Scientists figure out how Tonga volcano caused a tsunami on a global scale

(ORDO NEWS) — In August 1883, a mountainous island in Indonesia called Krakatoa, or Krakatau, self-destructed. Episodic volcanic eruptions resulted in an explosion that threw debris to a height of 80 kilometers and covered 800,000 square kilometers of the earth’s surface with caustic ash.

When most of the island was blown to pieces and fell into the sea, a tsunami rose and hit the nearby islands of Java and Sumatra, killing most of the 36,000 people.

While Indonesia bore the brunt of the damage, the Krakatoa eruption had bewildering repercussions around the world.

Somehow, small tsunamis hit the shores of countries in the Pacific and Atlantic oceans, although it would seem that the Krakatoa tsunami could not jump over the continents from the Indian Ocean and get into other ocean basins. With no other explanation, scientists of the day blamed these distant tsunamis on coincidental earthquakes.

But geophysicists in subsequent decades continued to puzzle over these data. For example, a 1955 study showed that distant tsunamis correlated with the arrival of a pressure wave that propagated through the air from the eruption.

The authors of the study suggested that there was some connection between these atmospheric disturbances and water.

Computer simulations in 2003 confirmed this suggestion, showing that the main Krakatau tsunami, even if it made its way through gaps in the continents and reached the Pacific and Atlantic oceans, lagged behind smaller tsunamis in places like Hawaii, California and Alaska, which were synchronized with more rapid pressure wave from the explosion. (A pressure wave whose frequency is in the range of audibility is called sound.)

To validate the speculative idea that sound or pressure waves from volcanoes can cause tsunamis, scientists need to see another real-time version of Krakatoa in the modern era – an awkward wish, of course.

Scientists figure out how Tonga volcano caused a tsunami on a global scale 1An 1888 lithograph depicting the 1883 eruption of the Krakatoa volcano

Then, on January 15, 2022, an almost completely submerged volcanic cauldron in the South Pacific called Hunga-Tonga-Hunga-Hapai let out a nightmarish roar. Its mushroom cloud of ash and localized tsunami devastated the archipelagic kingdom of Tonga.

Despite the fact that there were very few victims, this volcano broke all kinds of records: It threw debris two-thirds of the way into space; its ash cloud generated up to 200,000 lightning strikes per hour; and the explosion itself was one of the most powerful ever recorded.

In terms of the scale and energy of the explosion, the Tongan Volcano was “virtually like Krakatau 2,” said Matthew Haney, a geophysicist at the USGS Alaska Volcano Observatory.

A crackling sound like a gunshot could be heard in Anchorage, where Haney is located. “We’re 6,000 miles away. Can you hear the volcanic eruption? Wow! It just blows my mind.”

Not only the sound of Hunga-Tonga-Hunga-Ha’apai resounded all over the world. A tsunami burst tens of centimeters high hit distant shores in different ocean basins.

“We did not expect such a tsunami signal in the Caribbean,” says Paul Fanelli, an oceanographer with the National Oceanic and Atmospheric Administration (NOAA).

This time, scientists believe they have found a solution. When wave height sensors around the planet were matched with corresponding air pressure sensors, it became clear that the pressure wave from the explosion must have connected with the surfaces of many oceans and seas, transferred energy to the water and created myriads of tsunamis.

This explanation simultaneously solved the 139-year-old Krakatoa mystery. “Given what was observed in 1883 … it is quite logical that this could have happened then,” said Greg Dusek, NOAA physical oceanographer. “Both sets of observations are in good agreement.”

But, as with all great scientific discoveries, new questions have arisen. When and why do volcanic pressure waves dance the tango with ocean waves? Why did tsunamis in Tonga only occur along certain coastlines? And how powerful and potentially destructive can these tsunamis be?

Usually, in order to generate a tsunami, it is necessary to push a large mass of water into a body of water to force it out. Earthquakes do it in a simple way. “Earthquake tsunamis are very, very simple,” says Emily Lane, a hydrodynamicist at New Zealand’s National Institute of Water and Atmospheric Research.

“An earthquake occurs underwater, which causes deformation of the seabed, this deformation propagates to the surface of the water, and then radiates in the form of a tsunami.”

Volcanic tsunamis are more complex. Debris thrown into the water or spray, partial or complete destruction of the volcano itself, as well as underwater explosions – all this can lead to the displacement of water.

Research on the seafloor around Hunga-Tonga-Hunga-Khapai will reveal in the coming months which process or combination of processes caused the classical-type regional tsunami.

But almost three hours before this massive tsunami crossed the Pacific Ocean and reached Japan, small wave peaks appeared on the Ogasawara Islands, about 1,000 kilometers south of Tokyo.

On the same day, similar peaks appeared in the Caribbean Sea from Puerto Rico to Mexico and even in the Mediterranean Sea, 18,000 kilometers from the site of the eruption.

These small, fast waves have reminded some scientists of a less traditional way the Earth can create tsunamis: through the atmosphere.

Storms can sometimes cause persistent and significant atmospheric disturbances. In 1929, the British mathematician and oceanographer Joseph Proudman suggested that if a disturbance moves at a certain speed over a body of water, then it can cause what is now called the Proudman resonance.

