What is El Nino?

El Niño is the warm phase of the El Niño Southern Oscillation (commonly called ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including off the Pacific coast of South America. It refers to the cycle of warm and cold temperatures, as measured by sea surface temperature (SST) of the tropical central and eastern Pacific Ocean.

El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. The cool phase of ENSO is called "La Niña" with SST in the eastern Pacific below average and air pressures high in the eastern and low in western Pacific. The ENSO cycle, both El Niño and La Niña, causes global changes of both temperatures and rainfall. Mechanisms that cause the oscillation remain under study.

Developing countries dependent upon agriculture and fishing, particularly those bordering the Pacific Ocean, are the most affected. Recently, a robust tendency to more frequent extreme El Niños has been reported to have occurred in agreement with a recent model prediction for the future. El Niño is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value.

The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode". The first signs of an El Niño are a weakening of the Walker circulation and strengthening of the Hadley circulation. It also sees a rise in surface pressure over the Indian ocean, Indonesia and Australia.