So What is an El Niño, Anyway?

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.

What causes it?
What makes it stop growing?
What effects does it have?
How long does it last?
How often do we get them?
How well can we predict El Niño?
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.

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. After a while they switch directions. Everything happens very slowly and inside the ocean, and you can't even see them on the surface. These things are so slow, they can take months or years to cross the oceans. If you had the patience to sit there while one was going by, you'd hardly notice it; the water would be moving 100 times slower than walking speed. But they are large, hundreds or thousands of kilometers in length (not height! Remember, you can hardly see them on the surface), so they can have an effect on things. Another wave you rarely hear about is called a Kelvin wave, and it has some characteristics in common with Rossby waves, but is somewhat faster and can only exist close to the equator (say, within about 5 degrees of latitude around the equator).

El Ninos often start with a Kelvin wave propagating from the western Pacific over towards South America. Perhaps you saw, on the TV news, the movie (produced by JPL) for the El Nino of 1997/98? It showed a whitish blob (indicating a sea level some centimeters higher than usual) moving along the equator from Australia to South America. That's one of the hallmarks of a Kelvin wave, the early part of the El Nino process.

When an El Niño gets going in the middle or eastern part of the Pacific, it creates Rossby waves that drift slowly towards southeast Asia. After several months of travelling, they finally get near the coast and reflect back. The changes in interior ocean temperature that these waves carry with it "cancel out" the original temperature changes that made the El Niño in the first place. I'm being deliberately vague here becuase it's complicated; look at the "For Further Reading" link or the "More Technical Explanation" link for more information. The main point is that it shuts off when the these funny interior-ocean waves travel all the way over to the coast of Asia, get reflected, and travel back, a process that can take many months.

What effects does it have?
A strong El Niño is often associated with wet winters over the southeastern US, as well as drought in Indonesia and Australia. Keep in mind that you aren't guaranteed these effects even though there is an El Niño going on; but the El Niño does make these effects more likely to happen.

How long does it last?
A strong El Niño can last a year or more before conditions return to normal. If you read the bit above about Rossby and Kelvin waves (you did, didn't you?) then you know that it lasts more or less as long as it takes the interior-ocean waves to travel all the way over to the coast of Asia, get reflected, and travel back. You can also look at the Historical El Niño section, which has a plot showing the last 30 years of El Niños, and judge for yourself.

How often do we get them?
El Niños happen irregularly, but if you want to impress people at cocktail parties, you might mention that we usually get one every three to seven years. Note the word "usually": sometimes they turn up more frequently, sometimes less. You can also look at the Historical El Niño section, which has a plot showing the last 30 years of El Niños, and judge for yourself (deja vu).

How well can we predict El Niño?
On average, complex computer models designed to predict El Niño can successfully do so 12 to 18 months in advance. However, it seems to vary by episode; sometimes El Niños are predicted quite well, with plenty of advance notice from the models, while other times they are predicted poorly, with the models not picking them up until the El Niño has already started. Trying to fix up the models is one of our research topics here at Scripps.