What causes El Niño?
Usually, the wind blows strongly from east to west along the equator in the Pacific. This actually piles up water (about half a meter's worth) in the western part of the Pacific. In the eastern region, deeper water (which is colder than the sun-warmed surface water) gets pulled up from below to replace the water pushed west. So, the normal situation is warm water (about 30 C) in the west, cold (about 22 C) in the east.
During an El Niño period, the winds pushing that water around weaken. As a result, some of the warm water piled up in the west slumps back down to the east, and not as much cold water gets pulled up from below. Both these tend to make the water in the eastern Pacific warmer.
But it doesn't stop there. The warmer ocean water then affects the winds by lessening their strength! So if the winds get weaker, then the ocean continues to get warmer. This positive feedback is what makes El Niño grow.
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