The rate of ocean waters warming measured with sound waves
The idea of using sound waves to measure the ocean’s temperature and structure emerged as long as 30 years ago. Since then, scientists are trying to find the way to measure the temperature of the deepest oceans abyss, not available for measurements in any other way, without affecting the wildlife at seas.
As it turns out, the nature itself comes to the rescue – it was observed that the artificial sources of sound, such as subwoofers, could be replaced with natural ones, such as underwater earthquakes. As a result of such quakes, sound waves arise.
When shakes affect the ocean floor, some of the produced energy transforms into acoustic waves. We know that sound travels faster in water that is warm and dense, hence based on the speed of sound waves travel time from sound source point to a receiver, scientists can assess its’ density and temperature.
By measuring the sound waves caused by earth quakes, scientists estimated that the Indian Ocean is warming by about 0.044 K per decade in 2004 – 2016.
The article (https://www.sciencemag.org/news/2020/09/ocean-s-hidden-heat-measured-earthquake-sounds) describes bumpy history of this extraordinary undertaking, including the use of the first autonomous floater Argo and recent research that took place in the Indian Ocean. It also indicates the possible future development of ocean acoustic thermometry.