The hidden heat of the ocean measured by the sound of an earthquake. Science

By Paul Vossen

In 1991, scientists launched large subwoofers into the waters on the icy volcanic island of Herd Island in the Indian Ocean. The speakers emitted low-frequency sounds that drifted across the oceans like a whale song. Picked by receivers sourced from the coasts of California and Bermuda, the signals contained important information about the water they extracted: how hot it was. It was a promising way to monitor Earth’s warming oceans, but concerns about how underwater noise could affect marine life were quickly sidelined, with only a few dedicated scientists surviving the technology Kept. Now, it is back – only at this time, the Earth itself is providing noise.

A team of seismologists and oceanographers has shown that small earthquakes that recur frequently from the same place below the sea level can replace subwoofers. Beyond the reach of other techniques, earthquakes produce reliable acoustic signals for measuring ocean temperatures at depths below 2000 m. Approach, if valid, published today ScienceOne can open up an entirely new ocean observation system to understand past and future climate change, says Frederick Simmons, a geophysicist at Princeton University, unaffected by the study. “Possible treasures of data waiting to be analyzed.”

Oceans absorb more than 90% of the energy trapped by global warming, and how rapidly any change in the rate at which they absorb heat affects the external environment. Two decades ago, robotic floats from the international Argo array began monitoring ocean warming to a depth of about 2000 meters. But the float array, now 4000 strong, could not detect large volumes of water at greater depths. “The inability to determine what is happening in deep water is still a major obstacle to understanding the ocean and climate today,” says Carl Woonch, a retired oceanographer at the Massachusetts Institute of Technology.

In 1979, Wench and Walter Munk, an oceanographer at the Scripps Institution of Oceanography, who died the previous year, first proposed using sound waves to measure the ocean’s heat and composition. The sound increases rapidly as the water is heated or condensed, making the travel time a reliable gauge of temperature and density if the sound source and receiver are at fixed locations.

The technique did not require particularly loud sources. At a depth of about 1000 m, the speed of sound hits a minimum, forming a conductive channel between warm water above and dense water below. This waveguide enables sound waves to coast across ocean valleys, with Bruce Cornuel, a Scripps Oceanographer who has worked with Munk. “It’s like grabbing a 5-year-old wrapping paper tube and yelling in your brother’s ear.”

In addition to checking the full width of the ocean, sound waves – along with vertical dimensions of thousands of meters – capture all conditions from shallow water to the abyss. As a result, they average small-scale natural temperature fluctuations, revealing fundamental changes of a few thousandths of 1 ° every year. “This makes it very easy to extract the global warming signal,” says Jarn Callidge, an oceanographer at California Institute of Technology (Caltech) and co-author of the new study.

After a 1991 performance on Heard Island, Munk won the Department of Defense Finance for a follow-up experiment in the Pacific Ocean called Acoustic Thermometry (ATOC). But it was engulfed in controversy by its two man-sized speakers, with the coasts of Hawaii and California placed in prime whale territory. “It became a political nightmare,” says retired oceanographer Brian Dushav, who works on the ATOC. The ATOC’s signals were no louder than whale calls and ship traffic, but much of its $ 35 million budget went to study the impact of sound on marine mammals.

Military secrecy also got in the way. To listen to the signals, the project relied on classified naval hydrophones normally used to detect submarines. Scientists have not even published the locations of receivers, Woonch says. “We didn’t tell the Navy that if you published the signal, which we did, you could find out where the receivers were,” Vanch said. The aerial source, from Kauai, lasted until 2006, provides 10 years of warming data. But by then, oceanographers had overtaken acoustic thermometry and were relying on Argo, Dussehra says.

Until 1 year ago, when Wenzhou Wu, a Caltech seismologist, realized that repeating earthquakes that slowly creep below sea level could provide an alternative sound source. When earthquakes shake the ocean floor, some energy is converted into acoustic waves. Wu and his co-writers had to find just the right source.

His discovery went back to the Indian Ocean. In earthquake records, they identified more than 4000 earthquakes in the sea level west of Sumatra in Indonesia from 2004 to 2016, many of them ranging in magnitude between 3.5 and 5. Triangulated at the source, the team identified fault patches less than 100 meters apart, repeatedly broken, says Geodesi of the Chinese Academy of Sciences and co-author of the Institute of Geophysics and seismologist Sidou Ni. This resulted in sound waves reaching Diego Garcia, a remote atoll in the south of India through the sea, where they struck the ground and came back in seismic waves, picking up on the island’s seismic kilometers.

By converting those travel times to temperatures, Wu and his colleagues found that the eastern Indian Ocean heated to 0.044 ° C in a decade. The annual fluctuations coincided well with Argo’s measures from the same time, but the warming signal was almost double what Argo floats found. The disparity suggests that Argo is missing some heat, Callidge says, at least for this basin for a short time. Approximately 40% of their heat measurements have come from water below 2000 m, suggesting some warming, working deep into the sea beyond the Argo’s current reach.

“This work is quite extraordinary and very promising,” says Susan Wijfels, an Argo leader at the Woods Hole Oceanographic Institution. If expanded globally, it could provide an independent check on Argo measurements, especially when producing a new line of Argo that can descend up to 6000 meters, currently deployed only in dozens, ramp up. . Even more attractive to Wijffels, the detection of repeaters in older seismic records is likely to increase the global warming trend in the time before Argo. “Is the climate a gift to the community,” she says.

The team feels that it can capture earthquake-based sounds more cleanly with a hydrophone than a land-based seismometer. This would allow them to use fewer electrical earthquakes, and they should be able to take signals from repeaters in the oceans around the world, using a global network of hydrophones deployed as part of the Comprehensive Nuclear Test Ban Treaty.

Hydrophones deployed under Arctic sea ice can measure the water temperature at a place where Argo floats cannot reach. It may also be possible that snow collision accidents are used in nearby Greenland-glacial earthquakes, as they are known as sound sources. “It’s free data,” says Dasav. “There is no question that someone will implement a system to take advantage of it.”

New bright possibilities for ocean acoustic thermometry are also a recognition for Munk, says Cornuel, who was muting his global acoustic dreams. “I wish Walter would have been around to see it. He will be very happy. ”

* Correction, September 17, 4:25 pm: A previous version of this story stated that 40% of the measured warming came from below 2000 m. Although 40% of the measured temperature comes from water below 2000 meters, the technology cannot yet say where the warming has occurred in the water column.