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Meteorological Causes of Harmattan Dust in West Africa ⁎ Wolfgang Schwanghart , Brigitta Schütt

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Meteorological Causes of Harmattan Dust in West Africa ⁎ Wolfgang Schwanghart , Brigitta Schütt Available online at www.sciencedirect.com Geomorphology 95 (2008) 412–428 www.elsevier.com/locate/geomorph Meteorological causes of Harmattan dust in West Africa ⁎ Wolfgang Schwanghart , Brigitta Schütt Institute of Geographic Sciences, Freie Universität Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany Received 11 October 2006; received in revised form 24 April 2007; accepted 15 July 2007 Available online 20 July 2007 Abstract We investigated the temporal dynamics of dust entrainment in the Bodélé Depression, Central Sahara, to better understand the intra-annual variability of aerosol emission in the world's largest dust source. The linkages between dust entrainment and large- scale meteorological factors were examined by correlating several meteorological variables in the Mediterranean and Africa north of the equator with the aerosol concentrations in the Bodélé Depression separately for winter and summer. The methodological tools applied are NCEP/NCAR reanalysis data and the aerosol index of the Total Ozone Mapping Spectrometer (TOMS-AI), available for 15 years from 1978 to 1993. We found that dust mobilisation during the Harmattan season is highly dependent on air pressure variability in the Mediterranean area. High pressure to the north of the Bodélé intensifies the NE trade winds, leading to an increased entrainment of dust in the Bodélé Depression. In summer, dust mobilization cannot be explained by the large scale meteorological conditions. This highlights the importance of local to regional wind systems linked to the northernmost position of the intertropical convection zone (ITCZ) during this time. © 2007 Elsevier B.V. All rights reserved. Keywords: NCEP/NCAR reanalysis; TOMS-AI; Synoptic climatology; Bodélé; Aerosol; Dust; West Africa; Aeolian dynamics 1. Introduction (Herrmann et al., 1999; Brooks and Legrand, 2000; Goudie and Middleton, 2001; Prospero et al., 2002; Mineral dust is a crucial factor in the Earth's climate Giles, 2005; Washington et al., 2003). Dust entrained in system and impacts on nutrient dynamics and biogeo- this basin is transported over great distances as far as the chemical cycling of oceanic and terrestrial ecosystems Caribbean Sea, the Amazon Basin, the United States and (Péwé, 1981; Arimoto, 2001; Middleton and Goudie, Europe (Schütz et al., 1981; Prospero, 1999; Dunion and 2001; Griffin et al., 2004; Engelstaedter et al., 2006). Velden, 2004; Koren et al., 2006). Entrained from sediments or soils, it can be transported In Sahelian and Saharan Africa aeolian dust transport by the wind for several thousands of kilometres. The is accomplished by several wind systems (Jäkel, 2004; Bodélé Depression in the Republic of Chad has been Engelstaedter et al., 2006). One of the most prominent found to be the world's largest source of mineral dust wind systems is the Harmattan (Fig. 1), a ground level stream of dry desert air which is part of the African continental trade wind system that sweeps far southward ⁎ Corresponding author. Tel.: +49 30 838 70439; fax: +49 30 838 70755. from a consistent NE direction during the boreal winter E-mail address: [email protected] (Hastenrath, 1988). During the winter months the (W. Schwanghart). seasonally variable Harmattan current transports large 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.07.002 W. Schwanghart, B. Schütt / Geomorphology 95 (2008) 412 – 428 Fig. 1. Mean annual atmospheric mineral dust concentrations quantified by TOMS aerosol index (dimensionless) and NCEP/NCAR horizontal wind vectors at 925 hPa in winter (December to February) and summer (June to August) during the years 1978–1993. The black square indicates the location of the Bodélé Depression. 413 414 W. Schwanghart, B. Schütt / Geomorphology 95 (2008) 412–428 amounts of mineral dust at irregular intervals from the In order to prevent or mitigate the negative ecological Chad Basin to the Sahel and Guinean coast where it and economical effects of mineral aerosols on popula- reduces visibility, relative humidity and temperatures tions in deserts and desert-fringe environments, the (Kalu, 1979; McTainsh and Walker, 1982; Adedokun understanding of the geomorphological processes et al., 1989; Stahr et al., 1996; Breuning-Madsen and governing atmospheric dust dynamics is of vital interest Awadzi, 2005). The increased aerosol concentrations (Goudie and Middleton, 2001). Geomorphological cause irritation of respiratory tracts, and visibility research of aeolian systems largely focuses on the reduction in car and air traffic that represent a serious production of the fine-grained material, the mechanisms problem during dust