Simulating the evolution of the Asian and African monsoons during the past 30 Myr using an atmospheric general circulation model Frédéric Fluteau, Gilles Ramstein, Jean Besse
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Frédéric Fluteau, Gilles Ramstein, Jean Besse. Simulating the evolution of the Asian and African monsoons during the past 30 Myr using an atmospheric general circulation model. Journal of Geo- physical Research: Atmospheres, American Geophysical Union, 1999, 104 (D10), pp.11995-12018. 10.1029/1999JD900048. hal-03012020
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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D10, PAGES 11,995-12,018, MAY 27, 1999
Simulating the evolution of the Asian and African monsoons during the past 30 Myr using an atmospheric general circulation model
Fr6d6ricFluteau, 1'2 G. Ramstein2 and Jean Besse 1
Abstract. At geologictimescales, many proxy data suggest a contrastingevolution of Asianand African monsoonssince the Oligocene.The Asian summermonsoon increases drastically around 8 Ma, whereasthe African summermonsoon gradually weakens during the Miocene.Using an atmosphericgeneral circulation model, we simulatemost of the spatialevolutions of both monsoonsonly accountingfor the changesof paleogeography,including continental drift, orogeny, and sealevel change.The paleogeographicchanges modify drastically the climateover central and southernAsia betweenthe Oligoceneand the present.The retreatof an epicontinentalsea warms centralEurasia in summer.The heatingof thisarea and the uplifts of theTibetan plateau and of the Himalayasdeepen the Asianlow-pressure cell anddisplace it northwest.This thenshifts precipitationfrom Indochinatoward the southernflank of the Himalayas.This is in good agreementwith proxydata. Therefore our modelingstudies support a shiftand a strengtheningof the Asianmonsoon during the late Tertiary ratherthan a real "onset".We suggestthat the increase in seasonalprecipitation and the strengtheningof thenumber of dayswith heavyrainfall over the Himalayasfrom 30 Ma to thepresent may be of criticalimportance to explainthe long- term evolutionof physicalerosion of this area.We alsoinvestigate the respectiveimpact of the Paratethysshrinkage and of the Tibetanplateau uplift throughsensitivity experiments and prove that theParatethys retreat plays an importantrole in monsoonevolution. The northwarddrift of the African continentconfines summer monsoon precipitation to a thinbelt whichfavors the stretching of the subtropicaldesert, in goodagreement with data.We finally showthat duringthe Oligocene, the African andAsian monsoon systems are clearlyseparated by the Tethysseaway. The closureof this seawayand the evolutionof the Asianmonsoon induce a connectionbetween both monsoon systemsin the low andmiddle troposphere.
1. Introduction Although driven by a commonmechanism, the rainfall intensity of the Asian monsoon is stronger than that of the African The monsoondominates the large-scaletropical circulationon monsoon. This difference is due to the Asian continental size and both the African and Southeast Asian continents. It results from to the presenceof orography(Tibetan plateau and Himalayan the thermal contrastbetween the continent and the surrounding range) [James,1994]. ocean [Hastenrath, 1985; Webster, 1987]. In winter, the Monsoonspresent a large time variability, rangingfrom years continent is colder than the ocean and induces northeasterly to thousandsof yearsand more [Prell et al., 1992]. At geologic winds toward the Arabian Sea (Asian monsoon)and toward the timescales(more than a million years), paleoclimaticindicators Gulf of Guinea(African monsoon), producing weak precipitation suggestan abruptstrengthening of the Asianmonsoon during the because of the little moisture brought by these atmospheric late Miocene, about7.5 Ma [Quade et al., 1989; Prell et al., masses [Hastenrath, 1985]. In summer, overheated continents 1992]. Althoughsparse, paleoclimatic indicators suggest the become warmer than the ocean. A low-pressurecell is then developmentof a subtropicaldesert over northemAfrica to the generatedover the warm continents,and a high-pressurecell over detrimentof the monsoon[Axelrod and Raven, 1978;Ruddiman the cold ocean. This differential heating induces advectionof et al., 1989; Maley, 1996]. moist air, producing heavy precipitation in a tropical belt At geologictimescales, monsoon evolution may be influenced stretchingacross Africa, India, and SoutheastAsia. The release by differentforcing factors such as paleogeographicchanges, of a huge amountof latent heat contributesto the strengthening oceanic circulation changes, variations of carbon dioxide of the monsoon circulation [Hastenrath, 1985; Webster, 1987]. concentration and variations in insolation [Barron and Washington,1984; Frakes et al., 1992]. The impactof someof these forcing factors was investigatedthrough numerical 1Laboratoirede Pa16omagn6tisme et G6odynamique, Institut de experimentsusing an atmosphericgeneral circulation model Physiquedu Globede Paris. (AGCM) [Barronand Washington,1984; Kutzbach et al., 1989; 2LaboratoiredesSciences duClimat etde l'Environnement, CE-Saclay. Ramstein et al., 1997a]. Until now, mostexperiments on the monsoonhave focusedon Copyright1999 by theAmerican Geophysical Union. theimpact of a mountainuplift without accounting for changesin Papernumber 1999JD900048. platemotions or sea-landdistribution. Sensitivity experiments to 0148-0227/99/1999JD900048509.00 the Himalayanand Tibetanuplift were doneusing full or half
11,995 11,996 FLUTEAU ET AL.: SIMULATING THE EVOLUTION OF MONSOONS altitude of the mountain rangesin a presentday environment in their relative positions. For this, we use the finite [Kutzbach et al., 1989; Ruddirnanand Kutzbach, 1989; Prell and reconstructionparameters of Olivet et al. [ 1984] for the North Kutzback, 1992; Kutzbach et al., 1993]. Major resultswere that and Central Atlantic Ocean, of Niirnberg and Muller [ 1991] for the uplift of the Tibetanplateau significantly increases the Asian the South Atlantic Ocean, and of Royer and Sandwell [ 1989] for monsoonintensity over mostof southernAsia (+50% in summer the Indian Ocean. The paleomagneticsynthetic apparent polar precipitation) [Prell and Kutzbach, 1992] and modifies the wanderpaths (APWPs) of Besseand Courtillot [ 1991] were then climate over adjacentareas (Siberia, central Asia, Middle East, used to fix the paleolatitude grid. These APWPs were Mediterranean basin) [Kutzbach et al., 1989; Ruddirnan and constructedfrom a careful selectionof the best paleomagnetic Kutzbach, 1989]. polesavailable for Asia, Europe,America, and Africa, transferred Other factors such as atmospheric carbon dioxide onto a single referenceframe and averagedover 20 Myr time concentration, oceanic circulation, and orbital parameters windows. In each of these independentwindows, data from influence the climate. The atmosphericcarbon dioxide plays a individual plates are almost always found to be in agreement, significantrole. The modelingcurve of Berner [ 1992] points out whichverifies the qualityof the originalpaleomagnetic data, the an atmosphericCO 2 concentrationdoubled with respect to the cinematicmodels, and the axial dipole assumption.The 20 Ma present day in the past 30 Myr. Prell and Kutzbach [1992] sequencesof paleomagneticpoles define a "synthetic"APWP, in showedthat a CO2 doublingenhances monsoon precipitation by excellent agreementwith the "classical"paths from each plate. only 7%. Conversely,this CO 2 doublingcould lead to a 6øC The 95% level uncertaintieson pole positionsare of the orderof warmingat middle and high latitudes[Ruddirnan et al., 1997a]. 3ø for the 10 and 30 Ma poles,leading to rather well constrained Oceaniccirculation patterns are induced by the evolution of paleolatitudes. oceanic basinsand the closureor opening of seaways(another The hot-spot referenceframe [Morgan, 1983] was not used consequenceof plate motions).Despite tectonic drift, the major becausesignificant inter-hot spot motion [Chase, 1983; Molnar features of the oceanic circulation in the late Cenozoic were and Stock, 1987] and true polar wander [Besseand Courtillot, relatively similar to present day circulation. A thermohaline 1991] cast some doubt on the use of this method as a global oceanic circulation began to be establishedduring the early reconstruction tool. Miocene [Kennett and Scott, 1990]. Oxygen isotopic analyses The proceduredescribed above can be used as long as show a weak cooling of sea surfacetemperature (SST) at high continentsare not stronglydeformed, or separatedby destructive latitudesduring the late Tertiary but hardly any changesin SST active margins. This is, of course,not the case for the regions occurred in the tropics [Frakes et al., 1994]. Finally, using an borderingthe Tethys, where a collision has occurredsince the AGCM coupledwith a mixed oceaniclayer allowing to take into end of the Mesozoic.In particular,central and southeastAsia are account the thermal responseof the ocean [Ramstein et al., characterizedby intensedeformation linked to the Indian-Asian 1997a], we have shownthat the changesin SST for theseperiods collision. Since the collision of India with Eurasia at about 50 were not inducing drastic modification compared to Ma, roughly north-south intracontinental convergencehas paleogeographicchanges. occurredat a rate of around 5 cm/yr. A combinedanalysis of The changesin insolationat the top of the atmosphereare magnetic anomalies from the Indian Ocean [Patriat and driven by orbital parametersof Earth and solar constant. Achache, 1984] with paleomagneticdata on the northernedge of However, we are not able to predict their values beyond5 Myr India and southernedge of Eurasia [Achacheet al., 1984; Besse [Laskar, 1988]. Moreover, the fluctuationsof these parameters et al., 1984; Chen et al., 1993] has showna N-S convergencein vary on a timescaleshorter than the evolutionsof the Asian and excessof 2600 km. The paleomagneticstudies on Cretaceous African monsoonsdepicted here. Thus we do not change the rocksfrom Tarim [Li, 1990], the Junggarblock, and Tibet (for a orbital parametersin our simulations. review, see Chen et al. [1993]) indicate distributed deformation Among the causesdriving climate changesduring the late during the Tertiary within central Asia and confirm the total Cenozoic, we will focus on the impact of the paleogeographic amplitude of convergence.The relative motion of India with changeson the Asian and African monsoonsusing an AGCM. respectto Siberia is well constrainedby the classicalcinematic During the late Cenozoic,orogenic episodes formed the great plate circuit Asia-North America-Africa-Indiausing the North mountainranges and plateaus(Tibet, Himalayas,Andes, Rockies, and Central Atlantic and the Indian Ocean, respectively.The Alps) observedtoday. Also duringthis time, significantchanges paleopositionof the Indochinablock, expulsed southward during occurredin the land-seadistribution such as the shrinkageof the the collision, is restoredusing the South China Sea kinematics ParatethysSea, an epicontinentalsea stretchingfrom Europe to parametersof Briais et al. [ 1993]. westernSiberia, and the closureof the Tethys seaway.We will investigatethe climatic impact of the paleogeographyof the 2.2. Land-Sea Distribution Oligocene(30 Myr ago) and the middle late Miocene (10 Myr ago). Theseperiods represent two different statesin the Tibetan We restoredthe paleoshorelines,using mainly the geologic and Himalayan uplifts. We have also analyzed the respective mapsof the Tethysprogram [Dercourt et al., 1993]. Changesin impactof Paratethysshrinkage and Tibetan plateau uplift through the distributionof seaswere not only due to globaleustatic sea sensitivityexperiments. level fluctuations[Haq, 1984], but also to the major orogenic 0 phaseswhich occurredalong the southernEurasian margin in responseto the convergenceof Arabia and Africa, resultingin 2. PaleogeographicReconstruction the buildupof majormountain chains spanning from the Alps to Zagros. The collision between Arabia and Eurasia closed a 2.1. Plate Reconstruction seawayconnecting the Indian and Atlantic Oceansduring the Platereconstruction (Figures l a and lb) wasestablished using earlyMiocene. Another major feature of the late Tertiaryperiod the proceduredescribed by Besseand Courtillot[ 1988]. Oceanic is the shrinkageof the Paratethysepicontinental sea in Europe kinematicparameters are first usedto placethe major continents andAsia [Dercourtet al., 1993].We havetherefore reproduced a FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS 11,997
