Why are hurricanes like Dorian stalling, and Is global warming involved?

(Inside Climate News, 3 Sep 2019) Hurricanes Harvey and Florence also stalled, leading to extreme rainfall. Research shows it's a global trend.

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.

With the storm still over the islands on Sept. 2, the magnitude of the devastation and death toll was only beginning to become clear. "We are in the midst of a historic tragedy in parts of our northern Bahamas," Prime Minister Hubert Minnis told reporters.

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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Inside Climate News, 3 Sep 2019: Why are hurricanes like Dorian stalling, and Is global warming involved?