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. El Niño Southern Oscillation 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.