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Figure 1. Paleogeogmphicreconstructions at (a) 30 Ma, (b) 10 Ma, and(c) PD. Isolinesare for 1 km, 3 km, and 5 km. Gray arearepresents oceans. The figuresare representedat our modelresolution. 11,998 FLUTEAU ET AL.: SIMULATING THE EVOLUTION OF MONSOONS large Paratethyssea in our Oligocenereconstruction stretching France-Lanord eta!. [1993]. We have used the work of from Europe to westernSiberia (Figure la), as shown in the Tapponnieret al. [ 1990] and Meyer et al. [ 1998] to constrainthe OligoceneTethys paleogeographic map of Lorenzet al. [ 1993]. elevationof the Tibetanplateau. The topographyis characterized During the Miocene, tectonicactivity and eustaticchanges by a progressivenorthward shift of the areas affected: the contributedto the shrinkageof the Paratethyssea, which was elevationof the southernTibetan plateaureaches some 3000 m localized in a confined area of central Eurasia (Figure lb) as the result of continental subduction within the ancient (Alpine-Carpathianand Ponto-Caspianbasins) until the middle Bangongsuture. The elevation to the north, closeto the Jinsha Miocene [Orszag-Sperberet al., 1993]. Pathwaysconnecting region, has a lower elevation of some 1000 m. This situation this inland sea with open oceanswere progressivelyclosed in correspondsto a progressivebuildup of the plateauat its northern responseto the evolution of the Alpine-Carpathian-Dinaridic edge. orogenic belt [R6gl and $teininger, 1984; $ztano, 1995]. Sea In North America, the mountain belt existed since at least the level regressionfurther isolatedthe ParatethysSea during some middle Cretaceous. Nevertheless, the elevation of this chain is shortperiods, favoring the onsetof endemicfaunas [see Sztano, muchhigher today than it wasduring the last 30 Myr [Ruddiman 1995, Figure 2]. The last connectionwith the MediterraneanSea et al., 1997b]. We prescribefor this mountainbelt, a low mean occurredbetween the late Burdigalian(18 Ma) up to the middle altitude(less than 1000 m) at the Oligocene[Bird, 1988] (Figure Seravallian (14 Ma) through the Slovenian corridor la), remnantof the Laramideorogeny (75-45 Ma). The tectonic [Nagymarosy, 1990, in Sztano, 1995], whereas the marine evolution of the western continentalpart was driven by the connectionwith the Indian Oceanwas alreadyclosed since the subductionof the Pacific plates during the Cenozoic[Oldow et late early Miocene (18 Ma) [R6gl and $teininger, 1984]. Thus, al., 1989]. The segmentationof this subductionzone has induced duringthe middleMiocene (14 Ma), the ParatethysSea became a transitionfrom compressiveto extensiveregimes in western entirely isolated, the salinity decreasedby about 30%, and America. This transitionled to the openingof grabensin the endemic fauna appeared [Hallare, 1994]. In the 10 Ma Basinand Range areas and to its gravitationalcollapse [Stewart, reconstruction,we only considerthe deeper basins and the 1977]. Consequently,this structure was higher during the flooding zones representedby shallow lacustrine sediment Oligoceneperiod than at present,but due to its surfaceand to the deposits[Orszag-Sperber et al., 1993] (Figurelb). interpolationon the grid model, this feature does not appear clearly in our reconstruction.The uplift of the Rocky Mountain 2.3. Topography range and of the Colorado plateau occurredafter 24 Ma, as deducedfrom paleodrainage systems suggesting an oppositeflow The deformationof Asia and the Tibetanplateau have been directionsince the Miocene [Scott, 1977]. We have assigneda the subjectof an ongoingcontroversy over the last decade.End- meanheight close to 1500 m for the Miocenetopography (Figure membermodels involve crustal or lithospheredoubling [Powell lb). and Conoghan,1975; Barazangiand Ni, 1982], homogeneous Sincethe late Oligocene,the increasedsubduction rate of the crustal or lithosphericthickening [Houseman and England, Nazcaoceanic plate has driven the formationof the Andes,along 1986a, 1986b],inhomogeneous strain concentrated along a few the PacificCoast ISmbrieret al., 1988]. Nonetheless,this uplift majormountain ranges, or suturesor strike-slipfaulting leading is not homogeneousover the full range length and also not to lateralescape of blocks[Tapponnier et al., 1986; Peltzer and continuousthrough time [Hoorn et al., 1995]. Compressiveand Tapponnier,1988]. The climatemay have also playeda role in extensivephases alternated within the centralAndes during the this orogenicprocess, due to complexfeedback between sea-land lateCenozoic. A linearuplift is usedin ourmodels to buildup distribution,orography, and monsoon[Molnar and England, the SouthAmerican topography. The Oligocenetopography is 1990]. In all cases,the elevationswith respectto time are not characterizedby a low orography(less than 1000 m) with a well constrained,and for instance,the period when the Tibetan discontinuousmountain range along the PacificOcean (Figure plateaureached its present-dayelevation is dated betweenthe la). At 10 Ma (Figurelb), we affecta continuousrange with a middle Miocene [Turner et al., 1993; Colemanand Hodges, half altitudewith respectto thepresent day. The Andesis not, at 1995] and the late Miocene [Harrison et al., 1992]. any rate, well depicted on our map becauseof its limited We have first assumed(Figure l a) that a mountainrange of longitudinalextent with respectto themodel resolution (Figure moderateelevation (2500 m) was locatedalong the Yarlung- lc). Zangbo suture (boundaryof the Indian-Eumsianblocks) at 30 EasternAfrican regions have experienced several phases of Ma(Oligocene reconstruction) andrejuvenated theGangdese uplift during the late Tertiary [Baker, 1971; Partridge, 1997]. batholith[Yinet al., 1994]. The rest of the future Tibetan plateau ' We kept the relief nearly unchanged withrespect tothe present remainedalow land (about 500 m), with probably smooth relief day (Figures laand lb). This mountain range does not exhibit ascontinned by paleodatasuggesting the presence of subtropical highelevation, with theexception of theEthiopian traps dated at flora, found for examplein the Lumpolabasin, centralTibet 30 Ma [Hofmannet al., 1997]. [Ren, 1981]. A rapid pulse of uplift of the Gangdesebatholith Sincethe early Cenozoic,Australia has driftednorthward with [Copelandet al., 1987;Richter et al., 1991] andHigh Himalayas the Indian plate. During the Oligoceneperiod, the Australian crystallinechain [seeAmano and Taira, 1992; Harrison et al., croton was greatly emerged [Lorenz et al., 1993]. It also 1992; Hodgeset al., 1992; Yin et al., 1994 for reviews]lead to underwentuplift in its southeasternpart, which is notrepresented an importantthrusting along the Main Central Thrust (MCT) because of the low resolution of our model. duringthe lower Miocene(17 Ma). A secondimportant uplift phaseof the High Himalayasthen occurredduring the middle- 2.4. North and South Glaciations: Antarctica and Greenland late Miocene (around 10 Ma) [Harrison et al., 1992], with the creationof a new thrustzone, the Main BoundaryThrust (MBT). The onset of Antarctic ice sheet formation [Ehrmannand We chose a mean elevation value of 4000 m in our l0 Ma Mackensen, 1992] is linked to the intensificationof the circum- reconstructionthat is in agreementwith the conclusionof Antarcticcirculation [Kennett et al., 1975]due to theopening of FLUTEAU ET AL.: SIMULATING THE EVOLUTION OF MONSOONS 11,999
the Drake Passageduring the Oligocene [Diester-Haassand Table l a. The Set of RealisticExperiments Zahn, 1996; Rack, 1993]. Since this epoch, this ice sheet underwentimportant fluctuations,although it never entirely Experiment Age,Ma Comparisons Impact disappeared.The progressivebuildup of the ice sheet has influenced the global climate, especially the pole-equator 0 PD Exp0-Exp1 paleogeography gradientin the southernhemisphere. Because this study concerns 1 10 Exp1-Exp2 paleogeography 2 30 the impact of paleogeographyon climate changes,we have consideredthat the Antarcticice sheethad alreadybuilt up since the Oligocene. In the northernhemisphere, the glaciationstarted at 3 Ma. However, evidenceexists for seasonalsea ice in the Arctic Ocean prescribedboundary conditions.This set includes the global in the early Miocene [Zubakov and Borzenkova, 1990]. paleogeographic reconstruction (land-sea distribution and Glaciationof the Greenlandcontinent occurred during the end of orography) as previously described. The other prescribed the Pliocene[Shackleton and Opdyke, 1977]. Consequently,we boundary conditions are SST, vegetation, CO2 atmospheric removed the Greenland ice sheet in both simulations(10 and 30 content, and orbital parameters.For the SSTs, we attributed Ma). The Greenland orography (Figures l a and lb) has been present-dayvalues when possible.When continentalgrid points adjustedaccounting for the isostaticresponse due to its absence becomeoceanic grid pointsdue to plate motions,we assignthe of ice sheet. value of the closestgrid point of an equivalentnature at the same latitude. For example, we use the present-daySSTs of the 3. Model and Run Descriptions Caspianand Black Seas for the ParatethysSea. This procedure hasbeen preferred to an easierone consistingin usingPD zonal 3.1. The LMD Model averagedvalue for SST, inducing a stronglyzonal atmospheric circulation and thus, cooling deeply the inner continent To perform our numericalexperiments, we used the [Ramsteinet al., 1997b]. Recentclimate investigations[Dong et Laboratoirede M6t6orologieDynamique (LMD) version5.3 al., 1997; Kutzbachet al., 1993] use an AGCM coupledto a slab AGCM [Harzallahand Sadourny, 1995]. This three-dimensional ocean but prescribethe meridionaloceanic heat fluxes. We have (3-D) modelsolves the dynamicand thermodynamicequations, alsoperformed such experiments with the LMD AGCM at lower continuityequations for mass,energy, and water vapor transport resolutioncoupled to a slab ocean(LMD4ter, 48x36 grid points) usinga finiteelement scheme. This is a grid-pointmodel with a to quantifythe influenceof changesin thermalresponse of the standardresolution corresponding to 64 pointsregularly spaced ocean. The conclusions on the Asian monsoon and the Eurasian in longitude,50 pointsdistributed in sineof latitude.The surface climate remain similar to those obtained with an AGCM not of thecells is equaland corresponds to 400 by 400km 2 at coupledto a slab ocean [Ramsteinet al., 1997a]. Moreover, as midlatitudes and reaches a finer latitudinal resolution at low discussedin section 1, there is no drasticvariation of the global latitudes(250 km at 30øN and 220 km at the equator).The model oceaniccirculation since the Oligocene. has 11 vertical levels in normalizedpressure coordinates; four According the modeling curve of Berner [1992], the CO2 lower levelsare devotedto the boundarylayer, four for the rest atmosphericcontent decreased by half from the Oligoceneto the of thetroposphere, and three for the stratosphereup to 30 km. It present day. This is in agreement with atmosphericCO2 includesa full seasonalcycle but no diurnalcycle for insolation. concentrationfor the past 10 Myr deducedfrom stomatadensities The radiativeschemes for solarand infraredradiation are adapted of fossil leaves [Van der Burgh et al., 1993], suggesting from Fouquart and Bonnel [ 1980] and Morcrette [1991], fluctuationsbetween 280 ppm and 370 ppm. However,we chose respectively.The AGCM includesa soil model,accounting for to keep it unchangedin order to focus only on the impact of the impactof a prescribedvegetation [Ducoudrd et al., 1994]. paleogeographicchanges. Thus we specifiedPD value (345 ppm) Each simulationlasts 16 years,and averagesof parametersare in all our simulations. We also use the PD values for Earth's madeon the last 15 yearsto representthe climaticequilibrium orbital parameters.Finally, for the soil-atmosphereinteraction, reachedby the model. the LMD5.3 AGCM includes an interface scheme [Ducoudrd et al., 1994] that accounts for the influence on climate of a 3.2. Set of Simulationsand Boundary Conditions prescribedvegetation. Vegetation changes are able to modify the Three experimentswere performed (Table 1) including a climate locally [De Noblet et al., 1996; Otto-Bliesner and simulationfor the present day (PD) (which is our control Upchurch, 1997]. We have, however, kept the present-day experiment(Exp0)), a simulationof the middlelate Miocene(10 vegetationdistribution, except for the "newly" emergedland Ma, Expl) and finally, a simulationof the Oligocene(30 Ma, points, where we have used the nearestneighbor method as Exp2). For each period, the climate model requiresa set of alreadydescribed for the SSTs.
