Ocean acoustics confirm rising sea temperatures

Researchers devised a way to measure changes in ocean temperature using earthquakes. By mining historical seismic records, researchers got a glimpse of past ocean trends.
 

By Shi En Kim (@goes_by_kim)
 

Citation: Kim, S. E., 2020, Ocean acoustics confirm rising sea temperatures, Temblor, http://doi.org/10.32858/temblor.118
 

 

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

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

 

A new source of sound

To measure ocean temperature, researchers first need to make a noise that is loud enough to be heard above the ocean’s background hum. Receivers detect incoming sound waves and scientists can use the time the wave took to travel from its source to the receiver to calculate water temperature changes. Techniques originally proposed to generate a loud-enough source, when this idea was first introduced in the late 1970s, were expensive and potentially ecologically damaging. 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 Earth’s crust can be converted to sound waves in the water column when the seismic waves encounter the seafloor. Those waves can propagate over long distances underwater and some of that wave energy can be converted back to seismic waves upon reaching land, where seismometers situated near the coast can pick up their vibrations.

Wu and his team scoured a decade of historical records for similarly located earthquake pairs that had occurred in east Indonesia. Using earthquake pairs allowed the researchers to look at differences in ocean temperature at different points in time.

“Our method is relatively low cost and very accurate to about 0.006°C,” Wu says. “We didn’t deploy a new seismometer; we just collected data that were already out there and used them to derive ocean temperatures.”

The researchers extracted the waves’ travel times from the earthquake epicenters to the Diego Garcia seismic station in the Central Indian Ocean. Any differences in the arrival times or speeds of the wave pairs translate into changes in the ocean temperature in the intervening time. Although salinity and ocean currents may affect the sound speeds, the researchers determined that their effects were much smaller than temperature changes.

 

 

Frederik Simons, a geophysicist at Princeton University who was not involved in the study, says he appreciates the significant cost savings and creativity of Wu’s method. “[These researchers are] combing through records, finding gems in the archives of seismology and turning that into something that a whole other community is spending millions of dollars trying to measure,” Simons says. “This is a story of human ingenuity.”

 

Sound analysis

In wave speed measurements, getting the timing right is key, Wu says. The difference in the travel times of the sound waves were less than half a second, so even the smallest errors in the seismometer’s timing system would throw off the perceived wave speeds. To boost the accuracy, the researchers cross-checked each sound wave to the seismic waves registered by other stations around the world. All seismic and sound waves from the same earthquake event should have the same origin time. By verifying the timing of the sound wave against the seismic waves, the researchers pinpointed the relative origin time of the sound waves between paired earthquakes for an accurate speed measurement.

“Seismic ocean thermometry,” the term the researchers use to describe the new method, complements existing techniques for monitoring ocean temperatures, says Wu. One such method is the network of floats called Argo, which take temperature profiles locally in the oceans wherever they drift.

 

 

While the float network method measures absolute temperatures of the ocean with high spatial resolution, it suffers from large uncertainties when mapping trends on a global scale, because single-point measurements of a dynamic and vast environment such as the ocean are inherently noisy, Wu says. With this new seismic method, scientists average temperatures over long distances by listening for pairs of far-reaching sound waves. In the future, the researchers plan to measure across greater distances and expand their technique to other locations such as in the northwestern Pacific Ocean, using the acoustic data from earthquakes in Japan.

 

Echoes from the past

According to Simons, this new method is important not just as a tool to monitor the present, but also for what it can tell us about the past. After all, humans have been collecting seismic data for a century, whereas other global ocean thermometry methods such as the floats have only begun a few decades ago.

“The promise is that [scientists] can extend the record of the temperature variations in the ocean to as many decades as we can put our hands on such observations,” Simons says.

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

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

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

 

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Further reading

Wu, W., Zhan, Z., Peng, S., Ni, S., Callies, J. (2020). Seismic ocean thermometry. Science, 369. https://doi.org/10.1126/science.abb9519

Munk, W. H., Forbes, A. M. G. (1989) Global Ocean Warming: An Acoustic Measure? 19, 11. https://doi.org/10.1175/1520-0485(1989)019<1765:GOWAAM>2.0.CO;2