Satellite images of the Tongan eruption of 15th January

Tongan eruption sends atmospheric shockwaves around the world

by Kirsty McCabe, FRMetS


The violent volcanic eruption in the Kingdom of Tonga on 15th January 2022 was so powerful it not only devastated the island nation; its impacts were felt around the world. A tsunami spread across the Pacific in a matter of hours, with waves hitting Australia, New Zealand and Japan, as well as the west coasts of North and South America. People as far away as Alaska and Canada heard the sonic boom, and barometers around the world recorded pressure changes from atmospheric shockwaves that rippled around the globe.



Part of the Pacific “Ring of Fire”, Hunga Tonga-Hunga Ha’apai is a submarine volcano nearly 2 km high, with only its top 100 metres poking out of the sea. It has erupted regularly over the past few decades, with small eruptions over recent weeks. But the blast on 15th January was extremely violent, generating a pressure wave that blasted through the atmosphere at more than 1000 km/h. The amount of lightning strikes above the volcano increased dramatically a day or two before the cataclysmic eruption itself sparked almost 400,000 lightning bolts.



The volcanic eruption created a colossal mushroom cloud of ash, steam and gas that reached up to 30 km high, roughly three times the altitude at which commercial aircraft fly. Preliminary data from the University of Oxford suggests the volcanic cloud may even have reached an altitude of 39 km (128,000 ft).



Thunderstorm clouds flatten out at the tropopause, or top of the troposphere, since a lid of warm air suppresses their upward development. However, the plume of ash was so buoyant it was able to penetrate this layer and reach into the stratosphere. The ash cloud radiated out to a massive 260 km in diameter before being distorted by the wind and sweeping more than 3000 km west to Australia. Satellite imagery captured atmospheric gravity waves rippling outward from where the plume punctured the tropopause. A gravity wave is simply a wave moving through a stable layer of the atmosphere.



Twenty minutes after the eruption, tsunami waves up to 15 metres high hit the islands of Tonga, including the main island of Tongatapu. Less than an hour later, the volcano was raining ash and tiny pebbles on the Polynesian archipelago, coating everything with a layer of fine ash resembling grey snow. It took less than 5 hours for smaller tsunami waves to reach New Zealand and about 10 hours to reach Alaska.



Meanwhile, the shockwave was traveling almost as fast as the speed of sound and causing noticeable jumps in atmospheric pressure as it reached the UK on the opposite side of the world about 15 hours after the eruption. Numerous weather stations recorded astonishing 2 to 3 millibar pressure changes during the passage of several waves. Shockwaves are formed when a pressure front moves at supersonic speeds and pushes on the surrounding air, usually heard as a loud crack or snap noise. During a shockwave, air pressure increases more than usual, as a result of the air being quickly compressed, and then decreases again.



Volcanic eruptions can release enormous amounts of sulphur dioxide and other aerosols, which can produce a measurable cooling effect on the Earth’s climate by reflecting incoming sunlight. But initial data suggests the amount of sulphur dioxide the Tongan eruption blasted into the air was about 2 per cent of the amount produced by the eruption of Mount Pinatubo in 1991. This means the eruption is unlikely to lower global temperatures. And it’s worth noting that the Pinatubo aerosols only had a short-term impact of a year or two, so the Tongan eruption is not going to buy us time in our battle against climate change.


Satellite image of eruption's ash cloud illuminated by moonlight
Satellite image of the eruption's ash cloud, illuminated by moonlight


However, the plume has already spread over Australia, with extremely high concentrations of sulphur dioxide above the Pacific Ocean and some becoming wrapped into cyclone Cody, east of New Zealand. Sulphur dioxide is potentially harmful to human health, worsening conditions such as asthma and causing acid rains.



More pressing is the impact of the volcanic ash on water supplies on the Tongan archipelago and getting aid to the islands. So far, the ash cloud has hampered aid efforts, and there is also concern over aid workers bringing the Covid-19 virus. Meanwhile, communications with the Kingdom of Tonga have been severely disrupted making it difficult to assess the scale of the devastation.



Scientists are now starting to take the measure of the monster volcanic eruption that rocked the South Pacific kingdom of Tonga and made our atmosphere ring like a bell, albeit at a frequency too low for us to hear. A recent NASA assessment found that the Tonga volcanic eruption was one hundred times more powerful than the atomic bomb that was dropped on Hiroshima, Japan, in 1945. As for the height of the ash cloud, it would appear it reached even higher into our atmospher than first thought. In other words, the ash plume entered three layers of our atmosphere: the troposphere, the stratosphere and the mesosphere.


Categories: In the Spotlight Weather
Tags: Atmospheric Dynamics Dust Emissions Extreme Weather Observations Weather Wind WorldWeather

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