Table 1b. The Set of SensitivityExperiments
Experiment Age,Ma PaleogeographicChanges Comparisons Impact
3 PD reducedTibetan plateau elevation Exp0-Exp3 Tibetanuplift at PD 4 10 reducedTibetan plateau elevation Exp1-Exp4 Tibetanplateau at 10 Ma 5 10 reducedParatethys Sea Exp5-Exp1 partialParatethys retreat at 10 Ma 6 30 reducedParatethys Sea Exp6-Exp2 partialParatethys retreat at 30 Ma 7 30 no ParatethysSea Exp7-Exp2 full Paratethysretreat at 30 Ma 12,000 FLUTEAU ET AL.: SIMULATING THE EVOLUTION OF MONSOONS
3.3. Ability of the LMD AGCM to Simulate the Present-Day subtropicalvegetation are replacedby conifersin Siberia and by Monsoon steppe in central Asia during the Miocene [Traverse, 1982; Zubakov and Borzenkova, 1990]. The LMD5.3 AGCM reproducesthe main featuresof the The strengtheningof the land-sea temperature gradient Asian and African monsoons[De Noblet et al., 1996]. Asian increasesthe monsoonadvection and thusenhances precipitation precipitationisrelatively faithfully simulated by theAGCM, with from Oligoceneto PD [Ramsteinet al., 1997a]. Figure 2 shows theexception of an overestimateover the eastern Himalayas and the summer (June-July-August)precipitation distribution for southernChina. The rainfall is directly linked to both the each period. The simulatedmonsoon rainfall in the Oligocene westerlyand the Somalijet, whichbring moisture from the experiment(Exp2) (Figure 2a) is mainly locatedover Indochina, IndianOcean. The windintensity is relativelywell simulated,but whereasthe intensity of precipitationis rather weak over the thewesterly does not converge enough toward India and induces Himalayas. For the middle Miocene (Expl) (Figure 2b), the a too dry climate over northernIndia. This is a common rainfall is enhanced over the Himalayas but reduced over drawbackof severalAGCMs. In Africa, rathergood agreement is Indochina. These trends are further enhanced in the control obtainedfor the locationsof heavyrainfall, greater than 5 mm/d experiment(Exp 0) (Figure 2c), with increasedprecipitation over (Guineacoast, Nigeria). Strong rainfall values simulated over the India, over the Himalayas, and on the easternmargin of the Ethiopianhighlands are excessive.On theother hand, the light Tibetanplateau. These changesare statisticallysignificant at the precipitation,weaker than 2 mm/d, simulatedby the LMD 95% level in a T-test. We compute the mean summer AGCM overthe southernpart of Saharais unrealistic.Although precipitationchange over Indochina(90øE-110øE and 0ø-25øN). most of the African monsoon precipitation is correctly The mean summer precipitationreaches 19 mm/d during the represented,comparison with EuropeanCentre for Medium- Oligocene(Exp2), only 15.2 mm/day(-3.8 mm/d) for the middle- RangeWeather Forecasts (ECMWF) analyses clearly shows an Miocene (Expl), and 11.3 mm/day (-3.9 mm/d) for PD (Exp0). overestimationof the simulatedwind intensity.This is again a This decreaseis mainly explainedby a weakeningof the heavy commonflaw to many AGCMs [Worm Climate Research rainfall events (greater than 10 mrn/d). The changein seasonal Program,1993]. Although slight discrepancies exist, the LMD contrast is rather weak. The decrease of the monsoon AGCM is able to simulate most of the characteristics of the precipitationover Indochina is not seen in data which show a present-dayAfrican and Asian monsoons both in spatialrainfall relativeclimatic stability [Ducrocqet al., 1994]. We suggestthat distributionand duration, realisticallyenough to be used for despite reduced precipitation over Indochina, the monsoon paleoclimaticpurposes. climate prevailed since the Oligocene. The decrease in precipitationover Indochina does not agree with the abrupt 4. Simulated Asian Monsoon Since 30 Myr increaseof the terrigeneousaccumulation rate filling the adjacent basins[Mgtivier, 1996]. This sedimentaryevolution is probably 4.1. Evolutionof Temperatureand PrecipitationSince 30 more influencedby tectonic activity and sea level fluctuations Myr than by climaticchanges. Based on paleoenvironmentchanges in lakes, Chenggaoand Renaut [1994] suggestedthat a monsoon The main cause of the Asian monsoon is in the thermal circulationexisted during most of the Cenozoicperiod in south gradientbetween the continentand the Indian Ocean.In the China and migrated northwardsince 30 Ma. Their idea is also Oligocenesimulation, the ParatethysSea in Eurasiarestricts supportedby changesin vegetation[Leopold et al., 1992; Wang, heatingof the inner continent.The progressiveretreat of the 1994]. The progressivenorthward drift of the monsoonis clearly Paratethysenhances the continentalclimate over central Eurasia simulatedin our experimentsand is directly induced by the [Ramsteinet al., 1997a], characterizedby a winter cooling(- evolution in elevation of the eastern margin of the Tibetan 10øC) and a summerwaxming (+8øC) from Oligoceneto PD, plateauwhich strengthensthe advectionover this area. increasingthe land-oceantemperature gradient. We alsoobserve Figure 2 also shows an important feature: the progressive in this areathat the summerwarming is strongerbetween the PD separationbetween monsoon rainfall and the annual belt of and 10 Ma (Plate l a) thanbetween 10 Ma and 30 Ma (Plate lb) equatorialprecipitation over southernIndia (5øN-15øN; 80øE) whereasthe coolingpresents the oppositespattern (Plates l c and due to the northwarddrift of India. A singleprecipitation center ld). However,the amplitudeof thesechanges relies partially on was simulatedover southernIndia at 30 Ma (Figure 2a) whereas SST valuesthat we prescribedfor the ParatethysSea at 30 Ma. we observetwo areasof precipitationat PD (Figure 2a), one over The uplifts of the Tibetan plateauand Himalayasinduce an southernIndia (15øN, 80øE) and the otherover the Equator.This annualcooling over theseareas consistent with a 6øC/kmlapse latter is less sensitive to monsoon changes. The climatic rate. The wintercooling simulated over the Tibetanplateau and evolutionin southernIndia is consistentwith paleodatawhich overEurasia is partly due to the albedofeedback superimposed suggest the decreaseof tropical forestsand the emergenceof to the impactof the uplift. Focusingon the summerperiod at savanna [Traverse, 1982]. The shift from C3 to C4 midlatitudes,we suggestthat the strongwarming between 10 Ma photosyntheticpathway during the late Miocene [Quade et al., and PD results from both the final retreat of the ParatethysSea 1989; Cerling, 1997, Cerling et al., 1997] may also explain this and the uplift of the Tibetanplateau [Ramstein et al., 1997a]. vegetationchange. This climatic change over central Asia is marked by an To go onestep further, we haveanalyzed more accurately the atmosphericcirculation diverted southward due to the Tibetan precipitationchanges over the Himalayas(84øE-106øE and uplift [Hahn and Manabe, 1975;Kutzbach et al., 1993]. 25øN-30øN).This areaincludes both the mountain range and the The atmosphericcirculation changes and the Paratethys forelandbasin. We then compareour climaticparameters with shrinkageinduce a dryingof this area(Figures 2a and 2b) and a the evolution of the Bengal fan sedimentation.It is indeed weakeningof cloudinesswhich acts as a positivefeedback on the generallythought that the terrigeneous sediments of thisarea find summertemperature. These climatic changesare confirmedby theirmain source in the erosionprocesses of the Himalayas.This vegetation changes observed in Asia. The caduceus and interpretationis supported by theirisotopic signature similar with FLUTEAU ET AL. ' SIMULATING THE EVOLUTION OF MONSOONS 12,001
AT(øC) PD minus10 Ma JJA AT(øC) 10 Ma minus30 Ma JJA
12.00 1200 8OO 8OO 4.00 4.00 -4.00 -400 -8,00 -8.00 -12.00 I -•4.4e -23.95 --'1 -12.00 T 1 3O 60 90 120 -30 0 30 6O O0 120 I.ONGITUDE I.ONGITUDE
AT(øC) PD minus10 Ma DJF d AT(øC)10 Ma minus30 Ma DJF
i i ß i i a
12 81
12.00 12.00 8.00 800 4.00 4.00 ß -4.00 -4.00 -8,OO -12.00
ß -18.74 •...... • ...... •...... i...... '•...... •a...... i...... :;.i!' -18.63 0 3o 60 oo 120 -30 0 30 60 90 120 LONGITUDE LONGITUDE
Plate 1. Mean temperaturedifferences (øC) over Eurasia' (a) PD (Exp0) - 10 Ma (Expl) in summer,(b) 10 Ma (Expl) - 30 Ma (Exp2) in summer,(c) PD (Exp0) - 10 Ma (Expl) in winter,and (d) 10 Ma (Expl) - 30 Ma (Exp2) in winter. Cooling is in greenand blue, warmingis in orangeand red, and no changeis yellow.
