Bodélé Depression

Coordinates: 16°57′22.4″N 17°46′51.2″E / 16.956222°N 17.780889°E / 16.956222; 17.780889
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Bodélé depression
)
Bodélé Depression
MODIS) flew over on NASA's Aqua satellite. The full-sized image has a resolution of 250 meters per pixel.
LocationNorth Central Africa
AgeFew thousand years
Formed byDrying up of Lake Chad

16°57′22.4″N 17°46′51.2″E / 16.956222°N 17.780889°E / 16.956222; 17.780889

NASA World Wind
to complement the MODIS imagery and provide coordinates that make this important geographic feature easier to locate.

The Bodélé Depression (pronounced

above sea level. The dry endorheic basin is a major source of fertile dust essential for the Amazon rainforest
.

ephemeral lakes, many of which were last filled during wetter periods of the Holocene
.

Diatoms from these fresh water lakes, once part of the prehistoric Mega-Lake Chad, now make up the surface of the depression and are the source material for the dust,[2] which, carried across the Atlantic Ocean, is an important source of nutrient minerals for the Amazon rainforest.

Drying up of Lake Chad

tonnes of dust each day (Todd et al., 2007).[5]

Bodélé Depression, showing how mountains cause a low level jet "windtunnel" to be formed

Research published in the 25 March 2004 edition of Geophysical Research Letters, which used images taken by the Moderate Resolution Imaging Spectroradiometer (MODIS), aboard NASA's Terra and Aqua satellites, indicated that storms move across the Bodélé Depression at about 47 km/h (29 mi/h)—two times faster than previously believed. The research also found that winds have to whip across the region at a minimum of 36 km/h (22 mi/h), to kick up a dust storm.[2][6] The pattern of air flow is so common that the winds have scoured a straight path in the ground, marking its southwesterly flow.[3]

Complementary research published in

Ennedi massif, which lie 2600 m and 1000 m above the flat terrain in the Djourab Desert of Chad, respectively. The effect of the Tibesti massif is clearly evident in creating a split in the low-level easterly flow north and south of these mountains. While the jet feature is pronounced over the Bodélé, it is absent from other longitudes over west Africa along 18 N. It is therefore a feature which uniquely overlies the greater Bodélé region, downwind of the mountains of Chad.[7]

The Bodélé Low Level Jet undergoes a marked seasonal cycle. It is active in the October–March period and relatively inactive from June–August. This timing closely matches the seasonality of dust emission from the Bodélé. Individual dust storms in the Bodélé were also shown in this research to coincide with a major strengthening of the Bodélé Low Level Jet which, in turn, is associated with the ridging of the Libyan High, a feature of the subtropical High Pressure belt.[2][7]

The same researchers who in 2004 more accurately determined the speed of wind through the depression also published in 2006 work showing that more than half of the dust needed for fertilizing the

east coast of the United States originates from a single source—the Bodélé depression.[11]

Bodélé Dust Experiment

In February 2005 the first field experiment, the Bodélé Dust Experiment or BoDEx 2005, was carried out in the Bodélé Depression. The experiment measured the surface winds and near surface winds, dust concentrations, and the influence of dust on the radiation budget in the Bodélé Depression for the first time.[2] This work coincided with a major dust emission event during which the Bodélé Low Level Jet sustained surface wind speeds of around 16 m/s. The core of the Bodélé Low Level Jet was also mapped for the first time from the wind data, and was shown to undergo a very marked diurnal cycle with peak winds occurring mid morning. During night time, the Bodélé Low Level Jet flows over a near-surface inversion, but quickly mixes down to the surface a few hours after sunrise once the intense surface heating induces turbulence in the lowest layers.

Dust from the Bodélé may be seen as a simple coincidence of two key requirements for deflation: strong surface winds and erodible sediment. But recent research has argued that long-term links exist between topography, wind, deflation and dust, and that topography acts as the controlling agent ensuring the long term maintenance of this source. The spatial co-location of strong winds and dust is not simply fortuitous, but results from a set of processes. Specifically:

  • Contemporary deflation from the Bodélé is delineated by topography, such that wind stress, the maximum in dust output and the topographic depression are colocated.
  • The topography of the Tibesti and Ennedi Mountains plays a key role in the generation of the Bodélé Low Level Jet.
  • Enhanced deflation from a stronger Bodélé Low Level Jet during drier phases, such as the Last Glacial Maximum, was probably sufficient to create or enhance a shallow lake populated by diatoms during wetter phases, such as the Holocene pluvial.

Wind conditions which deflate the erodible sediment now may have created the depression necessary for generating the erodible

diatomite in the past. Instead of a simple coincidence of nature, the world's largest dust source results from a system of processes operating over paleo timescales.[12]

The largest town associated with the Bodélé dust source is Faya-Largeau (17°55′00″N 19°7′00″E / 17.91667°N 19.11667°E / 17.91667; 19.11667), located just to the northeast of the depression.[13]

References

  1. ISSN 0169-555X
    .
  2. ^ a b c d e Washington R, et al. Dust and the Low Level Circulation over the Bodélé Depression, Chad: Observations from BoDEx 2005, J. Geophys. Res.Atmospheres Vol. 111, No. D3, D03201.
  3. ^ a b c "Dust Storms from Africa's Bodélé Depression Archived October 5, 2006, at the Wayback Machine". Natural Hazards, Earth Observatory, NASA. URL accessed 2006-12-29.
  4. ^ "Dust in the Bodélé Depression[dead link]". Natural Hazards, Earth Observatory, NASA. URL accessed 2006-12-29.
  5. ^ Fisher, Richard. "Amazon forest relies on dust from one Saharan valley". NewScientist Environment online. 3 Jan 2007. URL accessed 2007-01-04.
  6. S2CID 130653223
    . Retrieved 2007-01-01.
  7. ^ a b "Washington R., and Todd M.C., Atmospheric Controls on Mineral Dust Emission from the Bodélé Depression, Chad: The role of the Low Level Jet, Geophysical Research Letters 32 (17): Art. No. L17701, 2005.
  8. ^ "Dust to gust". EurekAlert!. AAAS. 28 Dec 2006. URL accessed 2006-12-29.
  9. .
  10. . Retrieved 1 November 2013.
  11. ^ "New Earth Science Journal Highlights the Work of Climate and Radiation Branch Scientists". Climate News. NASA GSFC. 1 Nov 2006. URL accessed 2006-12-29.
  12. , 2006.
  13. ^ Prospero, Joseph M et al. "Environmental Characterization of Global Sources of Atmospheric Soil Dust Identified with the Nimbus 7 Total Ozone Mapping Spectrometer (Toms) Absorbing Aerosol Product Archived September 26, 2006, at the Wayback Machine". Reviews of Geophysics. 40(1). Feb 2002.


External links