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  3. What is an El Nino An El Nino is a temporary change in the climate of the Pacific Ocean, in the region around the equator. Y

What is an El Nino An El Nino is a temporary change in the climate of the Pacific Ocean, in the region around the equator. Y

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问题                          What is an El Nino
    An El Nino 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 strong 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 muses 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 sunwarned surface water) gets pulled up from below to replace the water pushed west. So, the normal situation is warm water (about 30 ℃; ) in the west, cold (about 22℃ ) in the east. In an El Nine, the winds pushing that water around get weaker. As a result, some of the warm water piled up in the west slumps hack 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 signs of an El Nine. But it doesn’t stop there. The warmer ocean then affects the windsit 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 Nino 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 is 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 leisurely 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 had some characteristics in common with Rosaby waves, but is somewhat faster and can only exist closer to the equator (say, within about 5 degrees of latitude around the equator).
    El Ninos often start with a Kelvin wave spreading from the western Pacific over towards South America. Perhaps you saw, on the TV news, the movie (produced by JPL) for the El Nine of 1997/98? It showed a whitish blob(白色团状物) (indicating a sea level some centimeters higher than usual) moving along the equator from Australia to South America. That is one of the hallmarks of a Kelvin wave, the early part of the El Nino process.
    When an El Nino 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 traveling, 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 Nino in the first place. The main point is that it shuts off when these funny interiorocean waves travel all the way ever to the coast of Asia, get reflected, and travel hack, a process that can take many months.
What effects does it have?
    A strong El Nine is often associated with wet winters over the southern U.S., as well as drought in Indonesia and Australia. Keep in mind that you’re not guaranteed these effects even though there is an El Nino going on; but the El Nino does make these effects more likely to happen. How long does it last?
    A strong El Nino 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 interiorocean waves to travel all the way over to the coast of Asia, get reflected, and travel back.
How often do we get them?
    El Nino happens 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.
How well can we predict El Nino?
    On average, complex computer models designed to predict El Nino can successfully do so 12 to 18 months in advance. However, it seems to vary by periods; sometimes El Nino is 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 Nino has already started. Trying to fix up the models is one of our research topics here at Scripps.
A strong El Nino can last two or three years ______.
选项
答案before conditions return to normal
解析 文章倒数第三段第一句话指出“A strong El Nino can last a year or more before conditions return to  normal．”强烈的厄尔尼诺现象在恢复正常之前可能要持续一年或更长时间，由此可知此处该填 before conditions return to normal。
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