SeaWiFS Dust to Dust
The north Atlantic Ocean borders one of the most
active arid and semi-arid regions in the world: the northwest
African continent, home of the Sahara and Sahel deserts and
source of the majority of dust storms
that are frequently transported aloft for many thousands of miles
across the tropical Atlantic.
The most intense dust storm activity is generally considered to occur
from late spring to late summer. However, dust outbreaks
can also take place during the non-dusty season, such as was the
case on February 26, 2000 (image at top). SeaWiFS
captured this massive dust cloud and monitored its spatial and temporal
evolution as it moved over the Atlantic Ocean. Most of the dust was carried
southwestward toward South America, but a portion of the dust cloud was
transported northward toward Britain and eventually toward the east coast of the United States.
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The figure above shows the aerosol radiance at 865 nm for the same dust storm. A partial cloud mask (white) has been applied. The color palette shows
the corresponding radiance values.
On March 11 the dust in this dust plume reached the northeastern coast of
South America. The image below was acquired at the HRPT receiving station at
the University of Puerto Rico in Mayaguez. The bright specks of color that can
be seen in this image (about halfway down) are due to brief loss of signal from
the spacecraft while the data was being acquired. The curved edges are the
boundaries of the SeaWiFS scanning range. Country borders and the coastline
have been mapped on this image.
Dust particles from such plumes are one type of
"natural" component (other components are sea salt and volcanic
ash) that comprise atmospheric aerosols. The other
constituents are primarily anthropogenic in nature, and are
ejected into the atmosphere as a result of fossil fuel combustion
or biomass burning. Aerosols in general, and dust in particular,
are of growing concern because of their potential influence on climate.
As dust can both directly (by absorbing and scattering solar radiation)
and indirectly (by altering cloud properties) affect climate,
global coverage monitoring the intensities of dust storms and
their spatial/temporal patterns is of great interest.
There are several other reasons to study aeolian events such as
these. One reason is that dust particles, which are derived from the Earth's
crust, can contain elements such as iron which are known to
be the limiting micronutrient to phytoplankton growth
in some parts of the world's open ocean regions. Although the
Atlantic Ocean is generally not considered to be
iron-limited, uptake of iron by surface-residing phytoplankton
can result in increased rates of carbon dioxide assimilation,
which in turn may result in climate change effects.
Aeolian aerosols may also be viewed as a transport vehicle by which other
substances are carried long distances. For example, pollution products,
pathogens, or agronomical pests can be brought from one part of the world to
another, all within a matter of a few days. The increased appearance of
certain diseases has raised the question of a possible link to alterations in
climate such as prolonged periods of rainfall or drought. (Harvard Medical School
Center for Health and the Global Environment) Also, studies are underway
to test the hypothesis that dust storms emanating from the African continent
and crossing the Atlantic may in fact carry pathogens or other substances
that are responsible for the demise of some coral reefs in the Caribbean. (USGS Center for Coastal
Geology) And finally, there may even be a connection between dust storm
activity in the Sahel and hurricane intensity in the Atlantic. (Third World
Foundation)
Although the main mission of SeaWiFS is to look at ocean
color, this sensor is clearly capable of monitoring large-scale
dust and pollution plumes on a global scale, thus providing
a platform from which ocean-atmosphere interactions may be
studied.
(Text and images contributed by Dr. Petra Stegmann,
University of Rhode Island Graduate School of Oceanography. (Dr. Stegmann is now with the NOAA Pacific Fisheries Environmental Laboratory.) Additional images
provided by Norman Kuring, SeaWiFS Project, NASA Goddard Space Flight Center.
The "Ocean Color Spectrum" article in the Summer 2000 issue of
Backscatter magazine, published by the Alliance for Marine Remote Sensing,
also discussed aerosol dust monitoring.)
Saharan dust outbreak over the Canary Islands observed on June 22, 1998. On the following day, atmospheric dust collectors located at Izaña on the island of Tenerife recorded the highest dust load ever measured.
The Mars Global Surveyor captured an image of
a dust storm off of the northern polar cap on August 29, 2000. The shape
of this Martian dust storm is remarkably similar to the shape of the Saharan
dust storm observed by SeaWiFS in late February 2000.
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