El Niño–Southern Oscillation (ENSO) Mechanism

ENSO is a coupled ocean-atmosphere phenomenon originating in the tropical Pacific Ocean. Under normal conditions, easterly trade winds push warm surface water westward, raising sea surface temperatures (SSTs) in the western Pacific and Indonesia. El Niño events occur when these trade winds weaken or reverse — warm water surges eastward toward the central and eastern Pacific, disrupting the normal Walker Circulation (the east-west atmospheric overturning cell above the Pacific, named after Gilbert Walker who discovered the Southern Oscillation in the early 20th century).