Ex-tropical cyclone Gabrielle has left several people dead, displaced thousands and caused billions of dollars in damage, making it one of New Zealand’s worst – if not the worst – weather disasters. Its incredible evolution from a tropical depression to a destructive monster, meanwhile, is likely to be studied for years to come. Jamie Morton explains what created Gabrielle, and why it packed such a punch.
February 4 offered a somewhat cheerier forecast for flood-ravaged Auckland.
Emerging from its wettest day and month in observed history – and a second moisture-packed atmospheric river that doused the city once more – weary residents welcomed the prospect of a spell of settled weather.
With hundreds of homes condemned, thousands more properties being assessed, and landscapes transformed by landslides and slips, the clean-up job had barely begun.
The odds of something even more destructive than that freak January 27 deluge rolling over our northern horizon might have seemed unfathomable.
But, so it was that weekend, meteorologists cast their eyes to a certain “tropical disturbance” in the Coral Sea, thousands of kilometres to the north of New Zealand.
‘Tropical low 14U’
Forecasters closely tracking this ominous depression were careful to couch their language with uncertainty.
While it was likely to develop into a cyclone within days, it was too early to lock in a southward track, let alone New Zealand’s second hit of the season.
Nevertheless, early model runs showed the threat to our country was apparent, even as far as eight to 10 days out.
Right from its origin as a westward-rolling tropical low pressure system titled 14U, Gabrielle packed the potential to cause serious trouble.
Its formation 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.
Gabrielle, as it was soon named by Australia’s Bureau of Meteorology, was born over Coral Sea surface waters measuring close to 30C – nearly 5C hotter than the oft-cited threshold for tropical cyclone development.
Deeper down, that region of the Coral Sea was packing enough heat in its upper layer that satellites measured it at tens of kilojoules (per square centimetre) more than was needed to spawn a new system.
This area fell within the so-called Western Pacific Warm Pool – a vast swathe of ocean where, even 150m below the surface, sea temperatures had been measuring up to 4C hotter than normal.
Under an incredible three years of La Nina, and against a backdrop of global heating, this sprawling blob of water had expanded and heated to the point it’d become a potent source of moisture for big rain-makers.
“It’s worth considering that, with three years of La Nina, we’ve just seen things come to a head for extreme weather systems forming up in the tropics and then coming here,” Niwa meteorologist Ben Noll told the Herald.
In Gabrielle’s case, it took mere days to power up – it formed on February 8 and reached category 3 strength with 185km/h wind gusts on February 10 – into one of the Southwest Pacific’s biggest cyclones in recent years.
“So, it didn’t linger too long over those warmer seas before it moved out of the tropics – but it was still able to intensify considerably in that short window,” he said.
“At that time, we were also seeing low levels of wind shear, and also what we call outflow – which basically provides an outward exhaust for the system – and this allowed it to intensify further.”
‘Our perfect storm’
As Gabrielle began pushing southward into the subtropics, two atmospheric factors helped form a corridor straight toward New Zealand.
One was a building area of high pressure to the north-east, in the direction of Fiji; the other was a separate upper low incoming from eastern Australia.
Later, around Saturday, and while packing winds gusting higher than 100km/h, and sea level air pressure bottoming out at 958hPa, Gabrielle transformed into something else.
That was when it underwent “extra-tropical transition” in the colder waters of the Tasman Sea – and the strong winds in the upper atmosphere took over as its driving force.
“It basically got captured by the subtropical jet stream,” said scientist and Climate Prescience director Dr Nathanael Melia.
“And when it went on to impact New Zealand, in some ways, it’d become a stronger system than it was when it was a category 3 tropical cyclone; I really can’t imagine a system that would transition as powerfully as it did.
“It was very strong, and very violent: our ‘perfect storm’.”
As it approached, Melia was alarmed by the degree of agreement between weather forecast models, all of which pointed to a deep, inbound tropical cyclone by February 7.
On February 8, he couldn’t have picked a worse forecast for our relatively small tropical cyclone zone.
”Weather forecasters and models don’t get the credit they deserve, the warnings resulting from these will have saved lives.”
Gabrielle first hit Norfolk Island with raging gusts of up to 155km/h, knocking out power and bringing down trees, before veering deeper into the Tasman.
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.
Overnight on Sunday, the Far North was blasted with gale-force winds and rain that brought down slips, felled trees and left tens of thousands of homes without power.
Whangārei became New Zealand’s first main centre to experience the immense amount of moisture packed within the system, with 348mm falling in just 24 hours – its second wettest February day on record.
By Monday, Gabrielle – appearing in satellite images as a gigantic, swirling mass of white – had enveloped the entire North Island with its high cloud and rain.
As the day wore on, Auckland was pummeled with storm surge, heavy gusts and rain that reached torrential levels that night, dousing Waitākere with 160mm in 12 hours.
Coromandel, already battered from the last two storms, was slammed with gale-force winds and heavy rainfall that cut the peninsula off, knocked out power to a third of its residents and drove widespread slips and flooding.
