How can the ocean's temperature be measured with sound?
Sound travels faster in warm water than in cold water. By measuring the time it takes for sound to travel a known distance through the ocean, the average temperature of the water can be calculated with great accuracy. The technique is called acoustic thermometry of ocean climate (ATOC).
It takes sound about an hour to travel through 5,000 miles (8050 km) of ocean. On the way, it is continually refocused by the structure of warm and cold layers of water at different depths, so that the signal remains strong. In a recent experiment, high-intensity, low-frequency sounds (fewer than 100 cycles per second) were generated at Pioneer Seamount in the Pacific Ocean, 939 meters (3081 feet) deep. Later, the sound was picked up by microphones at Hawaii, Christmas Island, and New Zealand.