新GRE阅读内容：厄尔尼诺现象

新GRE阅读内容：厄尔尼诺现象 　　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 int 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, that 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. 　　数学模型最近对“厄尔尼诺”的出现所作出的显然是成功的预测使研究者们兴奋不已。所谓厄尔尼诺，即一股沿南美洲太平洋海岸周期性发展的海洋暖流。早在20年之前，雅各·皮叶克尼斯指出了风何以可能在东部赤道太平洋地区形成反常温暖或反常寒冷的水域。虽然如此，在上述数学模型得以发展之前，还没有任何人能够解释情况为什么应该有规律地从一种转变为另一种，正如在暖厄尔尼诺和冷厄尔尼诺出现之间周期性的交替变化中所发生的那样。问题的——至少如果目前将海洋的行为与大气层的行为联系起来的模型是正确的话——应从海洋中寻找。 　　长期以来众所周知，在厄尔尼诺出现期间，两个条件必须存在：格外温暖的水流沿着东太平洋延伸，主要是沿厄瓜多尔和秘鲁海岸;风从西部吹向在东部温暖的海水上空升起的较暖的空气。这些风倾向于造成一种反馈机制，迫使较为温暖的层水形成一“堆状物”，阻挡住东部较深层的、寒冷的水的正常上涌，并进一步使东部的水温度上升，从而更进一步增强风的力度。该模型的贡献是旨在证明，致使海平面在东部海域升高的厄尔尼诺水流的风会同步向西部海域发送一信号，导致海平面降低。按照该模型，那一信号作为一种负罗斯比波而被产生形成，即一种致使海平面沉降、或负增长的波浪，此波浪会以每天85公里的速度平行于赤道西行，此海盆被模拟为一堵平滑的墙，但实际上却是由极不规则的群岛系列构成，如菲律宾群岛和印度尼西亚群岛。