El Niño

El Niño is a recurring warming of surface waters in the central and eastern equatorial Pacific Ocean that changes rainfall and winds far beyond the Pacific. It is the warm phase of the El Niño-Southern Oscillation (ENSO), a coupled ocean-atmosphere phenomenon involving changes in tropical Pacific temperatures, atmospheric pressure and circulation.

How El Niño reorganises tropical circulation

Normally, easterly trade winds push warm surface water towards the western Pacific, supporting rising moist air and heavy rainfall near Indonesia. Cooler water rises in the eastern equatorial Pacific, where conditions are relatively dry.

• During El Niño, the trade winds weaken, allowing warm water to spread eastward and reducing the upwelling of cold water in the eastern equatorial Pacific.
• The Walker circulation, the east-west tropical atmospheric circulation, weakens and reorganises as rising air and rainfall shift towards the central and eastern Pacific.
• Ocean warming and weakened winds can reinforce each other through positive feedback, helping the event develop.

How its influence spreads globally

The displacement of tropical rainfall changes where heat is released into the atmosphere through condensation. This disturbs large-scale atmospheric waves and alters winds beyond the tropics.

• These teleconnections can shift jet streams and storm tracks, changing the routes and intensity of weather systems far from the Pacific.
• Indonesia and parts of Australia often experience drier conditions, while coastal Peru and Ecuador can receive unusually heavy rainfall and flooding.
• During Northern Hemisphere winter, the southern United States often becomes wetter, while parts of Canada experience warmer conditions.
• El Niño generally increases Atlantic vertical wind shear, tending to suppress hurricane activity there; it does not prevent all hurricanes.

Implications for India and global temperatures

El Niño often favours a weaker Indian summer monsoon by altering tropical circulation and creating conditions less favourable for rainfall over India. However, this is a probabilistic relationship, not a rule that every El Niño produces drought.

• The Indian Ocean Dipole, other ocean conditions and internal atmospheric variability can modify the monsoon response.
• El Niño tends to raise global mean surface temperature temporarily as the tropical Pacific releases more heat to the atmosphere.
• Its effects occur alongside human-driven climate change: El Niño is natural variability, not the underlying cause of long-term global warming.