thoseof the High Himalayancrystalline series [France-Lanord et littlemoisture [Hastenrath, 1985]. The meanannual precipitation al., 1993; Derry and France-Lanord, 1997]. increasesby +1.52mm/d from 30 Ma to 10 Ma anddecreases by Accountingfor the variability of the signal, the summer 0.38 mm/d from 10 Ma to PD. precipitation (Figure 3a) increasesquite linearly from the The sedimentaryevolution of the Bengal fan remains Oligocene(5.8 mm/d) to the middle-lateMiocene (10.2 mm/d) to complex. On one hand, there is a clear increase of the mean the PD (14.2 mm/d). However, the strengtheningof mean sedimentaccumulation rate integratedover the entire basin summerprecipitation is twice as strongin the past 10 Myr (+0.4 [Mdtivier,1996] startingbetween 10 and 5 Ma. This studyis mm/d/Myr) as from 30 Ma to 10 Ma (0.22 mm/d/Myr). The mainly based on proximal deposits.One the other hand, the temporal distribution of rainy events may give further analysisof threevery well datedcores from the OceanDrilling information.We onlypresent the evolution of thenumber of days Program(ODP) leg 116 and 117 [France-Lanordet al., 1993] with the strongestrains in summer (Figure 3b) which are show a sudden diminution of the accumulation rate from 7 Ma responsiblefor strongerosion processes [Kutzbach et al., 1997]. (late Miocene)to 1 Ma. Our simulationsdo not displayany The numberof days marked by intenserains clearly increases abruptchanges in thesummer monsoon intensity from Oligocene with time, reaching+31% (days/month)between 30 Ma and 10 to PD. This characteristicis also observedby other sensitivity Ma and 50% (days/month)between 10 Ma and PD. The increase studiesusing a gradualmountain uplift [Kutzbachet al., 1989; is also more conspicuousin the past 10 Myr (+0.42 day/Myr) Valdes, 1997]. The increases of both seasonal contrast which thanbetween 30 Ma to 10 Ma (+0.13 day/Myr).This evolutionis may have contributedto the C3/C4 vegetationshift in the associated with an increase in summer rainfall concentration Siwaliks[Quade et al., 1989;France-Lanord and Derry, 1994], representing33% of the annual value at 30 Ma, 43% at 10 Ma, and strongrainy eventdays agree well with the brutalincrease of and64% today(Figure 3c). The last 10 Myr are characterizedby the mean accumulationrate integratedover the entire basin. an intensificationin seasonalitywhich is 4 times as strongas Conversely,they do not fit the sedimentationslow down in the between 30 Ma and 10 Ma. These results point out an distal Bengalfan. This last signalappears more similar to the intensificationof the monsoonprecipitation and alsoa significant evolutionof our simulatedmean annual precipitation. The effect strengtheningof the seasonalcontrast which may drive peak of ourstrong increase of seasonalityis to favorrapid transport of dischargesof sediment[Summerfield, 1991]. We then compute theeroded products toward the Ocean and a majoraccumulation the annualmean precipitationover the Himalayas(Figure 3d). in proximalzones is likely,while the ODP leg correspondsto a Previous studies [Kutzbach et al., 1989; Prell and Kutzbach, distal position, receiving thus the finest terrigeneousfraction. 1992; Kutzbach et al., 1993; Ramsteinet al., 1997a] only We, however,recall that the relationshipsbetween monsoon and focused on the summer monsoon because winter monsoon carries orogenyare very complex [Molnar et al., 1993; Rea, 1992], and 12,002 FLUTEAUET AL.' SIMULATINGTHE EVOLUTION OF MONSOONS
•) 30 M• summerprecit•tion (mm/d•y)
! ...... I .... I ,. . I...... '":"•••_'-'•"'•'"'•'"'...'•'' :. •:ili '""'"•ii:iiii!iiiiiiiii!•iiii!•i•i?J':'•i.--.::-.----•::•i:11::•:.•0.)i '"'•i:•i:::
58.83 20.00 10.00 7.50 5.00 0 2.50 1.00 ' I" I ...... 02 -30 0 30 60 90 120 LONGTTUD•
b) 10 Ma summerprecipitation (mm/d•y)
I I I I
' iiiiiii•i...iiii•ii•iii•s•iiiiii•iii....:•iii...iiii...•i•i•ii•:.iiiiiiii..`..ii*;zi•iii•iii•i.•.:i•iii:!:..`...... :!!...•!*i!*i•:.:•.':"':•:•Z'!.:: -i•-•,-'•:;;•:'-:: **:%-'z::.-.'-*-::- -..:::.-i -:.-.-::- ...... ,ii;::i:i•i•i•::;i'"ii..•F•!t•...:::::ii::::ii::':::.
....:::::::::::::::::::::::::::::::::::::::::::::::::" ...... •!";D0 ' ' ======...;., I '•.' ...... ?';** ...... ::**i:i,i:?:....,...... -.-.:•:0•0.:.: . •o.oo .' lO.OO ß "'*:.:,i:**•**!**•:,,,***'"""'"'"'"""''• **•"'• 5.oo
1.00 .01
ø30 0 $0 60 90 • 20 LONGITUDE
c) PD summerprecipitation (mm/day) ...... :,,:;•i;i:.i;i,:•::L<;.:.•:":•"":'? ' ?i!$i i":'•::•...... :i'"":::?:.•::'•; !:;"•"";:""'•'ii•i ::::-'-i;i:-;."'::".-.:...... •:"?:'"' ':" ...... '...... :': {•0--•-•:•:.•-:::•:•:•:::-•:•:'•:•' ::-....' ..::' ( '"•-"-"!*'.- '..... ""' •:.r.'-:.."•.:.•"....:•-;i*;':.:i:i::•i;; ...... •-'•-•-•:*:'•::-•::i-iii;i:::..'".::•:'•*'-•::i'•i::i:i:.•:.:'•:::h:.':'...... '-'-.'-.•::-:s:•::-.:-•-•:'...:-"•* ";';}:::': ..:;ii-:;::.::::--*'::*:***.'..- ..... -.:": -- •.0o - ... -::.:.•:•%:--.. '...... '..:-:,•::::'•':':i:'-'......
•...... 30- ..... ß..... p--,•.,.,::://:::•::.•::•..; ...... - .,•.."'* •::*;-'""* "s•i•...... ::::::::::::::::::::::::::::::::::::::: ...... :.;..:,:•'•:•:• ...... 40.28 20.00 10.00 7.50 :,..:..:::::--::.:::.; .....:..$.• '$•$,...... ::..':, .:-::-.-...... • ß•.•'*' 5.00 2.50 1.00 :•:'-"'•f'•::•'•-i :':•..'."."c:;.:--,.--.--,-:..---.•.•...... :. .00 I -30 0 30 60 90 120 LONGITUDE
Figure 2. Mean summerprecipitation (mm/d) over Eurasia at (a) 30 Ma (Exp2), (b) 10 Ma (Expl), and (c) PD (Exp0).Isolines are plotted for 1, 2.5, 5, 7.5, 10, 20 mm/d.White represents rainfall lower than 1 mm/d(add area), and the dark grayscale represents the wettestareas (> 10 nun/d). FLUTEAUET AL.' SIMULATINGTHE EVOLUTIONOF MONSOONS 12,003
a) Mean summer precipitation. c) Percentageof summer precipitation.
65
ß•. 55
o. 50
E E ::3 45
C• 40
• 35
4 30 ExpO Expl Exp2 PD 10Ma 30Ma
b) Number of summerdays with P_>10mm/day. d) Mean annual precipitation.
E '• 6.5 E E E 6 A
. o '• 5.5
o. 5
c: 4.5 o
E . .
z 5 .... • .... I .... • .... • .... I .... • .... 3.5 ExpO Expl Exp2 PD 10Ma 30Ma
Figure3. Precipitationevolution averaged over the Himalayas and the foreland at PD (Exp0),10 Ma (Exp1), and 30 Ma (Exp2).(a) Meansummer precipitation (mm/d), (b) numberof daysin summerprecipitation stronger than 10 mm/d(days/month), (c) percentageof meansummer precipitation relative to meanannual precipitation, and (d) meanannual precipitation (mm/d). The continuousline indicatesthe meanvalue; the dashedlines represent the signalaccounting for the variability.The valueson eachplot indicatethe meanamplitude of the changesper million yearsbetween 10 Ma and PD and between30 Ma and 10 Ma.
it seemsdifficult to separatein the sedimentaryrecord, the parts because,on the one hand, the Paratethyshigh-pressure cell limits linked to an enhancementof erosionresulting from increased its aorthwestwardextent and, on the other hand, the Tethys precipitationfro'm those resulting from tectonic activity. seawayseparates the Asian thermal trough from the one located over Arabia. For the middle Mioceneexperiment (Figure 4b), the 4.2. Evolutionof AtmosphericDynamics Asian thermal trough expandsonly from westernIndia to China. Today, the Asian low-pressurecell spreadsfrom northernChina The precipitationevolution over the Himalayasand Indochina to Arabia and is centeredover northwesternIndia (Figure 4c). sincethe Oligoceneexhibits opposite features. Although we The low-pressurecell shifts progressivelynorthwestward from simulate a decreaseof precipitationover Indochina, a wet southeastern Asia toward the Middle East. The center of this summermonsoon associated with an importantseasonal contrast systemfollows the same trends and shifts northwestwardover is maintainedsince 30 Ma. We pointout a northwestwarddrift of northern India. The changesin low-pressurecell result from the whole monsoonsystem. To explain such a drift, we will changes in land-sea distribution and from the orographic analyzethe evolutionof the large-scaleatmospheric circulation evolution. Among the paleogeographicchanges, the Paratethys usingsea-level pressure and wind fields.A deep low-pressure Sea abovewhich a high-pressurecell is located,controls strongly cell inducedby the warmingof the continentcharacterizes the the stretching of the Asian trough, whereas the minimum monsoon.The 1000 hPa isoline(indicated by the 0 hPa isoline pressureis driven by the releaseof latent heat over the flank of a on Figure 4) is usedto describethe low-pressurecell. For the mountainrange. Oligoceneexperiment (Figure 4a), the low-pressurecell is The 850 hPa winds also reflect the changesin atmospheric reduced. Its extent is limited to India and southern China circulation (Figure 4). The strongwesterly located over Bengal 12,004 FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS
a) 30 Ma sealevel pressure (hPa) and wind at 850 hPa (m/s) in summer
60-
22.78 12.00 30- 10.00 :. 8.00 6.00
. . 4.00 2.00 .00 -2.00 O-- -4.00 ll -6.00 -6.41
-3O
b) 10 Ma sealevel pressure (hPa) and wind at 850 hPa (m/s) in summer
I I I I ...... , ...... - ======::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::•:-..-...--••-••••:••..•.•••i..•,.-:...,..-•...• • ...... • .-:•:•.•.•.•:.....:,:::• -•i 60- .: ...... :: ======_ lo rrvs
23.60 12.00 30-
6.00 . 4.00 ' •'::':• ...... % ..:::':...• 2.00 .00 -2.00 O- -4.00 -6.00 -- -9.99 I I I I -3O 0 80 •0 •0 120 LONGITUDE
c) PD sealevel pressure (hPa) and wind at 850hPa (m/s)in summer
I
60-
ß
...... •::i•:,:,•:,':.:•:::::• ' ======26.94 1 12.00 30- 1 I 10.00 8.00 I '6.00 ! I 4.00 2.00 .00 -2.00 O- -4.00 -6.00 I I I I -12.06 -30 0 30 60 90 120 LONGITUDD Figure 4. Mean sealevel pressure (minus 1000 hPa) and wind field at 850 hPain summerat (a) 30 Ma (Exp2), (b) 10 Ma (Exp1), and(c) PD (Exp0). The pressureinterval is 2 hPa.Thick line representsthe 1000 hPapressure (noted0 hPa on the figure).An adjustmentfactor has been added to the Oligoceneand middlelate Miocenesea level pressureto compensatefor the differencein mean global elevation.This factor correspondsto the mean annualdifference of pressureaveraged over the globebetween the experimentsat 10 Ma or at 30 Ma and the presentday. Grey scaleindicates the low-pressurecell; whiteareas represent sea level pressure greater than 12 hPa. Vectorsrepresent the wind, bothin directionand amplitude. FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS 12,005
