Scientists Unearth Long Record of Past Hurricanes Digging down in a Caribbean lagoon, researchers go back in time |
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| Enlarge ImageResearchers extracted and examined cores of sediment from Laguna Playa Grande in Vieques, Puerto Rico, to look for evidence of intense hurricanes. (Photo by Jeff Donnelly, Woods Hole Oceanographic Institution) |
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| Enlarge ImageLaguna Playa Grande (left) sits behind a vegetation-covered barrier beach that is as much as 260 feet (80 meters) wide and 7 to 10 feet (2 to 3 meters) high. Surges from intense storms carry sand from the ocean beach over the dunes and into the lagoon. Such "over-topping" events leave distinctive layers of coarse-grained sands and bits of shell interspersed between the organic-rich silt usually found in lagoon sediments. |
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| Enlarge ImageGraduate student Jonathan Woodruff of the WHOI Geology and Geophysics Department works to sink a coring tube into the sediments beneath Laguna Playa Grande. (Photo by Erin Bryant) |
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Reaching down into the muck below a lagoon off Puerto Rico, two
geologists at Woods Hole Oceanographic Institution reached back 5,000
years to compile the longest record of strong hurricanes in the
Atlantic region.
The record showed that the dominant forces spawning heightened
hurricane activity appeared to be atmospheric conditions generated by
weak El Niños in the tropical Pacific and strong West African monsoons,
Jeff Donnelly and Jon Woodruff reported May 24, 2007, in the journal Nature.
Somewhat to their surprise, they also found extensive periods of
intense hurricanes in the past, when ocean temperatures were cooler
than they are now. Today, concerns about global warming have focused
public attention on warmer ocean waters as a prime suspect for
increased hurricane activity.
“Warm sea surface temperatures are clearly important in fueling intense
hurricanes,” Donnelly said. “Over the past several thousand years,
ocean temperatures have never been as warm as they are now, so we have
no analog to help predict how they will affect hurricane activity.”
“But our research demonstrates that the El Niño/Southern Oscillation
and the West African monsoon are certainly important,” he said.
“Understanding how they will change in a warming world could be
extremely important in determining the kind of hurricane activity we
will see in the future.”
Donnelly, an associate scientist at WHOI, and Woodruff, a doctoral
student in the MIT/WHOI Joint Graduate Program, began reconstructing
the history of Caribbean hurricanes in 2003, by driving 13-foot
(4-meter) cores through the bottom of Laguna Playa Grande on Vieques,
Puerto Rico. The lagoon is protected and separated from the ocean by
dunes during all but the most severe tropical storms. But storm surges
from intense hurricanes carry sand and bits of shells from the ocean
beach over the dunes and into the lagoon, leaving telltale layers in
the typically black, organic-rich silt on its bottom.
The interspersed layers of silt and sand recorded long stretches of
frequent hurricane strikes punctuated by lulls that lasted many
centuries. The team then compared their new hurricane record with other
climate influences, such as El Niño, the periodic diminishing winds and
buildup of warm waters in the eastern tropical Pacific. Other
researchers have established that El Niño can stunt hurricane activity
by causing strong high-altitude winds that shear the tops off
hurricanes or tip them over as they form.
The researchers also examined precipitation records from Lake Ossa,
Cameroon, and discovered that when monsoon rains increased, intense
hurricanes occurred more often on the other side of the Atlantic.
Researchers have theorized that storms over western Africa generate
atmospheric waves that move into the Atlantic and provide “seedlings”
for hurricane development.
“If we have few El Niño events and a strong West African monsoon,
combined with exceedingly high sea surface temperatures, we could
experience an active hurricane period that is unprecedented in the last
5,000 years,” Donnelly said. “Conversely, if we have more steady-state
El Niño conditions, it may reduce—but not stop—intense hurricane
activity in a warmer world.”
The research was funded by the
National Science Foundation, the Risk Prediction Initiative, the
National Geographic Society, the WHOI Coastal Ocean Institute, and the
Andrew W. Mellon Foundation.
Posted: June 13, 2007 [top] |
