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Tropical cyclones numbers declining in Australia

Ben Domensino, Thursday November 1, 2018 - 13:51 AEDT


Australia's tropical cyclone numbers appear to have declined over the last few decades, although the proportion of severe tropical cyclones could be on the rise.


Today is the first day of Australia's tropical cyclone season, which runs from November to April. On average, around 11-12 tropical cyclones form in the Australian region each year and four cross the coast.


Tropical cyclones form in areas where sea surface temperatures are at least 26.5 degrees and wind shear (the change in wind speed/direction with height) is low. These ingredients make the waters surrounding northern Australia an ideal place for tropical cyclones to develop during the warmer months of the year.


But according to the Bureau of Meteorology, there has been a decline in the number of tropical cyclones in the Australian region since the 1970. A number of studies have found a similar trend in cyclone frequency in other parts of the world, in both the northern and southern hemispheres.


While cyclones appear to have declined in numbers, sea surface temperatures near Australia's northern tropics have warmed by nearly one degree over the last 117 years, according to the Bureau of Meteorology.


This warming makes the reducing cyclone numbers paradoxical, as warmer water enhances tropical cyclone development. So, what's going on?


Like most long-term trends relating to our planet's climate, the apparent reduction in tropical cyclone numbers near Australia has a few elements that are worth pointing out.


Firstly, reliable satellite records of tropical cyclones near Australia has really only been available since the late 1970's. There's a good chance that some systems prior to this 'satellite era' were incorrectly identified as tropical cyclones.


Secondly, tropical cyclone numbers vary a lot from year-to-year, making it difficult to identify a clear trend using only a few decades of data. This variability is also influenced by El Nino and La Nina events, with more cyclones crossing the coast during La Nina years and less in El Nino years.


These caveats are likely to explain some of the observed changes in cyclone frequency in the past few decades, while changes to the environment in which cyclones develop are also likely to have played a part.


But despite evidence that the overall number of tropical cyclones has declined in Australia since the 1970s, this doesn't mean they are getting weaker.


A number of studies have found that there has actually been an increase in the proportion of higher intensity storms during the last few decades.


The International Panel on Climate Change (IPCC) predicts that globally, tropical cyclone numbers will either decrease or stay the same in the coming decades, although tropical cyclone intensity is expected to increase.


Another important feature of our planet's changing climate when it comes to cyclones is the 'expansion'; of our tropics. The warm air and seas that tropical cyclones require to exist are are spreading further away from the equator towards higher latitudes. Over time, this may allow tropical cyclones to venture further towards the poles before they decay.


Last year saw the decaying remnants of Tropical Cyclone Debbie pass directly over Brisbane a few days after it made landfall on the Central Coast of Queensland. It's possible that parts of southern Queensland, northern NSW and southwestern WA will be more affected by decaying tropical cyclones in the coming decades, particularly if the strongest tropical cyclones each season continue to get stronger.


A look at this year's hurricane season in the northern hemisphere reveals the potent atmosphere that these tropical systems are currently developing in. According to Meteorologist Philip Klotzbach from Colorado State University, this year produced the most Accumulated Cyclone Energy (ACE) of any hurricane season on record. The ACE is a measure of the total energy put out by all hurricanes and tropical storms in a season, based on the maximum wind speeds throughout their lifespan.


So what does that all mean for Australia's 2018/19 tropical cyclone season? With tropical cyclone numbers declining in Australia and the potential for an El Nino event in the month ahead, the odds favour fewer tropical cyclones than usual between November and April. However, the trend of the last few decades indicates that there could be a higher proportion of more intense tropical cyclones this season.


Visit http://www.bom.gov.au/cyclone for the latest information on tropical cyclones during the next six months.


- Weatherzone

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