Quirks & Quarks

Hurricanes are slowing down and settling in to do more damage

Cyclones are moving about 10 per cent more slowly, likely because of climate change

Cyclones are moving about 10 per cent more slowly, likely because of climate change

Hurricane Harvey on August 26, 2017, when it was near peak intensity and making landfall (NOAA)

Slowing typhoons and hurricanes

Hurricane Harvey's strange behaviour in 2017, when it parked over South Texas for days, and caused massive flooding, might be emerging as more typical behaviour for superstorms of the future. 

James Kossin, a researcher from the U.S. National Oceanographic and Atmospheric Administration, has found that tropical cyclones - hurricanes and typhoons - have been moving more slowly in recent years, likely the result of a warming climate.

Kossin, from the NOAA Center for Weather and Climate, studied tropical cyclones in the Atlantic and Pacific over the last 70 years and found that on average they had slowed by about 10 per cent. This was likely due to changes in the large-scale global atmospheric circulation patterns that  generate the winds that move the storms over the globe.

As a result, storms are lingering in place longer than in the past, which may contribute to how much damage they cause through storm surges, intense rainfall and flooding, and sustained winds.

A person walks through a flooded street with a dog after the area was inundated with flooding from Hurricane Harvey on August 28, 2017 in Houston, Texas. (Getty Images)

Harvey was slow and devastating

Hurricane Harvey might be an example of what to expect. The problem with Harvey was not that it was powerful - it was that it was slow. It hit Texas and like an unwelcome guest that wouldn't leave.

Harvey was a Category 4 storm when it made landfall, with winds exceeding 210 km/h, but it quickly diminished in power to an ordinary, if large, tropical storm. However Tropical Storm Harvey then refused to move - parking over Houston and south Texas for several days, and dumped an incredible metre-and-a-half of rain in some areas, causing more than 60 deaths and the astronomical $125 billion in estimated property damage.

While storms like Harvey can pack internal winds of more than 200 km/hr, as a whole they move only at the speed of the prevailing winds, which may be only 10 or 20 km/h. According to Kossin, tropical cyclones are pushed around, "a little bit like a cork in a stream," by relatively gentle prevailing winds, like the trade winds of the tropics.

This map shows much tropical cyclones have slowed in different regions in the past 70 years. Local tropical cyclone rainfall totals would be expected to increase by the same percentage due to the slowing alone. Increases in rainfall due to warming global temperatures would compound these local rainfall totals even further. (NOAA/NCEI)

Climate change might explain the slowdown

These winds may have been weakening due to climate change. Kossin explains that the strength of atmospheric circulation is driven in part by differences in temperature between the polar regions and the temperate and tropical regions, which create large zones of high and low pressure, which drive winds. Climate change, however, has been warming the polar regions more than other regions.

"That reduces the overall temperature difference between poles and tropics, and that reduces the gradient of temperature, and that reduces the gradient of temperature, and that reduces the winds," says Kossin. "So if there's changes in the global circulation, then we really should see changes in the way these storms are translating around the planet."

This, unfortunately, is one more way in which storms might be more destructive in a warmer world. Warmer temperatures are already expected to produce more powerful storms, since they're fuelled by warm water.

Similarly, warmer air can carry more water, which means future storms are predicted to produce more intense rainfall. Slower storms, says Kossin, will mean these extreme conditions will linger where the storm hits rather than passing by. 

Interstate highway 45 is submerged from the effects of Hurricane Harvey seen during widespread flooding in Houston, Texas, U.S. August 27, 2017. REUTERS/Richard Carson - RTX3DKUD

Like a bad guest who won't leave

"One way to think about it is all of the bad things that tropical cyclones bring to your neighborhood are going to last longer, and that's not good by any stretch," says Kossin.

More rain will fall while the storm lingers, leading to fresh-water flooding.

"There's also strong evidence that slower storms would have more (...) coastal storm surges, so now you have increased saltwater flooding."

Finally even if storm winds don't blow harder, they'll be blowing longer, and have more time to damage buildings or knock down trees.

This suggests one more way that storms will be more problematic in the future, says Kossin. "Every one of these hazards associated with tropical cyclones is amplifying over time, due to human factors, and none of them are really occurring in a good way.