Hurricane Dorian is the joint most powerful storm to make landfall in modern history, with sustained wind speeds of 185mph and gusts of up to 220mph at its peak, but despite this ferocity, the storm itself has been particularly slow-moving.
As Dorian battered the Bahamas, it stalled over land, with its speed of movement reducing to as low as just one mile per hour.
The slow-travelling system was one of the reasons why forecasters have had a tough time estimating its likely future direction.
In fact, research published in June this year shows Dorian’s sluggish movement, high winds and sustained heavy rain, are all in keeping with a trend for slower-moving cyclones that deliver extreme rainfall.
The average forward speed of North Atlantic tropical cyclones had decreased by 17 per cent from 11.5 mph in 1944, to 9.6 mph by 2017, according to research by scientists at Nasa and the NOAA.
It is a big problem – stalling tropical cyclones do more damage than those that pass through faster.
Writing in the journal Nature, the authors said the reasons for the trend are not fully understood, but theories include a “weakening of general atmospheric circulation patterns, including those of the tropics”, resulting in slower global wind speeds.
Other studies have suggested Arctic amplification – the disproportionate warming of the planet at its northern extreme as a result of increasing levels of greenhouse gases – may play a role in reducing airspeeds around the world.
The interplay of energy between the cooler polar regions and the warmer tropical areas are a key driver of winds, but as the contrast between the two has reduced, so too have wind speeds.
Furthermore, the scientists warned the trend for slower-moving tropical cyclones (TCs) result in more catastrophic rainfall, such as has been seen in the Bahamas during Dorian’s assault, and in Texas during Hurricane Harvey, in which some parts brought 60 inches of rain (1.5 metres) over four days.
The authors said: “Stalling TCs have the potential for depositing damaging amounts of rain. This trajectory-induced increase in rainfall is exacerbated by the climate-warming impact on the hydrologic cycle. Increased atmospheric moisture enhances the likelihood of extreme rainfall events of all types.”
A report in the New Scientist this month noted that “hurricanes are expected to intensify faster, to become stronger overall, to dump more rain and to move more slowly as the world warms, and that seems to be just what is happening”.
The study authors said their findings indicated the likelihood of increased rain and greater damage as hurricanes stall more frequently should be taken into account by those planning emergency response strategies.
