Jun 20, 1998 - Abstract. A daily analysis of African dust concentrations in the Mediterranean ... shift in transport location and the south-north gradients of dust ...
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 103, NO. Dll, PAGES 13,137-13,144, JUNE 20, 1998
Satellite climatologyof African dust transport in the Mediterranean atmosphere C. Moulin,1C. E. Lambert, 2U. Dayan,TMV. Masson, 1M. Ramonet, 1P. Bousquet, • M. Legrand, 5Y. J.Balkanski, • W. Guelle,l B. Marticorena, 6 G. Bergametti, 6 and F. Dula½2
Abstract.A dailyanalysisof Africandustconcentrations in theMediterranean atmosphere has beenmadebetweenJune1983andDecember1994usingthe IntemationalSatelliteCloud Climatology Project(ISCCP-B2)archiveof Meteosatvisible(VIS) channelimages.TheISCCPB2 archiveof Meteosatinfrared(IR) imageshasalsobeenusedto determine thefrequencies of dustmobilizationoverthe continent,northof 30øN.Despitea largedaily variability, climatological resultsshowa clearseasonal cyclewitha maximumduringthedryseason: dust
transport beginsovertheeastern basinin springandspreads overthewestembasinin summer. Thesepatterns areshownto berelatedto bothcyclogenesis overNorthAfricaandrainfallover theMediterranean Sea.Indeed,thefrequency of dustmobilization overthecontinent andof dust outbreaks overtheseaarestrongly relatedto theclimatology of depressions affectingNorth Africa.Precipitations appearto beanimportant factorexplaining boththeseasonal east-west shiftin transport location andthesouth-north gradients of dustconcentrations overthe Mediterranean.
1. Introduction
In the last 20 years, a large effort has been made toward the understandingof African dust transportover the Mediterranean Sea. Scientistsare now convincedthat dust transport,which was once consideredas a series of spectacularand unpredictable events, plays an important role in our understandingof the biogeochemical cycling and sedimentation within the Mediterranean
sea itself. The work on Mediterranean
dust has
developedin threedirections:(l) evaluationof the impactof dust transportthroughcontinuousmonitoringstations[Bergamettiet al., 1989a,b; Molinaroli et al., 1993;Kubilay and Saydam, 1995; Lo?e-Pilotand Martin, 1996; Guerzoniet al., 1996] or through the estimateof transportusingtrace elementsin seawater[Duce et al., 199I; Measures, 1996]; (2) tracingof the origin of dust stormsfrom meteorologicaland isotopicstudies[Berga•nettiet al., 1989a;Groussetet al., 1992]; (3) modelingdusttransportin seawater [Ruiz-Pino et al., 1990]. Recently, the European
programMediterraneanDust EYperiment(MEDUSE) [ 1996] has beensetto allow a predictionof dustlargeeventsthroughthe use of source models [Marticorena, 1995; Marticorena and Bergametti, 1996], meteorological transport [Nickovic and Dobricic, 1996; Schulz et al., 1996], and dust deposition [Balkanskiet al., 1996]. All thesestudiesconvergeto showthe chemicalimpactof African duston the precipitationsand surface waters in the Mediterraneanbasin. The radiative impact of dust has been demonstratedover the tropical Atlantic in the solar spectrumfrom satellitedata [Jankowiakand Tanrb, 1992] and in the infrared from model computations[Tegen et al., 1996], but remains poorly known over the Mediterraneanbasin. This is mainly due to the complexity of the synoptic meteorology: surrounded by lands and located at the boundary of the subtropicaland midlatitudeatmosphericcirculations,this region experiences very differentmeteorological conditionsthroughout the year [Dayan et al., 1989;La Fontaineet al., 1990]. Owing to their satisfactory spatio-temporal coverage, meteorological satellites enable long-term analysis over the western and eastern basins. A major interest of the satellite
•Laboratoire deMod61isation duClimat et del'Environnement, observation ofdesert aerosols istoenable thedailyobservation of
Commissariat &l'Energie Atomique, Direction des Sciences delaMati•re,largegeographical zones. Theretrieval oftheaerosol optical Gif-Sur-Yvette, France. 2Centre des Faibles Radioactivit•s, Centre National delaRecherche depth inthevisible spectrum ispossible over marine areas with Scientifique-Commissariathl'EnergieAtomique, Gif-Sur-Yvette, France.lowandfairlyconstant albedo because thecontribution fromthe 3Environment andRiskAssessment Section, Soreq Nuclear Researchatmosphere to thesatellitesignalis thendominant. Recently, 5 Centre, Yavne,Israel.