spell events (Middleton, 1997). As a of mobilization, transport and deposition, and the effects consequence, an airplane crashed in Ivory Coast because of wind-blown dust on landform evolution. A better the engines were affected by dust in February 2000 understanding of these processes requires observations, (http://www.nrlmry.navy.mil/aerosol/Case_studies/ analyses and modelling at different spatial and temporal 20000130_ivorycoast/, 20.09.2006). scales. Since dust entrainment, transport and deposition Fig. 2. Regional setting of the Bodélé Depression modified after Grove and Warren (1968), Mainguet (1996) and Pachur and Altmann (2006). Elevation data are based on GTOPO30. W. Schwanghart, B. Schütt / Geomorphology 95 (2008) 412–428 415 are closely connected to meteorological processes, the chronology of humid and dry phases exists for the last fifty investigation of these processes necessitates a multidis- thousand years (Busche, 1998; several authors in Pachur ciplinary approach. and Altmann, 2006). A domination of freshwater condi- The study presented in this paper is to be placed at the tions in these lakes caused the sedimentation of diatomite, interface of geomorphology and meteorology. We which today is either near the surface or partly covered by concentrated on the short-term variability of dust sands of the latest aeolian accumulation phase. According emissions of the Bodélé Depression and tried to explain to the Saharo-Sahelian Global Wind Action System of it with existing large-scale meteorological datasets. The Mainguet (1996) these sands are part of an aeolian sand underlying idea is that geomorphological processes of stream following the predominant NE-SW direction of the short-term and small-scale fluctuations in dust mobilisa- near-surface trade winds connecting deflation and accu- tionmainlydependonwindspeedandsurface mulation areas in the Sahara. roughness (Gillette and Chen, 1999). The small-scale meteorological processes, however, are related to the 3. Methods regional and large-scale meteorological conditions, which can be characterised by existing datasets and In order to understand the influence of large-scale can be forecasted. Due to the seasonality in the pressure meteorological factors on dust mobilisation we analysed and wind systems, the influences should differ during the correlation of two data sets covering the period from the most diverse seasons: summer and winter. Hence, 1978 to 1993. we investigated the seasonality in the meteorological Aerosol in the atmosphere was quantified using the impact on dust emission in the Bodélé Depression. The level-3 aerosol index (version 8) of the Total Ozone derived information on the relation between the large- Mapping Spectrometer (TOMS-AI) on the Nimbus scale meteorological conditions and dust entrainment in satellite. TOMS-AI is a measure of how much the a local area can contribute to predicting the hazardous wavelength dependence of back-scattered UV radiation weather events caused by atmospheric dust. from an atmosphere containing aerosols differs from that Most recently, Washington and Todd (2005) and ofapureRayleighatmosphere(Herman et al., 1997; Washington et al. (2006) showed that the low level jet McPeters et al., 1998; Chiapello et al., 1999). Because of (LLJ) is an important control on dust emission in the different absorptivity at UV wavelengths due to particle Bodélé Depression. In this paper we provide evidence size and form TOMS-AI allows us to distinguish between that day-to-day variability of dust emission in the mineral dust and smoke, both over land and sea. Negative Bodélé Depression can be explained by the LLJ using an index values indicate non-absorbing aerosols such as extensive dataset characterising large-scale meteorolog- sulphate aerosols (Chiapello and Moulin, 2002), whereas ical factors, defined as the state of the atmospheric atmospheric mineral dust concentrations are quantified by conditions with a duration of one to several days. positive values. An index value of zero stands for an aerosol free atmosphere. The spectrometer takes measure- 2. Regional setting ments almost every single day over most of the Earth's surface with a near-noon equator crossing time. The The Bodélé Depression is located in the Central spatial resolution of the level-3 data is one degree latitude Sahara around 17°N and 18°E northeast of Lake Chad. and 1.25 degrees longitude cell size, and the accuracy of With its lowest elevation at 153 m a.s.l., the Bodélé the data is set to one decimal place. The data that spans the Depression represents the lowest part of the Chad Basin period November 1978 to May 1993 are archived by (Fig. 2) and adjoins the Tibesti Mountains in the north, NASA and can be downloaded from http://toms.gsfc. the Erg de Bilma in the west and the Erg du Djourab in nasa.gov. To remove the effect of non-absorbing
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