If you ask "Why are there trade winds?", I will say "That's simple, there are trade winds because the water is cold in the east and warm in the west". But if you ask "Why is the water cold in the east and warm in the west?", I'll say "That's simple too, it's because there are trade winds". The ocean and the atmosphere are bound together in a system where each one affects the other.

When the trade winds (shown in cream) in the animation blow less, the waters warm up more to the east (shown in red). But no-one can say which causes the other. It is a chicken and the egg scenario!

Usually, the wind blows strongly from east to west along the equator in the Pacific. This actually piles up water (about half a meter) in the western part of the Pacific. In the eastern part, 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. In an El Niño, the winds pushing that water around get weaker. 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, which is one of the hallmarks of an El Niño. But it doesn't stop here. The warmer ocean then affects the winds--it makes the winds weaker! So if the winds get weaker, then the ocean gets warmer, which makes the winds get weaker, which makes the ocean get warmer ... this is called a positive feedback, and is what makes an El Niño grow.

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