years(1984-1988) of Meteosatimages[dankowiakand Tam'b,
4Now atDepartment ofGeography, Hebrew University ofJerusalem, 1992]and 4 years(1989-1992)of Advanced Very High Jerusalem, Israel. Resolution Radiometer (AVHRR)data[Swap etal.,1996]were
•Laboratoire d'Optique Atmosph•rique, Universit• des Sciences et analyzed toquantify theAfrican dust load over the tropical North
Technologies de Lille - CentreNationalde ia Recherch½ Scientifique,
Villeneuve d'A$cq, France.
Atlantic. Moulin etal. [1997a] used theresults ofan11.5-year
•Laboratoire Interuniversitaire desSyt•mes Atmosph•riques, Universit•(June1983to December 1994)dailymonitoring of dusttransport
de Paris7 et 12 - CentreNationalde la Recherche Scientifique, Cr6teil, retrieved France.
Copyright1998by the AmericanGeophysical Union. $09.00
Meteosat
International
Satellite
Cloud
In this work, we used the results of this Meteosat archive to
Papernumber98JD00171. 0148-0227/98/98JD-00171
from
ClimatologyProject(ISCCP-B2)imagesovertheMediterranean andthe Atlanticto evidencea large-scaleclimaticcontrolon the interannual variabilityof theexportof Africandust.
present andexplainthegeneralpatternof thedusttransport over 13,137
13,138
MOULINET AL.' SATELLITECLIMATOLOGY OFAFRICANDUSTTRANSPORT
the Mediterranean basin. We report here results of the first uncertaintyon the retrieveddustopticaldepthfor clearskypixels climatologyof dustopticaldepthover this poorlydocumented was estimated to be within +25%. The completedust field was reconstructed for eachday over region. The contrastbetweenhigh daily variability of dust by interpolating thevalueof eachcloudypixel, transportand "predictable"transportis highlightedby these the Mediterranean usingan inversedistanceweightedmean.As discussed by Moulin analyses. et al. [1997b], this interpolationis certainlyone of the major sourceof uncertainties, evenif it hassomephysicalbasisandif 2. Method too cloudyimagesare removedfrom the dataset. However,this step remains important for the representativity of our 2.1. Description climatological resultssincecloudsaremorefrequentin somepart
OnedailyMeteosatimageof thevisible(VIS) channelin the ISCCP-B2format(apparentresolution of 35 km by 35 km over the Mediterranean),takenat 1145 UT, was analyzedfrom June 1983 to December1994. These imageswere providedby the European Space OperationsCentre (ESOC, Darmstadt, Germany).Sincedustplumesoverthe Mediterranean generally last severaldays,it is expectedthat onedaily imagewouldbe sufficientlyrepresentative for a long-termanalysis,evenin the caseof !-day eventssuchas thosereportedby Dayan et al. [1991] comingfrom the Arabianpeninsulaover the eastern Mediterranean.During this period,four differentsensorswere used: Meteosat2 throughMeteosat5. Whereasthe spectral transmissionof the VIS channelis centeredaround0.7 pm for all
of the Mediterranean and could thus bias the results. Moulin et al.
