While a warming globe means hurricanes are likely to get stronger, it may also weaken a key protective barrier for the Southeast, new research shows.
That invisible buffer of wind shear, which acts as a "speed bump" for incoming storms, most recently helped shield South Carolina from Hurricane Matthew in 2016. The cyclone ripped through the Caribbean as a major hurricane before weakening to Category 1 strength upon landfall at the Cape Romain National Wildlife Refuge just south of McClellanville.
But this protection will likely be weakened in the future, said James Kossin, a climate scientist with the National Oceanic and Atmospheric Administration and an author of the new study.
In the North Atlantic, there's a slow, decades-long pendulum that has huge effects on activity during a hurricane season. Called the Atlantic Multidecadal Oscillation, this 20- to 40-year pattern means that tropical Atlantic sea surface temperatures alternate between slightly below average and slightly above average.
But even a small change can make a big difference; in warm periods, tropical storms turn into severe hurricanes twice as often as in cool periods, according to NOAA.
Since about 1995, the Atlantic has been in the midst of a warmer period. But it's becoming more challenging to distinguish that pattern from global warming.
During the hotter stretches, a natural barrier of wind shear forms along the Southeast U.S., with its center directly over South Carolina, Kossin said. He also laid out the phenomenon in a 2017 paper.
In addition to warm water, hurricanes need favorable winds that move in the same direction and the same speed throughout the atmosphere. Wind shear, however, moves in different directions and speeds at different heights. The phenomenon can weaken a hurricane significantly or entirely knock apart the structure of a storm, which is basically a tall, rotating cylinder, Kossin said.
"The U.S. East Coast is just very fortunate to have this natural protective barrier there," Kossin said. "When things are very favorable for hurricane formation and intensification in the tropics, they're unfavorable along the coast."
But the slow cooking of water and air by human-driven carbon emissions could "largely erode" that protection in the future, Kossin and three other authors found in an article published in May.
Meteorologist Jeff Masters, founder of the private forecasting company Weather Underground, said the finding is far from concrete, because not every computer model in the study agrees that the speed bump will weaken.
"We have to keep in mind that it's based on computer models for the climate going out decades in advance, so uncertainty is high," he said.
Some effects of climate change are far more certain, both Kossin and Masters said. For example, researchers have already established that a warmer atmosphere can hold more water, leading to wetter storms, and observed that tropical cyclones are moving more slowly, giving them a longer time to dump rain.
Other research has shown that the tracks of storms may nudge north as warmer water reaches higher latitudes and that the most intense cyclones will become more frequent.
Forecasters often say it only takes one storm to turn a hurricane season deadly, and it's entirely possible that such a storm arrives when the speed bump isn't in effect — 1989's Hurricane Hugo, for example.
The danger of a single storm was a strong takeaway from the National Hurricane Center's forecast for this season, which was filled with uncertainty. But removing the Southeast's speed bump will only intensify the risk that serious storm could come.
Right now, "those times when we need the protection the most is when the protection is greatest," Kossin said.