Source: Radio New Zealand
The track of Tropical Cyclone Vaianu as it moves south towards New Zealand. Screenshot / Zoom Earth
Tropical Cyclone Vaianu is making its way south from Fiji and appears likely to impact New Zealand sometime on Sunday.
It’s currently tracking as a category 2 storm with winds around 100km/h.
What does it take to make a cyclone, and what do the categories mean?
Tropical cyclone structure Supplied / MetService
What is a tropical cyclone?
Tropical cyclones usually form over large bodies of warm water – at least 26.5C, according to Australia’s Bureau of Meteorology. That heat fuels the developing storm.
Earth Sciences New Zealand (formerly NIWA) describes them as characterised by a low pressure centre, strong winds, and thunderstorms that produce heavy rain.
Those low pressure systems cause warm, moist air to rise upwards.
Think of them as a swirling oceanic engine, taking energy from the warm waters and generating enormous amount of energy.
They tend to move polewards – and can last about a week.
The tropical cyclone season in the Southern Hemisphere typically lasts from November to the end of April. An average of 10 tropical cyclones form in the South Pacific each season, with usually one of them affecting New Zealand in some way.
However, director of the Na Draki Weather Service in Fiji, Neville Koop, told the ABC this week that there is no climate normal anymore, and the cyclone season in the South Pacific is quite likely to continue well into May.
And of course, foremost in everyone’s mind with the current cyclone warnings is Cyclone Gabrielle in February 2023, which killed 11 people in New Zealand and was the costliest cyclone on record in the Southern Hemisphere, causing billions of dollars in damage.
For more, check out this video from Australia’s BOM:
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Are they still tropical cyclones when they hit New Zealand?
Typically, by the time a tropical cyclone reaches New Zealand waters, it actually becomes an ex-tropical cyclone due to the cooler waters, MetService says on its website.
“Re-classification as an ex-tropical cyclone does not necessarily mean the system has weakened, but rather that it has transformed into a completely different type of weather system,” MetService writes.
“Many of New Zealand’s most severe and impactful storms have been ex-tropical cyclones.”
“Although these systems will no longer be classified as tropical cyclones, we will often still refer to them by their given name e.g. ‘Cyclone Gita’ for communication purposes,” the forecaster notes.
Cyclone categories Supplied / MetService
What do the categories mean?
Under the scale we use, they range from category 1 to 5.
Category one tropical cyclones have sustained wind speeds from about 63 to 88 km/h, while category five having sustained winds surpassing 200 km/h.
Maximum gusts can be even stronger than that – theoretically 3-second gusts of over 280 km/h for category 5 cyclones.
Still, sometimes weaker tropical cyclones inflict more damage if they are slow moving or “stall out” over an island.
What’s the difference between tropical cyclones, hurricanes and typhoons?
They’re actually all the same, but have different names depending on where they form.
In the Atlantic, they are called hurricanes; in the Western Pacific, they are called typhoons; and in the Southwest Pacific, toward Australia and Indian Ocean, they’re called tropical cyclones.
How are cyclones named, anyway?
The naming list for cyclones in our region comes from Fiji’s Meteorological Service. They designated Vaianu as this storm’s name on Monday.
The Fiji Meteorological Service uses a list of names compiled by weather bureaus across the region, including Samoa, Cook Islands and Vanuatu, the ABC has reported.
“If there was a tropical cyclone to happen in the Cook Islands or Samoa, they shall rely on Fiji to provide the name,” Misaeli Funaki, Director of the Fiji Meteorological Service, told the ABC.
If a storm causes a great deal of damage and/or death, the name is retired – as is the case with Cyclone Gabrielle or Hurricane Katrina in the US.
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand
