A tropical cyclone headed our way is powering up over abnormally hot oceans – with important implications for how much rainfall it unleashes here.

Niwa meteorologist Ben Noll said Gabrielle was building force against a background of a regional marine heatwave – one of several reasons it was likely to reach category 3 strength.

MetService is expecting the cyclone to bring severe weather to the upper North Island from Sunday to Wednesday, just a fortnight after Auckland experienced its wettest day in recorded history.

Cyclones are natural features of the tropical atmosphere – about nine usually form each November-to-April season in our basin - the Southwest Pacific – and warm ocean water plays a critical part in transforming low-pressure systems into the more extreme beasts they are.

Gabrielle happened to be intensifying within an unusually hot Coral Sea, which itself sat near the “Western Pacific Warm Pool”.

That was a vast, balmy swath of ocean that’s expanded under three years of La Nina and background climate heating – and where much of the incredible amounts of moisture in Auckland’s anniversary weekend storm was drawn from.

“Waters in the Coral Sea – and not just at the surface, but also at depth, which is the important bit for tropical cyclone formation – have also been exhibiting marine heatwave conditions at times as well,” Noll said.

“Over the wider Western Pacific Warm Pool, we’ve been seeing sea temperature anomalies of 2C to 4C at 150m depth, which is tremendous, and spread over a large geographical area.”

Right now, satellite-observed sea surface temperatures where Gabrielle was tracking were hovering close to 30C – far warmer than the benchmark threshold of 26.5C known to favour cyclone development.

“We’ve been seeing sea surface temperatures anomalies between 0.5C and 2C in that area.”

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More importantly, the wider, upper ocean – or the layer 50m below the surface – was also abnormally warm.

Noll offered another oft-cited threshold for cyclone formation: 50 kilojoules of equivalent upper ocean heat content per square centimetre (kJ/cm2).

Gabrielle was intensifying over oceans sitting between 60 to 80 kj/cm2.

“Once you start getting closer to 70, 90 or 100kj/cm2, you’re talking about values that are certainly able to energise rapidly intensifying tropical systems.”

As Gabrielle built to category 3 strength within the next 24 hours – packing destructive gusts ranging from 165-224km/h - meteorologists would certainly be looking closely at sea temperature anomalies in its track zone, he said.

“Among other things, this is going to be a big boon for its intensification into a severe system.”

Later, as it travelled out of the tropics and south towards New Zealand, the system was likely to cross ocean surface waters running anywhere from 0.5C to 2C above average.

“Then, if it tracks west or east of the top of the North Island – such as into the eastern Tasman Sea – it’s headed straight into a marine heatwave.”

Current marine heatwave conditions around New Zealand happened to be some of the worst observed anywhere on the planet – and likely contributed to last month’s big deluges in the upper North Island, while causing myriad impacts elsewhere.

“So, if this cyclone travels here, it’ll have been influenced by both the ocean warming happening further afield and what’s occurring locally around our coasts,” he said.

“And all of this, of course, is happening against a long-term warming trend.”

Sea surface temperatures and ocean heat in parts of the Southwest Pacific, particularly, have been increasing at more than three times the global average rate.

At the same time, marine heatwaves around New Zealand – driven by a stew of ocean and atmospheric factors - have been increasing in frequency.

Niwa scientists have warned they could become a year-round phenomenon by century’s end, when local average sea surface temperatures may be measuring 3C warmer.

Already, marine heatwave conditions likely contributed to the havoc that cyclones Fehi and Gita wrought here in 2018, Noll said.

Just what difference extra ocean warmth would make to Gabrielle’s rain-making potential was difficult to quantify at this stage - particularly as it sat within a mix of determinants like wind shear and how the system interacted with surrounding pressure systems.

“But in a basic sense, having more ocean warmth means more evaporation into the atmosphere and more moisture for the system to take in,” he said.

“Based on those principles, what we have right now certainly isn’t a good combination.”