What does it mean that Cyclone Gabrielle is no longer classed as tropical?

Sunday, 12 February 2023

MetService says the cyclone bearing down on Auckland is no longer tropical, meaning the intense rain and wind it brings will be spread over a larger area.

Meteorologist Mmathapelo Makgabutlane said Cyclone Gabrielle will change shape as it moves into the cooler waters and windier climes of New Zealand.

This will change the shape of the cyclone, making it less concentrated around the eye of the storm and more spread out.

“It is not a tropical cyclone any more, but it is still an intense system,’’ she said.

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“The most intense weather will spread out over a larger area, rather than being concentrated around the eye.”

MetService has issued “red” heavy wind and rain warnings for Auckland and Northland with 200mm of rain expected and wind speeds of up to 130kph.

So many people sought out sandbags at stations across the city that supplies were depleted.

Cyclone Gabrielle is now tracking closer to New Zealand than previous projections suggested, leaving people racing to prepare for large waves, storm surges and coastal inundation that could be triggered.

Rain around Northland, Auckland, the Coromandel Peninsula and northern Gisborne is expected to cause dangerous river conditions and significant flooding.

Despite its changing shape, Makgabutlane said the cyclone was still a very intense weather system that would bring severe conditions for New Zealand.

A cyclone draws much of its energy from the warm oceans of tropical regions and remains stable thanks to the weaker tropical winds, she said.

But, as it moves into the cooler and windier New Zealand region, it will change form.

“The cooler waters affect the physical structure of the cyclone,” she said.

“The upper structure of the cyclone is influenced by the upper atmosphere. There are weaker winds in the tropics and stronger winds here.”

She said the cyclone spreading out was like an ice skater performing a pirouette.

“They spin faster when their arms are tucked in,’’ she said.

“When they extend their arms they slow down, but their energy is still the same – just spread out over a larger area.”