March 28, 2007
DID DUST BUST THE 2006 HURRICANE
SEASON FORECASTS?
A recent NASA study suggests that tiny
dust particles may have foiled forecasts that the 2006 hurricane season
would
be another active one.
In
June and July 2006, there were
several significant dust storms over the Sahara
Desert in Africa.
As this dust traveled westward into the Atlantic,
satellite data show that the particles blocked sunlight from reaching
the ocean
surface, causing ocean waters to cool. These cooler waters may have
impeded
some storminess since hurricanes rely on warm waters to form.
The
2006 Atlantic hurricane season
wrapped up on Nov. 30 with just four tropical storms and five
hurricanes,
relatively calm compared to the record number of 12 tropical storms and
15
hurricanes in 2005.
While
several factors likely
contributed to the sharp decrease in the number of storms, "this
research
is the first to show that dust does have a major effect on seasonal
hurricane
activity," said lead author William Lau, chief of the Laboratory for
Atmospheres at NASA's Goddard Space Flight Center, Greenbelt, Md. "Dust
concentrations may play as big a role as other atmospheric conditions,
like El
Niño, and offer some predictive value, so they should be
closely monitored to
improve hurricane forecasts."
Other
researchers, however, say that atmospheric dust may
have had relatively little influence on the 2006 hurricane season
compared to the
effects of underlying El Niño conditions.
Sea
surface temperatures in 2006
across the prime hurricane-breeding regions of the Atlantic and Caribbean were found to be as
much as 1 degree Celsius
(1.8 degrees Fahrenheit) cooler than in 2005. Most striking was how
quickly sea
surface temperatures responded to variations in the amount of Saharan
dust, Lau
said. Following the most significant dust outbreak, which occurred in
June and
July, ocean waters cooled abruptly in just two weeks, suggesting that
the dust
had an almost immediate effect.
The
dust worked to cool the ocean,
but it also warmed the atmosphere by absorbing more of the sun's
energy. This
temperature difference resulted in a shift in the large-scale
atmospheric
circulation. As air rose over West Africa and the tropical Atlantic, it
sank and
became less moist over the western Atlantic and Caribbean.
This pattern helped to increase surface winds that enhanced ocean
evaporation and
churned deeper, colder waters, causing the area of cool seas to expand.
Lau
and co-author Kyu-Myong Kim of
Goddard analyzed data on ocean temperatures, clouds, and water vapor
from
NASA's Tropical Rainfall Measuring Mission satellite and atmospheric
dust
levels from the Ozone Monitoring Instrument on NASA's Aura satellite.
The study
was published in the Feb. 27 issue of the American Geophysical Union's Eos.
The
research also considered the
role of El Niño by examining historical data on the
intensity and development
of tropical storms and hurricanes across the Atlantic basin. "We found
that Saharan dust may have a stronger influence than El Niño
on hurricane
formation in the subtropical western Atlantic and Caribbean, but that
El Niño
has a greater impact in the tropical eastern Atlantic,
where many storms are generated," said Lau.
El
Niño is the periodic warming of
the ocean waters in the central and eastern equatorial Pacific, which
in turn
can influence pressure and wind patterns across the tropical Atlantic.
"In
2006, it is quite
possible that the Saharan dust may have amplified or even initiated
pre-existing
atmosphere-ocean conditions due to El Niño," said Lau. But
other researchers
say that while the amount of atmospheric dust in 2006 was greater than
in 2005,
the increase may have been too insignificant to be influential on the
season. Instead,
they believe the atmospheric effects from the underlying El
Niño pattern in
2006 likely played a greater role.
Scott
Braun, a hurricane
specialist at NASA's Goddard Space Flight Center, said that in 2006, El
Niño brought
about broad changes to atmospheric conditions that likely had at least
some
influence on hurricane formation across much of the Atlantic.
Braun
noted that during most of
the hurricane season a large area of high pressure was located across
the
eastern Atlantic.
This steered disturbances
away from the warmest waters, so that they were less able
to mature into
tropical storms and hurricanes. At the same time, sinking motion
– an
atmospheric air mass that has cooled and is falling –
combined with enhanced
winds in the middle and upper atmosphere to minimize development in the
Caribbean
and western Atlantic and keep storms away from the U.S. These strong
upper-level
winds would contribute to a drastic change of winds with height, known
as
"shear" that can rip storms apart.
"This
large-scale pattern has
been associated with the effects of El Niño, suggesting it
may have played a
role in the seasonal activity," said Braun. "In fact, the last time
the Atlantic
produced so few storms was in
1997, when an El Niño pattern was also in place."
Braun
and another hurricane
researcher, Bowen Shen at NASA's Goddard Space Flight Center, agree
that factors
other than increased atmospheric dust may have contributed to cooler
ocean
waters in 2006.
"It
is arguable that stronger
surface winds over the tropical Atlantic
may
have cooled sea surface temperatures," said Shen. These winds likely
helped
to keep waters cooler by mixing the upper layers of the ocean and
sweeping
warmer waters westward. And although the waters were certainly cooler
in 2006
than in 2005, they were still at or slightly above normal, suggesting
other
conditions helped to shape the season.
Current
and future research efforts that examine how the
ocean responds to surface winds and dust should help clarify their role
in
hurricane development. Although seasonal atmospheric patterns may
increase the
amount of dust across the Atlantic,
the same
atmospheric patterns may also be responsible for creating stronger
winds at the
ocean surface. By modeling the oceans, winds, and dust, researchers
will generate
a clearer picture of how these conditions vary from season to season.
"Although we
continue to make significant strides in
forecasting hurricanes and understanding their development, it is
important to
remember that the atmosphere is a chaotic system and numerous
meteorological
variables influence individual storms and activity throughout the
season. NASA's
constellation of several Earth-observing satellites, including Aura, is
designed to provide coordinated measurements of these many variables
for future
research," said Lau.
For
more information and images,
visit:
http://www.nasa.gov/mission_pages/hurricanes/archives/2007/hurricane_dust.html
http://www.nasa.gov/vision/earth/environment/hurricane_dust.html
Writer: Mike Bettwy, NASA Goddard Space Flight Center
##
Contact:
Lynn Chandler
NASA
Goddard Space Flight Center
301-286-2806
This text is
derived from:
http://www.nasa.gov/mission_pages/hurricanes/archives/2007/hurricane_dust.html
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