Gabrielle appears to have been a stronger cyclone system than even 1988′s destructive Bola or 1968′s Giselle, with one early analysis showing it carried historically low pressure levels.
Having claimed at least 11 lives, displaced thousands of people and caused billions of dollars in damage, Gabrielle is already considered New Zealand’s worst weather event this century.
Meanwhile, one meteorologist has run a comparison with other historic ex-tropical cyclones to gain a clearer picture of the power it packed.
In a reanalysis of head-to-head pressure values, Gabrielle was found to be more intense than Bola – a system that similarly devastated the North Island’s East Coast – and also Giselle, largely remembered for creating ferocious storm conditions that sank the inter-island ferry Wāhine in Wellington Harbour.
At its minimum, Gabrielle’s low pressure plummeted to an estimated 963 hectopascals (hPa) and reached levels of 966.8hPa near Great Barrier Island.
That compared with Bola and Giselle’s respective lowest values of 982hPa and 967hPa.
“The lower the pressure, the faster winds are going to be blowing in toward the centre of the storm,” explained Niwa meteorologist Ben Noll, who calculated the values using the European Centre for Medium-Range Weather Forecasts’ (ECMWF) sophisticated ERA5 dataset.
The data is in: Cyclone Gabrielle, one of New Zealand's worst weather disasters in history, has been re-analyzed...
— Ben Noll (@BenNollWeather) February 20, 2023
Gabrielle (left) was stronger than Bola (1988) & Gisele (1968), reaching a minimum pressure of 966.8 hPa near Great Barrier Island as per ERA5.@Jamienzherald pic.twitter.com/SZP1EZUS4m
“As this happens, the winds come together and ascend upward through the atmosphere, which then lowers the mass at the centre of the storm.
“This creates a hole, if you will, in the atmosphere, and in order to fill that hole, the winds need to spin faster.
“Of course, one of the facets of Gabrielle was wind damage – and this was directly linkable to the storm’s strength, depth and lower air pressure.”
Those widespread, gale-force winds – which dramatically stripped entire forests in the Central North Island – came with extreme gusts of more than 130km/h in some places.
One powerful gust of 128km/h happened to be the fourth-strongest ever observed in New Plymouth Airport’s 50 years of records.
Severe winds were also disastrous features of Giselle - one gust at Wellington’s Hawkins Hill reached 235km/h - and Bola, in which the beachside Taranaki community of Oakura was blasted with peak gusts of up to 180km/h.
All three storms similarly brought torrential amounts of rainfall.
Gabrielle dropped nearly 560mm on the ranges above Gisborne in the space of a day and a half: values comparably intense as the 900mm that Bola delivered to the East Coast over 72 hours in March 1988.
In Gabrielle’s case, however, the system likely had more fuel for heavy rain, given total atmospheric moisture in the most recent decade happened to be 5.3 per cent higher than in the 1960s – and 3.2 per cent higher than in the 1980s.
“The atmosphere is moister now than it was when Giselle and Bola existed; if you were to re-run those two systems in the present climate, you’d expect them to produce more rain.”
The wild influence of climate change, certainly, was inter-weaved with many of Gabrielle’s driving factors.
Along with the compounded effects of three years of La Niña, it’d contributed to abnormal warmth in the tropical waters where Gabrielle formed up and quickly reached category 3 strength, before veering southward into the Tasman.
🌀A tracking comparison between Cyclone Bola and Cyclone Gabrielle.
— WeatherWatch.co.nz (@WeatherWatchNZ) February 14, 2023
As you can see, it doesn't matter if the centre makes 'landfall' or not when you're dealing with ex-tropical cyclones. The lopsided nature of them means wind & rain & severe weather extends far from the centre. pic.twitter.com/GLQ2FbNIbT
Another critical component of this month’s disaster also bore similarities with Giselle.
Over Wellington, around April 10, 1968, Giselle merged with another storm that’d driven up the South Island’s West Coast from Antarctica, intensifying it further.
Gabrielle, meanwhile, ingested a piece of vorticity or “spin” in the upper atmosphere that’d earlier been buried thousands of kilometres away, above the subantarctic Indian Ocean.
That re-energised the cyclone as it moved over the northeast of the North Island, while also slowing it and yanking it closer to land - with tragic results for the East Coast.
“It’s important to note that, by the time it reached New Zealand, Gabrielle wasn’t the very intense tropical cyclone it once was,” Noll said.
“It needed to be assisted by this other vorticity feature to deepen and become more intense once again – and of course, it did that right on New Zealand’s doorstep, and at the worst possible time.”
Noll expected Gabrielle would be the focus of scientific studies – not least to understand climate change’s precise contribution – for years to come.
“At this point, we just know the very basics: we’ve only really peeled back the first two or three layers of the onion.”
The ERA5-based reanalysis – able to draw on data stretching back to 1950 – at least offered a useful apples-to-apples comparison of past major events just days after Gabrielle had torn through.
Noll acknowledged that some past analyses had placed Giselle’s pressure readings as low as around 965hPa.
“But the key take-away message is that, when you run these storms on the same dataset, where the methodology is consistent, Gabrielle comes out more intense than the other two.”