Spinning south

Spinning south

The warming climate could bring more catastrophic hurricanes to ill-prepared areas of Australia’s coast

When Cyclone Seroja slammed into the West Australian holiday town of Kalbarri in April it carved a trail of destruction that is sadly familiar to many coastal communities in the nation’s north.

Roofs were peeled away from houses, windows smashed and debris scattered far and wide, leaving devastated residents to count the cost to properties and livelihoods.

But this storm was different from some that preceded it in a couple of ways: its formation under the rarely seen Fujiwhara Effect, in which it absorbed the weaker Cyclone Odette before shifting its trajectory south-eastwards; and its final destination.

Category 3 cyclones are rare visitors to Western Australia’s Mid West coast.

“It appears to be one of the more intense tropical cyclones to make landfall that far south on the Western Australian coast,” James Cook University Professor of Physical Geography Jonathan Nott told Insurance News in the days after the catastrophe.

Risk modeller AIR Worldwide noted it is “extremely rare for tropical cyclones to travel this far south”, with Geraldton, 155km south of Kalbarri, taking a hit after avoiding such storms since 1956.

Professor Nott, who specialises in reconstructing long-term natural records of extreme events, called it a possible wake-up call for Australia, and a sign of things to come.

That’s because in a warming climate, cyclones could track further south more often.

In September last year the National Centre for Atmospheric Research and insurer IAG released a research paper examining climate change impacts on “weather extremes that produce significant property, personal and economic damage and hardship” in Australia.

Among its key findings was that the frequency of tropical cyclones has declined slightly in recent decades “and this slow trend is projected to continue globally and for the Australian region”.

However, the good news ends there.

“Over the past 30 years, the proportion of the most intense tropical cyclones has increased at the expense of weaker systems, and this change is expected to continue,” the report says.

“Over the past two decades, the number of intense tropical cyclones making landfall on the Queensland east coast has increased substantially. The frequency of tropical cyclones making landfall throughout the western South Pacific region has also increased.

Flattened: damaged properties in Kalbarri.

Credit: DFES Incident Photographer Lewis van Bommel

“There is global evidence of a poleward shift in the latitudes where tropical cyclones reach their peak intensities. New research confirmed this shift is also evident for tropical cyclones in the Australian region.”

The report warns that warming seas to the east will enable cyclones to retain higher intensities further south and to penetrate further inland, with risk predicted to increase most rapidly in south-east Queensland and north-east New South Wales. “There is also a potential for increased risks in the coastal districts south of Shark Bay in Western Australia,” the report adds.

South of Shark Bay just happens to be where Seroja made landfall.

Valentina Koschatzky, Principal Risk Scientist at catastrophe modeller Risk Frontiers, tells Insurance News it is difficult to attribute the effects of climate change to a single event.

“The southerly track of Tropical Cyclone Seroja, although unusual, is not without precedent,” she says.

“Risk Frontiers’ tropical cyclone loss model includes a domain that extends south of Perth, recognising the existence of tropical cyclone hazard, albeit infrequent, for these areas under present-day conditions.

“Typically, tropical cyclones crossing the west coast of Australia so far in the south are infrequent and less intense. Winds speeds over 170kmh [as recorded in Kalbarri] have never been recorded south of Carnarvon, but the historical records are fairly recent and do not necessarily capture the occurrence of such extremes.”

Citing research by Professor Nott, she says there is palaeoclimatological evidence of tropical cyclones even more intense than Seroja in the same region in the past.

Dr Koschatzky says current scientific literature “suggests that similar events may become more frequent at some point in the future”. However, she notes there is room for doubt.

“To date, there is no evidence in the observational record to suggest a trend exists and there is low confidence in the climate model projections for a southerly shift in tropical cyclones in the Australian region.

“The climate models have a limited ability to accurately simulate the interaction of some of the tropical cyclone processes, such as the one between climate change and regional aspects of tropical expansion, that influence tropical cyclone formation, transport, decay and dissipation.”

Dr Koschatzky says Risk Frontiers recently “climate-enabled” all its models to “look at changes in financial losses on a portfolio of real assets under future climate scenarios”.

Future threat: cyclones could inflict such damage in communities further south.

Credit: DFES Incident Photographer Morten Boe

One concerning aspect of the damage in Kalbarri is that, under current building codes, the town’s buildings are constructed according to lower wind standards than those further north where the risk of cyclone impacts is deemed more extreme.

“Of the four windspeed zones into which Australia’s building standard divides the country, the central west coast is in wind region B – considered not prone to these events and with the second-lowest wind design speed values,” an AIR Worldwide blogpost says.

“As a result, the damage done by Seroja was severe.”

Professor Nott says it may be time to reconsider Australia’s “wind areas”.

This might involve moving more communities’ categories up from intermediate to cyclonic or severe cyclonic, and strengthening buildings accordingly.

Dr Koschatzky says that even under present conditions, the intermediate-rated areas of south-east Queensland and northern New South Wales have very high cyclone risk, in terms of loss.

“While the probability of tropical cyclone landfall in this region is low and events are weaker, the number of exposed properties is high and the building standards are lower than further north, driving the high risk and loss potential.

“As with most natural perils, this just highlights the important role of exposure and vulnerability in natural catastrophe risk. Catastrophe loss models can be used to assess how such a potential shift, combined with other possible changes in tropical cyclone activity, impacts the risk profile around the coast of Australia. This information can be used to inform future design levels.”

The National Centre for Atmospheric Research report from last year notes that a host of other climate-related factors are combining to change and enhance the threat from tropical cyclones.

Along with the possible poleward shift in the locations where tropical cyclones reach their maximum intensity, storm rainfall is already increasing and further rises are expected.

“For example, cyclone sensitivity studies have shown a near-doubling in the area experiencing greater than 600mm during a cyclone passage over south-eastern Queensland and north-eastern New South Wales has occurred in the past decade.”

And tropical cyclones’ speeds appear to be slowing at higher latitudes, which “combined with increasing intensity and rainfall, leads to a potential for substantial increases in cyclone impacts from wind, rain and water ingress into buildings”.

Sea level rise and increasing river run-off, when combined with more intense cyclones, point to “substantial increases in storm surge impacts and coastal erosion”.

Dr Koschatzky says Risk Frontiers’ models have it covered.

“For the tropical cyclone wind hazard, we modelled the change in frequency of occurrence for events of different intensities and locations under a range of climate scenarios and time horizons,” she says.

“This allows us to estimate the change in average annual losses on a market portfolio, say by 2050, under different [climate change pathways] at a postcode or other level.

“We are also able to model the effect of a southerly shift in tropical cyclone activity, and also consider the impact of this if the exposure and building standards were also to change in the future.”