Bodele Research Articles

ANALYSIS OF MAIAC DUST AEROSOL RETRIEVALS FROM MODIS OVER NORTH AFRICA

An initial comparison of aerosol optical thickness over North Africa for year 2007 was performed between the Deep Blue (DB) and Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithms complimeted with MISR and OMI data. The dust retrievals are performed using the model of spheroids. The new MAIAC algorithm has a better sensitivity to the small dust storms than the DB algorithm, but it also has biases in the brightest desert regions indicating the need for improvement. The quarterly averaged AOT values in the Bodele depression and western downwind transport region show a good agreement among MAIAC, MISR and OMI data, while the DB algorithm shows a somewhat different seasonality.

Fertilizing the Amazon and equatorial Atlantic with West African dust

Atmospheric mineral dust plays a vital role in Earth's climate and biogeochemical cycles. The Bodélé Depression in Chad has been identified as the single biggest source of atmospheric mineral dust on Earth. Dust eroded from the Bodélé is blown across the Atlantic Ocean towards South America. The mineral dust contains micronutrients such as Fe and P that have the potential to act as a fertilizer, increasing primary productivity in the Amazon rain forest as well as the equatorial Atlantic Ocean, and thus leading to N2 fixation and CO2 drawdown. We present the results of chemical analysis of 28 dust samples collected from the source area, which indicate that up to 6.5 Tg of Fe and 0.12 Tg of P are exported from the Bodélé Depression every year. This suggests that the Bodélé may be a more significant micronutrient supplier than previously proposed.

Remotely sensed dune celerity and sand flux measurements of the world's fastest barchans (Bodélé, Chad)

Quantifying sand flux with field measurements is an expensive and time‐consuming process. We here present an alternative approach using the COSI‐Corr software package for Earth surface deformation detection. Using pairs of ASTER satellite images, we detected dune migration in the Bodélé depression of northern Chad over time intervals of one month to 6.5 years. The displacement map can be used to automatically distinguish dunes from interdunes, which is a crucial step towards calculating sand flux. We interpolated a surface between the interdune areas and subtracted it from a digital elevation model, thus obtaining dune heights and volumes. Multiplying height with celerity yields a pixel‐by‐pixel estimate of the sand flux. We applied this method to large diatomite dunes in the Bodélé, confirming that these are some of the world's fastest moving barchans. Plotting dune height against inverse celerity reveals sand flux at the dune crest of >200 m3/m/yr. Average dune sand flux values for the eastern and western Bodélé are 76 and 99 m3/m/yr, respectively. The contribution of the dunes to the total area‐averaged sand flux is 24–29 m3/m/yr, which is ∼10% of the saltation flux determined by previously published field measurements.

Natural Minerogenic Dust and Human Health

Natural Minerogenic Dust and Human Health

Understanding the long‐term variability of African dust transport across the Atlantic as recorded in both Barbados surface concentrations and large‐scale Total Ozone Mapping Spectrometer (TOMS) optical thickness

The interannual variability of African dust transport over the north tropical Atlantic is monitored using in situ surface concentrations measurements performed at Barbados since 1966, along with the Total Ozone Mapping Spectrometer (TOMS) and Meteosat dust optical thickness (DOT) records covering the last two decades. Despite their differences in spatial coverage, the two dust records are in good agreement at both monthly and annual timescales over the 22 years of common operation. This demonstrates that the Barbados dust record is representative of the year‐to‐year variability of dust export over the north tropical Atlantic during both winter and summer. The satellite DOT are used to assess the characteristics of the impact of climate factors, i.e., North Atlantic Oscillation (NAO) and Sahel drought, on dust emission and export as a function of season, and in terms of spatial extend of their influence. The analysis shows a large regional impact of Sahel drought on dust emissions and transport both in winter and in summer, whereas the influence of the NAO dominates the winter export and is more geographically limited to the eastern Atlantic north of 15°N, and possibly some localized source regions (southern Mauritania and the Bodele depression). Overall, the combination of the 35 years of Barbados measurements of African dust with 22 years of satellite dust survey over the Atlantic highlights very high dust loads in the mid‐1980s related to the severe Sahel drought (maximum impact in 1983) and persistently high dusty conditions in the 1990s, most probably due to the continuation of relatively dry conditions in Sahel in the recent years.

Direct wind measurements of Saharan dust events from Terra and Aqua satellites

Wind speed and direction, during Saharan dust episodes are calculated by estimating the movement of the dust plume between the morning observations from Terra and the afternoon observations from Aqua by the MODerate resolution Imaging Spectroradiometer (MODIS). The difference in the position of the dust front (easily identified both near the dust sources and over the ocean) between the Terra and Aqua observations is used to derive the actual wind field of the dust layer. The analysis is used to estimate the near‐surface wind properties needed for dust mobilization in the Bodele depression. Comparison with NCEP reanalysis winds shows that NCEP underestimated by factor 2 the actual speed of the dust front progression near the sources and produced an azimuthal spread, three times wider than the measured one. Over the seashore, with more numerous meteorological observations, the NCEP wind field did match very well the dust measurements.

Long-term simulation of global dust distribution with the GOCART model: correlation with North Atlantic Oscillation

Global distribution of aeolian dust is simulated from 1981 to 1996 with the Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. The results are compared with in situ measurements and satellite data. An index is calculated from the model results and the satellite viewing angles to allow quantitative comparison with the Total ozone mapping spectrometer (TOMS) absorbing aerosol index. The annual budget over the different continents and oceans are analyzed. The simulated annual emission varies from a minimum of 1950 Tg in 1996 to a maximum of 2400 Tg in 1988. Of these emissions, 65% is from North Africa and 25% from Asia. It is found that North America received twice as much dust from other continents than it emits per year. There is no significant trend over the 16-year simulation. The inter-annual variability of dust distribution is analyzed over the North Atlantic and Africa. It is found that in winter a large fraction of the North Atlantic and Africa dust loading is correlated with the North Atlantic Oscillation (NAO) index. It is shown that a controlling factor of such correlation can be attributed to dust emission from the Sahel. The Bodele depression is the major dust source in winter and its inter-annual variability is highly correlated with the NAO. However, the long record of dust concentration measured at Barbados indicates that there is no correlation with the NAO index and surface concentration in winter. Longer simulation should provide the information needed to understand if the effects of the NAO on dust distribution is rather limited or Barbados is at the edge of the affected region.

The geographical application of some selected physio-climatic indices to Africa

Africa was seasonally analyzed by 4 different indices:Effective Temperature, Predicted 4-hr Sweat Rate, Relative Strain and Still-Air Temperature. These classifications were applied to approximately 800 station on the continent, resulting in 9 maps which were reduced from an original scale of 1:15,000,000.Some of the major findings indicate that areas often considered as "tropical" actually indicate lesser climatic stress than those in very hot-dry regions or transition areas between very hot-dry and hot-humid. The northern hemisphere summer is by far the most severe, physiologically, over vast areas of northern Africa. Regions of greatest stress are located along the southern Red Sea, interior southern Somalia and, to a lesser degree, the Djouf Basin and Bodele Depression. The year-round mildest areas occur most frequently in parts of the eastern highlands and Atlantic coast (those influenced by cold currents). Few really oppressive conditions during nighttime could be observed.