In the last week of August, the monster storm now known as Hurricane Dorian began its life as a bit of thunderstorm activity off the West African coast.
Within days it increased in size and intensity. The storm hit the Virgin Islands with strong winds and rain and dealt Puerto Rico a glancing blow.
But then as Dorian landed over the Bahamas, what NASA scientists call a "nightmare scenario"began to unfold: the Category 5 Hurricane Dorian stalled for nearly 40 hours over the Abaco and Grand Bahama islands, unleashing catastrophic 185 mile-per-hour winds and 20-feet of storm surge.
While a hurricane that fierce parked in one place for that long might seem freakish, a recent study published in the journal “Nature”shows that hurricane stalling has become more common over the last 70 years.
"There's definitely a trend upward," says study co-author Tim Hall, a senior scientist at the NASA Goddard Institute for Space Studies.
Using data from the National Hurricane Center, Hall and a fellow researcher looked all tropical cyclones in the North Atlantic between 1944 and 2017. They found that of the 66 storms within that time frame that stalled for more than two days, nearly two-thirds did so within the past 25 years.
To understand how hurricanes move, Hall uses the example of a leaf floating in a stream. Hurricanes don't move of their own accord; instead, they're pushed along by large-scale wind patterns in the atmosphere. When those wind patterns are steady and blowing in a consistent direction, the hurricane moves along quickly. But if wind patterns change and the storm has no guidance for a period of time, it becomes like a leaf caught in an eddy, swirling round and round.
So what's causing the increased stalling?
Hall says the scientific community is still debating the possible connection between climate change and a greater propensity for tropical cyclones to stall.
"Climate change simulations show pretty robustly that wind patterns in the atmosphere slow down as climate warms," he says. "And hurricanes are just responding to that slow-down."
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