[1997c] presenta first assessment of the uncertaintiesrelatedto
the interpolation by comparison with sunphotometer measurements:interpolated optical depths are found to be retrieved within a factor of 2. However, the number of Sun
photometermeasurementsavailable for this study is too small sinceit requiresboth cloudyconditionsfor Meteosatand clearskyconditionsfor the sunphotometer (i.e., the sunphotometer has to view betweentwo clouds).Only concentration measurements performedat the surfaceare thus suitableto complementour knowledgeon the representativity of the interpolated results. 2.2. Representativity of the Results
sensors, its shapefairly differsfrom oneto the next.In addition, We havetentativelyassessed how muchthe cloudcoverage the digitizationincreased from 6 significantbitsfor Meteosat2 and3 to 8 significant bitsfor Meteosat4 and5, yieldinga greater couldbiasour resultsby comparingour datasetwith concurrent sensitivityfor the lattersensors. daily groundmeasurements performedby Bergarnetti[1987] at a The methodusedto performa multiyearmonitoringof daily stationlocatedin the northwesternCorsicaat Capo Cavallo, African dust load over the Mediterranean has been described in duringa periodof 13 monthsbetweenApril 1985andApril 1986. detail by Moulin et al. [1997b]. First, pixels possibly Thisdatacollection consists in dailysampling of marineair and contaminated by highlyreflectivesurfaces suchasclouds,lands measurementsof tracers of mineral dust (A1, Si). We have and coastalwaters were eliminated. For eachclear-skymarine comparedthesedatawith a daily analysisof ninemarinepixels pixel,thesatellitesignal(numericcount)wasconvened intoa closeto the Corsicanstation(Figure 1). One doesnot expecta
radiance, takingintoaccount a detailed calibration of thedifferent perfect correlation between these two data sets since dust radiometers [Moulinet al., 1996].Theopticaldepthof desertdust transportmay sometimeshappenat high altitudes[Reiffet al., at a wavelength of 0.55 pm wasthenretrievedusinga radiative 1986; Martin et al., 1990], mainly in summer.In such cases, transfer model. The contributions of tropospheric and satellitedata will showdusttransport,but grounddatawill not stratosphericsulfateswere subtractedusing climatological detect any significant increase in dust concentrations.This information[Moulin et al., 1997a, b], and the mean optical occuredfive times duringthe coincidentperiod(in mid-July properties of desert dust were validated by coincident 1985,early August1985,beginningandendof November1985, sunphotometer measurements [Moulinet al., 1997c].Theaverage mid-February1986).The reverseis of coursenot true:largedust
,
,
,
, ,
,
'F M A
...........
M J
J
A
S
O N
D
lrl{
J F
M A
Month (since Feb. 1985)
Figure 1. Comparison of the Africandustloadsrecorded betweenFebma• 1985andApril 1986by Meteosat (opticaldepth;uppercume)andby surface measurements (siliciumconcentration; lowercurve[fromBergametti, 1987] at CapoCavallo,Corsica.For clarity,opticaldepthsbelow0.08 obtainedfrom Meteosatare fixed to 0.08. Peaksobservedin Meteosatdataare indicatedby ve•ical dashedlines.An a•ow indicatesthat a dusteventis registredusingsurfacemeasurements.
MOULIN ET AL.: SATELLITE CLIMATOLOGY OF AFRICAN DUST TRANSPORT
13,139
Table 1. Summary Statistics forthe 11-YearPeriod(1984-1994)of DailyAnalysis of Meteosat Images Numberof Days Period
Analyzed Retained
Interpolated Pixels,%
AverageOpticalDepthDue to Dust Western Med.
Central Med.
Eastern Med.
Total Med.
