Tibetan Plateau Summer Monsoon Northward Extent Revealed by Measurements of Water Stable Isotopes L

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Tibetan Plateau summer monsoon northward extent revealed by measurements of water stable isotopes L. Tian, V. Masson-Delmotte, M Stievenard, T. Yao, J. Jouzel

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L. Tian, V. Masson-Delmotte, M Stievenard, T. Yao, J. Jouzel. Tibetan Plateau summer monsoon northward extent revealed by measurements of water stable isotopes. Journal of Geo- physical Research: Atmospheres, American Geophysical Union, 2001, 106 (D22), pp.28081-28088. 10.1029/2001JD900186. hal-03100013

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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 GEOPHYSICALRESEARCH, VOL. 106,NO. D22, PAGES28,081-28,088, NOVEMBER 27, 2001

Tibetan Plateau summer monsoon northward extent revealed by measurementsof water stableisotopes

L. Tian,• V. Masson-Delmotte,2M. Stievenard, 2T. Yao •9 andJ ß Jouzel2

Abstract. A programof individualprecipitation events and river water samplingand of waterisotopic measurements (fiD,br80) was carried out during summer 1996 along a northeast/southwesttransect of theTibetan Plateau. The spatial distribution ofboth •80 anddeuterium excess (d=bD-8*b•80) ofthe precipitation reveals three distinct regions. Simulationswith a simpleisotopic model and seasonalisotopic variations measured at two extremesouth and north locations support our interpretationin termsof differentsummer moistureorigins: (1) Southof the Himalayanmountains, the moistureprovided by the Indianmonsoon has been recycled over the Indianpeninsula. (2) Betweenthe Himalayas andthe Tanggulamountains the oceanicmoisture is directlytransported from the Bay of Bengalalong the BrahmaptraRiver valley. (3) North of the Tanggulamountains, the moistureis not providedby the monsoonanymore but by continentalwater recycling.

1. Introduction September) along a northwest/southeasttransect of the Tibetan Plateau, across the Tanggula and the Himalayan The precipitationisotopic composition (6D, 61aO) mountains (Figure 1). Four locations have been chosen to representsan integratedclimatic parameter,reflecting the perform individual precipitation sampling: three evaporationand condensationhistory of an air mass.The meteorologicalstations (Delingha, Tuotuohe, and Lhasa) and World Meteorological Organization/InternationalAtomic the bottom of Xixiabangrnaglacier duringthe 1996 summer EnergyAgency (WMO/IAEA) networkset up to monitorthe glaciologicalfieldwork; in complement,water from the main global isotopiccomposition of the precipitationenables to rivers has also been collected, as well as fresh snow at define different moistureorigins for Asian precipitation Xixiabangrna(Figure 1 and Table 1). The total precipitation [Araguas-Araguaset al., 1998] but still lacksmeasurements amountsrecorded at Nyalam (a meteorologicalstation close in the TibetanPlateau region. In particular,the limit between to Xixiabangrnaglacier), Lhasa, Tuotuoheand Delingha are theIndian monsoon influence and the moisture transported by 566, 584, 278, and 170 mm, respectively,of water in 1996. In thewesterlies remains uncertain. The difficultyto accesssuch the souththese precipitations and their seasonalcycle reflect high-altitudesites strongly limits the abilityto run continuous an averagemonsoon year except for Lhasa, 30% above the precipitationsampling, especially during the monsoonseason. multiyearaverage (Table 1). However,high-altitude Tibetan glaciers offer uniquearchives of past tropical precipitation and several ice cores have In this paperwe focuson the informationbrought by the alreadybeen successfully retrieved there [Thompson e! al., precipitationisotopic composition. The 8•0 and fid of the precipitationdepend mainly on the degree of distillation of 1989; Thompsone! al., 1995]. To interpretpast changes in the air mass.They are relatedby the "globalmeteoritic water precipitationisotopic composition, it is necessaryto have a line,"8D=8 6•O+10 [Craig, 1961], where the slope of eight minimumknowledge of modemTibetan Plateau precipitation results from the differences in equilibrium fractionation isotopiccomposition seasonality and spatial distributions. The coefficientsfor the two isotopes.In temperateand polar isotopic compositionof the precipitationalso offers an regionsthe degree of distillation is mainly driven by the opportunityto distinguishbetween different water origins and therefore better define the northward limits of the monsoon progressivemoisture depletion as the air massesmove inland or cool downtoward the poles.As a result,local spatiallinear influence.A network monitoringindividual precipitation relationshipscan be empirically defined between surface eventswas setup in Tibet in 1991. Someprevious works have broughtinsight on local6280 distribution in September-temperature (a first-order indicator of rain-out degree, October[Aizen et al., 1996, Wakeand Stievenard,1995] and correlated to the cloud condensationtemperature) and onthe 8•80-temperature relationship [Yao et al., 1996; !999]. precipitationisotopic composition.In tropical regions the Here we show results from isotopic measurements distillationof the air massesis stronglydependent on vertical conductedduring the summer1996 monsoon season (May to movementsdue to convectiveactivity. In this case the first- ordermeasurement of the degreeof distillationis not the local surfacetemperature, which is not related to the condensation • Laboratoryof IceCore and Cold Regions Environment, Cold and Arid RegionsEnvironmental and EngineeringResearch Institute, temperaturebut the precipitationamount. Lanzhou, China. Simulationsperformed with a simpleisotopic model [Ciais 2Laboratoire desSciences duClimat et de l'Environnement, Gif-sur- etal, 1994]show the decrease of the slope between 15•80 and Yvette, France. surfacetemperature depending on the type of distillation (Figure 2). When a closed cloud is considered(all the Copyright2001 by theAmerican Geophysical Union. condensedphase stays in the cloud,no precipitation),which Paper number2001JD900186. is an idealizedconvective situation, this slopeis muchweaker 0148-0227/01/2001JD900186509.00 than in the caseof a classicalRayleigh distillation, where the

