It’s been a year since Gabrielle brought New Zealand’s worst storm in decades. What have we learned about it since? Science reporter Jamie Morton explains.
It was, as Niwa described it, among New Zealand’s worst weather disasters in modern history.
Gabrielle was the deadliest cyclone system – and weather event in general – to strike our country since the Wahine sank amid Cyclone Giselle half a century before.
It claimed 11 lives, injured 2000 people and displaced more than 10,500, and cost up to $14.5 billion in damage – leaving insurers with a claims bill comparable only to the previous fortnight’s deluge in Auckland.
The warm-and-wet climate background that Gabrielle formed against was exceptional, emerging in the third La Nina in as many years.
Its birth came at the riskiest part of a riskier-than-average cyclone season and in waters that were running abnormally warm, even for the tropics.
That was critical, given tropical cyclones – essentially giant, swirling, atmospheric heat engines – operate by drawing moisture from warm oceans as fuel, generating enormous amounts of energy as clouds form.
Then, there were the freakish characteristics of the system itself.
At 4am on February 14, the day it veered closest to our coast, Niwa’s climate station at Whitianga registered a minimum mean sea level pressure of 968 hectopascals – the second-lowest air pressure reading ever recorded in the North Island.
After Gabrielle moved out of the tropics and into the colder waters of the Tasman Sea, strong winds in the upper atmosphere took over as its driving force.
If effectively came out of this change – called an extra-tropical transitional – as a whole different beast, but one no less powerful.
Its arrival in New Zealand was foreshadowed by sea levels lifted up 30cm to 40cm by incredibly low pressure, southeasterly-driven waves towering more than 10m off the coast of Whangārei, and gusts as strong as 140km/h at Cape Reinga.
Later came another metamorphosis that had devastating consequences.
As the system veered by the North Island’s northeastern coastlines, it latched on to a piece of vorticity – or spin in the middle part of the atmosphere – and wrapped it into its furious circulation.
Gabrielle drew yet more energy from a subtropical jet stream draped across the northern Tasman Sea – and both of these boosts helped to squeeze it and tug it westward toward the North Island.
The system also slowed as it churned above the North Island, partly because the deepening pressure within it, but also due to another high-pressure system lying to the east of the country.
While it didn’t stall completely, it did move sluggishly enough to unload torrents of rain on the East Coast, while winds within the wider system fanned out hundreds of kilometres to lash areas like Taranaki with heavy gusts higher than 130km/h.
From the East Cape down to Hastings, power and communications networks went down as rivers flooded and entire communities were put underwater.
In the ranges north of Gisborne, nearly 560mm of rain had fallen in the space of a day and a half – intensity nearing the catastrophic totals of 1988′s Cyclone Bola.
Around 200mm fell in a high country station north of Napier in one night, while the city itself, soon cut off from all directions, recorded its second wettest February day ever.
Niwa meteorologist Ben Noll said those regions hit hardest had already been soaked by one of the North Island’s soggiest summers in history.
“If you’re going to put another couple of hundred millimetres of rain on top of that, along with winds in excess of 100km/h, the results are not going to be pretty.”
The scenes from Gabrielle’s immediate aftermath were unforgettable: seasonal workers trapped inside flooded orchards; beaches strewn with slash and downed trees; the picture-perfect Esk Valley put underwater, then left a basin of stinking silt.
A year on, scientists have used surveys and satellite imagery measure its impact on our landscapes and coastlines.
The cyclone carved away as much as 20m from the worst-hit beaches in the upper North Island, leaving some in their most eroded state in 80 years.
Many beaches in these regions have lost more than 10 metres to the storm, including well-loved tourist hotspots such as Whangamatā and Matarangi in Coromandel, Ōmaha beach in Auckland and Ruakākā beach in Northland.
“What set Gabrielle apart from other storms in recent decades is that it impacted such a large swathe of the North Island’s coast,” University of Auckland researcher Dr Murray Ford said.
“We’ve seen considerable erosion ranging from Northland all the way down to the Hawke’s Bay, as a result of the devastating path the storm took and the large swells it generated along this path.”
Beneath the water, in areas off the East Coast, scientists were still observing pollution from flushed-out sediment and woody debris.
“There is some evidence, in the very near-shore, of damage to the ecosystem,” Niwa marine geologist Dr Joshu Mountjoy said.
“What’s very difficult is to say just how much is caused by Cyclone Gabrielle, versus other sedimentation events.”
Much clearer, however, was the effect Gabrielle had on the East Coast’s hill country, which remained scarred by countless slips and landslides – including 2000 just in the Esk Valley.
“We’ve mapped over 140,000 landsides, the smallest roughly the size of a car, from 20 per cent of the area affected by Cyclone Gabrielle,” University of Canterbury researcher Dr Tom Robinson said.
“We can’t possibly map them all, but we believe there is about 750,000 to 850,000 in total.”
Untangling climate change’s role in what was one of New Zealand’s largest and most complex weather events wasn’t simple – but scientists have so far provided some evidence it worsened Gabrielle’s impacts.
One early analysis by the World Weather Attribution group found big deluges have become up to four times more common - and drop up to 30 per cent more rain - in the most-affected East Coast regions.
At the same time, however, extreme rainfall totals like those of Gabrielle’s remained rare at any given location in the studied regions, with a roughly 3 per cent chance – or less – of this occurring each year.
Scientists have pointed to other ways that climate change would have contributed, such as injecting more moisture into the system, or helping warm the region where Gabrielle formed.
As the planet continued to heat, it’s expected events like Gabrielle will be even more destructive in the future.
Niwa scientists recently estimated that by the last two decades of this century, the storms’ damage potential could worsen by about 10 per cent in a middle-of-the-road emissions scenario – or by as much as 26 per cent in a worst-case, though highly unlikely, scenario.
In any case, New Zealand’s extreme summer of 2022-23 made Kiwis care more about the prospect of a warmer, wilder future – as shown by surveys and Google search trends.
Researchers who recently asked nearly 300 primary sector leaders to rank the biggest issues affecting our land use in the coming decade found extreme weather events and climate change topped the list.
When it came to actually adapting to increasing climate impacts, however, some experts worried New Zealand was moving too slowly.
Adjunct Professor Judy Lawrence, of Victoria University’s Climate Change Research Institute, noted an adaptation act proposed by the last Government was still to be progressed.
“We have the tools and the knowledge to anticipate the risks, but fit for purpose institutional arrangements are missing in action.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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