Sound waves traveling thousands of kilometers across the ocean may help scientists to observe climate change.

As greenhouse gas emissions heat up the planet, the ocean absorbs large amounts of that heat. To monitor the change, a global ship of 4,000 instruments, known as argo floats, collects temperature data from 2,000 meters above sea level. But in some areas that data collection is very limited, including deep areas of the ocean and areas under sea ice.

So Caltech seismologist Wenbo Wu and colleagues reintroduce a decades-old concept: the speed of sound in ocean water is used to calculate ocean temperature. In a new study, Wu’s team developed and tested a method for using seismic sound waves traveling across the eastern Indian Ocean to estimate water temperature variations from 2005 to 2016.

Comparing that data with similar information from Argo floats and computer models showed that the new results matched well. Researchers report that this technology, called seismic ocean thermometry, promises to detect the impact of climate change in less well-studied ocean areas. September 18 Researchers in Science report.

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Sound waves are carried through water by the vibration of water molecules, and at higher temperatures those molecules vibrate more easily. As a result, the waves move a little faster when the water is warmer. But those changes are so small that in order to be able to measure them, researchers need to observe very distant waves.

Fortunately, sound waves can travel long distances across the ocean, thanks to a curious mass phenomenon known as the soffit channel, which is short for sound resolution and range. The soffit channel is a horizontal layer formed by different salinity and temperature layers in the water, which acts as a waveguide, and the optical fibers guide sound waves in the same way that light waves do, Wu says. The waves move back and forth up and down the channel, but can remain in effect for tens of thousands of kilometers (SN: 7/16/60).

In 1979, Walter Munk, a physicist and oceanographer at the Scripps Institution of Oceanography in La Jolla, California, and Carl Wunish, an Emeritus professor at MIT and Harvard University, planned to use these oceanic properties to measure water. The temperature from the surface to the beach is called “oceanic acoustic tomography”. They transmit sound signals through the sopher channel and measure the time it takes for the waves to reach the receivers at a distance of 10,000 km. In this way, the researchers hoped to compile a global database of ocean temperatures (SN: 1/26/1991).

Environmental groups have ultimately halted the experiment, saying man-made signals could have adverse effects on marine mammals, Wunsch said in a commentary on the same issue of science.

Forty years later, scientists have determined that the ocean is indeed a very serious area, and that specific man-made signals may be blurred compared to the effects of earthquakes, the belching of underwater volcanoes, and the roar of collapsing icebergs. Emily Ockle, a seismologist at Northwestern University in Evanston, did not participate in the new study.

Still, Woo and colleagues have devised an activity that addresses environmental concerns: they use earthquakes instead of man-made signals. When an undersea earthquake occurs, it emits energy in the form of seismic waves, known as P waves, and S waves that pulsate along the coast. Some of that energy enters the water, causing the seismic waves to slow down and become T-waves.

Those T waves can also travel through the soffit channel. Therefore, in order to detect changes in ocean temperature, Wu and colleagues identified “repeaters” – earthquakes that occur at different times, although the team determined that they occur from the same place. The East Indian Ocean was chosen for this Proof of Concept study mainly because it is seismically active, offering an abundance of such earthquakes. After identifying more than 2,000 repeaters from 2005 to 2016, the team measured the travel time differences of sound waves across the East Indian Ocean, approximately 3,000 km.

Data show that water experiences a slight warming of 0.044 degrees Celsius in a decade. That trend is similar, although slightly faster than the live temperature at which argo floats collect. Wu says the team is next to test the technology with remote receivers, including off the west coast of Australia.

O’Keefe says that extra distance will be important to prove that the new method works. “This is a cost-effective study,” he said, adding that the distance traveled by the T-waves is very small and the temperature changes are very small. This means that any uncertainty in matching the exact source of two recurring earthquakes can be translated into travel time uncertainty and therefore temperature changes. He says more far-reaching studies in the future will help alleviate this concern.

Frederick Simons, a geophysicist at Princeton University who did not participate in the research, says the new study is “really a new breakdown.” “They have really worked out a good way to ridicule very subtle and slow temporary changes. It’s really technical. ”

Simons adds that the seismic records are decades older than the temperature records collected by the Argo floats. This means that scientists can use seismic ocean thermometry to bring in new data on past ocean temperatures. “The hunt for high-quality archival records will continue.”

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