By Sid Perkins
The recent spike in hurricane activity in the North Atlantic—a trend that some scientists blame on climate change—actually reflects a return to normal frequency after a lull in the 1970s and 1980s, a new analysis confirms.
Between 1995 and 2005, meteorologists recorded an annual average of 4.1 category-3-or-stronger hurricanes in the North Atlantic and the Caribbean. Such hurricanes exhibit steady wind speeds exceeding 178 kilometers per hour. From 1971 through 1994, however, an average of only 1.5 such hurricanes swept through the same region each year, says K. Halimeda Kilbourne, a paleoclimatologist with the National Oceanic and Atmospheric Administration in Boulder, Colo.
Two factors thought to strongly influence hurricane formation are wind shear—an atmospheric phenomenon in which adjacent layers of air move at different speeds or in different directions—and sea-surface temperature. Strong wind shear tends to rip apart tropical storms before they strengthen into hurricanes, says Kilbourne. On the other hand, a sea-surface-temperature rise can provide more energy to a hurricane as it forms.
Kilbourne and her colleagues studied a variety of marine records to estimate year-to-year variations in wind shear back to 1730. For instance, the luminescence of growth rings in coral under ultraviolet light reveals how much organic matter has been washed from land by thunderstorms, which don’t form as readily or as often if wind shear is high. Also, the number of marine microorganisms in seafloor sediment—in particular, that of a species called Globigerina bulloides—indicates the upwelling of nutrient-rich waters, another measure of wind shear at the ocean’s surface.
When the researchers looked for correlations between wind shear, other scientists’ estimates of sea-surface temperature, and hurricane frequency, they found that wind shear has a much stronger influence in the North Atlantic than surface temperature does. They also found that large variations in hurricane frequency have been the norm, they report in the June 7 Nature.
Overall, between 1730 and 2005, the North Atlantic has experienced an average of 3.25 category-3-or-stronger hurricanes each year, says Kilbourne. However, at least six lengthy intervals since 1730 had hurricane activity comparable to today’s. In general, such boosts in hurricane frequency occurred when wind shear was weak. Most periods of low hurricane activity since 1730 were marked by strong wind shear, she notes. Some of these intervals even occurred when sea-surface temperatures were higher than normal.
Other analyses of long-term natural records bolster the connection between strong wind shear and reduced hurricane frequency, says Jeffrey P. Donnelly, a coastal geologist at Woods Hole (Mass.) Oceanographic Institution.
By studying sediments from a lake in Ecuador and a lagoon in eastern Puerto Rico, he and his colleagues compared the timing of hurricanes during the past 5,000 years with that of El Niños—weather phenomena that increase wind shear over the North Atlantic.
The researchers reported in the May 24 Nature that periods with strong, frequent El Niños experienced a lower-than-average number of hurricanes.
