ARI SHAPIRO, HOST:
It's been more than 10 years since the U.S. was hit by a major hurricane. Scientists mark that up to chance. But as NPR's Christopher Joyce reports, new research suggests a reason for our good fortune.
CHRISTOPHER JOYCE, BYLINE: Atlantic hurricanes are born in tropical, warm waters southeast of the continent. Over the past decade, there have been plenty of big ones out there - Category 3 or bigger. But they haven't hit the U.S., or else they've petered out by the time they do, like Hurricane Matthew, which started out last October as a major hurricane. Atmospheric scientist James Kossin with the National Oceanic and Atmospheric Administration says it didn't last.
JAMES KOSSIN: As it turned up and started to move over to the east coast of Florida, it began to weaken.
JOYCE: Down to a Category 1 hurricane by the time it hit the U.S. So what happened? Matthew was born in the tropical Atlantic. Two things allowed it to grow - very warm water and a lack of wind shear. Wind shear is when you get two adjacent layers of wind moving at different speeds. They break up hurricanes. As Matthew approached the U.S. coast, it encountered cool water and high wind shear, just the opposite of the conditions that created it. That weakened the storm.
Kossin says this is not an isolated case. He's looked at records back to 1947. It's happened before. His analysis shows that there's a sort of bipolar relationship between the tropical Atlantic and U.S. coastal waters.
KOSSIN: In a nutshell, when things are good for hurricanes in the tropics, they're bad for hurricanes near the coast. And when they're bad for hurricanes in the tropics, they're good for hurricanes near the coast.
JOYCE: Writing in the journal Nature, Kossin warns that when the conditions flip, when the Atlantic cools and gets windier, it still breeds hurricanes. And that's when conditions along the coastline tend to intensify hurricanes as they get closer. Kossin says this phenomenon has protected the U.S. in the past, but there's no guarantee it will last.
KOSSIN: This has been a very lucky thing for us. And we've had it in place now for a while, and we don't know how this phenomenon is going to be affected by climate change.
JOYCE: Climate change could strengthen the coastal buffer, he says, or eliminate it. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.
