The ocean is vast. If we wanted to find the average temperature of the Pacific Ocean between California and Hawaii using traditional methods (dropping a thermometer over the deck of a ship as you steam from San Francisco to Maui) it would require huge amounts of time and money. Science has come up with a better way using sound to effectively "see" heat. On average, sound travels at 1500 meters per second, more than four times faster than it travels in air. The speed of sound in water varies depending on the temperature, pressure, and salinity of the water it's traveling through. Increases in temperature, salinity, and pressure all result in faster travel times for sound waves.
Acoustic thermometry's success relies on the existence of the SOFAR channel, which lies at a depth of about 1000 meters. It is the Autobahn for sound. Sound waves normally bounce around between layers of the ocean but in the SOFAR channel they can travel great distances unscathed. Low frequency sounds below 100 Hz can travel thousands of kilometers. Scientists think that humpback whales may use the SOFAR channel to communicate with each other over the long distances that separate them during migrations. The blue whale emits sounds in the 5 Hz range allowing them to communicate with other blue whales a hemisphere away! By contrast, high frequency sounds greater than 1,000,000 Hz won't make it from one end of an Olympic-sized pool to another.
So how can we use sound and the SOFAR channel to measure average temperatures of the ocean? If you know the travel time of a pulse of sound and the distance it traveled, you can easily calculate the average speed at which it traveled (speed = distance/time). That speed will then directly correlate to a specific temperature. Does this give us a clear picture of temperatures at each point along the path? No, it only gives us information about the average temperature between the two points. If it took 1000 seconds for a pulse to travel 1,500,000 meters from California to Hawaii last year, but this year it takes 999 seconds, the sound must be traveling faster. That increased speed of sound might be a result of an increase in temperature. Small changes in water temperature are the result of the absorption of great amounts of heat because water has a large heat capacity. Changes in temperature in deep water can be a more accurate indicator of global warming than changes in temperature in the atmosphere because air has a small heat capacity.
- Sarah Quan