Was Yasi Australia's biggest cyclone?
We've heard Yasi called a 'monster cyclone'. Was it the biggest cyclone Australia has ever seen. Was it a super-cyclone?
Many people have called Cyclone Yasi a 'monster cyclone'. Was it the biggest cyclone Australia has ever seen? Was it a super-cyclone?
Severe Tropical Cyclone Yasi, which hit North Queensland last week, was one of the more intense cyclones in Australia's recent history.
A massive category 5 storm over 600 kilometres wide, it clocked wind speeds of 295 kilometres per hour at its greatest intensity.
But it was no super-cyclone, nor was it the biggest cyclone Australia has ever seen, says Dr Jeff Kepert, head of the High Impact Weather Research Team in the Bureau of Meteorology.
"To me super cyclone would imply something off the scales and you can't say that about Yasi."
"It was a larger than average storm, in the top five or ten per cent, but that's all. Storms of similar intensity have hit Australia and elsewhere in recent years."
"Events like Yasi are just an extreme part of normal weather patterns. The Western Australian coast and the Northern Territory tend to have more intense cyclones than Queensland, but Larry was a category 5 at sea before weakening to category 4 as it made landfall," says Kepert.
Professor Jonathan Nott from the Australasian Palaeohazards Research Unit with James Cook University agrees.
Nott says there's no formal definition for the term super cyclone and only uses it very loosely to describe an extremely powerful event with intensities below 910 hectopascals, a measure of atmospheric pressure.
"We don't get those often. The strongest in Queensland was 914 hectopascals Tropical Cyclone Mahina in 1899 which hit north of Cooktown."
Its 350 kilometre per hour winds killed over 400 people, the largest death toll in any natural disaster in Australian history.
"But for Queensland, Yasi is the most intense since Innisfail in 1918."
Cyclical cyclone seasons
How do we know Yasi was category 5?Australia uses a five category system based on wind speeds to grade cyclones.
Category 1: wind speeds up to 125 kilometres per hour, causing only slight damage.
Category 2: wind speeds up to 169 kilometres per hour, causing minor damage to buildings, severe damage to signs and trees, and heavy damage to crops.
Category 3: wind speeds up to 224 kilometres per hour, very destructive, causing structural damage to roofs and bringing down power lines.
Category 4: wind speeds of 279 kilometres per hour causing significant roofing and structural damage with a lot of airborne debris and widespread power failures.
Category 5: wind speeds over 280 kilometres per hour and capable of causing almost total destruction with buildings flattened and cars overturned.
Winds are measured by official weather bureau anemometers on the ground , although good estimates can also be made from satellite data.
Using this system, Yasi appears to be a category 5, although it could take officials six months to confirm that through studies, says Nott.
Cyclones are an annual event across the Australian tropics, but some years are worse than others due to seasonal weather patterns.
According to Kepert, Australia has more cyclones during La Niña weather patterns because of warmer sea surface temperatures, however this won't increase storm intensity.
Other contributing factors can help generate cyclones including a surge in easterly trade winds which could increase cyclonic rotation, he says.
"There's also the Madden-Julien Oscillation weather pattern blowing east along the equatorial belt slowly moving clouds and rain. Scientists are still debating the origin of these winds."
"The oscillation is linked to a doubling of cyclone activity, but is neutral at the moment, so we can't blame it for Yasi".
Nor can we link Cyclone Yasi to climate change, says Kepert.
"As for global warming, things are complicated.
"[Climate change] certainly increases sea surface temperatures, but other changes such as atmospheric circulation might possibly offset this.
"The theoretical evidence indicates a gradual increase of about 10 per cent in cyclone intensity. But it could mean a slow drop in the number of cyclones, especially the weaker ones."
Queensland goes through periods of intense cyclones following relatively quiet times lasting one or two centuries, says Nott.
"We have two types of records on past cyclone activity. Sand deposits going back five thousand years (the period sea levels have been at current heights) tell us how big and where a cyclone hit, based on the amount of sand left above the highest tides.
"The other uses limestone stalagmites in caves to tell us about different chemical signatures in tropical cyclone rain compared to normal and monsoon rain".
Using these techniques, Nott and colleagues have found that cyclones with winds of up to 300kmh cross the Queensland coast every 200 or 300 years.
Dr Jeff Kepert , head of the High Impact Weather Research Team in the Bureau of Meteorology and Professor Jonathan Nott from the Australasian Palaeohazards Research Unit with James Cook University were interviewed by Stuart Gary
Published 07 February 2011