El Niño–Southern Oscillation (ENSO)

ENSO describes linked changes in tropical Pacific Ocean temperatures and the atmosphere above them. Its name joins El Niño—the warm phase—with the Southern Oscillation, a seesaw in atmospheric pressure and circulation across the tropical Pacific. La Niña is the cool phase; periods that meet neither phase's criteria are called ENSO-neutral. The pattern recurs irregularly, rather than on a fixed calendar, and individual episodes vary in strength and duration.

In the tropical Pacific, easterly trade winds normally push surface water toward the west, helping maintain warmer water in the western basin and cooler conditions farther east. Winds and ocean temperatures influence one another: a change in one can reinforce changes in the other. During El Niño, the trade winds weaken and unusually warm surface water extends into the central and eastern equatorial Pacific. The deeper warm layer there makes the usual coastal upwelling less effective, reducing the supply of nutrient-rich water to the surface off western South America. During La Niña, stronger easterly winds accompany cooler-than-usual surface waters in the central and eastern tropical Pacific. These linked oceanic and atmospheric changes are what distinguish ENSO from a change in sea temperature considered by itself.

El Niño and La Niña are identified by sustained departures from typical sea-surface temperatures in parts of the equatorial Pacific, together with the expected atmospheric response. Neutral conditions describe the absence of a sufficiently developed warm or cool phase; they do not mean that weather is calm or that extreme events cannot occur. Monitoring centres compare ocean observations with winds, pressure and tropical rainfall to assess whether the atmosphere is coupled to the ocean signal. Definitions and operational indicators can differ among agencies, so an ENSO label is based on a defined set of criteria rather than a single short-lived temperature fluctuation.

By shifting where heat and moisture enter the atmosphere over the Pacific, ENSO can affect atmospheric circulation and seasonal temperature and rainfall patterns far beyond the tropical ocean. For example, El Niño is often associated with wetter conditions in parts of western South America and drier conditions in parts of Australia and Indonesia; La Niña often shifts the odds in the opposite direction. The response depends on the event's timing, duration and strength, the season, and other influences on climate. Typical patterns are probabilities, not guarantees: local weather can depart from them, and severe weather can occur during neutral ENSO conditions too.

Meteorological agencies use buoy, satellite and other ocean-atmosphere observations with climate models to monitor ENSO and estimate how an episode may develop. Because the pattern evolves over months, forecasts can provide advance information for seasonal planning in areas such as water management, agriculture, fisheries and disaster preparedness. They do not specify the weather on a particular day, and useful regional guidance must account for local conditions and other climate influences as well as ENSO.