Ocean acoustics confirm rising sea temperatures

Researchers have devised a way to measure changes in ocean temperature using earthquakes. By extracting historical seismic records, researchers have gained a glimpse into past ocean trends.

By Shein Kim (goes_by_kim)

Quotation: Kim, SE, 2020, Ocean Audios Affirm Rising Sea Temperatures, Tumble, http://doi.org/10.32858/temblor.118

The oceans act as a heat sink for the Earth. Scientists said they have discovered a new way to measure changes in ocean temperature. Credit: apasciuto (CC BY 2.0)

Sounds in the ocean reveal a wealth of scientific information for those who listen. Researchers have long used sound waves to explore and explore the oceans. Sonar, for example, relies on sound waves for underwater navigation and the detection of marine vessels. Scientists can also track the movement of marine organisms with a technique called acoustic telemetry.

Sound waves can be used to measure ocean temperatures, because their speed depends on the temperature of the water in which they are traveling. In a new study published in the journal Science, a team of geophysicists took this idea a step further and revealed how earthquakes can be used to measure changes in the average ocean temperature.

A new source of sound

To measure the ocean temperature, researchers first need to make noise loud enough to be heard over the ocean background hum. Receivers detect incoming sound waves, and scientists can use the time it took for the wave to travel from its source to the receiver to calculate changes in water temperature. The technologies originally proposed for generating a loud source were when this idea was first introduced in the late 1970s, were expensive and potentially environmentally harmful. But Wenbo Wu, a geophysicist at Caltech, and his collaborators discovered that they could use a natural source to record sound waves: earthquakes.

Seismic waves generated during an earthquake in the Earth’s crust can be converted into sound waves in the water column when seismic waves encounter the sea floor. These waves can propagate over long distances underwater and some of the wave energy can be converted into seismic waves upon reaching the ground, as seismographs located near the coast can capture their vibrations.

Wu and his team searched a decade of historical records for pairs of similar earthquakes that occurred in eastern Indonesia. Using pairs of earthquakes allowed the researchers to look at differences in ocean temperature at different time points.

“Our method is relatively low-cost and very accurate to 0.006 ° C,” says Wu. “We didn’t publish a new seismometer; we just gathered the data that was already there and used it to derive ocean temperatures.”

Researchers extracted wave travel times from the earthquake epicenters to the Diego Garcia seismic station in the central Indian Ocean. Any differences in arrival times or the speeds of the wave pairs translate into changes in the ocean temperature in the time lapse. Although salinity and ocean currents may affect sound speeds, the researchers determined that their effect was much less than changes in temperature.

There is a seismic station in Diego Garcia, which is a small island in the middle of the Indian Ocean. The researchers used ground motion data to measure ocean warming. Credit: NASA

Frederick Simons, a geophysicist at Princeton University who was not involved in the study, says he appreciates the significant cost savings and innovation with the Wu method. “[These researchers are] By combing through the records, finding gems in the seismology archives and turning them into something that another whole community spends millions of dollars trying to measure, “Simmons says.

Proper analysis

Wu says that in measurements of wave velocity, getting the correct timing is essential. The difference in the transmission times of the sound waves was less than half a second, so even the smallest errors in the seismometer timing system would do away with the perceived wave velocities. To enhance the accuracy, the researchers examined each sound wave with the seismic waves recorded by other stations around the world. All seismic and sound waves from the same earthquake event should have the same origin time. By investigating the timing of the sound wave versus the seismic waves, the researchers accurately determined the time of relative origin of the sound waves between the double earthquakes for an accurate measurement of velocity.

“Seismic ocean temperature measurement,” the term researchers use to describe the new method, complements existing technologies for monitoring ocean temperatures, Wu says. One such method is a network of buoys called Argo, which takes temperature profiles locally in the oceans wherever they are.

The Argo buoy at sea monitors ocean conditions, including temperature. Credit: Argo Data Management (CC BY 4.0)

While the floating net method measures absolute ocean temperatures with high spatial accuracy, it suffers from significant uncertainties when mapping trends on a global scale, because single-point measurements of a dynamic and broad environment such as the ocean are inherently noisy, says Wu. . With this new seismic method, average temperatures over long distances by listening to pairs of long-range sound waves. In the future, the researchers plan to measure over greater distances and expand their technology to other locations such as the Pacific Northwest, using acoustic data from earthquakes in Japan.

Echoes from the past

According to Simmons, this new method is important not only as a tool for observing the present, but also for what it can tell us about the past. After all, humans have been collecting seismic data for a century, while other global ocean temperature methods such as buoys only started a few decades ago.

“The promise is that [scientists] The record of temperature changes in the ocean could extend for many decades and we can get our hands on observations like this, ”Simons says.

As long as there is an earthquake that can be matched by a more recent earthquake, we can infer the trend of ocean temperatures from the excavated historical data, he says. Even with a decade’s passage of seismic data, the study results confirm what climate scientists already know: the ocean’s warming.

“The ocean regulates the pace of global warming,” says Wu. “More than 90% of the excess energy goes into the ocean.” An increase of 0.044 ° C in the ocean temperature over a decade, this study indicates, is alarming.

“It’s a big problem,” Wu notes, because even the ocean – Earth’s vast thermal reservoir, which covers 70% of the planet – is starting to feel hot.

Curious about earthquake risks? Check it out at Temblor.

Further reading

Wu, W., Zhan, Z., Peng, S., Ni, S., and Callies, J. (2020). Ocean seismic temperature measurement. Science, 369. https://doi.org/10.1126/science.abb9519

Munk, WH, Forbes, AMG (1989) Global ocean warming: a sound action? 19, 11. https://doi.org/10.1175/1520-0485(1989)0192.0.CO ;2

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