Acoustical Society of America
ICA/ASA '98 Lay Language Papers

Sound Measures Ocean Temperature

Robert C. Spindel spindel@apl.washington.edu
Applied Physics Laboratory
University of Washington
1013 NE 40th Street
Seattle, WA 98105

B. D. Cornuelle, M. Dzieciuch, W. H. Munk, P. F. Worcester
(University of California, La Jolla, CA 92093-0225)

A. B. Baggeroer, D. Menemenlis, C. Wunsch
(Dept. of Earth, Atmospheric and Planetary Sci., MIT, Cambridge, MA 02143)

T. G. Birdsall, K. Metzger
(Univ. of Michigan, Ann Arbor, MI 48109-2122)

C. Clark
(Cornell Lab. of Ornith., Bioacoustic Res. Prog., 159 Sapsucker Woods Rd., Ithaca, NY 14850)

J. A. Colosi
(Woods Hole Oceanographic Inst., Woods Hole, MA 02543)

D. Costa
(Inst. of Marine Sci., Univ. of Calif., Santa Cruz, CA 95064)

B. D. Dushaw, B. M. Howe, J. A. Mercer, R. C. Spindel
(Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98105-6698)

A. M. G. Forbes
(CSIRO, Marine Laboratories, Hobart, Tasmania 7000, Aus.)

Popular version of paper 1aA01
Presented Monday morning, June 22, 1998
ICA/ASA '98, Seattle, WA

Warming and cooling trends in the ocean have enormous impact on weather conditions as this year's El Nino has shown dramatically. California has been hit by a record series of rain storms and hurricanes resulting in damage to structures and crops totaling millions of dollars. Understanding the causes and effects of ocean temperature changes well enough to be able to predict them in advance is an important goal in weather and climate research.

Scientists are using underwater sound to help. The idea behind the project, known as Acoustic Thermometry of Ocean Climate (ATOC), is that sound travels slightly faster in warm water than in cold water. . Thus, precisely measuring the time it takes for a sound signal to travel between two points can reveal the average temperature along the path. Sound travels at about 1500 m/s in water (about 3,350 mph which is five times the speed of sound in air), while typical changes in the sound speed of the ocean as a result of temperature changes are only 5-10 m/sThe technique is very sensitive, and the experiment so far has shown that temperatures in the ocean can be measured to a precision of 0.01 degrees C which is needed to detect subtle variations and trends of the ocean basin.

The ATOC project, got off to a rocky start in 1994. As a result of misunderstandings and the difficulties of communication between scientists and the public, ATOC met with opposition by a wide variety of environmental organizations and concerned individuals. Two misunderstandings fueled the debate: a million-fold misunderstanding of the sound levels to be used, and a misinterpretation of the technical word "take" used in the permit applications prepared for the experiment. Many believed "take" meant "kill" and that thousands of marine mammals would be destroyed. Technically, however, in the sense used in environmental assessments, it means only that the animal will be in a region where it may hear the sound and may possibly respond to it in some way. Some people feared that the underwater sounds to be used would adversely affect marine life, particularly marine mammals such as whales that use sound for communicating, navigating and feeding.

. As a result, the experiment was stalled for almost two years while strict procedures were established for monitoring marine mammals in the vicinity of the proposed sound source locations off California and Kauai. Background marine mammal surveys were conducted to prepare for a before and after assessment of the impact of the sounds. Rules were developed for dealing with any observed responses to the sounds, and a research program planned to take advantage of the sound transmissions themselves to learn more about the effects of man- made sound on marine mammals was expanded. It wasn't until December of 1995 that the first ATOC transmissions were made.

Now, over two years later, the first results have come in. The sounds, at a frequency of 75 Hz, about the tone of the lowest note on a cello, are transmitted from a source on the seafloor about a half mile deep, at a site about 50 miles west of San Francisco. The sound level of the acoustic speaker is about 250 watts. The transmitted signals are received on U.S. Navy SOSUS and other listening arrays of hydrophones in the northeast Pacific. Signals are transmitted for about 20 minutes every four hours for about 4 days, and this cycle is repeated two to four times per month. The schedule is not exact because transmissions are permitted only when marine mammal observers are present, and circumstances such as poor weather sometime prevent observation. There is a second sound transmitter, also about a half mile deep off the north coast of Kauai, but it has been in operation only a short period of time..

While a year and a half is not long enough for long term ocean temperature trends to become evident, the data show the known warming of the Pacific in the summer, and cooling in the winter, thus giving confidence that temperatures are being measured faithfully. There are interesting deviations in the acoustic temperatures from those inferred from satellite altimeter data, presently the only measure of ocean temperature with global coverage. The satellite detects changes in sea surface height as small as 2 cm; these changes are attributed to expansion or contraction of the ocean as it warms or cools. The ATOC sound waves, which travel over all depths of the ocean, promise a more accurate means to measure the ocean's temperature. The differences between the altimeter and acoustic data are important because these data are used to initialize and test computer models that forecast future ocean and weather conditions, and the models are very sensitive to small differences in input data. If the input data are wrong, so too will be the forecast.

Results of the elaborate marine mammal monitoring and research program are also becoming available. So far there are no indications that any species shows any biologically significant adverse response to ATOC or ATOC-like sounds.