Winter (DJF)
990
409
76
0.06 + 0.02
0.08 + 0.02
0.06 + 0.03
0.06 + 0.02
Spring(MAM)
1011
696
67
0.10 + 0.04
0.15 + 0.06
0.14 + 0.05
0.13 + 0.05
Summer(JJA)
1011
952
48
0.14 + 0.04
0.18 + 0.06
0.10 + 0.04
0.14 + 0.04
Fall (SON)
1001
656
66
0.07 + 0.03
0.09 + 0.04
0.07 +_0.03
0.07 + 0.03
Annual
4013
61
0.10+0.03
0.13+0.04
0.10+0.03
0.11 +_0.04
2713
The western,central,and easternMediterraneanbasinswere arbitrarlydefinedby a first borderlinebetweenTunis, Tunisia,andthe tip of the Italianboot,anda secondborderlinebetweenAthens,Greece,andBenghazi,Libya. Resultswere retainedfor a given day only if clear-skypixels spreadover at least 10% of each basin. The averagepercentageof interpolated pixelsis computed onlyfromretainedimagesandincludesturbidcoastalwaterpixelsandcloudymarinepixels.
Surl•tces of thewestern, central, andeastern Mediterranean basins are1.035,0.757,and0.879x 106km2,respectively.
concentrationsat ground level are expected to be seen by Figure2 showsthatthecycleof dusttransportvariessignificantly Meteosat,exceptif too muchcloudiness prevails.Duringthe 13- from one basinto the next and that it is characterizedby both a monthperiodof observation,85 daysweretoo cloudyto allow us changein peakfrequencyand in backgroundlevel. Be•;garnetti et to recovera usefulMeteosatsignalcloseto the station.Usingall al. [1989a] consideredthat dust peakscontrol the quantity of coincidentdays of measurementsin Figure 1, we only missed mineralparticlesin the atmosphere, whereasthe baselinedepends threelargedusteventsdueto the cloudcoverage(mid-May 1985, mainly on the residencetime of the mineral particles in the mid-November1985, and beginningof February1986) among atmosphere.Thus the seasonalcyclesobservedwith Meteosat the 22 peaksrecordedat groundlevelby Bergarnetti[1987]. Two might resultfrom thesetwo combinedphenomena, that is, both othergroundeventsweremissedby Meteosat,onein mid-August changesin the amountof dustinjectedin the marineatmosphere 1985 and one in mid-March 1986. For these two events, we andof the atmospheric residencetime of dustparticles. Our long-termanalysisevidenceswell definedspatio-temporal analyzedthe otherimagesavailableduringthe day (at 0845 and 1445 UT). In both cases,one couldobservea small dustplume structuresof the African dusttransportoverthe Mediterranean,as whichreachedCorsicain the afternoonandhaddisappeared from shownby the monthly climatologicalmaps of the dust optical clearlyshowthat the regionby noonthe next day: it is thereforenot seenon our depthin Plate 1. The markedspatialstructures daily local recordsat 1145 UT. However, thesetwo eventshave the transportfollows a sufficientlyregularpatterndespitea large been observed and thus do not bias our estimates at the basin daily variability.The maximalnorthwardtransportshiftsfrom the scale. This qualitative comparisonshows that despite the easternto the westernbasin during March to August. During elimination of numerouscloudy days (mainly in winter), our autumnandwinter,thereis very little dusttransport.This general Meteosatanalysisseemto remain sufficientlyrepresentativeto patternof the monthlytransportwasverifiedfor individualyears. Indeed, despite large variabilities of the intensity of the dust observethe largemajorityof dustevents. Meteosat VIS window over the Mediterranean is of about exportboth from year to year for the last decade[Moulin et al., 2000 pixels,andmorethan4000 of thesewindowswereanalyzed 1997a] and from seasonto seasonfor a given year, the dust for the 11.5-yearperiod.Table I showsthatthe seasonaloptical transportalways follows the sametemporaldevelopmentas the depthdueto desertdustrangesfrom0.06 to 0.18 andthe standard one shown in Plate 1. We will now discussthe processesthat deviationrangesfrom 0.02 to 0.06. Becauseof an important explain these patterns of the monthly climatological dust cloudcoverage,abouthalf of the imageswere rejectedfrom the transport. databasein winter,andaboutonetenthwererejectedin summer. The average optical depth and its standard deviation are 3.2. Transport Processes
especiallylow in autumnandwinter.By contrast,in springand summer, both average optical depth and standarddeviation increase,especiallyoverthe centralbasin(Table 1).
