Global dust storm source areas determined by the total ozone monitoring spectrometer and ground observations



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Dust storms in Australia


Aeolian processes are of considerable importance in Australian deserts, as evidenced by large accumulations of sand dunes (Wasson et al., 1988) and extensive deposits of aeolian clay often referred to as ‘parna’ (Dare-Edwards, 1984). However, although almost three-quarters of Australia has a dryland climate, the continent’s desert landscapes produce relatively small amounts of dust. Nonetheless, desert dust in Australia has received considerable attention in recent years (Middleton, 1984; McTainsh et al., 1989; Nickling et al., 1999) and the area of greatest dust storm frequency, as determined from meteorological station data, has been shown to be broadly coincident with the area covered by the very large (1.3 million km2) internal drainage basin of Lake Eyre. Indeed, the TOMS analysis indicates that the present-day Lake Eyre, an ephemeral playa, is the continent’s main dust source, the only area where AI values exceed 11. The dustiness of the current playa bed itself can only be inferred from terrestrial data due to the absence of meteorological stations.
As an area of sediment supply, the Lake Eyre Basin has been compared to that of Lake Chad (McTainsh 1985), with deflation operating on alluvial spreads brought by the southward flowing Eyre, Diamantina and Cooper rivers.
Hydrologically, the state of Lake Eyre has varied in response to climate change in the late Pleistocene, from a perennial lake up to 25 metres deep to a groundwater-controlled playa, marked by substantial sediment deflation. The long history of deflation in this area is evidenced by wind-blown deposits, typically rich in gypsum and clay, found at a number of sites around the lake (Magee and Miller, 1998).
Throughout the months of maximum atmospheric dust loadings (October to March) the surface windspeeds over Australia reach a maximum over the Simpson Desert and the Great Victorian Desert (apart from the west coast of Australia) with a prevailing south easterly to southerly wind (not shown). The classic synoptic situation generating deflation in southern regions of Australia is an eastward moving midlatitude frontal system (Sprigg, 1982). Anticyclogenesis may follow the passage of the front producing marked horizontal wind shear in the easterlies to the south of a heat trough (Sturman and Tapper, 1996). Material raised by these systems is occasionally transported as far as New Zealand (Collyer et al., 1984). Dust storms in Australia are generally characteristic of the late spring and summer months.


Dust storms in South America

Information on the occurrence of dust storms in South America is relatively sparse. However, Johnson (1976) suggests that dust storms are a noticeable feature in the Altiplano of Peru, Bolivia and Chile, and Middleton (1986c) has mapped the occurrence of dust events in Argentina, and has noted their importance on the Puna de Atacama where salt basins – salars – appear to be an important source. The presence of extensive areas of closed depressions and of wind fluted topography, combined with the probable importance of salt weathering in the preparation of fine material for deflation (Goudie and Wells, 1995), suggest that the dry areas of the Altiplano should indeed be major source areas for dust storms. The TOMS-derived map (not shown) identifies one area in South America where AI values are relatively high (i.e. greater than 7, see Table 1). This appears to be centred on the Salar de Uyuni, a closed basin in the Altiplano of Bolivia which lies astride the 20 o S line of latitude and is located in an area with 200 to 400 mm of annual rainfall. This salar, the largest within the Andes, is possibly the world’s largest salt flat, though in the Late Pleistocene it was the site of a huge lake, pluvial Lake Tauca (Lavenu et al. 1984), which incorporated present day Lake Poopo, the Salar de Coipasa and the Salar de Uyuni itself (Clayton and Clapperton, 1997). It is bounded by shorelines at levels as much as 100 m, and many of these display impressive algal incrustations (Rouchy et al. 1996). The pluvial lake was more than 600 km long, and it is possible that the fine sediments from its desiccated floor are one of the reasons for the existence of high AI values in this region. It is of the same order of size as some of the other major basins that are major dust sources (e.g. Bodélé/Chad, Eyre, Tarim and Mkgadikgadi).



Dust sources in North America

North America is a classic area for dust storm studies, not least because of the severe dust bowl years of the 1930s (Worster, 1979), and human pressures on the land surface have led to severe dust episodes since that time (Wilshire et al, 1981; Gill, 1996). Details of dust generation have been undertaken in a number of regions, including the Owens Lake of California (Reheis, 1997), the Mojave and Colorado Deserts (Bach et al. 1996), and North Dakota (Todhunter and Cihacelik, 1999), while a detailed study of dust deposition for Nevada and California is provided by Reheis and Kihl (1995). Surface weather station observations have tended to suggest that for the USA as a whole, the greatest frequency of dust events occurs in the panhandles of Texas and Oklahoma, western Kansas, eastern Colorado, the Red River Valley of North Dakota and northern Montana. These areas combine erodible materials with a dry climate, and, most importantly of all, high values for wind energy (Orgill and Sehmel, 1976; Changery, 1983; Gillette and Hanson, 1989) (Figure 17).


The TOMS data (not shown) give a rather different picture, and show only one area with maximum AI values greater than 5 – parts of the Great Basin. The Great Basin is an area of fault-bounded blocks and troughs that is bounded on the west by the Sierra Nevada and Cascades and on the east by the Rocky Mountains. It contains over 150 basins separated from each other by north-south trending mountain ranges. Most of the basins were occupied by Pleistocene lakes, and they covered an area at least 11 times greater than the area they cover today (Grayson, 1993, p. 86). One of these lakes was Bonneville, which was roughly the size of present day Lake Michigan, and another was Lahontan, which covered an area roughly as great as present day Lake Erie. The desiccation of these large bodies, the presence of extensive areas of salty lake floor (Blank et al. 1999), and the existence of large expanses of alluvial fans running into the many basins, may account for the importance of this area as a source of dust storms.



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