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 “El Niño” – the boy – 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’t 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. Mysterious ‘cold blob’ in the Atlantic suggests the AMOC is weakening Under normal conditions, the sun’s heat and Earth’s spin fuel east-to-west trade winds around the tropics, which blow water into a “warm pool” near Indonesia. As this warm pool grows, it builds up energy.
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 roll across the Pacific Ocean, raising sea levels along the coasts of Peru, Colombia and Ecuador. These so-called Kelvin waves indicate that El Niño is coming.
A self-reinforcing cycle called the Bjerknes feedback kicks in. 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 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. 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.
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