The recent, apparently successful, prediction by mathematical models of an appearance of El 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 acro 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 El Nino and the cold so-called anti-EI 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: (1) unusually warm water extends along the eastern Pacific, principally along the coasts of Ecuador and Peru, and (2) 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 east, 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 dc1y. When enough Kelvin waves of sufficient amplitude arrive from the western Pacific, their negative sea-level signal overcomes the feed back 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.

21. The primary function of the passage as a whole is to

(A) introduce a new explanation of a physical phenomenon
(B) explain the difference between two related physical phenomena
(C) illustrate the limitations of applying mathematics to complicated physical phenomena
(D) indicate the direction that research into a particular physical phenomenon should take
(E) clarify the differences between an old explanation of a physical phenomenon and a new model of it



22. Which of the following best describes the organization of the first paragraph?

(A) A theory is presented and criticized.
(B) A model is described and evaluated.
(C) A result is reported and its importance explained.
(D) A phenomenon is noted and its significance debated.
(E) A hypothesis is introduced and contrary evidence presented.



23. According to the passage, which of the following features is characteristic of an El Nino?


(A) Cold coastal water near Peru
(B) Winds blowing from the west
(C) Random occurrence
(D) Worldwide effects
(E) Short duration



24. According to the model presented in the passage, which of the following normally signals the disappearance of an El Nino?

(A) The arrival in the eastern Pacific of negative sea-level Kelvin waves
(B) A shift in the direction of the winds produced by the start of an anti-El Nino elsewhere in the Pacific
(C) The reflection of Kelvin waves after they reach the eastern boundary of the Pacific, along Ecuador and Peru
(D) An increase in the speed at which negative Rossby waves cross the Pacific
(E) The creation of a reservoir of colder, deep ocean water trapped under the pile of warmer, surface ocean water



25. It can be inferred from the passage that which of the following would result fairly immediately from the cessation of the winds of an EI Nino?

I. Negative Rossby waves would cease to be generated in the eastern Pacific.
II. The sea level in the eastern Pacific would fall.
III. The surface water in the eastern Pacific would again be cooled by being mixed with deep water.

(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III



26. Which of the following, if true, would most seriously undermine the validity of the model of EI Nino,that is presented in the passage?

(A) During some years EI Nino extends significantly farther along the coasts of Ecuador and Peru than during other years.
(B) During periods of unusually cool temperatures along the eastern Pacific, an EI Nino is much colder than normal.
(C) The normal upwelling of cold water in the eastern Pacific depends much more on the local characteristics of the ocean than on atmospheri.c conditions.
(D) The variations in the time it takes Rossby waves to cross the Pacific depend on the power of the winds that the waves encounter.
(E) The western boundary of the Pacific basin is so irregular that it impedes most coastal Kelvin waves from beading eastward.



27. The passage best supports the conclusion that during an anti-EI Nino the fastest-moving signal waves are

(A) negative Rossby waves moving east along the equator
(B) positive Rossby waves moving west along the equator
(C) negative Kelvin waves moving west along the equator
(D) positive Kelvin waves moving west along the equator
(E) positive Kelvin waves moving east along the equator