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| Sahara dust across Atlantic, NASA, June 13-21, 2001 |
10/30/2006, "The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest," Environmental Research Letters, iopscience.iop.org, Ilan Koren1, Yoram J Kaufman2, Richard Washington3, Martin C Todd4, Yinon Rudich1, J Vanderlei Martins2,5,6 and Daniel Rosenfeld7
"Abstract
About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodélé depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodélé emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day....
3. Discussion
Using satellite data and reanalysis wind fields we have identified a remarkable connection between the Amazon forest and a single dust source in the Sahara: the Bodélé depression and its wind regime. A unique combination of global wind pattern and topography forms a vigorous dust source that emits an average of more than 0.7 Tg of dust per emission day and is active mostly during the winter–spring, which is different from most other Saharan dust sources. We estimate that between November and March, the Bodélé depression sends more than half of the dust that is deposited annually in the Amazon forest. Our direct measurements are consistent with a recent modelling study showing that the Bodélé is responsible for (more than) 40% of dust optical depth over the Amazon in the winter season [25]. The soil of the Amazon tropical rainforest is shallow, poor in nutrients and almost without soluble minerals. Heavy rains have washed away the nutrients in the soil obtained from weathered rocks. The rainforest has a short nutrient cycle, and due to the heavy washout, a stable supply of minerals is required to keep the delicate nutrient balance [26]. Kimmins [27] showed that any change in the nutrient supply will convert tropical forests to 'wet deserts'...
1. Introduction
Satellite observations show continuous dust transport across 5000 km from the Saharan sources to the Caribbean Sea and North America in the Northern summer and to the Amazon basin during the Northern winter [1, 2]. Due to the annual cycle in winds over the Sahara, the winter Saharan dust sources are different from the summer sources [3]. In the summer, dust fluxes reaching the Tropical Atlantic shore originate mainly from the northwest and central-west parts of the Sahara. During winter, strong surface winds (the Harmattan winds) occur along the southern border of the Sahara, activating sources on the border of the Sahel, notably the Bodélé depression in Northern Chad."...
"1 Department of Environmental Sciences, Weizmann Institute, Israel
2 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
3 Climate Research Lab, Oxford University Centre for the Environment, UK
4 Department of Geography, University College London, UK
5 Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, USA
6 Institute of Physics, University of Sao Paulo, Brazil
7 Institute of Earth Sciences, The Hebrew University, Jerusalem 91904, Israel"...
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Animated image above from NASA article, 6/26/2001, showing Sahara dust blowing across Atlantic, "All the World's a Stage...for Dust," science.nasa.gov, Sahara dust blows across Atlantic, June 13-21, 2001. (Not provided with ERL 2006 study above).
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NASA map below shows China dust blowing across the Pacific (right) to the US (left). A 2001 Asia dust cloud blew completely across North America and continued into the Atlantic Ocean:
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| China dust to US, NASA animation, April 2001 |
6/26/2001, "All the World's a Stage...for Dust," science.nasa.gov:
"This TOMS aerosol movie [above post], which spans the interval June 13 through 21, 2001, shows a cloud of African dust crossing the Atlantic and raining bits of the Sahara Desert over the Caribbean. Click on the image to view a larger 450 kb animation, which includes a color table and legend. See also the latest TOMS maps and more information about the color-coded aerosol index....
Airborne microbes and pollen in Florida or Brazil might have come from Africa. Mineral dust in the soils of India could have blown over from Iran....
The Americas frequently inherit huge volumes of dust from Africa and Asia, while countries of the Far East like India and China get sprinkled with dust from the Middle East.
African dust plumes begin their trans-Atlantic journey with storm activity in the Sahara Desert region. The dust, originating from fine particles in the arid topsoil, is transported into the atmosphere by winds and may be carried more than 10,000 feet high. Dust clouds cross the Atlantic Ocean and reach the Caribbean and the Americas in about 5 to 7 days....
