Unlike many other storm types, tropical cyclones can generate extreme winds, with gusts of more than 90km/h near their centre, or faster than 280km/h in the most severe cases. Photo / JMA
Unlike many other storm types, tropical cyclones can generate extreme winds, with gusts of more than 90km/h near their centre, or faster than 280km/h in the most severe cases. Photo / JMA
What are tropical cyclones?
They’re the ultimate storm system: sprawling, swirling areas of low pressure packing gale-force winds and stretching hundreds of kilometres across the tropics where they form.
We can think of them like giant atmospheric heat engines, drawing moisture from the warm ocean as fuel, and generating enormousamounts of energy as clouds form.
Rotating thunderstorms form spiral rainbands around their centre where the strongest winds and heaviest rain are found – creating a wildly-destructive “eye wall” around the eye.
This formation sucks heat 15km or higher into the atmosphere, while drier, cooler air at the top of the atmosphere becomes the exhaust gas of the heat engine.
Some of the cool air sinks into the low-pressure region at the centre – explaining why cyclones often have calmer eyes with light winds and often clear skies, tens of kilometres wide.
The rest of the cool air spirals outward, away from the cyclone centre, sinking in the regions between the rainbands.
As long as environmental conditions support this atmospheric heat engine, a tropical cyclone can maintain its structure and even intensify over several days.
While we know them as tropical cyclones in New Zealand’s wider neighbourhood – the southwest Pacific – such systems are called typhoons in South East Asia and China, and hurricanes in the North Atlantic, where they rotate in the opposite direction, or counter-clockwise.
Unlike many other storm types, tropical cyclones can generate extreme winds, with gusts of more than 90km/h near their centre, or faster than 280km/h in the most severe cases.
These winds can wreak devastation on buildings and turn airborne debris into potentially lethal missiles.
As well, torrential levels of rainfall and tide-raising storm surges can combine to cause disastrous floods.
Tropical cyclones are classified in five categories, ranging from category 1 – with 10-minute mean wind speeds (MWS) of 63-87km/h - to category 5, which can bring a 10-minute MWS of more than 200km/h.
Tropical Cyclone Gita, seen here via satellite on February 18, 2018, destroyed more than 170 homes in the Pacific and caused flooding across New Zealand. Image / JMA
Of an average 10 tropical cyclones that form in the Southwest Pacific basin each year, only a few ever reach category 4 strength.
Category 5 events in the region are rarer still, with only two dozen or so – and most recently Niran in 2021 and Yasa in 2020 – having reached that scale over the last four decades.
Vanuatu and New Caledonia typically experience the greatest activity, with an average of two or three named cyclones passing close to land each year.
How vulnerable is New Zealand to them?
An average one system tracks within 550km of New Zealand each November-to-April cyclone season, usually around February and March.
To get down here, they have to make their way over much colder waters, while hitting strong upper-level winds as they move out of the tropics.
In the process, they go through what’s called “extra-tropical transition” - and thus become “ex-tropical cyclones”.
But this doesn’t mean they’ve weakened or been downgraded, but rather have morphed into a completely different type of weather system.
This image shows cyclone, typhoon and hurricane tracks between 1985 and 2005. On average one system travels within 550km of New Zealand each season. Image / Wikimedia Commons
Under the right conditions, they can intensify and muster even lower pressures than they had before being re-classified.
After this transformation, cyclone systems lose their symmetric cloud pattern – meaning the strongest winds and heaviest rain can be found hundreds of kilometres from centre, usually in a large area south.
For meteorologists, the centre becomes a poorer indicator as to where the worst weather will be.
How do meteorologists try to track them?
Across the world, there are six regional specialised meteorological centres (RSMCs) and six tropical cyclone warning centres (TCWCs) responsible for putting out local advisories and bulletins.
MetService runs the Wellington TCWC, which monitors an area that stretches over the North Island and hundreds of kilometres east.
Tropical cyclone specialists track systems using several “ensemble” models that combine dozens of global and high-resolution regional models, with multiple “runs” each day.
Across the world, there are six regional specialised meteorological centres (RSMCs) and six tropical cyclone warning centres (TCWCs) responsible for putting out local advisories and bulletins. Image / MetService
The more that these different models “agree” with each other, the more confident forecasters can be in predicting where cyclones will move – but the further out they look, the less certain they can be.
How and why are they given names?
Naming developed cyclones helps different weather agencies to know they’re looking at the same system – and it’s also useful for communicating with the public and forecast users like pilots and sailors.
While cyclones have been named after dates, longitudes and latitudes and even politicians in the past, today’s titles are agreed in advance by various meteorological agencies around the globe.
Most of the regions have several lists of random, mixed-gender names and work their way through the alphabet taking the next name on the list.
If a tropical cyclone has been especially powerful or has caused a lot of damage, that name may be retired from the list and a new name picked in its place.
What have been some of the worst?
Some of New Zealand’s most disastrous storms of the 20th Century happened to be ex-tropical cyclones.
They notably included Bola in 1988, which caused more than $200m of damage; Gisele in 1968, during which 51 people lost their lives in the Wahine disaster; and Fergus in 1996, which pummelled Coromandel with 300mm of rain in 24 hours.
Flooding in Edgecumbe in April 2017. Photo / NZME
More recently, 2017′s Debbie brought a deluge that pushed a Rangitaiki River stopbank to breaking point, flooding Edgecumbe, while the following year’s Gita caused widespread damage across the country – resulting in more than $35m in insured losses.
Is climate change causing more cyclones?
Perhaps contrary to popular assumption, a warming planet isn’t creating more tropical cyclones and storms - but fewer.
The bad news: we can expect tropical cyclones that do form to become stronger, with lower central pressure, stronger winds and more rainfall, due to more moisture in the air.
That drop in frequency can be explained by changes in the state of the atmosphere gradually resulting in fewer storms being needed to maintain the flow of heat from the tropics to the poles.
Scientists don’t expect a dramatic decrease, but one that may lower the Southwest Pacific’s seasonal average of 10 systems to nine.
Sources: Niwa, MetService, Bureau of Meteorology, Professor James Renwick.