28,081 28,082 TIAN ET AL.: TIBETANPLATEAU WATER STABLE ISOTOPES

/ , /...... L.::•...... "7.,..'.::•_• , / B•.JM A • LegendsC•ty 2.• •,•,•Pakistan o Toxin 4 tJ•'k•stan / .... C.... • Bound•L,nc • n,j,k..... i B• of Bengal /:':.... [ake andRiver 6 Bhutan / Scale • /

' '...... '_...... Z_.L..... t

...... II . .[111I •œ Figure1. Geographicmapof western China and surrounding countries showing thelocation ofthe precipitation samplingsites (triangles) andthe river sampling sites (circles). The main mountain ranges are also indicated.

entirecondensed phase is precipitated(open cloud). In the averageof 10 [Craig,1961], d variesboth spatially and latter case, climaticconditions leading to droplettemporally [Rozanski et al., 1993].The valuesof d in the reevaporationbelow the cloudbase (dry air) or moisture precipitationreflect the nonequilibriumfractionation recyclingatthe land surface (dry ground before the rainfall) occurringat themoisture initial evaporation from the ocean both result in higher 8•So valuesand weaker 8•SO- (dependingonthe speed of theevaporation, therefore on the temperatureslopes (Figure 2). As differentphase change airrelative humidity, and the sea surface temperature) [Jouzel historiescan result in similar8•O behaviors,the first-order et al., 1982; Johnsen etal., 1989], at the land surface (when isotopicmeasurements are not sufficientto reconstructair mass histories. continentalrecycling is considered),and along the air mass trajectory(reevaporation of the droplets,formation of ice At the secondorder, slight differences between the two crystals).In particular,d increaseswith the moisturesource isotopescan arise during kinetic fractionation andjustify temperature(moisture source effect), increasesat cold usinga second-orderisotopic parameter, the deuterium excess temperatures(snow formation effect). Simulationswith a d=SD-88•SO[Dansgaard, 1964]. With a globalmean theoretical isotopic model (Figure 2) alsoshow that d TIAN ET AL.: TIBETAN PLATEAU WATER STABLE ISOTOPES 28,083

Table 1. Characteristicsof the SamplingSites (Latitude, Longitude, Altitude, Annual Mean Air Temperatureand Precipitation,Seasonality of the PrecipitationDefined As the RatioBetween the PrecipitationFalling Between May and Septemberto the AnnualMean Precipitation)and of the Water Sampling(Period of Collection,Number of Samples,Average May-August Isotopic Values). a

