Storm Norms: Caribbean Corals and Sediments Yield Clues to Hurricane Frequency
Perkins, S., Science News
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 seasurface 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. …