Bay, which producesheavy precipitationover Indochina during Verticalvelocity (m/s)at 30Ma the Oligocene(Figure 4a), slowsdown progressively.The low- pressurecell evolution since 30 Ma is accompaniedby an intensificationof wind over the Arabian Sea (Figure 4c) and by local feedback due to convective motion. An increase of the moisture convergencetoward the Himalayas results from this evolutionand thus explains the enhancementof precipitation •oo-•'"'"'"'•'"'•'"•"••'•-'••:•••• •:•:•::::::•:•:...... --:•>3•..• .-•,•- over the Himalayas. 4oo-• • "':::•½?••'•:*'• So far, we have analyzed the monsoon circulation at low i • 0- ..<.::::::::::::::::::::::::::::::::::::::::::::::::::::::'"":':':::;:;*;';:•; '::'• '-;•:•;:i•;;•:A'5•:•:3•-'-'•:•g:•:•;•< ••-:•;•;•: :•:•)• ;:;• atmosphericlevels. We show now that the paleogeographic changesalso contribute to changesin the upper atmospheric , .:5.•::;.:;...... • ::?:.-:--:-:,.:•-:•,....•:•;•:;•,""• • ,, ;:•!i?:::.;',:½'"-'"'•"•'•-'.'-:•:..-'-'•¾ ,.2- circulation. We investigatedthe evolution of the monsoon in altitude-latitudeplots (Figure 5) averagedbetween 75øE and -0.0 - -2.0 90øEcrossing the Indian subcontinent,southwestern Bengal Bay, I I I the Himalayas,the Tibetan plateau,and northwarduntil western -ao -•o 40 o •o •o • South North Siberia, to depict the major changes in vertical wind and atmosphericwater vapor content (not shown). At 30 Ma, the atmosphericrising motionover the Himalayasis weak and occurs b) Verticalvelocity (m/s) at 10 Ma on both sidesof the Himalayas(Figure 5a). The rising air mass over its northernflank reflectsthe releaseof latent heat brought from the ParatethysSea and may explain the presenceof - ß :•-. ' m•,-.'::.'' •:•:-•-•:•-.•'.:•$-"•:'•'"" '"•:•{•'•-•' subtropicalvegetation in Tibet in 30 Ma [Ren, 1981]. Moreover, 200 •,• -.•:•*.• ...... •*•--•':- ••"% ...... •---••••••:•....•••,•••••,.•;• the vapor contentwhich is relativelyuniform over both sidesof 300 the Himalayas in the Oligocene' experiment, increases progressivelyabove its southem flank and northem India in relation to the increaseof moistureconvergence and decreases over the northemflank. Conversely,water vapor decreasesover the northem flank, on the Tibetan plateau, and over Siberia becauseof the changesin atmosphericcirculation and the retreat
of the Paratethysthat previouslyprovided a large source of •.0- moisture.In the 10 Ma experiment(Figure 5b), the ascending 10• I I .... I I I I I BEL• •.0 •o -20 .,o o ,o motionover the southemHimalayas is stronglystrengthened. In South Noah the PD simulation(Figure 5c), two areas of rising air masses located above India (15øN) and the southern flank of the Himalayas(30øN) are simulated.They reflectthe releaseof latent c) Verticalvelocity (m/s) at PD heat by precipitation.Large vertical motion affectsthe upper troposphere(300 hPa) overthe Himalayas, inducing an important advection. lOO '•' 200 4.3. Moisture Advection Over the Arabian Sea • 300 The Somali jet blowing over the Indian Ocean and Arabian 4o0 Sea carries moisture toward southernAsia (Figure 4c). The • •oo Arabian Sea contributesmore than half of the atmospheric moisturetransported over the continentin the model. Thus the -0.2 - 0.2 locationof the strongestwinds over the Arabian Sea and their -•:• -1.0-0.,5 - -0.2 moisturecontent are of criticalimportance in the evolutionof the • -2.0-3.0 - -1.0-2.0 Asian monsoon. 1000I i I I I I I I • BELOW-3.0 -30 -20 -10 0 10 20 30 40 50 60 Over the Arabian Sea, the limit of confluence between the South North northwestefiiesand the southwestefiiesdefines the Inter-Tropical ConvergenceZone (i.e., ITCZ). The positionof this confluence Figure 5. Mean vertical velocity in summer averaged in providesfurther information on monsoondynamics. We estimate longitudebetween 80øE and 95øE at (a) 30 Ma (Exp2), (b) 10 Ma (Expl), and (c) PD (Exp0). The vertical scale is pressure the latitudeof confluenceusing the meridionalwind component (hPa) and representsthe atmospherefrom sea level to 30 km averagedbetween 55øE and 70øE (Figure 6). Negative values height. Light gray areasare attributedto subsidence(positive connote northwesterly (blowing from the central Eurasia), values),dark gray indicatesascending motion (negativevalues), whereaspositive values imply southwesterly(blowing from the and white representsthe orographiccontour (Himalayas and Indian Ocean).To completethis analysis,we have calculatedthe Tibetan plateau). mean amountof latent heat carriedin the boundarylayer (the first fourthlower atmosphericlayers in the LMD AGCM) in two areas located over the Arabian Sea. These areas stretch from the moisturecrosses both areas. The latentheat flux crossingthe 55øE to 70øE and from the Equatorto 32øN. The boundary Equator between southernIndia (75øE) and Indochina (100øE) between these zones is defined by the ITCZ latitude deduced affects southeasternAsia. In the 30 Ma experiment, the from Figure6. During the summermonsoon, the majorpart of southwesterlies/northwesterliesconvergence is locatedat 11ø5N 12,006 FLLFTEAUET AL.' SIMULATING THE EVOLUTION OF MONSOONS
10 et al., 1995]. The abruptincrease of foraminiferarecorded in the sedimentsof the Arabian Sea (site ODP 722, Owen ridge) occurredduring the late Miocene (7-8 Ma). It has beenrelated to an abruptdevelopment of the Asian monsoononset and may be 5 consideredas the most relevant evidence since these data only depend on the southwestefiiesintensity over the Arabian Sea [Kroon et al., 1991; Nigrini and Caulet, 1992; Prell and Kutzbach, 1992]. However, Sirocko [1991] showed that fluctuationsin upwellingintensity depend on the positionof the ITCZ during the Quaternaryand reflect the glacial/interglacial cycles.During the interglacialepochs, since the Asian monsoon and thus the southefiiesare stronger,setting the ITCZ in a ShiftofreIT••':: ...... "•'Y&z...... '":"<:'""'"'"'"' northern position over the Arabian Sea, upwellingsare clearly recordedin the sedimentcore. Conversely,during the glacial Shift of the ODP le1 periods,the northwestefiiesover the Arabian Sea (characterized by an increaseof Arabiandust dispersal) are intensified,shifting -lO the ITCZ southwardand inhibitingupwelling. o lO 20 30 40 We suggestthat this mechanismexplains the increaseof the LATITUDE North upwelling intensity during the late Miocene. The increaseof Figure 6. Meridionalwind velocity (m/s) in summeraveraged in upwelling deducedfrom sedimentcore (ODP 722) does record a band stretchingover the Arabian Sea (45øE-65øE).Values the northwardshift of the monsoonwind systemand the ITCZ equalto 0 m/sdefine the latitude of ITCZ. The ITCZ is locatedat during the Cenozoic. Accounting for the Arabia northward 11ø5N at 30 Ma (Exp2), 18øNat 10 Ma (Expl), and 25øN at PD motionof some6 ø at the latitudeof the drilling site,the sediment (Exp0). core was located from 30 Ma to 10 Ma under the ITCZ. The monsoonwind systemis locatedsouthward during both periods. The shift of the monsoon systemmay explain the absenceof upwelling developmentdetected in the sedimentcore until the (Figure 4a). The southwestefiiesadvect about 63% of the total late Miocene. Besidesthe locationof the drilling sitewith respect latent heat flux over the Arabian Sea (37% for the to the southwesterlies,we also observe a weakening of these northwestefiies).The closureof the Tethysand Paratethys as well winds over the Arabian Sea. Weak winds reduce the Ekman as the uplift modify the configurationof the monsoontrough current and may inhibit upwelling. Finally, the openedTethys (Figure 4b), shiftingthe ITCZ northwardup to 18øN (Figure 6). seawayand a surfacecurrent may also restrainupwelling despite The part of the latentheat transportcarded by the northwestefiies weak exchangebetween the Atlantic and Indian Oceans.These reaches 12%, whereas the southwestefiiesbring 88% of the climatic, paleogeographic,and oceanicfactors may interfere in moisture. The final Paratethysretreat and the end of Tibetan the developmentof endemicupwelling species. uplift lead to the westwardextent of the monsoontrough (Figure 4c). The PD confluencemigrates farther north to 25øN (Figure 6). The part of the total latent heat advected by the 5. Impact of Plateau Uplift and Paratethys southwestefiies reaches 97% versus only 3% for the Shrinkage northwestefiies.Superimposed to this redistributionof the latent heat transport,the southwestefiiesincrease by 20% between l0 Becausethe Tibetan uplift and the Paratethysshrinkage may Ma and PD (-65% for the northwestefiies).The configurationof have a strong impact on the summerAsian monsoonsince 30 the monsoon trough implies a redistribution of the advected Ma, we have performeda set of sensitivityexperiments (Table lb fluxes. and Figure 7) to quantify their respectiveactions. The AGCM is The differenceof the mean sealevel pressurebetween central forced with the paleogeographicreconstruction at PD, 10 Ma, Asia (Paratethyshigh) and northem India (monsoon trough) and 30 Ma, in which we remove a part of the ParatethysSea or reveals that the middle Miocene experiment has the strongest reducethe Tibetan plateauheight. Becausethe AGCM computes gradient(Figure 4b). For the Oligoceneexperiment (Exp2), the an atmosphericresponse in equilibrium with the prescribed eastwardlocation of the high-pressureadvects moisture toward boundaryconditions, the difference betweenthe "realistic" and the northwesternflank of the relief (Figure 4a). The shiftedhigh- sensitivity experimentsrepresents the impact of each forcing pressurecell over central Asia at 10 Ma brings more moisture factor on the climate. Thus, to quantify the impacton a climatic over India and the southernflank of the Himalayas than over the parameter(X) of each geologic event independently,we have northern flank. calculatedthe difference of values obtained in the sensitivity The northwardmotion of the ITCZ over the Arabian Sea may experiment(Xs) and in the realisticexperiment (Xr) for the same also be comparedto the upwelling evolution which reflects the geologicperiod. This difference(Xs-X0age is comparedto the summer monsoonwind field [Kroon et al., 1991; Nigrini and differencesbetween two "realistic" experiments (Xr)agel-(Xr)age2. Caulet, 1992]. The raisingof cold and nutrient-richwater masses To investigatethe role of the Tibetan plateau uplift on the led to the developmentof planktonic foraminifera (Globigerina strengtheningof the monsoon, we have performed two runs bulloides),This populationis thusan indicatorof the intensityof (Table lb). Contrary to previous studies[Kutzbach et al., 1989, monsoon winds [Prell et al., 1990; Curry et al., 1992]. At 1993], we have only modified the elevation of the Tibetan geologic timescales, the sediment core presents a fossil plateau and we have kept unchanged the Himalayas. The population which is a good recorder of the past monsoon differentiationbetween the Himalayasand the Tibetan plateauis variability [Kroon et al., 1991; Prell and Kutzbach, 1992; Erneis due to their distinct geologic evolutionsin time [Tapponnieret FLU'I•AU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS 12,007
a) PD b) PD, lowerTibetan plateau ?n ! , .! . t I
• ,;-.--:.-.::::•ß- ...... ::.::: ...... : ....-:,,..:• • • ...... -. '""•'"'•*'•'•:•--
3.• . 1.50 .00 10]10 ...... 4.50 - -I'•"'"•":•'"'• ...... 5...... i...... '...... •...... i'"" .oo
LONGITUDE LONGITUDE
c) 10 Ma d) 10 Ma, lowerTibetan plateau e) 10 Ma, reducedPavatethys sea 70 • I I t z0i.:.• ' : •J
/ '""'""""ae • ::::•::%!....• _• ao .:...... i:•:!-;--..-'.!:i:•i-i:i.::• •"'":"•?•...... _• •o' ...... ?•i...... • -
3.00 3.00 1.50 : .00 .00 : .00 40 [ p • t • .0• . ':...:-.::::..:-.':.::::s::•-:: :::'*.-•.•: ::--s::s::.:i:':' ::•:. :-::-:::::-•s::• s-:s:::'.':?.:.:•::s:.::'::•:•-. s::-'. &:::.'-5---:_•_ .•0 40 60 80 100 120 LONGITUDE LONGITUDE LONGITUDE
f) 30 Ma g) 30 Ma, reducedParatethys sea h) 30 Ma, without Paratethyssea 70- . . •.• •;:.::::-sI I"--] =) i:'?:.•.::...... ',,::,::'•':-:::'"""•'"':•"""/;•..--':•'•...... •:a ......
