How to take the temperature of an entire ocean

Want to know how to take the temperature of all the ocean between California and Hawaii? All you need is the right sound. A special acoustic technique lets people figure out the temperature of the ocean, and how it might be changing from year to year.

There's no real way to take simultaneous temperature readings of every section of ocean between continents, and so there should be no way to get an average ocean temperature. One technique manages to do just that. It's called acoustic thermometry, and it's another example of physics going poetic. Because water is denser than air, sound travels faster underwater. As we get down to very low frequencies, sound also travels more efficiently. Whales use sound below 100 cycles per second to communicate across thousands of miles.

Scientists do the same. Stations emit specific sounds on one continent and other stations pick it up thousands of miles away. Because the stations can communicate with each other in ways besides sound, the receiving station knows exactly when the emitting one makes the sound, and calculates the time it takes the sound to make the voyage. The faster the sound, the higher temperature the ocean. There are variables, of course. Changes in salinity can also cause chances in the speed of sound. But repeated signals between different stations can let scientists know the temperature of the ocean by the sound it makes.

Check out The Discovery of Sound in the Sea site for a sample of the sound that this acoustic thermometry uses. There's also an underwater gallery of different undersea noises. Haven't you always wanted to know what a narwhal sounds like?

Top Image: Tanaka Juuyoh