To monitor changing ocean temperatures, oceanographers use satellites for the sea surface and thermometers to measure the interior. Another less common approach called “ocean acoustic tomography” involves sending sound pulses from a source (essentially an underwater bell) and measuring the time for the acoustic signals to arrive at a distant underwater microphone (Munk and Wunsch, 1979). A recent study (Wu et al., 2021) demonstrates for the first time that acoustic waves from earthquakes (T waves) can be used instead of the large, expensive, and often controversial artificial sound sources that have hampered the widespread adoption of acoustic tomography into the global ocean observing system.
Because the speed of sound in seawater increases with temperature, scientists can relate shorter and longer acoustic signal travel times to warming and cooling ocean temperatures between transmitter and the receiver (panel a, b). Wu et al. (2021) calculated ocean temperature changes by analyzing 11 years of acoustic travel time data from earthquakes triggered by tectonic plate subduction off the coast of Sumatra in the East Indian Ocean. To validate their new record of ocean temperature changes, the authors compared their results with NASA’s latest multidecadal global ocean state estimate from the Estimating the Circulation of the Ocean (ECCO) project. Results show that the new method yields a reconstruction of the temperature change better than that from in-situ Argo measurements (panel c). ECCO state estimates are dynamically-consistent, full-depth reconstructions of ocean evolution that are created by fitting an ocean model to satellite and in-situ ocean observations. Fortunately, the ocean temperature fluctuations inferred using seismic acoustic tomography agreed closely with those from ECCO, opening the door for acoustic signals from earthquakes and other natural sources to be used to help measure changing ocean interior temperatures in other basins as well. To learn more, visit the storyboard on the ECCO website.
The ECCO datasets are now available through PO.DAAC, within the Earthdata Cloud in Amazon Web Services (AWS). To search and access (download) ECCO data, visit the Earthdata Search portal or ECCO landing pages at PO.DAAC, under ‘Data Access’ tab. Example data access resources are also available on the PO.DAAC Github.