Late that night – as a slip came down on a Muriwai house, killing volunteer firefighter Dave van Zwanenberg – Gabrielle’s pressure plummeted again, crashing below 970hPa at Whitianga.
A violent merger
It was around this time that the system went through another violent transition that energised it once more.
That was when it latched on to a piece of vorticity – or spin in the middle part of the atmosphere – and wrapped it into its furious circulation.
As late as Thursday and Friday, that slab of energy was located far away from the system, deep in the subantarctic Indian Ocean.
“That really goes to show why there’s such a high level of uncertainty with these cyclones when they essentially become mid-latitude systems, because there’s a lot of elements about that can increase or decrease their power,” Noll said.
“And this little piece of energy that was once thousands of kilometres away, buried deep in the Indian Ocean, actually contributed to one of the most destructive cyclones in New Zealand’s history.”
Melia likened Gabrielle’s ingestion of that vorticity to a viper slowing to pounce on its prey, then striking.
“That vorticity area was nowhere near as strong as the cyclone, but it did certainly help to recharge it.”
A “robust” subtropical jet stream, draped across the northern Tasman Sea, added another bout of power.
“This is something we’ve seen happen time and time again over the last few months, when the jet stream has helped to deepen low pressure systems as they’ve approached the North Island,” Noll said.
These processes combined had two devastating consequences for those north-eastern areas most exposed to Gabrielle.
Not only had that piece of energy reinvigorated Gabrielle, but squeezed and tugged it westward toward the North Island - explaining the odd pinball-like bounce in maps showing its track eastward.
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.
“Until that merger of these elements was complete, it couldn’t really move on – that’s what held up the show,” said Noll, who’d watched the same disastrous pattern play out in Hurricane Fiona’s punch into Nova Scotia last year.
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.
In the Central North Island, forests were stripped.
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 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.
In a flooded Esk Valley, stranded residents had to be plucked from their roofs by helicopter; in Puketapu, one family clung to a log amid floodwaters for eight hours as they waited to be rescued.
Already, the damage wrought by Gabrielle – this evening rolling toward the Chathams - was clearly likely to exceed that of Bola’s enormous bill.
As at Wednesday night, at least 10,500 people had been displaced; 144,000 properties remained without power; and the death toll stood at four, with potential to rise.
In a purely meteorological sense, the weather system was extraordinary in its own right, breaking a slew of rainfall records, while registering one of the lowest pressure readings ever documented in the North Island.
Fluke or red flag?
For climate scientists, an obvious lingering question was how climate change – something likely to have contributed 10 to 20 per cent of the rainfall that flooded Auckland on January 27 – might have beefed up Gabrielle.
Contrary to what many of might assume, over time, a warming planet meant few tropical cyclones like Gabrielle.
On the flip-side, however, the number of events that reached category 4 or 5 tropical cyclones were expected to increase - in part because the speed with which they intensified was also increasing.
“It’s likely that the low-pressure centre of the system will be slightly more extreme than what might have been in a world without climate change, with the associated winds therefore likely also slightly stronger,” Waikato University climate scientist Dr Luke Harrington explained.
“However, there’s uncertainty over the magnitude of this influence at current levels of warming.”
Harrington said the clearest signal of climate change for ex-tropical cyclones related to the amount of rain which fell from the system.
In the aftermath of 2017′s Debbie, which swamped the Bay of Plenty town of Edgecumbe, researchers estimated that around 5 to 10 per cent more rain fell as a result of climate change.
“This roughly corresponded to a doubling in the frequency of similarly wet events when compared to model simulations of a world without climate change.”
Another key part of the puzzle – and one that likely played a notable role with Gabrielle – was the influence of warmer waters.
Just as they’d been in the southwest Pacific, surface temperatures in the Tasman Sea, and also waters around New Zealand’s coasts, had been measuring unusually high amid an ongoing marine heatwave.
“While the seas around Aotearoa are still not hot enough to sustain a tropical cyclone, tropical cyclones like Gabrielle can maintain themselves much closer to us than before, and are not disrupted so much by cooler seas that are no longer there,” Niwa climate scientist Dr Dáithí Stone said.
“La Nina events also change the winds, bringing more hot and wet air from the tropics our way.”
Finally, Stone said, the warmer air of a warming world could hold all of that moisture until it met our mountains.
“In that sense, Gabrielle is very much part of the story this summer... of a warm nearby ocean, using a warm atmosphere to pump rain onto Aotearoa.”
Still, the cyclone had raised some other troubling questions.
Even with warmer waters close to our shores, Stone said it was unusual for tropical cyclones to make it this far south.
“Climate researchers have not been able to figure out yet if La Nina events will change under human-induced warming,” he said.
“Globally, it seems that climate change reduces the total number of tropical cyclones slightly, even while making them stronger.
“So, is Gabrielle’s track toward us a fluke then, against the grain of climate change, or does it portend the future?”