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Probing the Ocean Interior with Sound |
SOSUS, with its vast reach, also has proved instrumental in obtaining information crucial to our understanding of Earth's weather and climate. Specifically, the system has enabled researchers to begin making ocean temperature measurements on a global scale, measurements that are key to puzzling out the workings of heat transfer between the ocean and the atmosphere. The ocean plays an enormous role in determining air temperature--the heat capacity in only the upper few meters of ocean is thought to be equal to all of the heat in the entire atmosphere.
Given increasing evidence of global warming, scientists around the world are struggling to determine how much of the observed warming trend is simply part of the natural climate cycle and how much has been caused by the burning of fossil fuels and other human activity. Current numerical models that simulate the global climate and predict climate change are hampered by insufficient temperature measurements in many areas of the globe, and especially below the ocean surface.
In 1978, Walter Munk of the Scripps Institution of Oceanography and Carl Wunsch of MIT suggested using the methodology of computer-aided tomography--the CAT scan to study and monitor the ocean over distances of about 1,000 kilometers (600 miles). A medical CAT scan constructs a three-dimensional image by combining information from many different x-rays taken at different angles. The oceanic equivalent of a CAT scan--ocean acoustic tomography--would combine information from low-frequency sound instead of x-rays.
For sound waves traveling horizontally in the ocean, speed is largely a function of temperature. Thus, the travel time of a wave of sound between two points is a sensitive indicator of the average temperature along its path. Transmitting sound in numerous directions through the deep sound channel can give scientists measurements spanning vast areas of the globe. Thousands of sound paths in the ocean could be pieced together into a map of global ocean temperatures, and by repeating measurements along the same paths over time, scientists could track changes in temperature over months or years.
In 1983, John Spiesberger, now at Pennsylvania State University, and Kurt Metzger at the University of Michigan supplied the first experimental verification that tomography was possible across an entire ocean basin--much farther than Munk and Wunsch had proposed. Spiesberger and Metzger sent sound pulses 4,000 kilometers (2,300 miles) from a source on the seafloor off Oahu, Hawaii, to nine of the Navy's SOSUS listening arrays in the northeast Pacific. By repeating the experiment in 1987 and 1989, Spiesberger and Metzger demonstrated for the first time that extremely slight changes in acoustic travel time across an ocean basin reflect changes in water temperature along the sound path. A decrease of two-tenths of a second in travel time in this experiment was about equal to an average temperature rise of one-tenth of a degree Celsius.
In 1989, Munk and Andrew Forbes of the Commonwealth Scientific and Industrial Organization in Australia suggested transmitting sound globally on a regular basis for a decade to try to monitor climate change. To determine whether the signal would be stable enough to obtain measurements across half the globe, they placed a sound transmitter near Heard Island, an uninhabited Australian island in the southern Indian Ocean, with receivers in all oceans but the Arctic. For five days in January 1991, scientists from nine nations, led by the United States, transmitted sound from a ship off Heard Island. Sixteen  listening sites picked up the signals in the deep sound channel from as far away as 18,000 kilometers (11,000 miles). Although sound was detected at great distances, insufficient resolution was achieved in this experiment to measure temperature changes reliably at large distances.
Building on lessons learned in the previous experiments, the Acoustic Thermometry of Ocean Climate (ATOC) project involving scientists from 13 countries was launched in 1992. A key objective is to establish baseline ocean temperatures in the Pacific against which changes can be measured. Because of concern about the effects that the sounds might have on marine mammals, the ATOC transmissions were delayed until 1996. However, in April 1994, a team of U.S. and Russian scientists led by Peter Mikhalevsky at Science Applications International Corporation transmitted sound across the Arctic Ocean and made a startling discovery. This Transarctic Acoustic Propagation (TAP) experiment not only proved the feasibility of long-range acoustic thermometry in the ice-covered Arctic, but the travel-time measurements revealed an average warming of approximately 0.4 degree Celsius , when compared to historical temperature measurements, at the mid-depths of the Arctic Ocean along the propagation path. Extensive measurements by submarines and ice-breakers have subsequently documented this pervasive temperature change in the Arctic Ocean, which is now the focus of intensive new research. The TAP experiment launched the joint U.S. and Russian Arctic Climate Observations using Underwater Sound (ACOUS from the Greek, akouz, meaning "listen!") program in 1995. Although the pioneering ATOC program will end in 1999, ACOUS and other acoustic monitoring programs are continuing.
Researchers also are using other acoustic techniques to monitor climate. Oceanographer Jeff Nystuen at the University of Washington, for example, has explored the use of sound to measure rainfall over the ocean. Monitoring changing global rainfall patterns undoubtedly will contribute to understanding major climate change as well as the weather phenomenon known as El Niņo. Since 1985, Nystuen has used hydrophones to listen to rain over the ocean, acoustically measuring not only the rainfall rate but also the rainfall type, ranging from drizzle to thunderstorms. By using the sound of rain underwater as a "natural" rain gauge, the measurement of rainfall over the oceans will become available to climatologists.
In the centuries since Leonardo da Vinci's inspired suggestion for listening to ships underwater, many researchers have contributed to the development of techniques that take advantage of the way sound travels through water. From military uses such as submarine warfare and detecting underwater explosions to scientific endeavors such as monitoring climate change and studying ocean wildlife, we have seen how modern society benefits from the investigations of those who pursued the answers to basic questions of the workings of nature.
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