SAN DIEGO--A once-controversial experiment to take the temperature of the Pacific Ocean with powerful sound waves is much more precise than oceanographers had hoped. Readings from the first 15 months of the project are accurate to within 6 thousandths of a degree Celsius over ranges that span half of the Pacific basin, scientists said here today at the American Geophysical Union's biennial Ocean Sciences Meeting. Researchers are now more confident about the method's ability to detect possible long-term warming trends in the ocean.
Known as Acoustic Thermometry of Ocean Climate (ATOC ), the project is based on the fact that sound travels faster in warmer water. Climate models predict that global warming will heat the ocean a few thousandths of a degree per year, enough to accelerate the travel times of sound waves. ATOC, its proponents say, is the best way to detect that subtle signal. But the possiblity that the signals could harm marine wildlife or disrupt migratory patterns delayed the project for more than a year and forced scientists to extend their tests to see whether the sounds would disturb animals. To date, observations of animal behavior during the deep-water broadcasts have revealed no such pattern.
ATOC's low-pitched rumblings register surprisingly well on arrays of sensitive hydrophones as remote as Christmas Island, 5000 kilometers from the transmitter, about 80 kilometers west of Half Moon Bay, California. Indeed, the scientists can pinpoint the timing of each acoustic pulse to within 20 milliseconds out of the hour-long travel time, says oceanographer Peter Worcester of the Scripps Institution of Oceanography in La Jolla, California. Because oceanographers know how fast sound travels through water of a given temperature, they can deduce average ocean temperature along the sound wave's path to within 0.006 degrees. "That's far better than we expected," Worcester says. "We thought acoustic scattering within the ocean might wipe out the signal at those distances."
Few scientists are happier with the news of ATOC's precise readings than physical oceanographer Carl Wunsch of the Massachusetts Institute of Technology in Cambridge, Massachusetts, who proposed ATOC 20 years ago with Walter Munk of Scripps. In combination with altimetric measurements of the ocean surface via satellite, says Wunsch, "this is the only way we can hope to see warming trends on a time scale of a decade." However, ATOC's future is unclear beyond August, when the program's current marine-mammal research permit expires.