The El Niño Southern Oscillation is a see-saw in tropical sea level pressure between the eastern and western hemispheres. This see-saw causes the El Niño and the La Niña events that affect weather patterns throughout the world. This term is used to describe the full range of variability observed in the Southern Oscillation Index (SOI), including both El Niño and La Niña events.

El Niño: About every 3-4 years, the sea level pressure increases in the western tropical Pacific and Indian Ocean regions and unusually low sea level pressures develop in the southeastern tropical Pacific, causing higher sea temperatures in the in the eastern and central equatorial Pacific Ocean. It is also characterized by large scale weakening of the trade winds.

La Niña: This event is the opposite extreme of the 3-4 year cycle. It is characterized by unusually low pressures west of the date line and high pressures east of the date line, causing extremely cold equatorial Pacific sea surface temperatures.

Southern Oscillation Index (SOI): The Southern Oscillation Index is the normalized difference in surface pressure between Tahiti, French Polynesia and Darwin, Australia. It is a measure of the strength of the trade winds, which have a component of flow from regions of high to low pressure. High SOI (large pressure difference) is associated with stronger than normal trade winds and La Niña conditions, and low SOI (smaller pressure difference) is associated with weaker than normal trade winds and El Niño conditions.

In the southwestern US, the La Niña cycle causes drier than normal conditions from the late summer throughout the winter and the El Niño cycle causes wetter than normal conditions during the late summer and winter.

See the NOAA/OGP ENSO Experiment Research Archives or the Climate Prediction Center El Niño/La Niña Home for more information.