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  3. The recent, apparently successful prediction by mathematical models of an appearance of EI Nino—the warm ocean current that peri

The recent, apparently successful prediction by mathematical models of an appearance of EI Nino—the warm ocean current that peri

admin2020-12-14  68
问题     The recent, apparently successful prediction by mathematical models of an appearance of EI Nino—the warm ocean current that periodically develops along the Pacific coast of South America—has excited researchers. Jacob Bjerknes pointed out over 20 years ago how winds might create either abnormally warm or abnormally cold water in the eastern equatorial Pacific. Nonetheless, until the development of the models no one could explain why conditions should regularly shift from one to the other, as happens in the periodic oscillations between appearances of the warm EI Nino and the cold so-called anti-El Nino. The answer, at least if the current model that links the behavior of the ocean to that of the atmosphere is correct, is to be found in the ocean.
    It has long been known that during an El Nino, two conditions exist: unusually warm water extends along the eastern Pacific, principally along the coasts of Ecuador and Peru, and winds blow from the west into the warmer air rising over the warm water in the east. These winds tend to create a feedback mechanism by driving the warmer surface water into a "pile" that blocks the normal upwelling of deeper, cold water in the east and further warms the eastern water, thus strengthening the wind still more. The contribution of the model is to show that the winds of an El Nino, which raise sea level in the cast, simultaneously send a signal to the west lowering sea level. According to the model, that signal is generated as a negative Rossby wave, a wave of depressed, or negative, sea level, mat moves westward parallel to the equator at 25 to 85 kilometers per day. Taking months to traverse the Pacific, Rossby waves march to the western boundary of the Pacific basin, which is modeled as a smooth wall but in reality consists of quite irregular island chains, such as, the Philippines and Indonesia.
    When the waves meet the western boundary, they are reflected, and the model predicts that Rossby waves will be broken into numerous coastal Kelvin waves carrying the same negative sea-level signal. These eventually shoot toward the equator, and then head eastward along the equator propelled by the rotation of the Earth at a speed of about 250 kilometers per day. When enough Kelvin waves of sufficient amplitude arrive from the western Pacific, their negative sea-level signal overcomes the feedback mechanism tending to raise the sea level, and they begin to drive the system into the opposite cold mode. This produces a gradual shift in winds, one that will eventually send positive sea-level Rossby waves westward, waves that will eventually return as cold cycle-ending positive Kelvin waves, beginning another warming cycle.
According to the model presented in the passage, which of the following normally signals the disappearance of an El Nino?
选项 A、The arrival in the eastern Pacific of negative sea-level Kelvin waves.
B、A shift in the direction of the winds produced by an anti-El Nino.
C、The reflection of Kelvin waves reaching the eastern border of the Pacific.
D、An increase in the speed at which negative Rossby waves cross the Pacific.
答案A
解析 推断题。联系原文第二、三段内容，第二段介绍了厄尔尼诺现象存在的两种现象——异常的暖流沿着太平洋东海岸延伸和从西方来的季风吹进在东方暖流中产生的更暖的空气中。第三段指出．如果从西部来的“开氏波浪”达到足够的量．它们就会克服提升海平面的反馈机制，促使该机制变成相反的寒冷模式，进而导致季风的逐渐转变。最终促使“罗斯比波浪”向西流动．寒冷的“开氏波浪”结束循环返回。开始另一个暖流循环。这说明，“开氏波浪”的返回就预示着厄尔尼诺现象的结束。和选项A的表述一致。
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