Understanding the long-term variability of African dust as recorded in surface concentrations and TOMS observations

1. Understanding the long-term variability of African dust as recorded in surface concentrations and TOMS observations Isabelle Chiapello (LOA, Lille, France) Cyril Moulin (LSCE, Paris, France) Joseph M. Prospero (University of Miami, USA)

2. Introduction Mineral dust are emitted by wind erosion of arid and semi-arid areas. Particles are transported far from their source-regions over oceans (more than thousands kilometers). The main source is Africa (Sahara and Sahel). Mineral dust is thought to play an important role in climate processes, but their radiative effect is highly uncertain (sign unknown). Addidtionnally dust has an impact on biogeochemical cycles, atmospheric chemistry, health, … Mineral dust is characterized by a high spatial and temporal variability (daily, seasonal and interannual) The natural variability is so high that there is a lack of reliable estimates of the anthropogenic fraction of mineral dust (related to human pressure in the Sahel region)

3. MODIS January 26 2003 (14h25): Dust over West Sahara, Mauritania, and Senegal Louis Gonzales, http://www-loa.univ-lille1.fr/

4. 1st Objective Which climate factors control the natural varibility of the dust transport from Africa ? Prospero & Nees (1986) have shown a positive correlation between summer Barbados concentration and Sahel drought Changes in the intensity of the emissions Moulin et al. (1997) have shown a positive correlation between METEOSAT/VIS DOT over the Atlantic and the North Atlantic Oscillation (NAO) Changes in the tranport pattern Here we use 20 years of TOMS data to understand these impacts

5. 2nd Objective What is the impact of Sahelian population increase on African dust emissions (if it exists) ? Sokolik et al. (1996) suggest an anthropic contribution between 30 and 50% (dust from degraded soils) Prospero et al. (2002) show that the largest and most active dust sources are located in regions of Sahara where there is little or no human activity, suggesting a low anthropic component of dust Recent estimates from models (Tegen et al., 2004; Mahowald et al., 2004) vary from <10% to 0-50% !!! ➲ The analysis of long-term ground-based (35 years) and satellite (20 years) dust observations should allow to progress in our investigation of the natural component of African dust vs that related to human induced soil degradation

6. The satellites Satellite sensors allow atmospheric dust observations with a good temporal frequency (daily in general) and spatial coverage METEOSAT and TOMS are not accurate sensors but they provide daily observations since more than 20 years. Moulin et al., JGR, 1997

7. TOMS Sensor initially planned to retrieve atmospheric ozone based on UV measurements (300-400 nm). Contrary to METEOSAT, TOMS provides a qualitative index of aerosols (Absorbing Aerosol Index or AAI), which is available at global scale over both land and oceans. • 2 TOMS sensors have been used: • TOMS/Nimbus 7 (1979-1992) • TOMS/Earth-Probe (1997-2000) Herman et al., JGR, 1997 NASA/GSFC

8. TOMS Dust Optical Thickness Computed from the daily maps of AAI from TOMS/Nimbus-7 (1979-1992) and TOMS/Earth-Probe (1997-2000) http://jwocky.gsfc.nasa.gov/aerosols/aerosols.html The AAI is converted in Dust Optical Thickness (DOT) from a comparison of coincident TOMS AAI and METEOSAT DOT daily pixels over the Atlantic for 1986-88 (Nimbus-7) and 1997 (Earth-Probe) Chiapello & Moulin, GRL, 2002

9. Validation of TOMS DOT Comparison with ground-based sun-photometer measurements from the AERONET network and field campaigns in Africa (the size of a TOMS pixel is 1°x1.25°) The accuracy is not high but there is no bias with season or sensor! TOMS N7+ TOMS EP, Summer 400 pts, slope 1.08  0.02, R=0.82

10. TOMS DOT climatology over 20 years (1979-2000)  Set of African dust observations available over both land and ocean and for 20 years!

