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The Bodélé Depression (also Bodele), located at the southern edge of the Sahara Desert in north central Africa, is the lowest point in Chad. Dust storms from the Bodélé Depression occur on average about 100 days per year,〔 one typical example being the massive dust storms that swept over West Africa and the Cape Verde Islands in February 2004.〔"(Dust Storms from Africa's Bodélé Depression )". ''Natural Hazards'', Earth Observatory, NASA. URL accessed 2006-12-29.〕〔"(Dust in the Bodélé Depression )". ''Natural Hazards'', Earth Observatory, NASA. URL accessed 2006-12-29.〕 As the wind sweeps between the Tibesti and the Ennedi Mountains in Northern Chad, it is channeled across the depression. The dry bowl that forms the depression is marked by a series of ephemeral lakes, many of which were last filled during wetter periods of the Holocene. Diatoms from these fresh water lakes, once part of Mega-Lake Chad, now make up the surface of the depression and are the source material for the dust,〔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.〕 which, carried across the Atlantic Ocean, is an important source of nutrient minerals for the Amazon rainforest. ==Drying up of Lake Chad== As the Sahara dried out over the last few thousand years, Mega-Lake Chad receded to the current position of Lake Chad in the south-west corner of Chad. In the mid-1960s, Lake Chad was about the size of Lake Erie. But persistent drought conditions associated with the great Sahel drought coupled with increased demand for fresh water for irrigation have reduced Lake Chad to about 5 percent of its former size. As the waters receded, the silts and sediments resting on the lakebed were left to dry in the scorching African sun. The small grains of the diatomite are swept up by the strong wind gusts that occasionally blow over the region. Once heaved aloft, the Bodélé dust can be carried for hundreds or even thousands of kilometers.〔 In winter, the depression produces an average of 700,000 tonnes of dust each day (Todd et al., 2007).〔Fisher, Richard. "(Amazon forest relies on dust from one Saharan valley )". ''NewScientist Environment'' online. 3 Jan 2007. URL accessed 2007-01-04.〕 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.〔 The pattern of air flow is so common that the winds have scoured a straight path in the ground, marking its southwesterly flow.〔 Complementary research published in Geophysical Research Letters by Richard Washington from the University of Oxford and Martin Todd (University of Sussex) has shown that these strong winds are part of feature now called the Bodele Low Level Jet. In the Reanalysis data sets such as ERA-40 the wind shows up as a clear wind speed maximum at about 900 hPa (or roughly 1 km above the surface) near 18 N and 19 E. This jet maximum coincides with the exit gap of the North-easterlies between the Tibesti mountains and the 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 Bodele, it is absent from other longitudes over west Africa along 18 N. It is therefore a feature which uniquely overlies the greater Bodele region, downwind of the mountains of Chad.〔"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.〕 The Bodele 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 Bodele. Individual dust storms in the Bodele were also shown in this research to coincide with a major strengthening of the Bodele Low Level Jet which, in turn, is associated with the ridging of the Libyan High, a feature of the subtropical High Pressure belt.〔〔 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 Amazon Rainforest is provided by the Bodélé depression, depositing up to 50 million tonnes in South America per year.〔"(Dust to gust )". ''EurekAlert!''. AAAS. 28 Dec 2006. URL accessed 2006-12-29.〕 The research also shows that, contrary to what was previously thought, most of the Saharan dust that reaches the east coast of the United States originates from a single source—the Bodele depression.〔"(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.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bodélé Depression」の詳細全文を読む スポンサード リンク
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