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May 23, 2007
EL
NIÑO AND AFRICAN MONSOON HAVE STRONGLY INFLUENCED INTENSE
HURRICANE FREQUENCY
IN THE PAST
The
frequency of intense hurricanes in the Atlantic
Ocean
appears to be closely connected to long-term trends in the El
Niño/Southern
Oscillation (ENSO) and the West African monsoon, according to new
research from
the Woods Hole Oceanographic Institution (WHOI). Geologists Jeff
Donnelly and
Jonathan Woodruff made that discovery while assembling the longest-ever
record
of hurricane strikes in the Atlantic basin.
Donnelly and Woodruff began reconstructing the history of land-falling
hurricanes in the Caribbean in 2003 by gathering sediment-core samples
from
Laguna Playa Grande on Vieques (Puerto
Rico),
an island extremely vulnerable to hurricane strikes. They examined the
cores
for evidence of storm surges—distinctive layers of
coarse-grained sands and
bits of shell interspersed between the organic-rich silt usually found
in
lagoon sediments—and pieced together a 5,000-year chronology
of land-falling
hurricanes in the region.
In examining the record, they found large and dramatic fluctuations in
hurricane activity, with long stretches of frequent strikes punctuated
by lulls
that lasted many centuries. The team then compared their new hurricane
record
with existing paleoclimate data on El Niño, the West African
monsoon, and other
global and regional climate influences. They found the number of
intense
hurricanes (category 3, 4, and 5 on the Saffir-Simpson scale) typically
increased when El Niño was relatively weak and the West
African monsoon was
strong.
"The processes that govern the formation, intensity, and track of
Atlantic
hurricanes are still poorly understood,” said Donnelly, an
associate scientist
in the WHOI Department of Geology and Geophysics. “Based on
this work, we now
think that there may be some sort of basin-wide ‘on-off
switch’ for intense
hurricanes.”
Donnelly and Woodruff published their latest results in the May 24
issue of the
journal Nature.
Donnelly and his colleagues have pioneered efforts to extend the
chronology of
hurricane strikes beyond what can be found in historical texts and
modern
meteorological records and previously applied their methods to the New
England
and the Mid-Atlantic coasts of the United States.
Their research area, Laguna Playa Grande, is protected and separated
from the
ocean during all but the most severe tropical storms. However, when an
intense
hurricane strikes the region, storm surges carry sand from the ocean
beach over
the dunes and into Laguna Playa Grande. Such
“over-topping” events leave
markers in the geological record that can be examined by researchers in
sediment core samples.
The geological record from Vieques showed that there were periods of
more
frequent intense hurricanes from 5,000 to 3,600 years ago, from 2,500
to 1,000
years ago, and from 1700 AD to the present. By contrast, the island was
hit
less often from 3,600 to 2,500 years ago and from 1,000 to 300 years
ago.
To ensure that what they were seeing was not just a change in the
direction of
hurricanes away from Vieques—that is, different storm tracks
across the
Atlantic and Caribbean—the scientists compared their new
records with previous
studies from New York
and the Gulf
Coast.
They saw that the Vieques record matched the frequency of land-falling
hurricanes in New York
and Louisiana,
indicating that some
Atlantic-wide changes took place.
Donnelly and Woodruff, a doctoral student in the MIT/WHOI Joint
Graduate
Program, then decided to test some other hypotheses about what controls
the
strength and frequency of hurricanes. They found that periods of
frequent El
Niño in the past corresponded with times of less hurricane
intensity. Other
researchers have established that, within individual years, El
Niño can stunt
hurricane activity by causing strong winds at high altitudes that shear
the
tops off hurricanes or tip them over as they form. When El
Niño was less active
in the past, Donnelly and Woodruff found, hurricane cycles picked up.
The researchers also examined precipitation records from Lake Ossa, Cameroon,
and discovered that when there were increased monsoon rains, there were
more
frequent intense hurricanes on the other side of the Atlantic.
Researchers have theorized that frequent and stronger storms over
western
Africa lead to easterly atmospheric waves moving into the Atlantic
to provide the “seedlings” for hurricane
development.
Much media attention has been focused recently on the importance of
warmer
ocean waters as the dominant factor controlling the frequency and
intensity of
hurricanes. And indeed, warmer sea surface temperatures provide more
fuel for
the formation of tropical cyclones. But the work by Donnelly and
Woodruff
suggests that El Niño and the West African monsoon appear to
be critical
factors for determining long-term cycles of hurricane intensity in the Atlantic.
The research
by
Donnelly and Woodruff 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.
The Woods Hole Oceanographic Institution is a private,
independent
organization in Falmouth, Mass.,
dedicated to marine research,
engineering, and higher education. Established in 1930 on a
recommendation from
the National Academy of Sciences, its primary mission is to understand
the
oceans and their interaction with the Earth as a whole, and to
communicate a
basic understanding of the ocean's role in the changing global
environment.
##
Contact:
Joanne
Tromp
Woods Hole Oceanographic
Institution
508-289-3340
media@whoi.edu
This text
derived from:
http://www.whoi.edu/
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