11. Variability of dust transport over the Atlantic (15-30°N, 10-30°W) Monthly averages Summer peaks + winter peaks

12. NAO and winter transport The North Atlantic Oscillation depends partly on the intensity of the Açores high pressure center and controls the meteorological conditions in winter over north Atlantic. We use the winter NAO index of Hurrell (1995) updated each year High variability of the winter dust export over the Atlantic linked to the NAO r=0.51 Chiapello & Moulin, GRL, 2002

13. NAO and meteorology High NAO Low NAO • The NAO exerts a strong influence • on the large-scale variations of both • atmospheric circulation and • hydrological cycle in the NH + Trades - Trades Winter 1989 NAO index=5.07 Winter 1986 NAO index=0.5 Changes in strength and location of the Azores anticyclone exert a strong influence on winter dust transport

14. 0.1 1.25 0.4 1.0 But not summer export! 0.4 1.0 0.1 1.25 NAO controls winter export ...

15. Recent evolution of drought in Sahel We use a Sahelian Annual Drought (SAD) index derived from rainfall index of L’Hôte et al. (2002). Annual rainfall over Sahel Drought dominates since 1970 Only 3 wet years ( 1975, 1994 and1999), among which 2 are recent At the end of 2000 the drought continues, although it is less extended geographically, the wet years are still very isolated from each other L’hôte et al. (2002)

16. An impact of Sahel drought ? Sahel Atlantic Rain in the Sahel occur between July and September and control the drought conditions, and consequently dust emissions, for the rest of the year. Sahel/Atlantic: r=0.88 SAD/Atlantic: r=0.44 SAD/Sahel: r=0.56 Moulin & Chiapello, GRL, 2004

17. 0.1 1.25 0.4 1.0 … and winter export south of 15°N !! 0.4 1.0 0.1 1.25 Drought in Sahel controls summer export ...

18. Dust and climate : summary Correlation with NAO Correlation with drought DOT WINTER SUMMER

19. 35 years of in-situ measurements at Barbados (west Indies) Barbados Is this unique data set representative of the dust transport over the whole Atlantic ?

20. The Barbados is representative of the Atlantic dust export r=0.73 Winter Summer r=0.50

21. Barbados «records» NAO and Sahel drought Winter r=0.30 Summer r=0.65 Chiapello, Moulin & Prospero, JGR, 2005

22. Barbados 35-years record shows a residual increase in the dust loads We estimate the natural variability from a bi-linear regression with NAO, and Drought in Sahel SDCtheoretical=6.1(SAD) + 0.85(NAO) +10.4, R=0.71 ➲ There is a progressive increase of residual dust export at Barbados between 1966 and 2000 (~6 µg/m3 over 35 years, i.e. a factor 2)

23. Is this increase also recorded on 20 years of TOMS observations? For each TOMS pixel, we estimate the natural variability from a bi-linear regression with NAO, and drought in Sahel (if R>0.6) ➲ The residual TOMS DOT show a linear increase with year over Atlantic and some Sahel regions SLOPE/Year R/Year % OF INCREASE in 22 years

24. An increase coincident to semi-arid regions affected by land degradation due to human pressure Increase of population over Sahel (FAO) Wind erosion severity (UNEP/ISRIC) Mali and Niger Increase of dust in TOMS observations

25. Conclusions The ground-based and satellite long-term African dust records show: Intensity of dust transport is influenced by NAO (winter) and strongly controled by Sahel drought (summer and winter).  Prospero & Nees (1986) and Moulin et al. (1997) were right ! Most of the year-to-year variability of transport is related to emissions over Sahel not over Sahara When removing this natural variability, there is a progressive increase in the 35-years dust loads at Barbados, also recorded in the 20-years TOMS observations over Atlantic and some Sahel regions affected by soil degradation An human impact is highly suspected to explain an increase of 40-50% of the dust loads measured in the last 2 decades