temperatures to many areas in Europe including the UK. Globally, 1998 became the warmest year on record.