Why a warm patch of Pacific Ocean changes the weather everywhere.
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This is the Pacific Ocean. It is so big it covers about a third of the whole Earth. On the far left are Indonesia and Australia. On the far right is Peru, in South America.
The trade winds (the white streaks) blow from east to west, like blowing across a bath. They push the warm top water west, so it piles up near Indonesia (red). Warm sea makes lots of rain clouds there. Near Peru, cold water comes up from the deep instead (blue). That is called upwelling.
Let’s slice the ocean open and look from the side. The warm water is only a layer on top, like a warm blanket. Under it, the sea is cold and dark. The wind pushes the blanket west, so it’s thick near Indonesia and thin near Peru. Near Peru, the cold water is close enough to the top to come up.
The warm sea heats the air above it. Warm, wet air rises and builds giant thunderclouds, so it rains a lot near Indonesia. High up, the air flows east. Over the cold sea near Peru it sinks, so it’s dry there. Then it blows back west along the sea as the trade winds. Round and round!
Drag the slider to make the trade winds weaker or stronger. Watch the warm water, the rain clouds and the side view.
Fishermen in Peru noticed that every few years the sea got warm around Christmas. Fewer fish came, because the cold, food-rich water stopped coming up. They called it El Niño, "the boy", after baby Jesus. It happens every 2 to 7 years and usually lasts about a year.
El Niño moves the world’s rain around. Spin the globe to find all the changes. Places that are usually wet can get dry, and dry places can flood.
All that warm water lets heat out into the air, so El Niño years are often extra hot. global warming is already heating the planet, and El Niño adds a bump on top. After the big El Niño of 2023–24, 2024 became the hottest year since records began in 1850.
Scientists keep a close watch on one patch of sea, the Niño 3.4 box. When it stays at least 0.5 °C warmer than normal for months, they declare an El Niño. About 50 buoys (the yellow dots), tied to the sea floor, measure the water across the Pacific. Satellites check it from space.
The trade winds have gone weak. An El Niño is here!
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).