El Niño is a periodic warming of the central and eastern Pacific Ocean that disrupts weather patterns across the globe, bringing drought and wildfires to some regions while delivering storms and flooding to others. The phenomenon, whose name dates back to 17th-century Peruvian fishers who noticed warmer waters around Christmastime, is now recognised as one of the most powerful drivers of year-to-year climate variability.
Under normal conditions, the sun's heat and Earth's rotation fuel east-to-west trade winds around the tropics, which push water into a warm pool near Indonesia. As this pool grows, it builds up energy. Richard Allan of the University of Reading likens it to a coiled spring waiting to emerge. Every two to seven years, natural chaotic disturbances release that spring. Bursts of westerly winds slow the easterly trade winds, allowing swells of warm water to roll eastwards across the Pacific, raising sea levels along the coasts of Peru, Colombia and Ecuador. These Kelvin waves signal that El Niño is coming.
A self-reinforcing cycle known as the Bjerknes feedback then takes hold. The blob of warm water suppresses the upwelling of cold, nutrient-rich deep water that normally makes Peru's anchovy fishery so rich, further warming the surface. The warm sea heats the air above it, causing the air to expand. The resulting fall in atmospheric pressure slows the trade winds even more, since winds flow from high to low pressure, allowing still more warm water to slosh into the eastern Pacific.
The swathe of warm water stretching across the tropical Pacific releases trillions of joules of heat energy into the atmosphere, raising worldwide temperatures on top of global warming. This is why scientists expect 2027 to be the hottest year on record. But El Niño also drastically alters weather patterns. Normally, warm air rises over the warm pool, carrying moisture that falls as thunderstorms, which is why rainforests flourish in places like Indonesia. Denuded of moisture, this warm, dry air reaches the top of the troposphere and spreads out to the east and west. It descends over the eastern Pacific and the Horn of Africa, fuelling a belt of air loops around the tropics called the Walker circulation.
During El Niño, the warming of the eastern Pacific moves this upward convection eastward, shifting the entire Walker circulation. Dry, warm air that was sinking over the Horn of Africa and the eastern Pacific now sinks over Indonesia and the Amazon, encouraging drought and wildfires there. Over eastern Africa, warm air is rising, increasing rainfall and the risk of flooding. Meanwhile, the warming waters create a stronger temperature contrast between the tropical and northern Pacific, strengthening the jet stream and bringing more storms and wet weather to the southern United States. It can bring storms and rain to the UK as well, in certain circumstances.
Tim Stockdale of the European Centre for Medium-Range Weather Forecasts says that if deep convection in the atmosphere moves around, it changes things locally but also starts to change the whole circulation of the atmosphere. That is why El Niño is a big deal, not just some local process. The phenomenon was first linked to global weather by meteorologist Jacob Bjerknes in 1969, who showed that swells of warm water in the Pacific end up wreaking havoc on weather across the globe. Earlier, in the early 20th century, physicist Gilbert Walker had noticed that low atmospheric pressure and rainy weather over the western Pacific and dry, high pressure over the eastern Pacific periodically flipped.
Satellite data showed higher-than-normal sea-surface temperatures in the central and eastern Pacific in June 2026, a few days before El Niño was declared. The current event is expected to intensify through the year, with consequences for agriculture, water resources and disaster preparedness from South America to Southeast Asia and beyond.