"In some cases, such as in the Caribbean islands, much of the dust in the top soil is from Africa," says Jay Herman, principle scientist for aerosol detection by TOMS at NASA's Goddard Space Flight Center. "Over generations, lots of it piles up there. The top soil has been analyzed on these islands, and it's definitely of African origin."
Northern spring also brings massive dust clouds blowing east over the Pacific Ocean from China and Mongolia. The largest Asian dust cloud on record blew all the way across North America earlier this year (2001), traveling as far east as the mid-Atlantic Ocean before finally disappearing from view."...
"These springtime dust storms give visible evidence of a "conveyor belt in the sky" that ferries air from Asia to North America in the spring. Invisible pollutants are also carried across the Pacific Ocean by this conveyor, according to Daniel Jaffe, professor of atmospheric sciences at the University of Washington.
Based on air quality measurements at Cheeka Peak at the westernmost tip of Washington state and later airplane-based measurements, Jaffe's research group has shown that a steady trickle of air pollution comes across the Pacific from Asia -- at least in the spring -- punctuated by a surge of pollutants once or twice a month.
These surges are presumably due to the wholesale movement of air from Asian urban areas across the Pacific. This prevents the pollutants from being diluted by mixing with cleaner air. During these surges, the air entering the West Coast can have pollution concentrations as high as 75 percent of federal air quality standards, Jaffe said. "...
(continuing): ""The huge amount of dust blowing off Asia recently is kind of a new phenomenon," Herman says.
"It's apparently due to increased desertification brought about by a change in the dryness of the area and increased land use."
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The raining of dust into the Pacific Ocean may be important for the productivity of fisheries in the North Pacific, an important source of seafood for much of the world. Photosynthetic microbes in the top layer of the ocean form the foundation of the food chain. Often the concentration of dissolved mineral nutrients (particularly iron) controls the abundance of these microbes. Some of the minerals in the dust might help boost microbe populations and, therefore, fish populations. Scientists are still working to verify this effect, however.
Airborne dust clouds can also affect local weather by suppressing rainfall. Cloud droplets form around the minute dust grains, but some kinds of dust limit how big these droplets can become. Because droplets must reach a certain size before they'll fall to Earth as rain, dust clouds can leave dry weather in their path.
In addition to dust, the TOMS instrument detects volcanic ash, pollutants, smoke, ozone concentrations and ultraviolet light intensities. Most of these data are available online in near real-time....
A new TOMS instrument is scheduled to launch on August 10 aboard the QuikTOMS satellite. This satellite will replace the aging TOMS instrument currently in orbit, and while both instruments are functional, the redundant data will help NASA scientists to calibrate the new one. QuikTOMS's expected mission life is 3 years.
Thanks to TOMS and its successor, we'll continue to have a front row seat on the global stage of migrating dust for years to come."
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Caption for animated image at top of this post above 6/26/2001 article: In addition to Sahara dust blowing across the Atlantic, "This TOMS image shows a record-setting Asian dust cloud beginning its journey east across the Pacific. Click on the image to view a 440 kb animation of the dust cloud migrating to North America. Remember that as the dust disappears from the satellite's view, it's raining out of the air onto the ocean and the land. See a similar Asian dust cloud from 1998 in this animation [still image below]:"
Above image, 1998 dust cloud blowing from China to the US, NASA, animation.
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Source: China air entering US West Coast can have "pollution concentrations as high as 75 percent of federal air quality standards."
May 17, 2001, "The Pacific Dust Express," science.nasa.gov
"North America has been sprinkled with a dash of Asia! A dust cloud from China crossed the Pacific Ocean recently and rained Asian dust from Alaska to Florida."...
NASA caption for map showing China dust entering US west coast including Alaska: "While much of the dust cloud remains over the Pacific Ocean, the leading edge has reached as far east as the American Midwest by April 13 (2001)," NASA
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