His equations showed that a barometric pressure wave could transfer energy to water waves, making them bigger. And when these enhanced waves hit the coast, they are called meteor tsunamis.

As a result, Proudman’s mathematics showed that the transfer of energy from the sky to the sea is most efficient when the atmospheric disturbance moves at the same speed as the water waves. And the speed of water waves depends on the depth of the water.

Scientists figure out how Tonga volcano caused a tsunami on a global scale 2

According to Alexander Rabinovich, a weather tsunami expert at the Institute of Oceanic Sciences in Sydney, British Columbia, storms typically produce atmospheric pressure waves that travel tens of centimeters per second.

At such a low speed, pressure waves resonate with the same slow water waves that occur in shallow water bodies and cause large meteorological tsunamis.

That is why they occur dozens of times a year in the relatively shallow waters of the US East Coast, the Gulf of Mexico and the Great Lakes, sometimes with a fatal outcome: A 3-meter-high meteotsunami on Lake Michigan in 1954 claimed the lives of seven people.

On rare occasions, weather tsunamis can rival tsunamis caused by earthquakes: in 1978, 6-metre-high meteotsunami waves frightened the Croatian port city of Vela Luka, hitting it repeatedly for several hours.

The tsunami caused by the January eruption of Tonga was very similar to a meteotsunami – as numerous oceanographers, physicists and volcanologists realized almost in unison a few hours after the explosive zenith of the volcano.

“All the mechanics and physics are very similar,” said Eric Anderson, a researcher at the Colorado School of Mines who studies the interactions between the hydrosphere, cryosphere and atmosphere. But there is one key difference. “The whole globe is affected,” says Rabinovich.

The roar is heard all over the world

Researchers have not yet reached a consensus on the specific type of pressure wave or combination of types that are responsible in this case. The Tonga eruption caused many atmospheric disturbances, including a short-term shock wave and acoustic waves that spread around the world.

By pushing a large amount of air up out of its way, the explosion also created what are known as atmospheric gravity waves.

They occur when a cold air mass rises rapidly, then gravitationally falls back to sea level, essentially hitting the stratosphere like a gong and creating pressure fluctuations that travel horizontally in all directions.

Many researchers consider Tong’s atmospheric gravity waves to be the most likely source of distant tsunamis, given the specific locations of their occurrence. Gravitational waves swept across the planet at speeds reaching or exceeding 300 meters per second.

To get the Proudman resonance, the water waves have to travel comparably fast, and this requires an unusually large depth. “For an ocean wave to move at that speed, you need about 9,000 meters of water,” says Dusek, “and there aren’t many places where you can go that deep.”

Significantly, strong tsunamis arose in the Caribbean Sea and south of Japan, where the depth of the ocean trenches exceeds 8000 meters.

In these places, sea waves really move at a speed close to the speed of sound. It’s hard to imagine, but in the open ocean the waves are almost invisible. “Until you get into shallow water, the wave is not as noticeable on the surface of the sea,” says Dusek.

According to Eric Geist, a geophysicist at the U.S. Geological Survey’s Pacific Center for Coastal and Marine Sciences, while the Proudman resonance may have been most effective in these two troughs, even elsewhere, as atmospheric gravity waves moved across the open ocean, there was some resonance and amplification of sea waves. The underwater topography of the seabed near the affected shores could also amplify incoming tsunami waves.

Along with the theory of deep troughs, these ideas require further study before firm conclusions can be drawn. Other types of pressure waves may have further increased the height of the waves.

But even at this early stage, no one doubts that the dramatic eruption of Tonga caused the seas and oceans on the other side of the world to tremble, rear and convulse a revelation in the present that fully confirms a wonderful idea from the past.

Quirks of physics

Given the “strong similarities” between the events in Tonga and Krakatau, according to Rabinovich, the “cold case” about the worldwide tsunami in Krakatau can be closed. But, as in the case of Krakatoa, the pandemonium in Tonga raised new questions.

The distant tsunamis may not have been dangerous, but the local tsunami caused by the Tonga volcano was definitely dangerous.

There is a deep depression in Tonga, so it seems likely that the Proudman resonance occurred near the eruption as well. “Did it add a little more to the peak wave height than you would expect from the seismic signal alone?” Dusek asked.

The fact that the wave height never approached the wave height of some storm weather tsunamis was a relief. But will it always be like this?

Researchers are wondering how powerful an explosion would have to be to create a much larger weather tsunami. Volcanoes on land can also create large explosions, Lane noted, so are they capable of generating tsunamis from a distance?

Volcanic explosions, as terrible as those of Krakatau and Hunga-Tonga-Hunga-Hapai, are extremely rare by human standards.

And so far, it seems that such monstrous eruptions are not capable of causing serious wave peaks around the world. When it comes to atmospheric tsunamis, “weather-driven tsunamis are still a public safety concern,” Dusek said.

The sea manifestation of Tonga was not a terrible show. But it reminded scientists that nature’s use of even the smallest quirks of physics can lead to global consequences.

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