El Niño, A Non-technical Description

El Niño, A Non-technical Description

An El Niño is a temporary change in the climate of the Pacific ocean, in the region around the equator. You can see its effects in both the ocean and atmosphere, generally in Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees celsius. At the same time, the place where hefty thunderstorms occur on the equator moves eastward. Although those might seem like small differences, it nevertheless can have big effects on the world's climate.

o What causes it?

o What makes it stop growing?

o What effects does it have?

o How long does it last?

o How often do we get them?

o How well can we predict El Niño?

o A more technical explanation

What causes it?

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 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 there. 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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So what makes it stop growing?

The ocean is full of waves, but you might not know how many kinds of waves there are. There's one called a Rossby wave that is quite unlike the waves you see when you visit the beach. It's more like a distant cousin to a tidal wave. The difference is that a tidal wave goes very quickly, with all the water moving pretty much in the same direction. In a Rossby wave, the upper part of the ocean, say the top 100 meters or so, will be lesirely sliding one way, while the lower part, starting at 100 meters and going on down, will be slowly moving the other way.