Look at a map of Gabrielle’s track past New Zealand and it’s the first thing that sticks out: a sharp turn south toward Aotea-Great Barrier Island, followed by a pinball-like pivot to the east.
This shift - which began this morning and is likely to contribute to intense levels of rainfall for exposed north-east spots over the next 24 hours - might appear like a simple course change in Gabrielle’s march across the seas, but the factors behind it are much more complex.
After all, we’re talking about an incredibly powerful system, being constantly energised and reshaped by processes high in our atmosphere, as it moves through the dynamic weather environment that is Aotearoa’s home waters.
And meteorologists are at pains to point out that, while we refer to Gabrielle as a cyclone, it ceased to be one in the sense that most of us understand them as it underwent “extra-tropical transition” on its way here.
Having met colder waters and stronger upper-level winds on its southward path across the Tasman Sea, Gabrielle had morphed into a different weather system entirely, and not one necessarily any weaker.
Gabrielle's track, as at 7am Monday morning. Source / MetService
“It’s not determined by tropical dynamics anymore - it’s not really-warm waters giving it all of its convection and lowering the pressure – it’s now driven by the upper atmosphere,” MetService forecaster Lewis Ferris explained.
What was going on up at that level, tens of kilometres above the Earth’s surface, indeed explained that abrupt dog-leg turn in the system’s eastward course.
Niwa meteorologist Ben Noll singled out “a piece of vorticity – basically spin – in the middle part of the atmosphere that is coming in from the central Tasman Sea and getting wrapped around the cyclone’s circulation.
“This is causing two things to happen: one, the merging of these two pieces of energy is causing the system to intensify; two, it squeezes it, a little like a fist, and also tugs it back slightly to the west.”
It was also during this process, in tandem with a vigorous sub-tropical jetstream, that Gabrielle’s central pressure was expected to plummet to perhaps record values early tomorrow morning – perhaps just to the top of the Coromandel Peninsula.
“The guidance that we have at the moment shows it could come close to 960 hectoPascals, which could be potentially record-breaking for local climate stations.”
For extreme weather potential, that wasn’t good news.
“Put simply, the lower the air pressure the more powerful and severe the storm is,” WeatherWatch head forecaster Philip Duncan said.
“It makes the storm more unstable and will see wind and rain spread further out. The fact this intensification is going to occur as Gabrielle approaches the Auckland and Coromandel Peninsula regions makes it more problematic and complicated.”
Around Coromandel over the next 24 hours, MetService was predicting a further 250mm to 320mm of rain about the ranges on top of what had already fallen, along with 100mm to 150mm about the coast, and severe east-to-southeast gales gusting at more than 120km/h.
Noll said it didn’t help that an upper-level ridge of high pressure, sprawled out to the south-east of New Zealand, was causing the system to track through our neighbourhood more slowly.
“But certainly, the mechanism that’s causing the deepening, and that little pivot, is the interplay of the Tasman Sea energy coming into contact with the cyclone,” he said.
“Until this process finishes, it basically can’t keep away.”
Noll had seen this same pinball-like phenomenon play out before - but on the other side of the planet, in the North Atlantic Ocean in 2018.
That was when Hurricane Fiona interacted with a mid to-upper-level trough, accelerated to the north-east, transformed into an extratropical cyclone with hurricane-force winds, and slammed into the coast of Nova Scotia.
“And it’s the same kind of thing we see here: a little piece of energy that comes in, tightens the circulation, and pulls it off in another direction for 12 hours or so.”
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