It's been raining a lot

It was the explosion heard almost around the world, yet now all but forgotten. The Hunga Tonga/Hunga Ha'apai volcanic eruption and subsequent tsunami devastated much of the island kingdom of Tonga, displacing hundreds of people from the outer islands, and propelling a record-breaking amount of water vapour into the Earth's stratosphere -- enough to fill more than 58,000 Olympic-size swimming pools.

As an underwater volcano, it thrust an unprecedented amount of water into the sky, increasing total stratospheric water content by about 10% -- enough to cause a rapid change in atmospheric chemistry. That additional water vapour from the eruption is still up there, still decaying steadily, not expected to return to its pre-eruption range until around 2030.

Talking to a friend recently about the rain, the floods, the wet weather events in the last few years, you have to wonder whether that massive uptick in stratospheric water might still be playing a part?

Hunga Tonga erupted in January 2022. MetService recorded 53 severe weather events in 2022, and issued 182 severe weather warnings. In January 2023 Auckland had its worst flood in memory, a record 539mm of raining falling in January. Cyclone Gabrielle arrived in Feb 2023. Extreme rain events occurred throughout 2024, from extreme rainfall events occurred throughout the year, from Dunedin to Westland. The North Island got a Red Alert and the South Island a state of emergency in May 2025 for record rainfall and strong winds. And this week Wellingtonians were stuck with severe flooding and landslides after 77mm of rain fell in less than one hour, causing the worst flooding event since Wellington's disastrous 1976 storm.

Naturally, NIWA says "nothing to see here." (They don't seem to have even mentioned the eruption since 2024.) The influential 2025 Hunga Volcanic Eruption Atmospheric Impacts Report frustratingly focusses more on temperature than rainfall. But just as the atmospheric water increase still lingers, so too a few studies suggest some lingering interest in the question -- examining especially how much the eruption  may have nudged rainfall patterns in the Southern Hemisphere, including New Zealand.

A January 2026 study in Atmospheric Chemistry and Physics found some statistically significant extratropical regional climate responses to the eruption, driven by circulation changes. A recent finding from the Austrian Polar Research Institute), published just weeks ago, found that the eruption continues to influence stratospheric patterns, increasing chances of extreme precipitation events at mid-latitudes-- which includes New Zealand.

Yet while they're happy to go on the record about their modelled "causes" of recent rainfall events, the largest volcanic eruption this century remains the climate event no local climate scientists want to talk about. 

I can't imagine why.