Delingha Tuotuohe Rivers Lhasa Xixiabangma

Latitude 37.37øN 34.21 øN 36.41 øN to 29.70øN 28.45øN 28.18øN Longitude 97.97øE 96.43øE 97.70øEto 91.13øE 85.78øE 85.97øE Altitude 2981 m 4533 m 3658 m 5680 m Temperature 3.7øC -3.8øC 7.5øC Nyalam (3810m): 3.5øC Precipitation 219.9 mm 264.4 mm 444.8 mm Nyalam(3810m): 617.9mm Seasonality 91% 93% 98% 64% Start of'sampling May 12, 1993 May 26, 1996 July 13, 1996 May 27, 1996 July 29, 1996 End of sampling Sept. 1, 1996 August 28, 1996 July23, 1996 Aug. 23, 1996 August15, 1996 Number of 112 49 13 59 25 samples b•80 -6.5%0(+1.5%o) -9.0%o -14.8%o -16.8%o -19.2%o d + 11.8%0(ñ2.7%0) + 15.7%0 +9.5%0 +7.9%o + 17.9%0

aForDelingha the interannualstandard deviation is alsodisplayed. The river namesare Germud,Xiangride River., Tuotuohe,Yanshiping, Tanggula mountains., Amdo River, DangxiongRiver, Lhasa River., Yafiungzangbo,Xigaze, Tingd, LalongPass, and Nyalam River.

increases when the moisture provides from continental absolute value of the differences of 0.11%o; standard recycling,or when a convectivecloud is considered(closed deviation of the absolutevalues of the differences0.08%0). cloud), while it decreaseswhen droplet reevaporationtakes The error on d is evaluatedas the quadraticaverage of the placeduring the precipitation. errorson 15Dand 8 15•80,here about 1.9%o.For studies Thesetheoretical studies support the useof (15180,d) focusing on past variations in polar excess, the weak valuesto constrainthe origin of the moisturein continental amplitudeof the excessfluctuations (typically 5%0 along a regionssuch as the TibetanPlateau. glacial-interglacialcycle, Vimeuxet al. [1999]) requireda higheranalytical precision. However, the precisionused here 2. Measurements and Results is enoughto studythe largespatial and temporalfluctuations shownby the excessof Tibetanprecipitation (typically 10 to Deuterium measurements were performed at the 20%0). Laboratoire des Sciencesdu Climat et de l'Environnement, France,with an analyticalprecision of 1.0%o.The oxygen 2.1. Meteoric Water Line on the Tibetan Plateau isotopecomposition of the 312 water sampleswas measured at the Laboratoryof Ice core and Cold RegionsEnvironment, The local meteoric water line calculated from individual Cold and Arid Regions Environmental and Engineering precipitationevents at each sampling site (Table 2) is ResearchInstitute, China, with a precision of 0.2%0. An generallyclose to the globalvalue of 8, with the highestslope intercalibrationexercise conducted on 20% of Delingha15•SO at the most inland location (Delingha). To complementthe samplesmeasured in both laboratorieshas confirmed the precipitationdata obtainedat only four samplingsites, 13 analyticalprecision (mean of the differencesof 0.06%0;mean largerivers (from Germudin the extremenorth of the Tibetan

Table 2. MeteoriticWater Line CalculatedDuring the SamplingInterval for Each Site.

Slope Intercept Correlation Coefficient r 2

Delingha,3 years 8.4 15.0 0.98 May-August 1996 only 8.3 13.5 0.98

Tuotuohe 8.2 17.5 0.97

Lhasa 7.9 6.2 0.97

Xixiabangrnaprecipitation at 5900 m 8.2 21.5 0.99 Fresh snow between 6400 and 7000 m 8.7 29.9 O.99

Rivers 8.4 16.5 0.99 28,084 TIAN ET AL.: TIBETAN PLATEAU WATER STABLEISOTOPES

t t / / / / / -10 , • , I , 0 5 10 15 0 õ Temperature(øC) Temperature(Pc)

