What makes North Atlantic hurricanes more frequent may also soften the blow once they reach land.
By Nathan Collins
(Photo: Scott Kelly/NASA via Getty Images)
The relationship between climate change and extreme weather is pretty complex stuff. Case in point: hurricanes. Though climate scientists suspect hurricanes will get worse as oceans continue to warm, they aren’t sure about the details. It’s likely they’ll get more intense, and more likely that the most intense hurricanes will grow more frequent, even as the overall number remains the same or declines, according to the Environmental Protection Agency and National Oceanic and Atmospheric Administration.
Further complicating matters, according to a new study, is that the same factors that could make North Atlantic hurricanes more frequent may also help weaken them as they approach the Eastern seaboard—although what that means for the future isn’t entirely clear.
At issue, NOAA researcher James P. Kossin writes today in Nature, are two factors, sea surface temperatures (SST) and vertical wind shear (VWS), that shape hurricane formation. First, hurricanes need warm seas, which supply the energy and moisture needed to transform a little bit of bad weather into a vortex of doom. Second, they need relatively little VWS, or the degree to which wind speeds and direction change with altitude. High VWS essentially slices a nascent hurricane apart as energy and moisture move up from the ocean.
Fortunately for hurricanes (and unfortunately for coastal infrastructure), VWS tends to decline as SST increases, suggesting that global warming could make hurricanes more intense and frequent—and, indeed, that’s what researchers think will more likely than not occur.
When conditions are right for intense hurricanes to form, they are also right to protect the U.S. coast from those hurricanes.
But, Kossin noticed, there’s something else going on that could have an opposite effect, at least in the North Atlantic: When SST is unusually high and VWS is unusually low in the main-development region, where hurricanes first form, SST is unusually high and VWS is unusually low near the East Coast of the United States—a sort of buffer zone that could, in principle, starve a storm’s energy and rip it apart at the same time.
To test that idea, Kossin looked at the last 68 years of data on hurricane formation and intensification, divided into two “active” periods, 1947–69 and 1993–2015—during which hurricane frequency was higher but the buffer zone was in place; and one “quiescent” period, 1970–92, during which hurricane frequency declined and there was no buffer zone.
During the two active periods, wind speeds in major hurricanes—Category 3 and above—declined as they approached the East Coast, while corresponding wind speeds increased during the quiescent period. What’s more, hurricane intensification during the quiescent period were more variable. In particular, rapid wind speed increases, up to five or six miles per hour over the course of six hours, were much more likely during the quiescent period.
Taken together, those results hint that, when conditions are right for intense hurricanes to form, they are also right to protect the U.S. coast from those hurricanes, Kossin argues. Just as important is that the absence of a buffer zone seems to make hurricanes less predictable.
What the future holds is another story. While there’s been a strong trend toward more intense, more frequent hurricanes in the region where they form, there’s been no such trend along the Atlantic coast or the Gulf of Mexico, suggesting “the possibility that future warming will not strongly affect the control of VWS on these hurricanes,” Kossin writes. “Given the potential impacts on US coastal hazards and risk, this relationship merits further observational and modelling study.”