Is climate change causing slow-motion cyclones?

Global warming is changing our weather in so many ways – but one of the most concerning is its impact on cyclones.

On the plus side, research suggests climate change may lead to reduced frequency of tropical cyclones. But, worryingly, intensity is expected to rise, and many also believe cyclones and other weather systems are moving across the planet more slowly.

This creates obvious problems for people caught in their path – and for insurers.

When destructive winds and heavy rainfall sit over one location, the potential for damage and severe flooding soars.

Queensland’s Cyclone Debbie and Hurricane Harvey in the US, both in 2017, were examples of slow-moving systems that dumped extraordinary amounts of rain on communities.

The monsoonal low that brought devastating flooding to Townsville also stalled over the north Queensland city for more than a week.

A recent study published in the journal Nature shows the speed of tropical cyclones worldwide over land has decreased by 10% in the past 70 years.

But Thomas Mortlock, Senior Risk Scientist for catastrophe modeller Risk Frontiers, says while the phenomenon is clear in the northern hemisphere, it is less so in Australia.

“There are a lot of climate change projections relating to frequency and intensity [of cyclones],” he told insuranceNEWS.com.au. “There should be fewer landfalling events, but some research shows an increase in intensity and a higher capacity to hold moisture.

“There is also some global research that shows forward velocity is slowing down and it is quite apparent in the northern hemisphere.”

Dr Mortlock says Australia’s climate is so heavily influenced by El Nino and La Nina that long-term trends can be masked.

“The jury is still out. It can be hard to see, but that doesn’t mean it’s not there.”

He says there are good reasons to expect a slowdown.

“As the tropics warm, they expand, and as they expand circulation slows down. Townsville was quite remarkable, with a tropical monsoon hanging around for a very long time.

“Cyclone Debbie crossed the coastline at 7kmh – you could run faster than that.

“If we do have slow-moving cyclones, there is the capacity to rain down more volume on one location, and that could have a major impact.”

Dr Mortlock says climate change is also pushing cyclones further south, bringing more populated areas such as Brisbane into play.

James Cook University cyclone expert Stephen Williams appeared on ABC’s Q&A in Townsville last week to highlight the issue.

Professor Williams says the Townsville inundation was devastating, but entirely predictable.

“The sad part about it is that we have been saying it was going to happen for 30 years,” he told the audience. “There is just simply no denying the climate change impacts and the fact we are causing them.”

Professor Williams says slower-moving cyclones and weather systems are caused by “a global slowing down” of atmospheric and ocean currents.

“The theory is that a high-pressure system blocks the low-pressure system and basically maintains it in exactly the same position,” he said. “There is recorded evidence of all of those things starting to slow down with climate change.”

Writing in The Conversation, Steve Turton, Adjunct Professor of Environmental Geography at CQUniversity Australia, says there is a “plausible link” between climate change and slow-moving weather systems.

He points to the Tasmanian heatwave and bushfires earlier this year, and the Townsville floods.

“One common feature of many of these events… was that they were caused by weather systems that parked themselves in one place for days or weeks on end.

“It all began with a blocking high – so called because it blocks the progress of other nearby weather systems – in the Tasman Sea throughout January and early February.

“This system prevented rain-bearing cold fronts from moving across Tasmania, and led to prolonged hot dry northwesterly winds, below-average rainfall and scorching temperatures.

“Meanwhile, to the north, an intense monsoon low sat stationary over northwest Queensland for 10 days.

“It was fed on its northeastern flank by extremely saturated northwesterly winds from Indonesia, which converged over the greater northeast Queensland area with strong, moist trade winds from the Coral Sea, forming a ‘convergence zone’.

“Ironically, these trade winds originated from the northern flank of the blocking high in the Tasman, deluging Queensland while leaving the island state parched.”

Professor Turton says Australia has been hit by many extreme weather events already this year, with records broken. And the future could be bleak.

“If those extreme weather events travel more slowly across the landscape, their effects on individual regions could be more devastating still.”

Source [ http://insurancenews.com.au/analysis/is-climate-change-causing-slow-motion-cyclones ]