Originally defined in 1924 by Gilbert Walker as a low-latitude, planetary- scale “seesaw” in sea level pressure, with one pole in the eastern Pacific and the other in the western Pacific–Indian Ocean region. The pressure seesaw is associated with a global pattern of atmospheric anomalies in circulation, temperature, and precipitation. The primary timescale of the oscillation is interannual–multiyear, and it is now recognized to be primarily a response to basin-scale sea surface temperature variations in the equatorial Pacific arising from coupled ocean–atmosphere interactions, the opposite extremes of which are the El Niño and La Niña warm and cold events. See also ENSO.