NASA satellites have provided evidence that the chilling effect of dust was
responsible for one-third of the drop in North Atlantic sea surface
temperatures between June 2005 and 2006, possibly contributing to the
difference in hurricane activity between the two seasons.
Heat from warm ocean surfaces is known to fuel hurricanes, leading to
stronger and more frequent storms. During the hurricane season of
2006, however, sea surface temperatures in the North Atlantic remained
relatively cool and the season saw only five hurricanes, compared to 15
hurricanes in 2005 when the ocean surface was warmer.
Now, William Lau and Kyu-Myong Kim at NASA's Goddard Space Flight
Center, Greenbelt, Md., show that airborne Saharan dust over the
Atlantic was likely responsible for much of the temperature drop,
effectively blocking sunlight from reaching the ocean's surface.
According to their research dust accounted for 30 to 40 percent of
the drop in sea surface temperature between June 2005 and 2006, the
team reported this month in the American Geophysical Union's Geophysical
Research Letters. The finding provides the first quantitative estimate
of dust's role in cooling the entire North Atlantic and brings attention
to dust as a potentially important influence on hurricane activities.
In research published earlier this year, Lau first suggested that
Saharan dust blowing west over the Atlantic blocked sunlight from
reaching the ocean, which caused the ocean surface to cool and led
2006 to be a much weaker hurricane season than 2005. The idea sparked
controversy and scientists questioned the extent of dust's influence
on temperature shifts compared to the influence of El Niño.
"Previous studies have looked at how hot, dry air associated
with a Saharan dust outbreak affects an individual storm, but our
study is the first to focus on dust's radiative effect on sea
surface temperatures, which may affect storms for the entire season.
Nobody had suggested that link before," Lau says.
To investigate the dust and sea surface temperature link,
Lau and Kim calculated the cooling pattern that Saharan dust should
produce on sea surface temperatures from the sun-blocking effect and
compared the results to observed temperature patterns. The NASA team
first collected data from the National Center for Environmental
Prediction to find out how much solar energy enters Earth's
atmosphere and reaches Earth's surface free from the influence
of dust. As the energy radiates down toward the ocean's surface,
however, not all it survives the trip – a portion is either
absorbed or reflected by the dust and clouds.
Knowing the concentration of atmospheric dust as measured by the
Moderate Resolution Imaging Spectroradiometer (MODIS) instrument
onboard a NASA satellite, Lau and Kim calculated the amount of heat
that could penetrate the dust and reach the surface of the North
Atlantic. The team used a computer simulation of the upper ocean to
estimate how much the observed heat reaching the ocean would change
sea surface temperature.
The expected temperature pattern turned out to closely match
observations, supporting a link between dust and sea surface
temperatures. Results showed that the sunlight-blocking effect
of Saharan dust caused a cooling of the North Atlantic of 0.37
to 0.72 degrees Fahrenheit between June 2005 and 2006, approximately
one-third of the total sea surface cooling in the North Atlantic.
Lau and Kim also examined historical dust and sea surface
temperature records and found a strong link indicating cooler North
Atlantic surface temperatures in more dusty years, supporting their
satellite-based estimate.
As dust storms peak early in the hurricane season, researchers might
one day incorporate dust data into forecasts to better predict sea
surface temperatures and possibly the formation of hurricanes. Still,
dust in 2006 accounted for less than half of the sea surface temperature
change, leaving open significant influences from other factors such as
El Niño, surface evaporation and wind.
"We're not saying that El Niño does not have a substantial
influence on sea surface temperatures, but rather that dust is an
important factor that we cannot ignore anymore. The 2007 hurricane
season appears to be another one in which forecasts for an above normal
hurricane season have failed," Lau says.
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A massive dust storm covers hundreds of thousands of square miles of the eastern Atlantic Ocean with a dense cloud of dust. In this scene the dust has reached over 1000 miles from shore. Storms like this can lift the dust up to 15000 feet above Earth's surface. NASA Image.
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In this image the northern coast of South America is visible along with Puerto Rico and the Dominican Republic. The brownish haze over the water is a plume of African Sahara dust. It travelled across the Atlantic in about five days. NASA Image..
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Sea surface temperatures in June 2006 were cooler than the year
before in the North Atlantic, where dust storms from Africa can
stop sunlight from warming the ocean's surface. This global view
created from June 2006 data shows where sea surface temperatures
were cool (blue) and which regions were warm (light pink).
Credit: NASA
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A dust storm blankets the Mediterranean Sea, carrying a heavy haze of particles over Sicily and Greece. NASA Image..
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