Why slower cyclones are bad news for us

Rodiano Bonacci
Giugno 8, 2018

The center said the storm was likely to strengthen some more as it moved farther out into the Pacific, but predicted Aletta would begin weakening Saturday. Wind speeds within the storm remain high, but the whole system itself moves slower across the landscape, allowing punishing rains to linger longer over communities.

The study, released Wednesday in the scientific journal Nature, showed a 10% decrease in forward speed globally between 1949 and 2016, though there is some variation among ocean basins.

Study author James Kossin of the National Oceanic and Atmospheric Administration says Harvey is a great example of what he found.

Harvey, which killed at least 68 people, dropped a record 60 inches of rain in parts of southeastern Texas, according to the National Hurricane Center.

This analysis stopped at the end of 2016 and did not include any storms that formed in 2017, so Harvey was not factored in to the study.

Tropical cyclones have slowed more in the Northern Hemisphere, which is significant because that is where a majority of storms occur each year.

Unhurried hurricanes also mean strong winds blowing more often over the same place and possibly more storm surge, Kossin said.

Scientists expect climate change is going to make tropical cyclones - including hurricanes - more severe.

Kossin admits that there are probably both natural and manmade factors influencing the slowing of storms and recommends further studies using climate models to determine how much greenhouse gas emissions are responsible for affecting the storms' speeds. It is based exclusively on past observations, he said.

But Kossin made a decision to investigate it, based on the expectation that climate change is already altering the general, large-scale circulation of the atmosphere, within which hurricanes are embedded and by which they are steered.

Outside scientists were skeptical.

"Roughly 7 percent more water vapor per degree C of warming", Kossin said.

"Kossin is right that a 10 percent change in tropical cyclone motion would be an important change due to its effect on accumulated precipitation", Texas state climatologist John Nielsen-Gammon said in an email.

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