Why Are Hurricanes Like Dorian Stalling, and Is Global Warming Involved?

https://insideclimatenews.org/news/03092019/hurricane-dorian-climate-change-stall-record-wind-speed-rainfall-intensity-global-warming-bahamas

By Bob Berwyn, InsideClimate News
Sep 3, 2019

Hurricane Dorian's slow, destructive track through the Bahamas fits a pattern scientists have been seeing over recent decades, and one they expect to continue as the planet warms: hurricanes stalling over coastal areas and bringing extreme rainfall.

Dorian made landfall in the northern Bahamas on Sept. 1 as one of the strongest Atlantic hurricanes on record, then battered the islands for hours on end with heavy rain, a storm surge of up to 23 feet and sustained wind speeds reaching 185 miles per hour. The storm's slow forward motion—at times only 1 mile per hour—is one of the reasons forecasters were having a hard time pinpointing its exact future path toward the U.S. coast.

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Recent research shows that more North Atlantic hurricanes have been stalling as Dorian did, leading to more extreme rainfall. Their average forward speed has also decreased by 17 percent—from 11.5 mph, to 9.6 mph—from 1944 to 2017, according to a study published in June by federal scientists at NASA and NOAA.

The researchers don't understand exactly why tropical storms are stalling more, but they think it's caused by a general slowdown of atmospheric circulation (global winds), both in the tropics, where the systems form, and in the mid-latitudes, where they hit land and cause damage.

Hurricanes are steered and carried by large-scale wind flows, "like a cork in a stream," said Tim Hall, a hurricane researcher with NASA's Goddard Institute for Space Studies and author of the study. So, if those winds slow down or shift direction, it affects how fast hurricanes move forward and where they end up.

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"in the broadest sense, global warming makes the global atmospheric circulation slow down," said NOAA hurricane expert Jim Kossin, co-author of the June study.

He said scientists suspect the overall slowing of winds is at least partly due to rapid warming of the Arctic. The temperature contrast between the Arctic and the equator is a main driver of wind. Since the Arctic is warming faster than lower latitudes, the contrast is decreasing, and so are wind speeds.

"There is a lot of evidence to suggest this is more than just natural variability," Kossin said.

In a 2018 paper, Kossin showed that the increase in tropical cyclones stalling is a global trend. The magnitude varies by region but is "generally consistent with expected changes in atmospheric circulation forced by anthropogenic emissions," he wrote.

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Rising global temperatures also influence storms in other ways: A warmer atmosphere holds more moisture, which means hurricanes can bring more rain, and warmer oceans provide additional energy that can make them stronger.

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Dorian is the fifth Category 5 hurricane in just four years in the Atlantic, and only the 35th on record going back nearly a century.

Scientists have seen a trend toward stronger hurricanes in the Atlantic, but not an increase in the total number of storms, said Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research.

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