The eruption of the Tonga volcano has created confusing ripples in the Earth’s atmosphere

The United States National Oceanic and Atmospheric Administration’s GOES-West satellite captured the explosive eruption of Hunga Tonga-Hunga Haʻapai.Credit: NOAA

Scientists are racing to understand a puzzling series of massive ripples in Earth’s atmosphere triggered by the eruption of the Tongan volcano this weekend. Satellite data shows the event – which some fear devastated the Pacific island nation – caused an unusual pattern of atmospheric gravity waves. Previous volcanic eruptions have not produced such a signal, leaving experts puzzled.

“It’s really unique. We’ve never seen anything like this in data before,” says Lars Hoffmann, an atmospheric scientist at the Jülich Supercomputing Center in Germany.

The discovery was made on images collected by the Atmospheric Infrared Sounder (AIRS), mounted on NASA’s Aqua satellite, in the hours following the eruption of the Hunga Tonga-Hunga Haʻapai volcano on January 14.

They show dozens of concentric circles, each representing a wave moving rapidly through the gases of the atmosphere, extending over 16,000 kilometers. The waves reached the ocean surface to the ionosphere, and the researchers believe they likely circled the globe several times.

“Pretty concentric wave patterns”

“This instrument has been in operation for about 20 years now and we have never seen such beautiful concentric wave patterns,” adds Hoffmann.

Atmospheric gravity waves occur when air molecules in the atmosphere are disturbed vertically, rather than horizontally, in the air column. This can happen when the wind picks up speed as it rises on top of a mountain or as a result of convection in local weather systems.

Rising and falling waves transfer energy and momentum through the atmosphere and often show their effects in the way they cause clouds to form high in a series of ripples.

Hunga-Tonga-Hunga Ha'apai eruption seen by AIRS.

Atmospheric infrared sounder images from NASA’s Aqua satellite show dozens of concentric circles, which are fast-moving atmospheric waves.Credit: Lars Hoffmann, Jülich Supercomputing Center. AIRS level 1 data by NASA DES DISC

In theory, the rapid updraft of hot air and ash from an erupting volcano in the upper atmosphere could trigger gravity waves on a much larger scale. But nothing like this has been observed with previous flares analyzed since the launch of the AIRS instrument in May 2002.

“That’s what really puzzles us,” says Corwin Wright, an atmospheric physicist at the University of Bath, UK. “It must have something to do with the physics of what’s going on, but we don’t know what yet.”

He and his colleagues suspect that a “big, big, messy pile of hot gases” in the upper atmosphere could be what triggers the waves. The hot gas “rises high into the stratosphere and spills the air,” he says.

Eruption heard across the planet

The eruption of the Hunga Tonga-Hunga Ha’apai volcano was heard across the South Pacific, and even parts of the United States. Ash covered many parts of Tonga, but a loss of electricity, phone lines and internet connectivity made it difficult for aid agencies to assess the extent of injuries, deaths and damage.

Wright, who was the first to spot the wave patterns in the data provided by Hoffmann, says the images show what looks like a mix of wave sizes and types.

Convection in the atmosphere seems to be “very complicated and bumpy, and it’s doing a whole family of things at the same time,” he says. “That’s what we currently think, but we’ve only been looking at it for a few hours.”

The discovery was prompted by a Tweeter sent to Wright on January 15 by Scott Osprey, a climatologist from the University of Oxford, UK, who asked, “Wow, I wonder how big the atmospheric gravity waves are from this eruption?!

Explosive Speed

Osprey says the eruption could have been the only one to cause these waves because it happened very quickly compared to other eruptions. “This event appears to be over within minutes, but it was explosive and it is this impulse that is likely to trigger strong gravity waves,” he says.

The eruption may have lasted for a few moments, but the impacts could be long lasting. Gravity waves can interfere with a cyclical reversal of wind direction in the tropics, Osprey says, and this could affect weather patterns as far away as Europe. “We are going to watch very carefully how this develops,” he said.

The images and data collected from the eruption were “spectacular” and presented scientists with an exciting opportunity, says Vicki Ferrini, a marine geophysicist at Columbia University in New York. But she adds that she and others remain deeply concerned about the people of Tonga, especially given the absence so far of detailed information on the scale of the disaster.

New Zealand researchers say they are closely monitoring the volcano for further eruptions. “We just keep our ears to the ground,” says Shane Cronin, a volcanologist at the University of Auckland. The volcano could be replenished with large amounts of magma from deep underground and produce more explosive eruptions, he says. But if it has exhausted its main supply, it could only produce smaller eruptions, much of it hidden below the ocean surface.

Additional reporting by Smriti Mallapaty.

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