Low frequency sound aids in tracking temperature changes

From left, Jim Mercer, Robert Spindel, Bruce Howe and Brian Dushaw.

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.

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, according to Robert Spindel, director of the APL. 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, Spindel says. 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.”

Twenty scientists are co-authors on the Science paper, including the UW’s Spindel, principal oceanographer Bruce Howe, principal physicist Jim Mercer and oceanographer Brian Dushaw.

Scientists and engineers with the Applied Physics Laboratory designed, arranged for construction and deployed two sound sources and are responsible for the majority of the underwater hydrophones being used in the experiment.

Acoustic thermometry 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 about 55 miles off the coast of San Francisco. The low-frequency signals are 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. The needed analysis is done by researchers at the lab and Scripps. ¶

Sandra Hines, News and Information Services