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January
18, 2007
THE 2006 HURRICANE SEASON WAS NEAR NORMAL
After the record setting season of
2005 with 27 named tropical cyclones, many meteorologists and hurricane
specialists were forecasting another above average hurricane season for
2006,
but it didn't happen. NASA scientists have learned several reasons why.
Although hurricane cycles still
remain somewhat of a mystery that scientists are trying to solve, the
general
ingredients of what causes a hurricane to form are relatively well
known. You
need the winds to behave correctly, sea surface temperatures of at
least 82
degrees Fahrenheit, and no interference from African dust, among other
things.
The Atlantic hurricane season ended on Nov. 30, and wound up having
near-normal
activity with nine named storms, including five hurricanes, two of
which became
major hurricanes of Category 3 strength or higher. An average Atlantic
season
has 11 named storms, with six growing into hurricanes and two becoming
major
hurricanes. Unlike the past three years, in 2006, the stronger
hurricanes
stayed at sea.
In 2006, there were several factors that reduced the number of Atlantic Ocean storms from
forming compared to 2005,
while the eastern Pacific still saw 25 storms develop in 2006. In 2006,
the
Atlantic factors included lower sea-surface temperatures, the switch
from weak
La Nina to weak El Niño conditions and changes in the
large-scale steering flow
of winds associated with the Bermuda High (a semi-permanent high
pressure
system near Bermuda in the Atlantic
that moves
storms around it in a clockwise motion).
Persistent low-level easterly winds may have acted to keep the Atlantic Ocean sea surface
temperatures lower through
mixing of the upper ocean. When winds blow constantly, they push the
warmer
waters away, allowing cooler, deeper waters to come to the surface,
keeping the
ocean's surface cooler.
This past year, the Bermuda High steered systems away from the U.S. by steering them
northward into the cooler
waters of the Central Atlantic Ocean instead of into the warmer waters
of the
Caribbean Sea and the Gulf of Mexico
as it did
to storms in 2004 and 2005.
While dust activity from Africa
was somewhat
greater than in 2005, it probably played a much smaller role than these
other
factors.
Scott Braun, hurricane specialist
at NASA's Goddard
Space Flight
Center, Greenbelt,
Md. noted that the
sea surface temperatures in
the Atlantic Ocean
were much closer to normal
this year, and considerably cooler than last year. "Sea surface
temperatures in the Atlantic
were unusually
warm in 2005," Braun said. "Sea-surface temperature data from the
Tropical Rainfall Measuring Mission (TRMM) satellite showed that ocean
temperatures were up to two degrees warmer in 2005.
TRMM observed surface winds were persistently strong over the tropical Atlantic in 2006 while in 2005
they were somewhat weaker
and more variable. The stronger winds in 2006 likely helped to keep
temperatures cooler by mixing the upper layer of the ocean." The
persistent winds were a key in keeping the Atlantic waters cooler than
they
were in 2005. As the winds kept blowing, they swept the warmer water
west, and
it was replaced by cooler waters from below the surface. Hurricanes
need warm
waters of at least 82 degrees Fahrenheit to form and strengthen. The
winds
prevented those warmer conditions from happening. In addition, Braun
noted that
those winds may have also been indicative of strong vertical winds
called
"shear" that rip storms apart.
The Bermuda High Was Different
The Bermuda High is a fairly stationary high-pressure system that sits
near
Bermuda in the western Atlantic Ocean.
Surface
high pressure develops when air is being forced down from above. That
prevents
the formation of storms and usually brings sunshine to an area that
sits
underneath it. However, the "Bermuda High" is always strong enough to
act as a guide for the path hurricanes take when they move from the
eastern
Atlantic and head west toward the Caribbean and the U.S.
and storms go around it in a
clock-wise direction.
The position and the size of the Bermuda High is a key to where storms
will go.
In 2004 and 2005 the Bermuda High was situated more to west-southwest,
so it
steered systems west into the Caribbean and Gulf
of Mexico's
warmer waters. The warmer waters fueled the storms and strengthened
many storms
including Katrina, Rita and Wilma in 2005. This year, 2006, the Bermuda
High
was smaller and shifted to the eastern Atlantic, so it turned storms
like Florence,
Gordon, Helene and Isaac north into cooler
waters of the central Atlantic, and directed them eastward into colder
waters
in the North Atlantic Ocean.
The El
Niño Factor
The current El Niño started developing in July, 2006.
National Oceanic and
Atmospheric Administration (NOAA) scientists said that this hurricane
season's
activity was lower than expected due to the rapid development of El
Niño—a
periodic warming of the ocean waters in the central and eastern
equatorial
Pacific, which influences pressure and wind patterns across the
tropical Atlantic.
