The volcanic eruption in the Pacific island state of Tonga over the weekend has left a mysterious wave pattern in the atmosphere: ring-shaped gravity waves that become visible in satellite images. Experts still cannot explain what exactly triggered it. The only thing that is certain so far is that they are connected to the violent eruption of the volcano Hunga Tonga-Hunga Ha'apai. The eruption on January 14 may have devastated large parts of the islands and buried them under a layer of ash.
The atmospheric phenomenon is "really exceptional," says Lars Hoffmann, atmospheric researcher at the Jülich research center, "we have never seen anything like this in the data before".
The waves were discovered in pictures that the Atmospheric Infrared Sounder (Airs) made on board the Aqua satellite of NASA. In the hours after the volcanic eruption, he scanned the affected area. Its measured values show dozens of concentric circles. Everyone stands for a quickly moving wave in the atmosphere that ranges from the surface of the sea to the ionosphere. Experts assume that the waves have now circled the globe several times.
"The instrument has been in operation for a good 20 years now and we have never seen such a beautiful concentric wave pattern," says Hoffmann.
Atmospheric heavy waves arise when the air molecules in the atmosphere are moved vertically and not horizontally as, for example, with sound waves. Among other things, they form when the wind gains speed when climbing over a mountain summit, or as a result of convection in local weather systems.
The upward and downward movement transmits energy and momentum through the atmosphere. It sometimes becomes visible in the formation of clouds: The clouds at high altitude then trace the wave pattern.
Why weren't these waves noticed sooner?
Theoretically, the rapid rise of hot air and ash over an erupting volcano could trigger gravity waves on a much larger scale. However, these would have had to be discovered during earlier eruptions. The AIRS instrument has been in space since May 2002, but it has recorded nothing of the kind so far.
"This is what amazes us so much," says Corwin Wright, an atmospheric physicist at the University of Bath, UK. "It must have something to do with the physics behind this eruption, but we don't know exactly what yet.«
Wright and his colleagues suspect that a chaotic accumulation of hot gases triggers the waves in the upper atmosphere. The hot gas "climbs far into the stratosphere and whirls the air," he says.
The eruption of the Hunga Tonga-Hunga Ha'apai volcano was heard throughout the South Pacific and even parts of the United States. Many regions of Tonga are covered in ash. There is currently little information about the situation on the ground, because electricity, telephone and Internet have continued to fail. This makes it difficult for aid organisations to assess the extent to which people were injured or even died.
A complicated upper atmosphere process
Wright was the first to discover the wave patterns in the data supplied by Hoffmann. In his opinion, they show a mixture of different wave sizes and types, what complex conditions speak in the atmospheric convection. A lot of things would happen at the same time, says the researcher. “We are at least going out of that at the moment. But we could only see the phenomenon for a few hours. "
The trigger for the discovery was a tweet from Scott Osprey, a climate scientist at Oxford University. In it, he addressed Wright on January 15: "Wow, I wonder how big the atmospheric gravity waves of this eruption are!?"
The eruption may have triggered the waves only because it happened so quickly, Osprey thinks. The eruption "seems to have been over within minutes", but the force of the explosion could have caused the strong gravity waves.
As short as the outbreak was, it may be lengthy. Heavy waves can disturb the cyclical reversal of the wind direction in the tropics, according to Osprey, and that could affect the weather to Europe. "We will look very closely at how it develops," he says.
The "spectacular" images of the eruption and the data collected provide plenty of material for exciting research, says Vicki Ferrini, a marine geophysicist at Columbia University in New York. At the same time, like many others, she is concerned about the people of Tonga, whose situation is still uncertain.
Meanwhile, New Zealand researchers observe whether the volcano shows signs of a further outbreak. You continue to keep your ear on the ground, says Shane Cronin, volcanologist at the University of Auckland. The volcano may be supplied with large quantities of magma from the surface, then there could be further explosive eruptions. But if he exhausted his actual stock, there would be only smaller outbreaks. Most of them were then hidden below the surface of the sea.
