Tongan Volcano Pressure Bomb

Interesting times ahead for atmospheric scientists ...

It’s a long way from Tonga to Lauder. About 3000 km.

A huge underwater volcanic eruption on 15 January rocked Tonga and the surrounding region. The main blast was at about 5:30 pm local time. It spawned tsunamis seen around the world and sent an ash cloud more than 20 km high. In excess of 100,000 feet - about 30 km - according to one report. Or perhaps even higher according to a colleague at NOAA, who’s rushing to investigate further. It was BIG.

We were treated to fantastic satellite time-lapse images of pressure wave from the powerful blast spreading quickly around the globe.

Here in New Zealand, at NIWA’s Lauder Atmospheric Research Station, we saw a clear signature of the event in our pressure readings less than 3 hours later, around 19:15 NZST. That’s 8:15 pm summer time. The change - shown in the upper plot below - was much more rapid than the normal day-to-day variations. It also affected clouds and radiation. The lower plot shows that cloudless skies over Lauder that day were rudely disrupted by precursors to the main shock wave.

Data from Lauder at 1 minute resolution showing rapid fluctuations in atmospheric pressure as the shock wave passes around 19:15 NZST (at top), and rapid fluctuations in global radiation due to a build-up of clouds as the shock wave approaches (at bottom).

That arrival time means the disturbance must have sped through the atmosphere at more than 1000 km per hour (about the speed of sound - perhaps not surprisingly 😊). I think I may have even heard the sonic boom as it passed over our house in Alexandra a couple of minutes later. At the time I commented that it sounded as if a thunderstorm was brewing. But none eventuated.

All very interesting. But what’s all this got to do with Ozone, UV and Climate, I hear you ask.

Well, it’s all about that peak altitude reached by the ash cloud, and the residual chlorine that’s still in the atmosphere. The last time we saw a volcanic injection to those altitudes was after the eruption of Mount Pinatubo, way back in 1991. Then there were two main effects: one not so good and one potentially better.

Firstly, the injected aerosols provide surfaces for heterogeneous chemistry - like the ice crystals in the atmosphere above Antarctica - that have caused the ozone hole to recur every spring since the early 1980s. There, the amount of ozone depletion depends on the surface area of ice clouds present, and the amount of chlorine remaining in the atmosphere. Because of the success of the Montreal Protocol, chlorine concentrations are now gradually decreasing from their peak around the turn of the century. But the atmospheric lifetime is measured in decades, so concentrations are still close to their maximum and similar to those present when Pinatubo erupted. After that eruption, heterogeneous chemistry on the aerosol surfaces caused ozone depletion of a few percent, which would tend to increase UV at the surface (though few measurements were available that long ago to confirm those increases). The same may be expected this time. And this time, we’re prepared!

Secondly, the aerosols scatter sunlight back to space, resulting in a reduction in sunlight reaching the surface. This leads to a temporary cooling at the surface. Pinatubo caused a temporary global cooling of around 0.3 degrees Celsius. That is essentially the geoengineering ‘solution’ to global warming that’s currently being considered. I talked about the downsides of that in an earlier blog.

The first effect tends to increase UV, while the second tends to decrease it. We’ll have to wait and see the overall effect. We don’t yet know if the reduction from scattering losses will be larger or smaller than the increases due to ozone loss.

Effects can be relatively long-lived because there’s no rain-out or turbulent mixing at those altitudes. So it will probably be months or even years (depending on the size of the particles) before the aerosol cloud gradually dissipates through gravitational settling.

Early indications are that the amount of aerosol injected from Hunga-Tonga-Hunga-Ha'apai has been - so far - much smaller than from Pinatubo, so it probably won’t have as large an effect.

But, uniquely for the Tongan eruption, is the observation from satellites that the pressure waves from the explosion (they’re sometimes called atmospheric gravity waves) may have circumnavigated the globe several times. It’s been speculated that the induced pressure changes could disrupt seasonal changes in wind patterns - with unknown consequences.

Interesting times ahead for atmospheric scientists ….

Hopefully the event and its longer term consequences will provide fodder for debate on the merits and pitfalls of geoengineering solutions to mitigate climate change through stratospheric aerosol injection.

In the meantime, we spare a thought for our neighbours in the Friendly Isles. Their lives and livelihoods have already been devastated.

Updated March 2022: A description of the waves is given here.

Thanks for reading this. Previous posts on the intersection between Ozone, UV, Climate, and Health can be found at my UV & You area at Substack. Click below to share or subscribe for occasional free updates.