3. Climatologyof Dust Transport 3.1. Monthly Climatology
This annual cycle is evidenced in Figure 2 where the variations of both the daily dust optical depth over the three basinsare shownfor 1994, a year that was shownto be a mean year in terms of annual African dust export [Moulin et al., 1997a]. The variationsin optical depthare fairly different over eachbasin:most of the dusttransportoccursbetweenMay and November for the westernbasin and betweenJanuaryand July for the easternbasin.The large-scaleaverageopticaldepthduring dusteventsis not significantlydifferentbetweenthe threebasins.
Contrary to the African dust transportover the tropical Atlantic [Prospero, 1990, 1996], the average atmospheric circulation over the Mediterranean does not explain the northwardtransport of dustparticlesbecause windsmostlycome from the west or northwest[La Fontaine et al., 1990]. Thus African dustcanonly be transported overthe Mediterranean with the northwardwinds generatedby depressions. Numerouscase studieshave shownthat the frontalregionof thesedepressions can generateintensedustmobilization over North Africa and are
able to transportdustover long distancestowardItaly, France, and northernEurope [Prodi and Fea, 1979;Reiff et al., 1986; Franz•n et al., 1994]. The climatologyof the Mediterranean cyclones proposed by Alpert et al. [1990] using the meteorologicalanalysisof the EuropeanCentre for MediumRangeWeatherForecasts(ECMWF) enablescharacterization of the mostlikely synopticsituations allowingdusttransportover
13,140
MOULIN ET AL.: SATELLITE CLIMATOLOGY OF AFRICAN DUST TRANSPORT 0.6 WESTERN
=-05
•
0.4
•
0.3-
•D
,-
_
0.2-
tO
m
MED.
(1.035 106 knl2)
_
0.1-
Month (1994) 0.6 CENTRAL
o.5 t
MED.
(0.757 106km2)
_
0.4-
0.3-
0.2=
01.
_
o
Month (1994) 0.6 EASTERN
MED.
(0.879 106 km2)
0.40.3-
¸
o.1 o
, ..:1 .... .... v
J I F I MI AIM
I j Ij
..1[
IA I S t O I N I D
Month (1994)
Figure 2. Daily variation oftheoptical depth due todesert dust over thethree Mediterranean basins in1994 (fine black line). Wecomputed abackground value oftheoptical depth (wide grey line) asarunning mean over 5days after elimination ofpeaks characterized asdays with adust load larger than themonthly average and larger than the
twopreceeding andtwofollowingdays.
the seain springandsummer;it is indeedeasierto reducethe dust[Alpert andZiv,1989].Thetrajectories of theses cyclones
greatvariability of synoptic situations implying depressions over generally agree withtheimportant spring transport ofdust(Plate theMediterranean basinif onetakesonlyintoaccount themain 1) overtheeastern andcentralMediterranean basins. (2) Dust
zones ofcyclogenesis leading todustmobilization [Alpeft etal., 1990].
transport in summer is different becauseit is linked to two
synoptic situations described earlier byBetxametti etal. [1989a].
