by HansPeter Roesli and Jochen Kerkmann (EUMETSAT)Jump to images
Westward transport of desert dust from Africa to the Caribbean and Amazon Region is a rather well documented and a frequent feature of the subtropical circulation. Over West Africa dust is lifted either through outflow boundaries of thunderstorm clusters and frontal systems penetrating deep into the Saharan belt, or through turbulence created by low-level easterly winds between a subtropical high and and low pressure in the ITCZ. However, due to the lack of adequate imaging channels in geostationary orbit no continuous observations over longer time periods were possible until recently. With the advent of the MSG series and its IR window channels (in particular the IR8.7 channel) observation of major dust outbreaks has become almost routine, examples of which may be found under "See also" or on the real-time Derived Product Imagery web page.
It was intriguing, therefore, not only to give examples of dust outbreaks based on MSG data, but to also pick a good case that would document long-distance transport. Driven by a low-level southward moving wind front (see wind vectors at 925 hPa, PNG, 1159 KB) southeast of a high pressure area centred over the Azores, on 20 June dust was raised from the ground in two distinct areas of West Africa, both of which are common sources of West African dust (see map of dust hotspots from TOMS data, PNG, 95 KB, source: Sebastian Engelstaedter, Oxford University).
While the western dust cloud moved out into the Atlantic across the coastline between Western Sahara and Senegal, the more easterly one moved southward as far as Guinea-Bissau and Guinea (see Dust RGB from 22 June at 04:00 UTC, PNG, 901 KB) before reaching the Atlantic. Furthermore, the more easterly dust plume was bounded by low-level clouds that formed in the dust front and which dissipate later on (see Dust RGB from 21 June at 07:00 UTC, PNG, 892 KB). A third major dust plume was lifted over the Mauritania-Mali border around noon on 21 June 2007 (see Dust RGB from 21 June at 12:00 UTC, PNG, 1159 KB).
Both, this third plume travelling at approximately the same speed as the expanding convective clouds on 22 June 2007 (see animation (MPG, 2352 KB) and IR image (PNG, 295 KB)) and the dust clouds persistent colour shade, night and day, point to the dust having risen above the maritime boundary layer. Taking the period between the first dust front leaving the African coast (20 June at 16:00 UTC) and it reaching the Lesser Antilles (25 June at 19:00 UTC) the dust has travelled 4600 km in 123 hours, i.e. at an average speed of 10 m/s or 20 knots. Reference to ECMWF analyses of the wind fields it appears that the driving level of the dust cloud is somewhere below the 500 hPa level. On 23 June 00:00 UTC, at the midway point, and typical for most of the 7-day period, the speed is close to 20 knots in the lower troposphere (see ECMWF windbarbs at 700 hPa (PNG, 708 KB) and N-S vertical cross section at 40 deg W (PNG, 7 KB)). Actually, the third dust plume initially moved away from the coast of Guinea-Bissau much faster than the most westward dust front, at over 40 knots (on 22 June from 01:00 UTC to 07:00 UTC) over a distance of 500 km.
Many other features can be seen in the very long sequence of 15-minute images from 20 June 08:00 UTC to 26 June 12:00 UTC (see animation, MPG, 36555 KB). For example, towards the end of the sequence convective clouds are developing along the dust front (see animation, 25 June, 15:00-24:00 UTC, MPG, 3614 KB).
Finally, Saharan dust brings mixed blessings as it crosses the Atlantic. Although the dust may bring pathogens that harm Caribbean corals and aggravate asthma symptoms among humans, the dust is also beneficial. Without Saharan dust, islands in the Caribbean might be little more than hulks of barren rock since the dust supplies the islands with valuable nutrients. A study published in 2006 identified the Bodele Depression as the source for soil nutrients in the Amazon (see paper from Koren et al, 2006, PDF, 1063 KB).
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