D
The recent, apparently successful, prediction by mathematical models of an appearance of El Nino—the warm ocean How that periodically develops along the Pacific coast of South America has excited researchers．Jacob Bjerknes pointed out years ago how winds might create either abnormally warm or abnormally cold water in the eastern equatorial （赤道附近的） Pacific．Nevertheless, before the creation of the models, no one could explain why conditions should regularly change between appearances of the warm El Nino and the 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 and winds blow from the west into the warmer air rising over the warm water in the east．The contribution of the model is to show that the winds of an El Nino, which raise sea level in the east, send a signal to the west lowering sea level at the same time．According to the model, that signal is created as a negative （负的） Rossby wave, a wave of lower sea level, that moves westward parallel to the equator at 25 to 85 kilometers per day．Taking months to move across the Pacific, Rossby waves march to the western boundary of the Pacific basin, which is modeled as a smootli wall but in reality consists of quite irregular island chains．
When the waves meet the western boundary, they are reflected, and the model predicts that Rossby waves will be broken into many coastal Kelvin waves carrying the same negative sea-level signal．These eventually shoot toward the equator, and then head eastward along the equator drove by the earth at a．speed of about．250 kilometers per day．When enough Kelvin waves of adequate amplitude （振幅） arrive from the western Pacific, their negative sea-level signal overcomes the feedback mechanism, raising the sea level, and they begin to drive the system into the opposite cold mode．This produces a gradual change 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．
67．What is the passage mainly about?
A．How Rossby waves are found．
B．Where El Nino is formed．
C．What the models predict．
D．How the models work．
68．Where does El Nino often appear?
A．Along the western coast of the USA .
B．In the Pacific in the south, of the equator．
C．In the Pacific in the north of the equator．
D．Along the southern coast of South America．
69．What is right according to the passage?
A．Rossby waves and Kelvin waves move in opposite directions along the equator．
B．People could explain El Nino before the creation of the mathematical models．
C．Adequate sea-level waves can produce westward positive cold cycle．
D．The speed of Rossby waves is faster than that of Kelvin waves．
70．What does the underlined word "waves" in Paragraph 3 refer to?
A．Kelvin waves．       B．Sea-level signals．   C．Coastal waves．   D．Rossby waves．