10 4.,5O 3.00 ß 3.00 1.,50 1.50 .00 .00 EE .... .00 ... -10 ...... • ! I I ::....•j•4.5O.00 10 '1 "' "="'- .00 40 60 8o lOO 1•( 40 60 8o lOO 12o LONGITUDE LONGITUDE LONGITUDE
Figure 7. Paleogeographyused for (a, c, f) "realistic"and (b, d, e, g, h) sensitivityexperiments for (b) presentday, for (d, e) middle-lateMiocene, and for (g, h) Oligocene.Light greyrepresents oceanic areas, white represents the plain andrelief lowerthan 1000 m. Grey scaleshows the relief. (a) PD - Exp0, (b) PD - Exp3 - reducedTibetan plateau,(c) 10 Ma - Expl, (d) 10 Ma - Exp4- reducedTibetan plateau, (e) 10 Ma - Exp5 - partialParatethys retreat, upliftedHimalayas, (f) 30 Ma - Exp2, (g) 30 Ma - Exp6 - partialParatethys retreat, and (h) 30 Ma - Exp7 - no ParatethysSea.
aL, 1986; Harrison et al., 1992; France-Lanordet al., 1993]. ParatethysSea until its dimensionat 10 Ma andreplaced it by a Exp3(Figure 7b) teststhe PD impactof theTibetan plateau. The low plain.Finally, the ParatethysSea in Exp7 (Figure7h) is meanelevation of the Tibetanplateau of some5000 m in thePD completelyremoved from the 30 Ma mapwith the exception of run (Exp0)is reducedto 1000m in Exp3. In Exp4 (Figure7d), oceanicbasins corresponding to the PD Caspianand Black Seas. we have also reduced the elevation of the Tibetan plateau at The goals of these sensitivityexperiments are (1) to about1000 m but on the 10 Ma paleogeographicmap. investigatethe impact of theTibetan uplift (Exp3 and Exp4), (2) The shrinkageof theParatethys epicontinental sea leads to the to seewhether the amplitudeof the shrinkageinduces a linear strengtheningof a continentalclimate regime over much of responseof themonsoon (Exp6 and Exp7), and (3) to showif Eurasia[Ramstein et al., 1997a]. To quantifyits impacton the localfeedback over the Himalayasis a functionof the elevation monsoon,we have performed three sensitivity experiments of thischain and is ableto amplifyclimatic changes (Exp5 and -(Tablelb). In Exp5 (Figure7e), we haveperformed a simulation Exp7). usingthe 10 Ma paleogeographicmap (Expl), in which we We have used the "monsoonindex" defined by Prell and nearlysuppress the Paratethys Sea. In Exp6 (Figure7g), we have Kutzbach[ 1987] to quantifythe intensity of themonsoon, which removedfrom the 30 Ma map (Exp2) the eastempart of the can be approximatedby the pressuregradient between the 12,008 FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS
Differenceof the monsoonindex (hP•) sensibleheating, and the other over the eastern flank of the Tibetan plateau, resulting from latent heat release. With a reducedTibetan plateau in a PD land-seadistribution (Exp3), the low-pressureminimum located over its eastem flank weakens 1.4 •' ' • "- '"' (becauseof lessprecipitation), whereas the Indian trough remains o_ unchanged.Precipitation increases over India becauseadvection 1.2 . towardthe easternTibetan flank is reduced.The high-pressure cell locatedover the Tibetan plateau also weakens.In a 10 Ma 1 ...... •--- o:• ...... •• -•': ..... •"• ...... '•' experiment(Exp 1), the elevationof the Tibetanplateau and the 0.8 • ..... x ...... • ...... " ...... • ..... presenceof a smoothedslope inhibit the emergenceof a low- pressureminimum over its eastern flank. There is only a
0.6 ...... : .... • .... _ ...... : . minimum located over northwestern India. With a reduced , • • . • - Tibetan plateau(Exp4), the whole low-pressurecell increases. 0.4 ...... •,-- The impactof the uplift is widely linked to the developmentof the monsoonlow-pressure cell. 0.2 .... = .- ' .. , , Theseresults point out that two thirds of the monsoonindex
, . increasefrom the Oligoceneto PD is due to the combinationof the full retreatof the ParatethysSea and of the Tibetanplateau Figure 8. Differences of monsoon index in responseto uplift. Althoughthis resultsuggests that both the shrinkageand paleogeographicimpact. Vertical scale is in hPa. the Tibetanuplift are the main causesof the monsoonchanges, theretreat of theParatethys Sea alone plays a majorrole in large- scaleatmospheric changes. continent(Asia) and the adjacentocean (Indian Ocean).This We haveshown previously in the "realistic"experiments that index equalsthe differenceof mean sea level pressurebetween the precipitationshifts northwestward from Indochinatoward the the Asian continent and the Indian Ocean, averagedin an area Himalayas.To quantifynow the influencesof thegeologic events stretchingbetween 45øE-120øEand 15øS-45øN.This zone on the precipitationevolution, we have definedtwo areasof land includesboth the Indian monsoonlow-pressure cell, advecting points. The first stretches over southern India and Indochina wet air masses,and the Indian Oceanhigh-pressure cell. (area A: 65øE-120øE;0ø-20N), regionsin which precipitation We show in Figure 8, the computed monsoon index mainlyoccurs during the Oligocene.The secondcovers northern differences for the different simulations: the "realistic" India, the Himalayas,and the Tibetan plateauup to southern paleogeographicchanges between Oligocene, Miocene, and PD China(area B: 65øE-120øE;20øN-40øN) in whichheavy rainfall inducea nearly similar (strong)increase of the monsoonindex occurs for the PD run. (1.57 hPa and 1.5 hPa for Exp0-Expl and Expl-Exp2, To showthe impactof the differentforcing factors, we have respectively).The sensitivityexperiments show that the strongest plottedthe mean summerprecipitation over areasA and B, as a increaseof monsoon index (1.22 hPa) results from a full retreat functionof themonsoon index (Figure 9a). We pointout thatthe of the ParatethysSea in the Oligoceneenvironment (Exp7-Exp2). intensification of the monsoon index correspondsto a In comparison,a partial Oligocene Paratethysretreat (Exp6- redistribution of the rainfall over both areas. An increase in the Exp2) strengthensthis index by only 0.31 hPa. The retreatof this monsoonindex strengthensthe precipitationover the northern seain the Miocene (Exp5-Exp1) increasesthis index by a higher area (box B, the lower group of points) and depletesthe southern 0.51 hPa value.The uplift of the Tibetanplateau at 10 Ma (Exp 1- part (box A, the uppergroup of points). Exp4) enhancesthe monsoonindex by 0.79 hPa. Finally, the To further investigatethe impact of these geologic events, effect of the altitude of the Tibetan plateauappears negligible in Figure 9b showsthe variationof precipitationas a functionof the the PD experiment(Exp0-Exp3), with an increaseof only 0.13 monsoon index changes for the different forcing factors. No lipa. forcing factor tested in our sensitivityexperiments is able to A full shrinkage of the epicontinental sea (Exp7-Exp2) accountfor the increaseof precipitationover the northernpart inducesstronger heating of Eurasiaand the extent of the thermal (box B) (Exp0 [PD]-Exp2 [30Ma], + 1.65 mrn/d) or the decrease low pressuretoward central Asia, and westernSiberia (and also of precipitationover the southernpart (box A) (-2.25 mrn/d).The Arabia). The full retreatof the ParatethysSea inducesa stronger impact of the Tibetan plateau uplift in the 10 Ma runs (Expl- climaticchange than a partial retreat(Exp6-Exp2) due to a more Exp4) is strong enough to produce about one half of the efficientsummer heating. precipitationdecrease from 30 Ma to PD (1.2 mm/d) in the The uplift of the Tibetan plateau mainly modifies the southernarea and one half of the precipitationincrease from 30 amplitudeand location of the minimum of the low-pressurecell Ma to 10 Ma over the northem area (0.5 mm/d). Moreover, this withoutmodifying entirely its spatialdevelopment (Exp 1-Exp4). increasemainly affects the easternflank of the Tibetan plateau We notice that the uplift of the Tibetan plateau in a PD and northernIndochina, and lessthe Himalayas. experiment(about 4000 m) increasesthe monsoonindex by only The Tibetanplateau uplift in the PD run (Exp0-Exp3) induces 0.13 hPa, 6 times less than in the 10 Ma run (Expl-Exp4, 0.79 weak precipitationdecrease over the northernarea (-0.2 mm/d) hPa), despite a weaker uplift (about 1500 m on average). To and relativelystrong changes over the southernpart (-1.7 mm/d). attempt to solve this contradiction, we have examined the However, as in the 10 Ma run, precipitation increasesover the developmentof the monsoonlow-pressure cell, with and without eastern flank of Tibet, while a broad decrease occurs over India the Tibetanplateau in the PD (Exp0 and Exp3) and 10 Ma (Expl and Indochina.This change reflects the slowing down of the and Exp4) simulations.The PD run (Exp0) presentstwo sealevel westerliesover India and the increaseof moistureconvergence pressureminimums, one over northwesternIndia, resultingfrom alongthe easternflank of the Tibetan plateau. FLUTEAU ET AL. ßSIMULATING THE EVOLU•ON OF MONSOONS 12,009
a) strongerthan in responseto the Tibetan uplift (Exp 1-Exp4) (+0.5 mm/d). We suggestthat the impact of the ParatethysSea retreat 14 ...... :...... :...... !...... (Exp5-Expl) on precipitation in the northern box depends 12 ...... : ...... i...... i ...... strongly on dynamic feedback over the Himalayas. The Paratethys partial shrinkage (Exp5-Expl) strengthens the : 10 ...... :...... moisturetransport over northernIndia and the Himalayas,while : increasingprecipitation over its southern flank. An elevated range compels air masses to rise and to produce heavy : : : precipitationalong the slope. The latent heat releaseimplies importantconvective motions, which act as a positivefeedback 6...... i...... ,...._.•+_ to increasemoisture convergence. This feedbackmay explainthe amplificationin precipitationobserved in both Figures9a and 9b.
