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　　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�瞔urrent 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�瞫cale 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�瞔onsuming, 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. (460 words)

　　1. 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] density variations in the ocean are mathematically predictable.

　　[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] low-frequency sound waves are well described mathematically.

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

　　[A] The use of earthquake shock-wave data collected at several different locations and combined to create a three-dimensional image of the Earth’s interior.

　　[B] The use of ocean-current meters to determine the direction and velocity of the ocean’s mesoscale fields.

　　[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.

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

　　[A] An argument is advanced, then refuted, and an alternative is suggested.

　　[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] A theory is proposed, considered and then amended.

　　4. 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] Ships are another possibility, but they would need to stop every 50 kilometers to lower measuring instruments.

　　[B] To understand how ocean acoustic tomography works, it is necessary to know how sound travels in the ocean.

　　[C] Timekeeping in medical tomography must be precise because the changes in travel time caused by density fluctuations are slight.

　　[D] These variations amount to only about 2 to 3 percent of the average speed of sound in water, which is about 1500 meters per second.

　　5. The author mentions EL Nino primarily in order to emphasize .

　　[A] the difficulty of measuring the ocean’s large�瞫cale currents.

　　[B] the variability of mesoscale phenomena.

　　[C] the brief duration of weather patterns.

　　[D] the possible impact of mesoscale fields on weather conditions.

　　核心詞匯

　　circulation n. 流通,循環(huán),發(fā)行量

　　identify v.識別;(with)把…和…看成一樣,打成一片

　　dominate v. 支配,占優(yōu)勢

　　mesoscale n. & a.[氣] 中尺度(的) ,中等規(guī)模的

　　fluctuate v. 變動,上下,動搖 v. 使動搖

　　velocity n. 速度,速率

　　phenomenon n. 現象(pl.phenomena)

　　equatorial a. 近赤道的,赤道的

　　prohibitive a. 禁止的,抑制的,類同禁止的

　　conventional a. 普通的,常見的,習慣的,常規(guī)的

　　feasible a. 可行的,可能的

　　tomography n. 斷層攝影術

　　salinity n. 鹽濃度,鹽分

　　density n. 密度

　　acoustic a. 美聲的，聽覺的,音感教育,音響學的

　　難句剖析

　　1. 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.

　　【解析】 這是一個復合句。 本句的主干結構為researchers began to employ… techniques and equipment…; that move… signals 和that record data… in the ocean 均為定語從句，先行詞分別是floats 和meters，floats 和meters 為并列成分。

　　【譯文】 但是到了二十世紀五六十年代，研究者們開始使用新技術和新設備進行研究，其中包括：能夠跟隨洋流移動并發(fā)射出辨識信號的水下漂流儀器和能夠在海洋中的一個固定地點持續(xù)數月紀錄數據的洋流儀表。

　　2. Mesoscale phenomena may, in fact, play a significant role in oceanic mixing, air�瞫ea 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.

　　【解析】 這是一個復合句。 句子結構為Mesoscale phenomena may, in fact, play a significant role in…;in fact 是插入語;interactions和climate events 是并列成分;that affects global weather patterns 是定語從句，修飾disturbance.

　　【譯文】 事實上，紊流現象可能在空氣和海洋的互相影響中扮演了重要角色，也在一些偶爾發(fā)生但卻影響深遠的氣候現象中扮演重要角色，比如厄爾尼諾就是這樣一種在赤道太平洋地區(qū)通過大氣和海洋的相互干擾來影響全球氣候的現象。

　　3. 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.

　　【解析】 這是一個復合句。句子主干為 It is …increase…that accounts for …attraction…; obtained from…signals 為過去分詞短語作定語，修飾data;冒號表示解釋說明。

　　【譯文】 從信號的多條傳輸路徑中獲得的數據可以成倍增加，正是這一點說明了X射線斷層射影技術引起海洋學家注意的原因：它可以使用較少的設備測量廣大的區(qū)域。

　　文章類型：科普類——紊流以及測量方法和意義

　　這篇文章介紹了海洋中的紊流以及測量這一現象的意義和方法。