August 27, 1998
Novel approach to measuring ocean temperatures proved successful
An experiment to devise a new method for tracking large-scale changes in ocean temperature associated with events such as El Niño and with global warming indicates that scientists can successfully use low-frequency sound transmissions to measure the temperature of vast expanses of ocean.
Analysis of the initial 15 months of data from the Acoustic Thermometry of Ocean Climate (ATOC) project proves that a technique called acoustic thermometry is a valuable tool for monitoring changes in ocean climate, according to a consortium of researchers that includes physicists and oceanographers with the University of Washington’s Applied Physics Laboratory.
Originally conceived by Walter Munk of the Scripps Institution of Oceanography at the University of California, San Diego, and Carl Wunsch of the Massachusetts Institute of Technology, the idea behind ATOC is to send sound signals from underwater speakers and track how long it takes them to reach receivers moored to the floor of the Pacific Ocean thousands of miles away. Because sound travels faster in warmer water than cool water, a long-term series of tests that recorded increasingly faster travel times would indicate the ocean is warming.
During the first 15 months of the experiment, which began in 1995, the scientists were able to detect variations as small as 20 milliseconds in the hour it takes pulses to travel some 3,000 miles between the underwater speakers and receivers. Those subtle shifts allow them to estimate average ocean temperatures along the signals’ pathways to within .006 degrees Celsius.
If global temperatures rise in the future as some scientists predict, researchers anticipate large jumps in the amount of heat stored in the planet’s oceans. Indeed, such changes in oceanic heat storage are key components of computerized global climate models used to predict future global change. Yet results from the ATOC experiment published in the Aug. 28 issue of Science indicate that current ocean temperature data derived from satellite measurements and used in such models may not be as accurate as expected.
Because satellites cannot measure ocean temperatures directly, scientists have relied on satellite recordings of sea level to calculate estimated ocean temperatures. Satellites can detect changes in sea surface height as small as 2 centimeters, with these changes being attributed to the expansion or contraction of the ocean as it warms or cools.
ATOC scientists, however, found that about half of the season-to-season changes in sea level aren’t related to changes in temperature, according to Robert C. Spindel, director of the UW’s Applied Physics Laboratory and one of the authors of the Science article. Instead, such things as large fluctuations in water mass and changes in salinity appear to affect sea levels.
“The ATOC sound waves, which travel over all depths of the ocean, promise a more accurate means to measure the ocean’s temperature,” Spindel said. “The differences between the sea level measurements and acoustic data are important because these data are used for computer models that forecast future ocean and weather conditions. If the input data are wrong, the forecasts will be wrong.”
For decades, scientists have relied on dropping instruments from ships in order to take ocean temperature readings. In addition to being expensive and time consuming, this technique provides incomplete coverage of the vast oceans.
Acoustic thermometry, however, capitalizes on sound channels in the deep sea capable of trapping and transmitting sound over very long distances. The channels are created by the variation of pressure and temperature with depth. Located about 3,000 feet below the surface, these deep sea super-highways act almost like a lens in focusing the sound and guiding it over thousands of miles.
The acoustic signals are sent intermittently through the sound channel from underwater speakers also about 3,000 feet deep. The sound sources are deployed off the coast of Kauai and on the Pioneer Seamount, about 55 miles off the coast of San Francisco. The low-frequency signals are then picked up thousands of miles away by sophisticated underwater hydrophones scattered around the Pacific, where the transmissions are so faint that scientists must rely on elaborate computer programs to distinguish them from the ambient ocean noise.
Scientists and engineers with the Applied Physics Laboratory designed, arranged for construction and deployed the two sound sources and were responsible for the majority of the underwater hydrophones used in the experiment. Data gathered were analyzed by researchers at the lab and Scripps.
Twenty scientists are co-authors on the Science paper, including the University of Washington’s Spindel, principal oceanographer Bruce Howe, principal physicist Jim Mercer and oceanographer Brian Dushaw, as well as researchers from eight other institutions: Scripps, MIT, University of Michigan, Cornell University, Woods Hole Oceanographic Institution, University of California, Santa Cruz, and CSIRO in Hobart, Tasmania.
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For more information:
Robert C. Spindel, University of Washington, (206) 543-1310, or Jim Mercer, University of Washington, (206) 543-1361.
Scripps Institution of Oceanography media contacts:
Janet Howard or Cindy Clark, (619) 534-3624
