A NASA satellite caught Hunga Tonga–Hunga Ha‘apai’s explosive eruption. Credit: NASA Worldview/NOAA/NESDIS/STAR

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The blast from a South Pacific volcano at Tonga was strong enough to jiggle the atmosphere from the surface to the ionosphere, according to preliminary analyses by scientists scattered around the world.

“This is something we have never seen in 20 years of AIRS data,” said Lars Hoffmann at the Jülich Supercomputing Centre in Germany, who analyzed atmospheric infrared sounder (AIRS) measurements from NASA’s Aqua satellite in the days after the blast.

The Hunga Tonga–Hunga Haapai eruption, which killed at least three and inflicted widespread damage, may be one of the most powerful eruptions of the past 30 years.

Atmospheric waves from the explosion shot across the globe in less than 24 hours. The waves even shifted local air pressure: In Seattle, the wave briefly lifted the city’s fog, said the local National Weather Service office.

The colossal detonation sent many types of waves through Earth’s atmosphere: infrasound waves, Lamb waves, acoustic-gravity waves, and signals from tsunamis across the Pacific Ocean.

While scientists are still parsing the eruption’s effects, the magnitude and extent of the waves reveal the explosive power of the volcano. “It’s this really almost unimaginable amount of energy,” said Mathew Barlow, a professor of environmental, Earth, and atmospheric sciences at the University of Massachusetts Lowell. “On the ground, this is a tragedy for Tonga.”

The Eruption Felt Around the World

The eruption was “strong enough to make the whole atmosphere vibrate, just like a musical instrument.”

On the day of the explosion, Barlow, in Massachusetts, had a curious idea: Could he see the eruption affecting the surrounding atmosphere right after the blast?

He turned to data from NOAA’s GOES-West satellite, which are accessible online in high resolution. Unlike true-color satellite images that showed the eruption in photographs, the images he was most interested in looking at showed the electromagnetic wavelength best for viewing clouds in the atmosphere.

The initial atmospheric response to the eruption was captured by Mathew Barlow using NOAA’s GOES-West satellite infrared radiance data (band 13). This sequence is based on images taken 10 minutes apart, and colors show the difference in infrared radiance between each time step. Credit: Mathew Barlow/University of Massachusetts Lowell

When Barlow stitched the images together, he saw waves shoot across the globe through the atmosphere in a matter of hours. “As this wave moved around the world, it essentially gave a little shove or a jiggle to the whole atmosphere,” Barlow said. The compiled images show how the wave disturbed characteristics of the atmosphere, like air temperature and water vapor, as it traveled.

Barlow posted the video to Twitter and watched as others posted similar images with data from additional satellites.

Nedjeljka Žagar, an atmospheric dynamicist at the University of Hamburg in Germany, simulated the wave in her team’s model. She believes the massive initial wave is a Lamb wave, a special type of wave named after English mathematician Horace Lamb. The eruption was “strong enough to make the whole atmosphere vibrate, just like a musical instrument, with a note being the Lamb wave,” she said.

Nedjeljka Žagar created a simulation of the atmospheric response triggered by the eruption in the first 24 hours. The colors of the wave represent a perturbation of the height of the stratosphere (positive is red, negative is blue). The signal is proportional to the perturbation in pressure seen at Earth’s surface. The simulation was performed by the TIGAR (Transient Inertia-Gravity and Rossby) model of the atmospheric dynamics and predictability group at the University of Hamburg. Credit: Nedjeljka Žagar

Waves of this size come from nuclear detonations, huge eruptions, or meteors that explode in Earth’s atmosphere, according to research professor David Fee of the Geophysical Institute in Fairbanks, Alaska. NASA scientists estimated that the Tongan eruption was 500 times as powerful as the nuclear bomb dropped on Hiroshima, Japan.

Scientists at the Comprehensive Nuclear-Test-Ban Treaty Organization registered the initial wave at all 53 of the organization’s infrasound international monitoring system stations. The stations are at distances ranging between 1,800 and 18,000 kilometers from the volcano, said the organization’s analysts. (The extent of the waves beat the last big atmospheric event to rattle Earth—the 2013 Chelyabinsk meteor—which registered at only half of the organization’s stations.)

Shrewd observers noticed a perturbation in local pressure, too. “A number of places around the globe reported at least six returns of the wave signal,” Žagar said.

Ripples in Earth’s Upper Atmosphere

While some scientists tracked waves racing across the globe, others looked at the reach of the waves into the upper bounds of our planet’s atmosphere.

In the United Kingdom, Oxford climate physicist Scott Osprey retweeted a GIF of the eruption and tagged colleague Corwin Wright. “Wow, I wonder how big the atmospheric gravity waves are from this eruption?!” Osprey wrote. Gravity waves regularly travel through the atmosphere and are one driver of general atmospheric circulation.

Wright turned to images from NASA’s Aqua satellite, which were processed and posted automatically online by Hoffmann. On the day after the eruption, a series of large concentric waves radiated from the volcano, like ripples in a pond. One image showed the wave in the stratosphere, which stretches from the upper limits of commercial airliners to 5 times a jet’s cruising altitude. The waves represent lows and highs in temperature caused by the wave.

Ripples in temperature in the stratosphere above the volcano are thought to be acoustic-gravity waves that travel just below the speed of sound. Credit: Lars Hoffmann/Jülich Supercomputing Centre, data from AIRS Level-1 via NASA GES DISC

Joan Alexander, a Colorado-based senior research scientist at NorthWest Research Associates, sees ripples like this every day above thunderstorms around the world, “but they are never this large of an extent of a pattern,” she said. “They’re never as strong as this.”

While it’s clear that the volcano generated the waves, scientists are curious about which specific phenomenon was responsible. It could have been the volcano’s massive belch of heat into the atmosphere, the sudden onslaught of latent heat by condensation, or the hurtling vertical velocity from the explosion (the volcanic cloud shot 39 kilometers into the atmosphere, preliminary calculations estimated). “It’s probably a mixture of things,” Alexander said.

Physicist Cathryn Mitchell at the University of Bath even saw the waves reaching into the ionosphere, a region that extends between the mesosphere and the thermosphere. She pulled from multiple ground and space-based sensors to plot the ionosphere’s total electron content over time.

In a video she created, the ionospheric disturbances propagate away from the direction of the volcano over New Zealand for several hours after the eruption. The waves are seen as both positive and negative excursions in the filtered total electron content (colored dots in the video). Five hours later, the waves reached the continental United States, said Mitchell.

Mysteries Remain

“I do not think we have a good understanding of how audible sound is generated at such long distances from the source.”

While scientists have made huge strides in understanding waves from the Tongan eruption, some aftereffects are still inexplicable.

“People, including myself, also heard this eruption in Alaska at almost 10,000 kilometers away, which is remarkable,” said Fee. “I do not think we have a good understanding of how audible sound is generated at such long distances from the source.”

Hoffmann wonders whether high-resolution atmospheric models would be able to reproduce such an event. The models currently do not simulate volcanic eruptions.

Indeed, Alexander said, “Because of the wealth of measurements that observed different aspects of this event and its global extent, there will likely be research continuing for years.”

—Jenessa Duncombe (@jrdscience), Staff Writer

Citation: Duncombe, J. (2022), The surprising reach of Tonga’s giant atmospheric waves, Eos, 103, https://doi.org/10.1029/2022EO220050. Published on 21 January 2022.
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