Saturday, March 2, 2024

Tonga eruption and the weather: too soon to call it

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NIWA and WeatherWatch weigh in on whether the Tonga eruption is affecting the weather.
The Hunga Tonga–Hunga Ha’apai eruption in 2022 propelled a record-breaking amount of water vapour into the Earth’s stratosphere.
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There’s been a lot of chatter over whether the unprecedented amount of rainfall and subsequent flooding the upper North Island experienced in January is linked to climate change or the Tonga eruption last year.

The Hunga Tonga-Hunga Ha’apai (HT-HH) volcano eruption in January 2022 propelled a record-breaking amount of water vapour into the Earth’s stratosphere – enough to fill more than 58,000 Olympic-size swimming pools, according to research from NASA

Recent weather reports warn Aucklanders and other parts of the country – specifically areas that took a beating from Cyclone Hale – to batten down the hatches as it starts to look more likely that Cyclone Gabrielle will make landfall in New Zealand this week, bringing even worse conditions than Hale.

Cyclone Gabrielle, now a “severe category three storm”, is forecast to be one of the most serious storms to hit NZ this century, bringing up to 300mm of rain and 150km/h winds that could cause widespread damage to North Island regions. Large waves and a storm surge are expected to affect northern and eastern coastlines from Sunday. The Government issued a warning that people should “be prepared” and stock at least three days’ worth of supplies, including medication, water and food.

So is the wild and wet weather a result of the Tonga eruption? Experts say it’s unlikely, but also too early to tell.

Earlier this month, NIWA weighed in on the topic, saying: “We continue to see no direct, distinguishable linkage been [between] the eruption and tropospheric weather patterns. La Niña, a negative Indian Ocean Dipole and strongly positive Southern Annular Mode are the natural climatic factors that have driven our regional patterns in the last three months.”

WeatherWatch CEO and senior forecaster Philip Duncan agrees with NIWA.

“The Tonga eruption will likely have some influence on weather patterns but it may not be easy to spot it. It logically makes sense that what comes up must come back down, but it shouldn’t keep coming down,” Duncan said.

He says it’s likely there were localised immediate impacts to Tonga’s weather the day of the eruption (and weeks after it in parts of the South Pacific and Australia). But once the water and chemicals from that eruption were dispersed globally they thin out much more and it is much harder to then draw a direct conclusion to our weather pattern over a year later.

“La Niña has been the main driver for heavy rain in our part of the world so far this summer – and a marine heatwave in the NZ area added more heat. Unlike previous La Niña events, high pressure moved south giving the South Island the dry hot summer, but the North Island, tropics and eastern Australia had the classic wet summer, which was forecast from WeatherWatch/IBM and Australia’s BoM without any data from Tonga being included. 

Duncan says the pattern NZ is experiencing now is part of a normal La Niña set up, and with an even warmer climate and sea in the NZ area we’re seeing some much bigger rainfall totals in some of our flood events. 

As for whether the Tonga eruption is a small part of the equation, he says “I don’t see why not, but I don’t see it as the main ingredient”.

He says while there is no doubt that the eruption could have an impact on the climate, it may be years before there is enough data on the event to determine how it impacted the climate. Meantime, he doesn’t see any strong evidence that it’s changed our weather pattern in any noticeable way.

“I think if that was the case, you’d have NIWA and MetService scientists really bragging about it, as it would be fascinating to be able to link an underwater volcanic eruption to flooding or severe weather a year later. We would learn so much from that. The fact that isn’t the case so far, to me, highlights how little impact the Tonga eruption may have had on our weather in the big picture,” he said. 

“Either way it is a fascinating topic and until we know more there will be plenty of plausible well thought out theories, like [a recent] piece from Steve Wyn-Harris.

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