2 ...... i...... : ...... : ...... • ...... Conversely, a low mountain range reduces this dynamic . , ' , , feedback. Thus we are able to reproduce partially the precipitation 8 8.5 9 9.5 10 10.5 11 11.5 changesoccurring in the realisticexperiments with the sensitivity Monsoon index (hPa) runs (-2.25 mm/d in the southern area, +1.65 mm/d in the northern area). However, no obvious dominant forcing factor emerges as a sole explanation for the monsoon precipitation changes.Both the Tibetan uplift and Paratethysretreat should be A . ,W•' , .'•/'__-.'•/' 1 •, 2 ...... •'- ...... •'- --':- -- •'- ...... -%:--i ...... _•.... ?---.%'--•:...... taken into accountto reproducethe precipitationchanges over southernAsia. However, we suggestthat the impact of the Paratethys Sea retreat constitutesthe most efficient factor to , ...... i...... ,1,...... ;-;...... enhance precipitation over the Himalayas,but it dependsstrongly I:: ß on the elevation of this chain. .e + : v ß 0 ß '
.--
, 6. African Monsoon o • -1 ...... ; ...... At present, the northern part of the African continent ic divided into two main climatic areas: desert northward of 20øN n -2 ...... and savanna/tropicalforest south of 20øN, where the summer monsoon occurs. The long-term evolution of north African -3 , , • , I , , , , I , , , , I , , , , climate is only based on rare and poorly dated paleoclimatic 0 0.5 1 1.5 2 indicators [Ruddiman et al., 1989a]. Nevertheless, the Differenceof monsoonindex (hPa) progressivearidification of this area during the Cenozoicperiod is usuallyaccepted and leadsto the emergenceof desertin north Figure 9. (a) Mean summerprecipitation averaged over northern Africa in the late Miocene [Axelrod and Raven, 1978; Sarnthein and southern Asian areas as a function of monsoon index. ']'he horizontal scale is in hPa; the vertical scale is in mm/d. ']'he et al., 1982; Robert and Chamley, 1987; Maley, 1996]. upper (lower) groupof pointsrapresents the meanprecipitation averagedin box A (box B). (b) Mean precipitationvaz•ation as a 6.1. Simulationsof African MonsoonChanges function of the mean vaz•ations of monsoon index. ']'he horizontal scale is in hPa; the vertical scale is in mm/d. For each We have first simulated climatic changes between the experiment,the upper (lower) point representsprecipitation Oligoceneand PD, over a band of longitudecovering 0ø-30øE. changein box B (box A). Samenotation as Figure 8. This band does not accountfor the northerliesblowing along the Atlantic Coast and the area of heavy precipitation on the Ethiopianhighlands, which may hide the monsoon,changes over inner Africa. We use values averaged in June-July-August- In comparisonto the Tibetan plateauuplift, the full retreatof September(JJAS) with the aim of coveringmore accuratelythe the ParatethysSea (Exp7-Exp2) strengthensprecipitation over monsoon season. During JJAS over this area, precipitation the northernpart (+0.6 mm/d) and especiallyover the Himalayas, decreases by 30% and evaporation by 17% between the whereasthere is no apparentrainfall changeover the southern Oligocene(Exp2) and PD (Exp0). Besidesthe weakeningof the part (0 mm/d). In fact, a differencemay be observedbetween the monsoon, a time redistribution of the monsoon rainfall occurs. In center of India (intensification) and Indochina (weakening) the Oligocene experiment (Exp2), we observe two distinct within box A (Figure 9) as reportedby Ramsteinet al. [1997a] phases,interrupted by a slight break in June. Only one phase becauseof the westwardshift of the monsoonlow-pressure cell existsat PD (Exp0). This break is amplified if we observethe resulting mainly from the Paratethys retreat. Actually, the precipitationminus evaporation(Figure 10) becauseof the high influenceon precipitationof the ParatethysSea is also closely evaporation rate and the weaker precipitation decrease. To connectedto orographic elevation. The rainfall strengthening compensate this effect, the local recycling of moisture (+0.58 mm/d) over the northernarea (mainly over the Himalayas) strengthensby 10% duringthis breakat the expenseof advection. in responseto a partial shrinkageof the ParatethysSea in the 10 The zonal mean average JJAS precipitation shows that the Ma experiment(Exp5-Exp 1) is as strongas in responseto a full monsoon is centered at 11.2øN for the PD simulation and at 14øN retreat (+0.6 mm/d) in a 30 Ma configuration(Exp7-Exp2) or for the Oligoceneexperiment (Figure 11). This northwarddrift of 12,010 FLUTEAUET AL.: SIMULATINGTHE EVOLUTIONOF MONSOONS
monsoonintensity (Figure 10) (this does not occur in the PD simulation (Figure 12d)). The following months undergo a southernposition of the insolation maximum: thus the thermal low-pressurecell drifts southwardand deepens.We observea new phaseof heavyprecipitation lasting from the monthsof July to October, with an intensification of advection. However, the low-pressurecell in the Oligocene experiment (Figure 12e) is weaker than the one developedfor the PD (Figure 12f), although it advectsmore moisture.We attribute this apparentparadox to the characteristicsof the low-pressurecell and to its link with the Asian low-pressurecell. In the Oligoceneexperiment (Figure 4a), the Tethys seawaygenerates a large ridge, separatingthe Asian thermal low-pressurecell from the African one, whereasin the PD experiment,the Asian-African monsoon systemis strongly J F M A M J J A S O N D connected(see section7).
Month These paleogeographic changes also modify the midtropospherecirculation and may induce feedback on the Figure 10. Monthly meandifferences between precipitation and evolution of precipitation patterns. According to Flohn and evaporation(mm/d). The solid (dashed) line representsthe PD Nicholson[ 1980] and Ruddimanet al. [ 1989], a strongertropical (30 Ma) experiment. Negative values indicate periods with easterlyjet over Africa shouldenhance both rising motion and evaporationexceeding precipitation. the precipitationover tropical Africa, and subsidenceand drying over northern Africa. We do not observe all these features simultaneously.In our case, we observe an increase of the 2.8ø reachesmore than 3ø during August, which defines the tropical easterly jet over Africa at 700 hPa between the precipitation maximum simulated during the Oligocene Oligocene(4m/s) (Figure 13a) and the PD (8 m/s) (Figure 13b). monsoon.The northernlimit of the precipitationbelt, definedby However, this jet is associatedwith a spatial shrinkageof the the 2.5 mm/d thresholdaveraged on JJAS,is locatedat 15.3øN ascendingmotions over tropical Africa covering 7.5ø in latitude for PD and 22.1øN for the Oligocene(Figure 11). The JJAS in the PD experimentand 17ø for Oligocene,whereas we do not northernlimit shifts northwardby 6.8ø, and >8ø in September. observeany changesin the jet intensity.For the Oligocene,a dry These results show that the Oligocene monsoon stretches and smaller in size subsidence is located above the northern northward without significantly shifting the position of the African Coast and the Tethys seaway(Figure 14a), whereasin strongestprecipitation. the PD run, this subsidencecovers a large part of the northern Usingthe evaporationover precipitation ratio, we may show Africa and the Mediterranean Sea with a maximum over western that the Asian monsoon precipitationis mainly driven by Egypt (Figure 14b). The paleogeographicchanges also disturb advectionof moisture,whereas local recyclingplays only a minor the westerliesin the upper troposphereat 300 hPa. This jet is role. For the African monsoon, we observe a strong local located at 50øN for the P D and at 30øN in the Oligocene.The recyclingfor PD and 30 Ma. Betweenthe Oligoceneand PD, southwardposition of the westerliesnear the tropical easterlies precipitationweakening (-30%) is almostequally shared between increasesboth the wind gradientin the upper troposphereand the advection(- 17%) and local recycling(- 13%) decreases. shearstress. Although thesefeatures may play a role in African
6.2. Link BetweenAtmospheric Circulation and PaleogeographicEvolutions Mean JJAS precipitation(mm/day) At largespatial scales, the sealevel pressureevolution (Figure 12) explainsmonsoon precipitation and advection changes. In the Oligocene experiment,because the continent is in a more , , , southernposition, the low-pressurecell, locatedover southern equatorialAfrica, movesover westernAfrica as early as March (Figure12a), whereas this position is reacheda monthlater in the PD experiment(Figure 12b). As soon as the trough reaches \ \ 30Ma westernAfrica, the winds reverseand advect moisture from the Gulf of Guinea and the Atlantic Ocean, which induce the first /I /h \ Shiftof thenorthern /] __/ \ '..... \ of monsoon periodof precipitation.At the end of the springseason in the ii t,u \ x/ Oligocenesimulation, the monsoonlow-pressure cell shifts northward,following the locationof the strongestinsolation. The insolationmaximum migratesabove northern Africa close to / X '-.,. • .•.•.•.• i/ 20øN, inducing a strong heating of this area as well as the northwarddrift of the monsoonlow-pressure cell (Figure 12c). However, becauseof the more southerlyposition of Africa, the ,'o ;o low-pressurecell is blockedby the high-pressurebelt of the LATITUDE Tethys seaway,which in turn, leads to a weakeningof the monsoontrough (Figure 12c). Thus the moisture advection Figure 11. Mean JJAS zonal precipitation(ram/d)averaged weakens, and the month of June experiencesa decreasein between0 ø and 30øEat PD (solidline) and 30 Ma (dottedline). FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS 12,011
a) 30 Ma sealevel pressure (hPa) in march b) PD sea levelpressure (hPa) in march
27.7O 18.00 14.00 12.00 10.00 8.00 6.00
c) 30 Ma sealevel pressure (hPa) in june d) PD sealevel pressure (hPa) in june
i
26.10 18.00 14.00 14.00 12.00 ,-3 12.00 10.00 8.00 e.oo 0- 6.00 4.0O ...... 4.00 2.00 2.00 -1.68 -30 30 60 -30 0 30 60 LONGITUDE LONGITUDE
e) 30 Ma sealevel pressure (hPa) in september f) PD sealevel pressure (hPa) in september
21.89 21.82 18.00 18.00 14.00 14.00 12.00 12.00 10.00 10.00 8.00 8.00 6.00 6.00 4.00 4.00 2.00
-30 0 30 60 30 60 LON(•ITUDE LONGITUDE Figure12. Sealevel pressure (minus 1000 hPa) over Africa for the Oligocene (left) and PD (right). The gray areas representthelow-pressure cell,and white indicates pressure greater than 18 hPa (i018 hPa): (a) 30 Ma- Exp2 in March,(b) PD - Exp0in March,(c) 30 Ma- Exp2 in June,(d) PD - Exp0in June,(e) 30 Ma - Exp2in September, and (f) PD - Exp0 in September. monsoonevolution, further analysis would be beyondthe scope remotesensing images [Burke and Wells, 1989]. The stretching of thispaper. of the monsoonover northAfrica is drivenby the continental drift of westernAfrica. Accordingto Axelrod and Raven [ 1978], 6.3. ModelResults and Proxy Data tropicalforests extend both northward and southward, whereas Pickford[ 1992] proposesthat the southerncontinental drift is the Rain forestsand savannamay have existed over a largepart of causeof a northwardsstretching of the monsoonand of a drying northemAfrica duringthe Oligoceneand early Mioceneperiods of PD equatorialAfi'ica (Congo, Zaire). [Axelrodand Raven, 1978; Maley, 1996], although evidence is Usingour numerical experiments, weinvestigated theimpact limited [Ruddimanet al., 1989]. During the Oligocene,because of continentaldrift on climate using the following climatic Africa wasmore to the souththan today, these forests have been parameters: precipitation, temperature, and K6ppen's associatedwith a strongermonsoon invading this continentfar classification[Kb)9pen, 1923]. This climatic classificationis inland[Axelrod and Raven, 1978]. The presenceof an important basedon the comparisonof the meanmonthly precipitation and drainingsystem during the Cenozoic has been observed through meanmonthly temperature with empiricalcriteria [K6ppen, 12,012 FLUTEAUET AL.' SIMULATINGTHE EVOLUTION OF MONSOONS
a) Verticalvelocity and zonal wind at 30Ma Oligocene (Exp2) and the PD (Exp0). Drastic reductionsalso occur during spring (-30%) and autumn (-36%), which are related to a shorteningof the monsoonseason by a month in . spring and in autumn. In the Oligocene experiment, heavy rainfall is simulated until the month of October. Between the Oligoceneand PD, temperatureaverages increase by +2.2øC for the annualmean and by +3.3øCduring the monsoon.Less than +0.7øC of this changemay be explainedby the differenceof mean elevation (using a mean lapse rate equal to 6øC/km). However, a large part of this warming is due to cloudiness --'!'•":--':::.-ß ABOVE 0.25 .-;.,,• o.oo- 0.25 reduction,especially in convectiveclouds, and reflectsa decrease •
Verticalvelocity (m/s)in summer-30 Ma I I I I 60-? ' '%'::'"'::'"•'•':•'
30-
.75 • 1.00.50 .25 -.25 -1.00 '""•iii...... "'"'•:•"":-' -•.•o -5.00
i -30 0 30 60 90 120 LONGITUDE
b) Verticalvelocity (m/s)in summer-PD -> •- I I I I 60' " o
...... :.--:q: ß '"•-'•:•'---"• .....*. -,-'--.-'.-.-',.-?--.•:•:•:•--'--:- •:- t-':--'.--•-;.'--.'--'•:•,-- :'•....-:•-•':- .:•..•--k-•.•--•/
1.13 1.00 .75 • 30- .50 •--• .25 -.25 ?...... k -1.00 O- -2.50 25..... '.... "- • /' • ...... ' -5.00 " I ...... •..... I ':•...... I''":::' ...... '...... I "'•-- -5.42 -30 o 30 60 90 120 LONGITUDE
Figure14. Meanvertical velocity (m/s) in summerat 500hPa (•4500 m) at (a) 30 Ma (Exp2)and (b) PD (Exp0). Isolinesare plotted for -5, -2.5, -1, -0.25,+0.25, +0.5, +0.75, and +1 m/s.Gray color represents vertical motion greaterthan _+0..25 m/s. Dotted line indicates ascending flow, and solid line shows subsidence.
