# How El Niño Happens How weakening trade winds let warm Pacific water drift east, shifting rain and cutting off the cold, nutrient\-rich upwelling that feeds sea life off South America\. By sunny\-panda\-8b922f66 Topic: Earth & Geography [Open the interactive Jig](https://ask.jig.com/j/9K9hTkYNUKPE) ## Transcript ### 1. A Restless Ocean This is the Pacific Ocean along the equator, cut open like a slice of cake, from Indonesia on the left to South America on the right\. Every few years its warm water drifts east and changes weather around the world\. That shift is called El Niño\. Drag to look around\. Here the Pacific stretches nearly halfway around the planet\. ### 2. Trade Winds Push West Steady winds called trade winds blow from east to west along the equator\. They push the sun\-warmed surface water toward Asia, where it piles up into a deep warm pool\. The sea near Indonesia stands about half a metre higher than near Ecuador, and its surface is about 8 °C warmer\. ### 3. Cold Water Rises Near South America, the winds sweep surface water away, so cold water rises from the deep to take its place\. This is called upwelling\. The cold water is full of nutrients that feed plankton, fish, seabirds and sea lions\. The thermocline is the boundary between the warm top layer and the cold water below\. ### 4. A Loop of Air Over the warm pool, moist air rises, builds tall clouds and pours rain on Indonesia\. High up, the air flows east, then sinks over the cool water near South America, where skies stay dry\. Near the surface it flows back west as the trade winds, closing the loop\. Scientists call this loop the Walker circulation\. ### 5. The Winds Ease Every few years the trade winds weaken, and bursts of wind may even blow the other way\. Without that push, the piled\-up warm water slides back east as a slow, giant wave beneath the surface\. Press the button to ease the winds\. This underwater wave is a Kelvin wave\. Scientists are still studying exactly what makes the winds weaken\. ### 6. Warm Water Reaches the Americas The warm layer now spreads across the ocean and stops cold water rising off South America\. With fewer nutrients, plankton and fish become scarce\. A warmer east weakens the winds even more, so the ocean and the air keep El Niño going\. El Niño is declared when the central Pacific stays at least 0\.5 °C warmer than average for several months, with matching changes in the winds\. ### 7. Rain Moves East The rising air and its rain clouds follow the warm water toward the middle of the Pacific\. Indonesia and Australia can turn dry, while Peru and Ecuador get heavy rain\. Weather shifts far beyond the Pacific too\. In strong El Niño years, some normally dry places get nearly twice their usual rain\. ### 8. Back and Forth El Niño usually fades within about a year, and the winds return\. Sometimes they blow harder than usual and cool the east even more: that opposite pattern is La Niña\. Slide the trade winds to explore the swing, which comes back every two to seven years\. Fishermen in Peru named it El Niño, Spanish for “the boy”, because the warm water often arrived around Christmas\.