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

observationsobservations

Isabelle Chiapello (LOA, Lille, France)Isabelle Chiapello (LOA, Lille, France)

Cyril Moulin (LSCE, Paris, France) Cyril Moulin (LSCE, Paris, France)

Joseph M. Prospero (University of Miami, Joseph M. Prospero (University of Miami, USA)USA)

IntroductionIntroductionMineral 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)

MODIS January 26 2003 (14h25): Dust over West Sahara, Mauritania, and Senegal

Louis Gonzales, http://www-loa.univ-lille1.fr/

11stst Objective ObjectiveWhich 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

22ndnd Objective ObjectiveWhat 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 dustRecent 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

The satellitesThe satellitesSatellite sensors allow atmospheric dust observations with a good temporal frequency (daily in general) and spatial coverageMETEOSAT and TOMS are not accurate sensors but they provide daily observations since more than 20 years.

Moulin et al., JGR, 1997

TOMSTOMSSensor 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.

Herman et al., JGR, 1997 NASA/GSFC

2 TOMS sensors have been used:

-TOMS/Nimbus 7 (1979-1992)

-TOMS/Earth-Probe (1997-2000)

TOMS Dust Optical TOMS Dust Optical ThicknessThickness

Computed from the daily maps of AAI from TOMS/Nimbus-7 (1979-1992) andTOMS/Earth-Probe (1997-2000)

http://jwocky.gsfc.nasa.gov/aerosols/aerosols.html

The AAI is converted in Dust OpticalThickness (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

Validation of TOMS DOTValidation of TOMS DOT

Comparison with ground-basedsun-photometer measurements from the AERONET network and field campaigns in Africa(the size of a TOMS pixel is1°x1.25°)

The accuracy is not high but there is no bias with season or sensor! 0,0

0,4

0,8

1,2

1,6

2,0

0,0 0,4 0,8 1,2 1,6 2,0

Sun-Photometer AOT

TO

MS

DO

T

Cape Verde IslandDakarBanizoumbouGaoM'Bour

TOMS N7+ TOMS EP, Summer

400 pts, slope 1.08 0.02, R=0.82

TOMS DOT climatology TOMS DOT climatology over 20 years (1979-over 20 years (1979-

2000)2000)

Set of African dust observations available over both land and ocean and for 20 years!

Variability of dust transport Variability of dust transport over the Atlantic (15-30°N, 10-over the Atlantic (15-30°N, 10-

30°W)30°W)

0,0

0,2

0,4

0,6

0,8

1,0

Mea

n D

iust

Op

tica

l Th

ickn

ess

Mean METEOSAT DOTMean Nimbus 7 TOMS DOTMean Earth-Probe TOMS DOT

Monthly averages

Summer peaks + winter peaks

NAO and winter transportNAO and winter transport

Chiapello & Moulin, GRL, 2002

-10

-8

-6

-4

-2

0

2

4

6

1980 1985 1990 1995 2000

DO

T

NA

O Index

Year

0.1

0.2

0.3

0r=0.51

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

NAO and meteorologyNAO and meteorology High NAO Low NAO

Winter 1986 NAO index=0.5Winter 1989 NAO index=5.07

+ Trades - Trades

Changes in strength and location of the Azores anticyclone

exert a strong influence on winter

dust transport

The NAO exerts a strong influence on the large-scale variations of both

atmospheric circulation and hydrological cycle in the NH

NAO controls winter export ...NAO controls winter export ...

0.1 1.25 0.4 1.0

But not summer export!But not summer export!

0.1 1.25 0.4 1.0

Recent evolution of drought Recent evolution of drought in Sahelin Sahel

Anual rainfall index in Sahel

-1,5

-1

-0,5

0

0,5

1

1960

1962

1964

1966

1968

1970

1972

1974

1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

Nicholson, 1993Lhote, 2002 r=0,9

Start of TOMS observations

L’hôte et al. (2002)

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

Annual rainfall over Sahel

We use a Sahelian Annual Drought (SAD) index derived from rainfall index of L’Hôte et al. (2002).

An impact of Sahel An impact of Sahel drought ?drought ?

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

1980 1985 1990 1995 2000

DO

TSR

D Index

Year

-1.5

-1.0

-0.5

0.0

0.5

0.2

0.4

0.6

0.8

Atlantic

Sahel

Drought in Sahel controls summer Drought in Sahel controls summer export ...export ...

0.1 1.25 0.4 1.0

… … and winter export south of 15°N !!and winter export south of 15°N !!

0.1 1.25 0.4 1.0

Dust and climate : summaryDust and climate : summary

DOTCorrelationwith NAO

Correlationwith drought

WINTER

SUMMER

35 years of in-situ 35 years of in-situ measurements at Barbados measurements at Barbados

(west Indies)(west Indies)

0

10

20

30

40

50

60

70

Mo

nth

y S

DC

(µ

g/m

3)

0

5

10

15

20

25

An

nu

al S

DC

(µ

g/m

3)

Is this unique data set representative of the dust transport over the whole Atlantic ?

Barbados

The Barbados is The Barbados is representative of the representative of the

Atlantic dust exportAtlantic dust export

0

10

20

30

SD

C (

µg

/m3

)

0

0,1

0,2

0,3

0,4

Sa

tell

ite

DO

T

Atlantic (satellite)

Barbados (ground)

r=0.73Winter

0

10

20

30

40

50

60

1966 1970 1974 1978 1982 1986 1990 1994 1998

SD

C (

µg

/m3

)

0

0,2

0,4

0,6

0,8

1

Sa

tell

ite

DO

T

Barbados (ground)

Atlantic (satellite)

Sahel (satellite)

Summer r=0.50

Barbados «records» NAO and Barbados «records» NAO and Sahel droughtSahel drought

0

10

20

30

40

50

60

1966 1970 1974 1978 1982 1986 1990 1994 1998

SD

C (

µg

/m3

)

-1

-0,5

0

0,5

1

1,5

SA

D I

nd

ex

Winter r=0.30

Chiapello, Moulin & Prospero, JGR, 2005

0

5

10

15

20

25

30

1966 1970 1974 1978 1982 1986 1990 1994 1998

SD

C (

µg

/m3

)

-6

-4

-2

0

2

4

6

NA

O I

nd

ex

Summer r=0.65

Barbados 35-years record Barbados 35-years record shows a residual increase in shows a residual increase in

the dust loadsthe dust loads

➲ 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)

-10

0

10

20

30

1965 1970 1975 1980 1985 1990 1995 2000Year

SD

C (

µg

/m3

)

MEASUREMENTSESTIMATED NATURAL VARIABILITYRESIDUAL TRENDINCREASE 0,17 µgm-3/Year, R=0,50 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

Is this increase also recorded on Is this increase also recorded on 20 years of TOMS observations?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

An increase coincident to semi-An increase coincident to semi-arid regions affected by land arid regions affected by land degradation due to human degradation due to human

pressurepressurePopulation Sahel

0

5000

10000

15000

1965 1970 1975 1980 1985 1990 1995 2000 2005

SenegalMauritaniaMaliBurkina FasoNigerChad

Increase of population over Sahel (FAO)

Wind erosion severity (UNEP/ISRIC)

Increase of dust in TOMS observations

Mali and Niger

ConclusionsConclusionsThe 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 degradationAn human impact is highly suspected to explain an increase of 40-50% of the dust loads measured in the last 2 decades