Using this!nformation, wehavefound three different major Saharan lows stilldevelop, butthepresence ofahigh overLibya
cyclogenesis situations whichcan explainnorthward dust preventsthem from followinga northeastern direction.This transport respectively in April,June,andAugust,asshownin meteorological situationinducesviolentsouthor southwestern Plate2. (1) Springandearlysummerarethe mostfavorable
winds between thetwosystems andischaracterized byastrong
periods forthedevelopment ofSaharan thermal lows(alsocalled dusttransport fromTunisiaandwestern Libya.Sucha Sharav cyclones) south ofAtlas, under theinfluence ofthestrongconfiguration isingoodagreement withthesummer maps ofthe
thermiccontrast betweenthe temperature of thecoldmarine averagedustopticaldepthdeducedfrom Meteosatdatawhich waters andofthewarmcontinental surfaces. Such cyclones move showa maximal transport northof AlgeriaandintheGabes Gulf eastward alongthis thermalgradient just southof the North (Plate1).(3) Towardtheendof summer, Balearic Islands become
African coast andfinallycross theMediterranean between Libya an activedepression centerwhereAtlanticlowsare reinforced andEgyptwheretheytransport largeconcentrations of desert beforethey go acrossthe Mediterranean in the directionof
MOULIN ET AL.: SATELLITE CLIMATOLOGY OF AFRICAN DUST TRANSPORT
13,141
0.30 0.20 0.15
'--'-'
0.10
0.05 0.00 O.D.
Plate 1. Monthly I l-year averageof dustoptical depths(O.D.). Data from availableMeteosatimagesbetween 1984 and 1994 (seeTable 1).
CorsicaandItaly. Thismeteorological situationexplainsthehigh provide a sufficient spatio-temporal coverage to assess dustopticaldepthsover the AIboranSea in July and August climatologicaldust sources.We thusrelied on infrared(IR) (Plate 1). It is noteworthythat duringsummer,a semipermanent Meteosatimagesto observethe major dustsourcesover the usingthemethodof Legrandet al. [ ! 994].This low, centeredoverCyprus,takesplaceovertheeasternbasinand Africancontinent methodis basedon the observationof a significantdecreaseof prevents dusttransport by imposing southward winds. and thusof the IR numericcounts Previouscasestudiesbasedon air masstrajectoryanalysisfor the brightnesstemperature bothwestern[e.g., Bergamettiet al., 1989a;Dulac et al., 1992; during daytime over arid region when dust concentration in the atmosphere, providedthatthereis no cloud.This Prodi and Fec• 1979;Reiffet al., 1986]andeastern[e.g.,Dayan, increases of the 1986;Dayan et al., 1989;Kubilayand Saydam,1995] basins apparentcoolingof the surfaceis dueto the attenuation follow thesegeneralfeaturesof the meteorological situations infraredradiance,emittedby the hot arid soil, by the colder dustlayer.This dusteffecton infraredimageryis which generatethe most frequent dust transportover the atmospheric Mediterranean. enhancedwhenonecomparesdaily imagesto a compositeimage containing thewarmestpixelsof theweek.ForeachMeteosatIR imageduringthe 11.5-yearperiod,we computed thisqualitative 3.3. Location of Sources infrareddifferencedust index (IDDI) for every clear-skypixel Contrary to the principle schemesshown in Plate 2, case over North Africa. studies as cited above enable more or less determination of the The monthlyanalysisof the frequencyof dustmobilization locationof sourceregionsof dustparticles.However,theydo not over the North African continentusingIR Meteosatdata set is
13,142
MOULINETAL.:SATELLITE CLIMATOLOGY OFAFRICANDUSTTRANSPORT 3.4. Dust Removal by Precipitations
Althoughtile atmosphericcirculationobviouslyplaysa major role in determining the frequency of dust plumes, it does not explain the relatively strong south-nortl• gradients of dust transport intensity (Plate 1). Precipitations are an important constrainton the extent of dust transportbecausewashoutby raindropstraps particlesand depositsthem to the surface[Gatz, 1977]. Bergamettiet al. [1989a] have shownthat during the wet 0.30 June ' period in Corsica (winter and autumn) the average interval 50 0.20 ---between two rain events is shorter than the average interval -' '" •--' : '"' 30 between two dust events, so that, as a first approximation,the 0.15 atmospherehas no time to be loaded again with dust particles. 