When an underwater volcano in the Pacific island nation of Tonga erupted violently in mid-January, it spawned a tsunami that devastated many of its islands and struck far-off shores across the ocean.
But the huge volcanic explosion also generated something that scientists hadn’t seen in more than half a century: a planetary-scale pressure wave, or shockwave, in the atmosphere.
The wave circled Earth for days.
Scientists will be studying this event for years.
As shown in this visualization, based on a simulation created by Ángel Amores, a physical oceanographer at the Mediterranean Institute for Advanced Studies in Majorca, Spain, the shockwave took about 36 hours to circumnavigate the globe, spreading out in concentric rings from the volcano known as Hunga Tonga-Hunga Haʻapai and traveling at the speed of sound. The simulation was published in the journal Geophysical Research Letters in March.
Dr. Amores was checking data from local weather stations from home when he first saw the signature of the wave. Local instruments showed sudden pressure changes when the shockwave made its first pass over Majorca, about 15 hours after the eruption.
“Then I was waiting and I said, OK, it should take like 36 hours to come back,” he said. “And then it passed again.” After another 36 hours it passed a third time.
“This is the first time that I see something like that,” he said.
“It’s super spectacular,” Peter W. Brown, a physicist at the University of Western Ontario, said of the shockwave, which traveled around the world several times at the speed of sound. “Everybody who studies atmospheric waves are all quite, I would say, awe-struck.”
In Japan, the company Weathernews maintains a network of thousands of low-cost weather sensors that collect data every minute. Many of their sensors detected nearly simultaneous spikes in air pressure as the shockwave passed:
Source: Weathernews Soratena sensor network.
The visualization shows minute-to-minute changes in air pressure measurements in Japan. For example, the data visualized for 9:01 p.m. shows the change in air pressure between 9 p.m. and 9:01 p.m. Japan time.
Weather stations across the globe detected similar spikes in pressure as the wave passed, including those across the United States, Britain, Germany, India, China and Australia. As it traveled, the shockwave caused small disturbances in local atmospheric properties such as the temperature of water vapor, creating faint ripples that could be seen in satellite images and in video footage at an observatory in Hawaii.
Shockwaves are generated by rapid movement that compresses the surrounding material, which in this case, was air, said Mark Boslough, a physicist at Los Alamos National Laboratory in New Mexico.
“You’ve got a compression wave moving into a material, and it’s moving faster than the material can get out of the way,” Dr. Boslough said. “So everything kind of piles up.”
A sonic boom is a familiar type of shockwave, caused by the buildup of pressurized air molecules when an aircraft reaches and then exceeds the speed of sound (about 650 miles an hour at a jet’s cruising altitude).
But a sonic boom is a localized event, experienced briefly on the ground along a path that is at best 50 miles wide. The Tonga explosion was so big its shockwave encompassed the whole planet.
“This was like a giant global sonic boom,” Dr. Boslough said.
The Jan. 15 eruption killed at least three people in Tonga; destroyed or damaged homes, roads and other infrastructure; and damaged crops and reef fisheries. The damage, which the World Bank estimated at $90 million, was caused by volcanic ash and by the tsunami.
As with earthquakes, volcanic eruptions can sometimes generate tsunamis by rapidly displacing a huge amount of seawater. In the Tonga event, the tsunami traveled across the Pacific, generating waves as high as four feet along the North American coast and higher in South America.
Source: Tide data from NOAA’s Center for Operational Oceanographic Products and Services
To account for tides, these charts show the difference between the water level each minute and a 60-minute rolling average. The times shown are in Greenwich Mean Time.
For some Pacific locations, the tsunami arrived during high tide, resulting in the highest water levels since the 1950s, according to Greg Dusek, a physical oceanographer and chief scientist of the NOAA office that monitors ocean tides.
Volcanologists are still studying the eruption, which occurred underwater at a depth of less than 1,000 feet when superhot magma rose up and out of the volcano. By itself that would be a very explosive event as carbon dioxide and other gases within the magma rapidly expanded. But the magma also reacted with seawater, causing it to flash violently into steam.
A plume of hot gases and ash rose more than 20 miles into the atmosphere. At its peak, the plume rose 36 miles, extending beyond the layer of the upper atmosphere known as the stratosphere. According to a NASA report, this was “likely the highest plume in the satellite record.”
Tonga Geological Services via Reuters
The type of shockwave the eruption generated is called a Lamb wave, after Horace Lamb, a British mathematician who first described them in the early 20th century. “It’s really only present when there’s a really big explosion,” Dr. Brown said, one that “can make the entire atmosphere basically vibrate like a bell.”
Dr. Amores and other scientists studying it had never seen one before because the last time there were explosions this big was decades ago, when the United States, the Soviet Union and other countries tested nuclear weapons in the atmosphere. Aboveground tests were largely banned in the early 1960s, although a few small ones were conducted until 1980.
Dr. Brown said the Lamb wave generated by the eruption was similar in scale to one from the largest atmospheric test ever conducted, of a Soviet weapon known as “Tsar Bomba.” It was detonated over the Soviet Arctic in 1961 and released energy equivalent to about 50 million tons, or 50 megatons, of TNT.
The Tonga explosion certainly released more than that amount of energy, Dr. Brown said. “We can say that comfortably.”
The changes in atmospheric pressure observed as the wave traveled around Earth were relatively small, a deviation of well under 1 percent from standard pressure. But the changes persisted for tens of minutes, Dr. Brown said.
That resulted in another kind of tsunami, called a meteotsunami, in places far removed from the volcano. Meteotsunamis are most commonly caused by fast-moving weather systems, when under the right conditions the change in air pressure above a lake or other body of water can cause potentially damaging waves to develop.
After the eruption, meteotsunamis were seen in Japan, arriving hours before the “classic” tsunami waves caused by seawater displacement reached the country. That’s because the pressure wave in the atmosphere traveled faster than the tsunami in the Pacific.
Meteotsunamis were also observed much farther from the Pacific, in the Caribbean and even in the Mediterranean.
Source: Tide and air pressure data from NOAA’s Center for Operational Oceanographic Products and Services
The air pressure chart shows the change in air pressure over six-minute intervals, and the water level chart shows the difference in water level from a 60-minute rolling average, to account for tides. The times shown are in Greenwich Mean Time.
When Dr. Dusek’s colleagues at NOAA detected the signature of a tsunami in the Caribbean, they were initially surprised. “We were like, well, that doesn’t seem likely,” he said. “And what we noticed is that it was immediately following the arrival of this pressure wave or shockwave.”
The Krakatau shockwave, which shattered the eardrums of sailors on a ship 40 miles away, was recorded by barometers around the world and circled the globe at least three times. “This is the first time, though, that we’ve seen it happen in real time,” Dr. Dusek said.
The shockwave eventually degraded, Dr. Boslough said, as all waves do. “As you knock molecules together from the compression wave, a little energy gets sucked out by heating up the air,” he said. “So eventually they just die out, just like sound waves don’t travel forever.”
Dr. Boslough’s primary focus at Los Alamos is on protecting the planet from collisions with objects from space, studying the potential effects of, say, an asteroid explosion in the atmosphere.
The Tonga explosion “is highly related,” he said. “The phenomena are very similar.”
Dr. Boslough is also developing a simulation of the explosion. “This is really an opportunity,” he said. “One of the reasons we are working on this is its relationship to planetary defense, and understanding what a big shockwave in the atmosphere can do to the Earth.”