Severe tropical cyclones, called 鈥渟uper-cyclones鈥�, hit the Great Barrier Reef coast every 200 to 300 years 鈥� 10 times more often than previously thought.
Moreover, 鈥渨e鈥檝e gone a long, long time without having a big one,鈥� says Jonathan Nott, from James Cook University in Cairns, Queensland.
Nott, who produced the new research with colleague Matthew Hayne, says the increased knowledge about super-cyclone frequency has 鈥渟erious implications for the vulnerability of coastal human settlements鈥�.
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Tropical cyclones affect people living across the north coast of Australia, in Queensland and northern New South Wales. Globally, they are second only to drought in their human and economic impact. In Australia they can also flatten rainforest and damage coral reefs.
Storm surge
The instrument-based record of tropical cyclones in north-east Australia only goes back 100 years, preventing a long term analysis of their frequency.
Nott started by studying recent storm surge deposits on the Queensland coast and correlating them with cyclone data. He then used radiocarbon dating on shells and coral fragments to give the ages of older deposits, with some being over 5000 years old.
Finally, he compared three existing numerical models to estimate the magnitude of storm required to produce such deposits 鈥� extremely intense cyclones were the most likely cause. Tsunamis could cause similar ridges but 鈥渨e don鈥檛 know of any that deposit coral ridges in this way,鈥� says Nott.
John Chappell, an expert on tropical cyclones at the Australian National University in Canberra, agrees. He adds that the sites studied by Nott and Hayne have ridge structures that match those built by recent cyclones, and that the settings are well-protected from possible tsunami sites.
Generation gap
Tropical cyclones are common in Australia, occurring about 10 times a year, usually between November and April. They form when a combination of a low-pressure system and moist ocean air develops into a spiraling cloud with a central 鈥渆ye鈥� of very low pressure.
The super-cyclones inferred by Nott and Hayne are likely to have been category five or severe category four cyclones on the Saffir-Simpson scale. This means the pressure in the calm centre was less than about 920 hectopascals, with winds gusting at over 280 kilometres per hour. According to Nott, his estimates of intensity were conservative.
Jeff Callaghan, from the Bureau of Meteorology鈥檚 Severe Weather Systems office in Brisbane, says it is almost a generation since there was even a category three cyclone in Queensland.
He adds that recent climate patterns suggest a major cyclone is likely: 鈥淲e鈥檝e had two or three La Ni帽a events, one after the other. Normally, we鈥檇 expect a pretty big event by now.鈥�
Journal reference: Nature (vol 413, p 508)