NOAA scientists noted that the El Niño produced a sinking
motion in the middle
and upper atmosphere and increased wind shear in the Caribbean.
Those conditions minimized thunderstorm activity from the western coast
of
Africa across the Atlantic to the Caribbean,
and suppressed tropical storm formation. They also believe that a
strong low
pressure system with enhanced westerly winds in the jet stream across
the
eastern U.S.
helped to steer hurricanes out to sea.
Oceanographer Bill Patzert, of NASA's Jet Propulsion Laboratory, Pasadena,
Calif.
said "Forecasting the hurricane season in the spring is a high-risk
business. This is because much can happen during the summer. Over the
summer of
2006, a late-developing, modest El Niño arrived to shift
upper level winds.
Over the past few decades these forecasts have really improved, but
there is
still much we don't understand about these great storms. Looking to the
future,
NASA and NOAA scientists are on the case and, I'm confident that more
reliable
forecasts are in our near future."
The African Dust Effect
African dust was found to "contaminate" several fledgling storm
systems emerging off western Africa,
according
to Jeffrey Halverson, research meteorologist at the University of
Baltimore
County, Baltimore. Halverson participated in the NASA African Monsoon
Multidisciplinary Analyses (NAMMA) hurricane research mission in the Cape Verde Islands
off Africa's west
coast this past summer.
During the mission scientists from NASA, the National Oceanic and
Atmospheric
Administration and universities looked at how African dust affects
storms.
Data from NASA's Moderate Imaging Spectroradiometer (MODIS) that flies
aboard
NASA's Terra and Aqua satellites revealed that both 2006 and 2005 had
active
dust storm seasons. Although 2006 was somewhat more active, and the
dust played
more of a role in whether or not individual storms were able to
intensify,
rather than limiting the overall number of storms that formed during
the entire
season.
The dust plumes contain very dry air, and also had high wind shear
(winds that
blow storms apart). Both of these factors knock the proverbial wind out
of a
storm's sails. Halverson said that out of the 12 or so seedling storms
coming
off the African west coast several had a veil of dust associated with
them,
which hindered their development.
The African dust storms happen because there's a big temperature
difference
over the continent, between the tropical, moist, vegetated areas in the
south
and the very hot, dry desert in the north. That actually helps to
create some
of these dust storms which can be the size of the continental U.S.
and as
much as 3 miles high into the atmosphere.
More Storms Made Landfall in Mexico in
2006
Although the Atlantic Ocean had a near normal hurricane season in 2006,
the
eastern Pacific had a greater number of named storms than occurred in
2005, and
more of them made landfall in Mexico.
Braun said that the steering currents in the eastern Pacific were
responsible
for bringing several storms into Mexico
in 2006. "Normally in
any given year, there is some percentage of storms that turn northward
into Mexico,"
he said. "In 2005 there were 17 storms in the Eastern Pacific, while in
2006 there were 25 storms, so there was a greater chance for more
storms to
steer into Mexico
this year."
Carlotta, John and Paul were three of the Eastern Pacific storms that
affected Mexico
in the
2006 season. Carlotta was a category 1 hurricane on the Saffir-Simpson
Hurricane Scale. She brought modest coastal rains but otherwise did not
affect
land. Hurricane John struck southern Baja California
as a category 2 hurricane and was
responsible for five deaths.
Hurricane Paul was a category two hurricane over the open waters of the
eastern
North Pacific. Paul weakened to a tropical storm as it passed south of Baja California, then made
landfall along the coast of
mainland Mexico
near the
southern end of Isla Altamura and later dissipated inland over mainland
Mexico.
Paul
caused significant rainfall and floods in the Mexican state of Sinaloa
and was
responsible for four deaths.
The primary factors that made 2006 so different from 2006 were changes
in sea
surface temperatures in the Atlantic
Ocean,
the developing El Niño and changes in large-scale winds like
the Bermuda High.
African dust is being intensively investigated and is thought to be a
secondary
factor that mostly affected the strengthening of storms.
Although the Atlantic Ocean experienced a much calmer hurricane season
in 2006
than it did in 2005, some of the factors that made it that way may or
may not
be in place during the 2007 season, and NASA scientists will be keeping
a close
eye on the Atlantic.
For
more information and images, visit:
http://www.nasa.gov/mission_pages/hurricanes/archives/2006/normal_2006.html
For more
information
on the NAMMA research mission, visit:
http://namma.msfc.nasa.gov/
Feature
story on El
Nino and the hurricane season:
http://www.noaanews.noaa.gov/stories2006/s2748.htm
##
Contact:
Rob Gutro
NASA
Goddard Space Flight Center
301-286-4044
Robert.J.Gutro@nasa.gov
This text is
derived from:
http://www.nasa.gov/mission_pages/hurricanes/archives/2006/normal_2006.html
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