A powerful volcano eruption may have a cooling and rainy influence on parts of Australia for up to eight years

Lasting impacts from an enormous volcanic eruption a year ago may have a cooling and rainy influence on parts of Australia for up to eight years, according to scientists.

• The Hunga Tonga-Hunga Ha’apai undersea volcano erupted last January
• It pumped a record-breaking amount of water vapour into the stratosphere
• Scientists are tracking the impact on Australia’s weather

In January 2022, the underwater volcano Hunga Tonga-Hunga Ha’apai erupted, shooting ash and other particles more than half way to space.

The eruption triggered a tsunami that reached heights of more than 19 metres above sea level, and created long lasting and vivid sunsets for several months following.

But the powerful explosion also caught the intrigue of scientists around the globe, fascinated by its potential impacts on climate.

Of particular interest was the record-breaking amount of water vapour, a strong greenhouse gas, which it pumped into the stratosphere.

So a year on, what do we know about its influence on Australian weather?

Volcano potentially added to rain on east coast

Martin Jucker, from the University of New South Wales Climate Change Research Centre, is leading a research paper exploring the impacts of the eruption’s water vapour on Australian weather.

Dr Jucker said while it was too early to provide solid answers with the paper still under preparation, they did have some idea about what the impacts might be on weather in Australia.

These included a potential increase in rainfall over the east coast of Australia, south of about Brisbane, between mid-February 2022 and mid-April 2022.

“So we can say, around March, April last year, the volcano would probably have favoured rain in Australia on the east coast,” he said

This was by moving a large band of cloud, known as the South Pacific Convergence Zone (SPCZ), closer to Australia.

“The South Pacific Convergence Zone is where it rains a lot,” Dr Jucker said. “It’s basically constantly cloudy. “And so if that moves further south-west, that move means this band of rain moves closer to the (Australian) coast.”

Dr Jucker said they were “quite confident” about the correlation between the eruption and the shift in position of the SPCZ. But he said they could not put a figure on how much of an influence on rainfall it had during that time. This was because it was too hard to untangle from other factors, such as La Nina, climate change and natural variability. He said they also could not link it to a specific weather event, such as the Lismore flooding which occurred during this period, for the same reasons.

Possible cooling in WA and northern Australia

Dr Jucker said there could also be impacts to temperature for up to eight years because of how long it takes for particles to clear out of the stratosphere.

While our day-to-day weather occurs in the troposphere, the bottom layer of the atmosphere, conditions in the stratosphere can have flow-on impacts to how weather systems at the surface behave.

Dr Jucker said they expected a very slight warming influence, on average, across the globe and a strong warming influence for parts of North America during their winter for the next seven years.

But he said in Australia they were expecting the opposite, due to the way heat circulates around the globe. This included a cooling effect for Western Australia during the summer months, and northern Australia during winter.

“For WA, we are expecting a cooling in summer of about 0.3 degrees on average over seven years,” he said. “So if that’s the average over such a long time, it’s something which you will feel, probably.” He said northern Australia had an even stronger cooling influence. “So that’s almost a degree [on average over seven years],” he said. “As a comparison, with the Paris Agreement, 1.5 degrees Celsius warming is still okay, but 2 degrees Celsius starts being dangerous, and that difference is half a degree.”

Larger ozone hole behind cool, rainy influence

Dr Jucker said the changes to weather were created by a chain reaction of events that filtered down from the stratosphere into the troposphere.

“So water vapour is a very powerful greenhouse gas, it’s very important for radiation,” he said. “But, probably more importantly, in the stratosphere, [water vapour] actually cools the stratosphere. “And that means that the polar vortex will be stronger in winter, and there will be a larger ozone hole, and all of these things have an effect on surface weather.”

Potential rainfall increase in south-east Australia

He said, through a similar process, there were also theories that the eruption will boost rainfall in south-eastern Australia during summer for the next few years.

But he said this influence was a lot less clear, because of the capability of their modelling. “So [this theory], it’s more relying on past research,” he said. The theory boiled down to the influence of a belt of strong westerly winds below the continent, known as the Southern Annular Mode (SAM). Dr Jucker said the extra water vapour was expected to cause the SAM to trend toward a “positive” phase. When the SAM is in a positive phase in summer, there is typically above average rainfall in south-eastern Australia. But Dr Jucker said it was unlikely to have had an influence this summer, because the water vapour was not yet in the part of the stratosphere it needed to be to create the change.

Changing ocean conditions not considered

Dr Jucker said there was a caveat to his research. He said the modelling was “very good for the stratospheric chemistry”, but they were not able to consider changing ocean conditions. “If we included the ocean, we would not have been able to run it out for 10 years, or we would not have had sufficient statistics to get clear results,” he said. He said it meant the results reflected the likely influence of the volcano on its own.

But he said it was also important to remember several factors influenced weather each day and the eruption was just one piece of the puzzle.

Originally published at https://www.abc.net.au on February 15, 2023.