of the subtropical area between 15øN and 20øN. Climatic point out the role of the Tethysseaway to separatethe Asian and variationsin theseareas confirm the shrinkageof the monsoon. African thermal low-pressurecells. For the Oligocene, the These results are in favor of Axelrod's assumptionwhich African low-pressure cell develops two pressure minima suggesteda stretching[Axelrod and Raven, 1978] of the African associatedwith two heatingareas, one locatedover northwestern monsoon. In response to the northward motion of Africa, Africa and the other over northeasternAfrica (Figure 4a). These precipitationchanges mainly affect subtropicalAfrica (20øN), two maxima of heating advectmoisture from the Atlantic Ocean whereasrainfall in equatorialAfrica (area C) is not greatly and westernEurasia, whereas in the PD experiment,moisture is reduced.However, because of thepresence of a dry season(when onlyadvected from the Atlantic Ocean x)ia the African trough and evaporationexceeds precipitation), the formationof evaporitesis by the minimum of pressure,also associatedwith a maximum of possible,as shown by Pickford [1992] in Uganda for the heating over Arabia (Figure 4c). This low-pressure cell Miocene.Although paleogeographic changes are able to reduce configurationexplains the'weak convergence toward the inner stronglythe monsoon area and replace it by an aridclimate, other continent.This split of the monsoonsystem from the PD to the mechanismssuch as albedo evolution and CO2 decrease also play Oligoceneis also found using anotherLMD version (LMD4ter) an importantpart. at lower resolution 48x36 coupled or not with a slab ocean [Ramstein et al., 1997a]. 7. Links Between the African and Asian Monsoon Flohn and Nicholson [1980] and Ruddirnan et al. [1989] suggestedthat the strengtheningof the African tropical easterly Systems jet is linked to the Tibetan plateau uplift becauseit strongly In section 6.2, we comparethe seasonalevolution of the diverts the midlatitude westerliessouthward, which then merge African low-pressurecell betweenthe Oligoceneand PD, and with the tropicaleasterly jet over Arabia. This mechanismis also 12,014 FLUTEAU ET AL.: SIMULATING THE EVOLUTION OF MONSOONS
Table 2a. The ClimaticEvolution Over TropicalAfrica Table2c. TheClimatic Evolution Over South Tropical Africa
Climatein AreaA PD (Exp0),% 30 Ma (Exp2),% Climatein AreaC PD (Exp0),% 30 Ma (Exp2),%
Warm and everwet 0.9 2.7 Warm and everwet 4 0 Warm and summerwet 31.6 53.1 Warm and summerwet 92 76.7 Warm temperateand winterwet 0 0.9 Warm temperateand summerwet 0 10 Warm temperate 2.6 8.8 Semiarid 0 3.3 Semiarid 14.9 26.6 Arid 4 10 Arid 50 7.9
al., 1991; Nigrini and Caulet, 1992; Prell et al., 1992]. An found in our PD simulation(not shown).The easterlyjet located abrupt increasein the foraminiferapopulation during the late above the Indian subcontinentand the Arabian Sea mergeswith Miocene has been attributed to the "onset" of the modem the midlatitude westerly jet diverted by the Tibetan plateau monsoon [Kroon et al., 1991]. However, the foraminifera in the toward Arabia, feeding the tropical easterlyjet at 500 hPa over sedimentcore is an indicator of the evolution of winds, only Africa (Figure 13b). In the Oligocene experiment, the mid- abovethe drilling site, whereasour simulationsare able to point latitude westerlyjet is poorly diverted for two reasons,namely, out the global spatial pattern of the wind field evolution. Our the absenceof Tibetan plateau and the presenceof inland seas experimentsreveal a northwardshift of the monsoonwinds over (the Paratethys Sea and the Tethys seaway) which drives the Arabian Sea. Thereforewe suggestthat this apparentonset of atmosphericflow locatedat 20øN zonally (Figure 13a). The weak the monsoonobserved in the upwellingrecord reflec[s the release of latent heat over the Himalayas reducesthe rising air northward shift of the monsoon winds over the Arabian Sea. masses(Figure 5a). For thesereasons and also in responseto a Besidesthe evolution in the spatial pattern of rainfall, our strong westerly jet driven by the Tethys seaway, the simulations reveal changes in the seasonal distribution of India/Himalayaneasterly jet is not generated(Figure 13a). precipitationand in the monthly heavy-rainfrequency during summerover the Himalayas.We have also introducedthe runoff, 8. Conclusions the seasonaldistribution of precipitation,and the monthlyheavy- rain frequencybecause they play importantroles and theyhelp to In thispaper, we haveshown that paleogeographic changes, better explain the sedimentaryprocess. The role of heavy rainy orogenicepisodes, plate motion, and sea level changes are able to eventsappears to be predominantin physicalerosion. reproducethe major trendsof the monsoonevolution in Africa We quantifythe respectiveimpact of the Paratethysshrinkage and in Asia during the past 30 Myr. Until now, the Tibetan and of the Tibetan uplift on the Asian summermonsoon by plateauuplift was consideredto be the major causeof the performing sensitivity experiments.These experimentsreveal summer Asian monsoon [Kutzbach et al., 1989; Prell and that both the Paratethysshrinkage and the Tibetanuplift increase Kutzbach,1992]. In this paper, we highlight the role of the precipitationover the Himalayas, northern India, and northern progressiveshrinkage of the ParatethysSea on the Eurasian Indochina,whereas the southernpart of Asia becomesdrier. climateand the Asianmonsoon. The Paratethysretreat stretches, These experimentsalso reveal that a completeretreat of the deepens,and shifts northwestwardthe monsoonlow-pressure Paratethys Sea induces a precipitation increase over the cell, modifyingthe large-scalecirculation and thusthe monsoon Himalayasstronger than the precipitationincrease induced by the precipitationpattern. Precipitation decreases over Indochinaand sole Tibetan plateau uplift. The precipitationincrease over the increasesover the southernflank of the Himalayasand central Himalayas is weakly linked to the elevation of the Tibetan India.These changes are in agreementwith proxydata [Ducrocq plateauas long as the Himalayasare alreadyuplifted. We also et al., 1994] and imply that Indochinahas experiencedno show that the impact of the Paratethys shrinkage on the significantclimatic changes since the early Miocene,while the precipitationdepends on the elevationof the Himalayasbecause Himalayashave experienced an increasein precipitationduring of feedback mechanismssuch as the latent heat release, which thelate Miocene. In additionto changesin precipitation,the 850 strengthensthe moistureadvection. hPa wind field revealsa strengtheningover the ArabianSea and For the African monsoon we show that the northward drift of a weakeningof windsover BengalBay from 30 Ma to PD. The the continentand the closureof the Tethysseaway explain a large locationof thestrong westerly jet overthe Arabian Sea may be part of the summermonsoon evolution during the late Cenozoic. compared with the upwelling intensity deduced from The southernlocation of Africa and the Tethysseaway splits the foraminiferadevelopment recorded in sedimentcores [Kroon et African and Asian monsoonsinto two separatesystems, which explainsthe moistureadvection far inland in Africa during the Oligocene. Moreover, comparing our simulationsto different Table 2b. The Climatic Evolution Over Northern Africa scenarios of monsoon evolutions [Axelrod and Raven, 1978; Pickford, 1992], we conclude that the northward drift of this Climatein AreaB PD (Exp0),% 30 Ma (Exp2),% continentleads to a shrinkageof the monsoonbelt as suggested by Axelrod and Raven [1978]. At last, we show that during the Warm and summerwet 0 15.9 Oligocene, due to the Tethys seaway, the Asian and African Warm temperateand winterwet 0 3.2 monsoonsystems were clearly separated,whereas for the present Warm temperateand summerwet 0 15.8 day, our simulationspoint out a link betweenthese monsoons at Warm temperate 1.2 1.6 Semiarid 13.6 44.4 low and middle troposphericlevels. Arid 85.2 19.1 The results presentedhere have been produced within a multidisciplinary approach between different fields of Earth FLUTEAU ET AL.' SIMULATING THE EVOLUTION OF MONSOONS 12,015
a) TropicalAfrica science.This approachwas necessary to allowthe inclusionof otherpieces of thepuzzle in futuresimulations. Therefore these
' i ' i , i , i ' i , i , I ' i , i , i , resultsare only expectedto reproducemajor climatic evolutions
. . inducedby paleogeographicchanges. Accounting for realistic . changesin carbondioxide, oceanic circulation, and vegetation shouldobviously help to improvethe model/datacomparison. Howeverat thisstage, we considerthat most of themain features of the monsoon evolution may be understoodusing realistic changesin paleogeography.
Acknowledgments. The authorswish to thank P. Bracconot,Y. Gaudemer,F. M6tivier, N. de Noblet, and L.E. Ricou for helpful discussions.We also thank S. Gilder and M. Kageyama,who helpedus to improvethe English.This workwas carried out using the IDRIS and CEA computingfacilities. We thankJ.Y. Peterschmittfor providingthe postprocessingpackage. This workis supportedby theFrench scientific programDynamique et TransfertTerrestre, INSU. IPGP contribution F M A M J J A S O N D 1581, LSCE contribution175, and DTT-INSU contribution157. Month
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