Before 1965 many scientists pictured the circulation of the

Before 1965 many scientists pictured the circulation of the ocean’s water mass as consisting of large, slow-moving currents, such as the Gulf Stream. That view, based on 100 years of observations made around the globe, produced only a rough approximation of the true circulation. But in the 1950’s and the 1960’s, researchers began to employ newly developed techniques and equipment, including subsurface floats that move with ocean currents and emit identification signals, and ocean-current meters that record data for months at fixed locations in the ocean. These instruments disclosed an unexpected level of variability in the deep ocean. Rather than being characterized by smooth, large-scale currents that change seasonally (if at all), the seas are dominated by what oceanographers call mesoscale fields: fluctuating, energetic flows whose velocity can reach ten times the mean velocity of the major currents.

Mesoscale phenomena—the oceanic analogue of weather systems—often extend to distances of 100 kilometers and persist for 100 days (weather systems generally extend about 1,000 kilometers and last 3 to 5 days in any given area). More than 90 percent of the kinetic energy of the entire ocean may be accounted for by mesoscale variability rather than by large-scale currents. Mesoscale phenomena may, in fact, play a significant role in oceanic mixing, air-sea interactions, and occasional—but far-reaching—climatic events such as El Nino, the atmospheric-oceanic disturbance in the equatorial Pacific that affects global weather patterns.

Unfortunately, it is not feasible to use conventional techniques to measure mesoscale fields. To measure them properly, monitoring equipment would have to be laid out on a grid at intervals of at most 50 kilometers, with sensors at each grid point lowered deep in the ocean and kept there for many months. Because using these techniques would be prohibitively expensive and time-consuming, it was proposed in 1979 that tomography be adapted to measuring the physical properties of the ocean. In medical tomography x-rays map the human body’s density variations (and hence internal organs); the information from the x-rays, transmitted through the body along many different paths, is recombined to form three-dimensional images of the body’s interior. It is primarily this multiplicative increase in data obtained from the multipath transmission of signals that accounts for oceanographers’ attraction to tomography: it allows the measurement of vast areas with relatively few instruments. Researchers reasoned that low-frequency sound waves, because they are so well described mathematically and because even small perturbations in emitted sound waves can be detected, could be transmitted through the ocean over many different paths and that the properties of the ocean’s interior—its temperature, salinity, density, and speed of currents—could be deduced on the basis of how the ocean altered the signals. Their initial trials were highly successful, and ocean acoustic tomography was born.

21. According to the passage, scientists are able to use ocean acoustic tomography to deduce the properties of the ocean’s interior in part because

(A) low-frequency sound waves are well described mathematically
(B) mesoscale phenomena are so large as to be easily detectable
(C) information from sound waves can be recombined more easily than information from x-rays
(D) tomography is better suited to measuring mesoscale phenomena than to measuring small-scale systems
(E) density variations in the ocean are mathematically predictable

22. The passage suggests that medical tomography operates on the principle that

(A) x-rays are superior to sound waves for producing three-dimensional images
(B) sound waves are altered as they pass through regions of varying density
(C) images of the body’s interior can be produced by analyzing a single x-ray transmission through the body
(D) the varying densities within the human body allow x-rays to map the internal organs
(E) information from x-rays and sound waves can be combined to produce a highly detailed image of the body’s interior

23. Which of the following is most similar to medical tomography as it is described in the passage?

(A) The use of ocean-current meters to determine the direction and velocity of the ocean’s mesoscale fields
(B) The use of earthquake shockwave data collected at several different locations and combined to create a three-dimensional image of the Earth’s interior
(C) The use of a grid-point sensory system to map global weather patterns
(D) The use of subsurface floats to map large-scale circulation in the ocean
(E) The use of computer technology to halt the progress of a particular disease within the human body’s internal organs

24. The author mentions El Nino (line 27) primarily in order to emphasize which of the following points?

(A) The brief duration of weather patterns
(B) The variability of mesoscale phenomena
(C) The difficulty of measuring the ocean’s large-scale currents
(D) The effectiveness of low-frequency sound waves in mapping the ocean
(E) The possible impact of mesoscale fields on weather conditions

25. Which of the following best describes the organization of the third paragraph of the passage?

(A) A theory is proposed, considered, and then attended.
(B) Opposing views are presented, elaborated, and then reconciled.
(C) A problem is described, then a solution is discussed and its effectiveness is affirmed.
(D) An argument is advanced, then refuted, and an alternative is suggested.
(E) A hypothesis is presented, qualified, and then reaffirmed.

26. The passage suggests that which of the following would be true if the ocean’s circulation consisted primarily of large, slow-moving currents?

(A) The influence of mesoscale fields on global weather patterns would remain the same.
(B) Large-scale currents would exhibit more variability than is actually observed.
(C) The majority of the ocean’s kinetic energy would be derived from mesoscale fields.
(D) Atmospheric-oceanic disturbances such as El Nino would occur more often.
(E) Conventional measuring techniques would be a feasible method of studying the physical properties of the ocean.

27. Which of the following, if presented as the first sentence of a succeeding paragraph, would most logically continue the discussion presented in the passage?

(A) Timekeeping in medical tomography must be precise because the changes in travel time caused by density fluctuations are slight.
(B) To understand how ocean acoustic tomography works, it is necessary to know how sound travels in the ocean.
(C) Ships are another possibility, but they would need to stop every 50 kilometers to lower measuring instruments.
(D) These variations amount to only about 2 to 3 percent of the average speed of sound in water, which is about 1, 500 meters per second.
(E) The device used in medical tomography emits a specially coded signal, easily distinguishable from background noise.
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