0.10 Over the Mediterranean,the probability for a dust plume to 0.05 encounter rain increases toward the north, explaining the 0.00 relatively stronggradientsin dust optical depth observedin the O,D. principal(south-north)directionof the transport(Plate 1). Plate3 shows a comparison between seasonal climatologies of precipitations and of our desert dust optical depths. The Mediterraneanis heavily affectedby precipitations in winter and autumn, which, together with the abundanceof rain over the Maghreb,explainthe absenceof dusttransportduringthisperiod. During spring, precipitationsare low over the easternbasin, where dusttransportis more intense,whereasthe westernbasin Plate 2. How the main meteorologicalsynopticsituations remains affected by intense precipitations, especially in its generatea recurrentdusttransportduringspringandsummer. northernpart. During summer,the whole Mediterraneanbecomes Locationsof the majorlows(L) andhighs(H) are shown.The dry, and the dusttransportcan then developefficiently. Low dust cyclogenesis is adapted from,41pert et al. [1990].It showsthe concentrations over the easternbasinare thendue to the presence Sharavcyclones in April;thecouplingbetween Saharan lowand Libyanhighin June;andtheeffectof theBaleariccyclogenesisof the semipermanentdepressionover Cyprus.This goodspatioin August.The frequency (Fr.) of dustmobilization overNorth temporal correspondancebetween dust optical depth and Africa during these monthshas been estimatedfrom IDDI precipitationfieldsexplainswell the strongcontrolexertedby the computedfrom Meteosatinfraredimagesbetween1984and North Atlantic Oscillation on the interannualvariability of the 1994, usingthe methodof Legrandet al. [1994]. Finally,our intensityof the dusttransportover the Mediterranean[Moulin et monthlydustopticaldepths(O.D.) overtheseasurface areshown al., 1997a]. Indeed, it is obvious from Plate 3 that the shift in as in Plate !. The lower and upper horizontaldashedlines latitude of the precipitations induced by this large-scale correspond to latitudesof 30ø and40øN,respectively. meteorological phenomenon mighthavea stronginfluenceon the geographicalextensionof the dusttransport.
ß•
...... •
15
Fr.(%)
4. Conclusion
also shown in Plate 2, togetherwith dust optical depth over the seaand main meteorologicalsituationsfor dusttransport.April is This work presents tile first comprehensive analysisof the shownto be a periodof intensedustuplift over North Africa, in African dust transportin the Mediterranean atmosphere. The relationto the frequentmovingcyclonetracks.Plate2 showsthat monthlyclimatology of Africandustopticaldepthsobtained from duringthismonth,the majorsourceregionsfor the Mediterranean 11 years of daily Meteosat ISCCP-B2 VIS imagesshowsa are south Tunisia
around 30øN for the western central basin markedannualcycleof thedusttransport. It beginsin springover [Bergamettiet al., 1989a];Libyandesertjust below30øNfor the the eastern basin, is maximum in summer over the western and easterncentraland easternbasin[Dayan et al., 1991;Kubilayand centralbasins,and stronglydecreases duringautumnand winter. Saydam,1995]; Egyptiandesertfor the far easternMediterranean The desertdust transportresultsfrom combinedactionsof the synopticcirculationwhichcontrolsthe frequencyof [Dayan, 1986]. In June, the extent of North African sources atmospheric decreases,while the dust export shifts westward.During this mobilizationand transport,and of the washoutby precipitation period,dustoriginatesmainlyfrom the northernSaharabelow which influences tile residence time of particles in the 30øN, from south Tunisia around 30øN, and from northern atmosphere. Comparison of the results with existing Algeria. Concerningthis last sourceregion,the meteorological climatologies of cyclogenesis and precipitation over the situationpresentedfor Junedoesnot seemsto fit well, andthe Mediterranean shows that over the Mediterranean, only sourcemight thus be activatedby the occurenceof summer perturbationsof the atmosphericsynopticcirculation generate situationsof the "August"type. An importantresultfor the June dusttransportand thatthe washoutis speciallyimportantbecause transport is thatthedusttransport overthecentralbasinis mainly it limits boththe westwardextensionof dusttransportduringthe related to northwestern African sources. In August, source springand its northwardtransportduring most of the year. The
regionsfor the westernMediterranean dusttransportare well definedandhavea surprisingly smallextent.They are locatedin Moroccoand northernAlgeria, as suggestedby the work of Bergametti et al. [1989a]basedonback-trajectories.
