Why New Zealand is getting pummelled by a 'perfect storm' of ex-cyclones

Thursday, 8 March 2018

Tropical Cyclone Hola was category 1 on Wednesday but has strengthened to a Category 4.

Tropical cyclones rely on the 'perfect storm of weather features' to steer one way or another, Niwa says.

The third cyclone to venture towards New Zealand since February, Tropical Cyclone Hola, is carving out a southward path.

Niwa meteorologist Seth Carrier said the 'increased frequency' of ex-tropical cyclones affecting New Zealand this year was unusual.

Tropical Cyclone Hola sits to the west of Vanuatu at 7am Thursday in the satellite image from Vanuatu's Meteorology & Geo-Hazards Department

'It's been a perfect storm of weather features that have been really conducive for dragging the weather masses from the subtropics and the tropics towards New Zealand,' he said.

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The forecast track of Tropical Cyclone Hola issued by RSMC Nadi Tropical Cyclone Warning Centre around 8.45am Thursday. Times are UTC which is 13 hours behind NZ daylight time.

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'But the same features that have brought us this warm, humid summer are also making it more likely for tropical cyclones to come near New Zealand. They're very related. And we just finished off the warmest summer recorded in New Zealand, so yes, this is unusual.'

Cyclone Fehi hit New Zealand at the beginning of February, Gita struck on February 20 and 21, and Hola is now on its way.

Fehi and Gita were mostly a product of the current weather set-up, he said.

'Tropical cyclones really can't steer themselves. They are reliant on other weather features around them to steer them wherever they may go.'

In an average November to April tropical cyclone season, one or two ex-tropical cyclones typically passed within 550 kilometres of Auckland, he said.

MetService meteorologist Brian Mercer agreed New Zealand normally had two interactions with ex-tropical cyclones a season, but he said having two in close succession was 'not unusual'.

'It's very hard to say 'normal' when discussing tropical cyclones because they're quite difficult to forecast for.'

Mercer said while Fehi, Gita, and Hola might have appeared to have occurred without much of a break in between, no long-term conclusions could be drawn from their timing.

He said 9-12 tropical cyclones usually developed in the Southwest Pacific Basin a season, but not all of them reached or affected New Zealand.