In the 1600s, fishers in Peru saw that every few years around Christmastime, the waters there warmed, and they caught fewer anchovies. They called this 鈥淓l Ni帽o鈥� 鈥� the boy 鈥� after the Christ child.

In the early 20th century, the physicist Gilbert Walker noticed that the prevailing low atmospheric pressure and rainy weather over the western Pacific and dry, high pressure over the eastern Pacific periodically flipped. But it wasn鈥檛 until 1969 that meteorologist Jacob Bjerknes linked the two, showing that swells of warm water in the Pacific Ocean end up wreaking havoc on weather across the globe.

Under normal conditions, the sun鈥檚 heat and Earth鈥檚 spin fuel east-to-west trade winds around the tropics, which blow water into a 鈥渨arm pool鈥� near Indonesia. As this warm pool grows, it builds up energy. 鈥淚t could be a bit like a coiled spring waiting to emerge,鈥� says at the University of Reading, UK.

Every two to seven years, natural chaotic disturbances release that spring. Bursts of westerly winds can blow against and slow the easterly trade winds, allowing swells of warm water to across the Pacific Ocean, raising sea levels along the coasts of Peru, Colombia and Ecuador. These so-called Kelvin waves indicate that El Ni帽o is coming.

A self-reinforcing cycle called the kicks in. The blob of warm water suppresses the upwelling of cold, nutrient-rich deep water that normally makes Peru鈥檚 anchovy fishery so rich, further warming the surface water. The warm sea heats the air above it and causes it to expand. The resulting fall in atmospheric pressure slows the trade winds down even more, since winds flow from high to low pressure. This allows still more warm water to slosh back into the eastern Pacific.

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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 it also drastically alters weather patterns, bringing drought and wildfires to some areas and storms and flooding to others.

Normally, warm air rises over the warm pool, carrying moisture that falls out in thunderstorms, which is why rainforests flourish in places like Indonesia. Denuded of moisture, this warm, dry air then 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 such air loops around the tropics called the Walker circulation.

During El Ni帽o, however, the warming of the eastern Pacific moves this upward convection eastward, shifting the entire Walker circulation. Now dry, warm air that was sinking over the Horn of Africa and the eastern Pacific is sinking 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 also 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 US. It can bring storms and rain to the UK as well, in certain circumstances.

鈥淚f this deep convection in the atmosphere moves around, it changes things locally, but it also then starts to change the whole circulation of the atmosphere,鈥� says at the European Centre for Medium-Range Weather Forecasts. 鈥淭hat鈥檚 why El Ni帽o is a big deal, not just some local process.鈥