useof coincidentMeteosatIR imagesenablesthe obtainmentof a coincident climatology of dust source regions Comparison between these two satellite climatologiesindicatesthat Lybian desert constitutesone major source of dust over the eastern
MOULINETAL.:SATELLITE CLIMATOLOGY OFAFRICANDUSTTRANSPORT
0.30 0.20
10
0.15
2.5
0.10
1
13,143
5
0.05
Pr. (mm/d)
0.00 O.D.
Plate3. (left)Climatological maps ofmonthly dust optical depths (O.D.)between 1984and1994(seePlate1)and (right)precipitations between 1979and1988(datafromAtmospheric ModelIntercomparison Project (AMIP) program[Schemm,1992]).
M6diterran6e occidentale: Aspects g6ochimiques et m6t6orologiques, Mediterranean and that some very local sources of the Ph.D. thesis,Univ. of Paris7, Paris,France,1987. northwestern Africa stronglycontributeto the dusttransportover Bergametti, G., A.-L. Dutot,P. Buat-Mfinard, R. Losno,and E. thewesternbasin.Finally,thissatelliteclimatology constitutes an Remoudaki,Seasonalvariabilityof the elementalcompositionof unequaled datasetfor three-dimensional dusttransport model atmospheric aerosol particles overthenorthwestern Mediterranean, Tellus,Set'.B, 41,353-36 !, 1989a. validationwhich will enableacc. urateestimatesof the deposition fluxes and of the radiativeimpactof African dust over the Bergametti,G., L. Gomes,E. Remoudaki,M. Desbols,D. Martin, and P. Mediterraneanregion.
Acknowledgments. We acknowledge thekindhelpof X. Schneider whoparticipated in ourcomputer testing experiments. Thisworkispart of a close collaboration between the LMCE and the CFR and was
Buat-M6nard, Presenttransportand depositionpatternsof African dusts to the northwesternMediterranean, in Paleoclimatology and Paleometeorologv: Modernand Past Patternso.!'GlobalAtmospheric
Transport, edited by M. Leinen and M. Sarnthein,pp. 227-251, Kluwer Acad., Norwell, Mass., 1989b.
Dayan,U., Climatologyof back-trajectories fromIsraelbasedon synoptic analysis,d. Clim.Appl.Meteorol.,25, 59!-603, 1986. CentreNationalde la Recherche Scientifique (CNRS). Additionalsupport Dayan, U., J.L. Heffter, and J.M. Miller, Meteorological and was providedby the CEC in the frameof the "Mediterranean Dust ClimatologicalData From Sur.!bceand UpperAir Measurements./br
supported bytheFrench Commissariat fi l'Energie Atomique (CEA)and Experiment" program andby INSU/CNRS andtheFrench Minist•rede
l'Environnement throughthe researchproject"ErosionEolienneen
the Assessmento.!' Atmospheric Transport and Deposition o.!' Pollutants in the Mediterranean Basin, MAP Tech.Rep. Ser., vol. 30,
R6gions Arideset Semi-Arides." C. E. Lambert andF. Dulacwouldlike U. N. Environ.Programme,Athens,1989. toexpress theirgratitude totheCEAforthecomputing facilities andthe Dayan, U., J.L. Heffter,J.M. Miller, and G. Gutman,Dust intrusion generous hospitality provided by theLMCE.Thisis contribution 418 events into the Mediterraneanbasin,d. Appl. MeWorol., 30, 1185from the LMCE and 2016 from the CFR. 1199, 1991.
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