What are El Niño and La Niña?
El Niño is a natural feature of the global climate system. Originally it was the name given to the periodic development of unusually warm ocean waters along the tropical South American coast and out along the Equator to the dateline, but now it is more generally used to describe the whole "El Niño - Southern Oscillation (ENSO) phenomenon", the major systematic global climate fluctuation that occurs at the time of an "ocean warming" event.
El Niño and La Niña refer to opposite extremes of the ENSO cycle, when major changes in the Pacific atmospheric and oceanic circulation occur.
When neither El Niño nor La Niña are present, (usually referred to as "neutral" or normal conditions), trade winds blow westward across the Pacific, piling up warm surface water so that Indonesian sea levels are about 50 cm higher than those in Ecuador. Cool, nutrient-rich sea water "wells up" off the South American coast, supporting marine ecosystems and fisheries. Relatively cold sea temperatures also extend along the equator from South America towards the central Pacific. High rainfall occurs in the rising air over the warmest water to the west, whereas the colder east Pacific is relatively dry.
During El Niño events, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of "up-welling" off South America. Heavy rainfall and flooding occur over Peru, and drought over Indonesia and Australia. The supplies of nutrient rich water off the South American coast are cut off due to the reduced up-welling, adversely affecting fisheries in that region. In the tropical South Pacific the pattern of occurrence of tropical cyclones shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.
During La Niña events, the trade winds strengthen, and the pattern is a more intense version of the "normal conditions", with an even colder tongue of sea surface temperatures in the eastern equatorial Pacific.