New Zealand has already received more than its usual quota of tropical cyclones from the Pacific this season. But a Hola hit could make the rate 400 per cent of normal, with potentially even more to come. Science reporter Jamie Morton explains.
What are tropical cyclones?
They're essentially low-pressure systems that form and build over warm waters in the tropics - but with extreme characteristics.
In less than a decade, and in the South Pacific alone, they've killed more than 100 people and cost nearly $2 billion in damage.
Gale-force winds - or those higher than 63km/h - are found at low levels near their swirling centres but can fan out for hundreds of kilometres.
Meteorologists consider a "severe tropical cyclone" one that blasts as hard as 118km/h.
Winds close to the centre of the Category 4 Hola were estimated to be about 165km/h as it raged toward Vanuatu.
How many usually hit New Zealand every year?
Each year, over the November-to-April season, about 10 tropical cyclones form in the southwest Pacific basin.
Only a few of those ever reach Category 4 strength, where mean wind speeds are more than 159 km/h, or higher.
Vanuatu and New Caledonia typically experience the greatest activity, with an average of two or three named cyclones passing close to land each year.
This season, it had been forecast New Caledonia, Fiji, Vanuatu and Tonga could see two or more, while three or four severe cyclones of Category 3 or higher were expected anywhere across the region.
At least one comes within 550km of New Zealand each year, usually around February and March.
To get down here, they've had to make their way over much colder waters, while hitting strong upper level winds as they moved out of the tropics.
By the time they arrived, they were almost always re-classified as "ex-tropical" cyclones.
That didn't mean they'd weakened or been downgraded, but had morphed into a completely different type of beast.
And ex-tropical cyclones could still pack the potential for severe weather.
Under the right conditions, they could intensify and even muster lower pressures than they had before being re-classified.
In the tropics, the strongest winds and most intense rain associated with a tropical cyclone usually occurred just outside the "eye", or cyclone centre.
But after it had been transformed in what's call an "extra-tropical transition", the systems lost their symmetric cloud patterns.
The strongest winds and heaviest rain could then be found hundreds of kilometres from the cyclone's centre - usually in a large area south of the centre.
For meteorologists, that meant the position of the cyclone centre was no longer a good indicator of where the most severe weather would hit.
In 1988's catastrophic Cyclone Bola, for example, the heaviest rain and strongest winds over New Zealand occurred well away from the centre.
Why has this season been different?
While the cyclone season was tracking near-normal across the southwest Pacific, New Zealand's activity this year was heading toward 400 per cent of normal.
"I mean, we only average two thirds of a cyclone every season – at least according to the 30-year averages between 1981 and 2010 – so we are certainly getting a bit fatigued here in New Zealand," Niwa meteorologist Ben Noll said.
Particularly, it had been a standout season in New Zealand for impact – if Hola hit us on Monday, it would be the third land-fall this year.
"We could be well above normal for the season."
Noll put the high rate down to three main factors: a La Nina climate system, a marine heatwave in the Tasman Sea, and less cyclone-killing wind-shear.
"La Nina is the over-arching theme here: its warming of the water in the west Pacific has basically encouraged rising motion in the atmosphere, which in turn spreads low-pressure systems like cyclones."
Unfortunately for New Zealand, La Nina's seasonal signature response favoured a track toward New Zealand.
"So, especially in the late season during La Nina, when you look at the hot spots on heat maps, we're right there in the red."
Why?
"It's all about sea surface temperature gradients – this is where warm water sets up with respect to colder water, and you're looking for these gradients to guide your cyclone tracks.
"This year, so we've certainly had a lot of warm water and that's also helped to maintain the structure of tropical cyclones as they've tracked south toward New Zealand."
That's where the Tasman Sea's record marine heatwave also came in to play.
The event had pushed sea surface temperatures in the Tasman several degrees above normal over summer, and, incredibly, had peaked three times: in early December, late January and mid-February.
The marine heatwave was blamed for effectively "energising" the big storms that had already rolled in across the Tasman this summer.
The third factor was a lack of those forces which typically tore apart cyclones over the ocean.
"The one thing that is a detriment to tropical cyclones is when winds are blowing very quickly in the jet stream or upper level of the atmosphere," Noll said.
"Over the last couple of months, we've had a weaker than normal jet stream over New Zealand, and that has allowed a more favourable pattern in the upper atmosphere for sustaining these systems."
What happens after Hola?
The tropical cyclone season is far from over – and last April's destructive double-hit combination of Debbie and Cook should remind us these systems can still hit at the tail end.
"I've been watching really closely what might happen in the late stages or March or early April, and I don't think we are done yet, to be quite honest with you," Noll said.
"There is still some favourability in the patterns here over the next couple of weeks and I don't think there will be anything in quick succession.
"But that said, the latest guidance this morning shows us there's another storm we need to watch, which is tracking down from the vicinity of the Coral Sea potentially as we head into next week.
"And that could move into the Tasman Sea, so, in short, it's not over yet.
"After all, the tropical cyclone season runs into April and we've still got another month and a half to watch the risks here before the polar jets poke up and shut the season off."
Is this our future under climate change?
With the extra visits from tropical cyclones, a record marine heatwave and our hottest summer in 150 years, many people are understandably pointing to the hidden hand of climate change.
While it's true we'll likely see more marine heatwaves, and obviously warmer temperatures, climate change didn't actually mean we'd see more cyclones.
Climate scientists predict there will be fewer of them.
As time went on, the number of tropical cyclones - and the number of storms generally - was likely to decrease slightly.
The drop could be explained by changes in the state of the atmosphere that would result in fewer storms being required to maintain the flow of heat from the tropics to the poles.
But that was only the good news; the bad is that they'll likely become stronger under climate change and cause more damage.
We could expect future cyclones to pack a lower central pressure, stronger winds, and much more rainfall because of the extra moisture in the air.