' I ' opencloud (Rayleigh), no dropletreevaporation closed cloud opencloud, 10% dropletreevaporation opencloud, mirror 10% recycling

/ / / -2 ! / I I -4 -10 -5 0 5•80(ø/oo) Figure2. Variationsof15•SO and d simulatedusing the mixed cloud isotopic model from Ciais et a.l [1994]along trajectoriesfrom an oceanat 20øCuntil a condensationtemperature of 0øC underdifferent parameterizations to take into accountthe cloudtype (opencloud, all the condensateleave the cloud;closed cloud, all the condensate remaininside) and the possiblereevaporations (during the dropletfall or at the land surface).The closedcloud system(representative of convectiveclouds) and/or the mirrorrecycling (assuming no landreservoir) is associated with high d valueseven with low temperatures.On the contrary,the dropletreevaporation leads to a larged decrease.Note that the closedcloud system also results in a weakerdependency of 15•80on condensation temperature[Yao et al, 2000].

Plateauto Nyalam River in the southernslope of the spatial15•80 gradient (from -5%0 in the north to -20%0 in south Himalayas)were also sampled for waterisotopes. In general, Tibet)results from different isotopic processes. In thenorth the river water isotopiccomposition results from combined the highisotopic composition of the precipitationmainly effects of precipitation,evaporation, glacial meltwater, reflectsthe impactof high summertemperatures. On the undergroundwater, and plant transpiration. In mostparts of contrary,the southTibetan precipitation depleted isotopic the Tibetan Plateau, snow accumulation and ablation occur compositionis mainly influenced by the precipitation amount duringsummer, and the short residence time of snowexplains effectdue to theheavy monsoon rains. It isnot possible from thatthe glacialmeltwater isotopic composition fits well with thespatial 15•80 distribution alone to delimitthe respective the precipitationisotopic composition.When the water extentsof themonsoon influence versus the temperate climate balance is mainly controlledby precipitation,the stable influence,as its spatialfluctuations are quitesmooth. The isotopiccomposition of the riversrepresents an integrated excellentagreement between the fiver water isotopic regionalmeasurement of the precipitation.In our case,the compositionand the seasonalaverages of precipitation 15D-/5•Soslope obtained from the fivers (8.4, Table 2) isclose supports,again, the use of the fivers to complementthe to the local meteofiticline and supportsthe useof the fiver informationobtained at discreteprecipitation-monitoring datasets to complementthe precipitationsampling network. stations. Thelocal 15D-/5280 slope being close to the global average of8 From north to southof the TibetanPlateau, d fluctuations justifies the use of the deuteriumexcess definition d = 15D-8 aremuch more abrupt than the progressive decrease in 15•So 15•80to extract the second-order isotopic information. values.Three differentregions can be clearlydefined from differentd levelsin bothprecipitation and fiver samples:(1) 2.2. MeanSummer 1996 Spatial Distribution of b•Soand d north of the Tanggulamountains, d has valuesabove 11%o; The spatialvariations of the mean summer1996 15•80of (2) betweenthe Tanggulaand Himalayanmountains, d theprecipitation (arithmetic average of all dailyevents) and decreasesto about7 %0;(3) on theHimalayas, d risesagain to river watersamples are displayedin Figure3. The large reacha maximumvalue at Xixiabangrna(17%o). Even if the TIAN ET AL.: TIBETANPLATEAU WATER STABLE ISOTOPES 28,085

- - ß - -excessO-• 180 I

TanggulaMts. Himalaya Mts. 20-- 4r _.---5 •, 16 -10 •

• 12 _ '•.•. •. •.•. • . - -20• 4 , • . , • , , , . , • . . . . • -25

Figure3. Geo•aphicaldis•ibution ofaverage su•er 1996isotopic composition ofthe precipitation from DelinghatoXixiaban•a, combining precipitation andriver water measurements. precipitationsampling period is quite short at Xixiabangrna (2 monsoon.During the monsoonseason, the moisture weeks),the measurementsare representativeof the local evaporatedin the Bay of Bengalwill movealong the precipitationas indicatedby d valuesmeasured along a Brahmaputra-YalongzangboRivers and bring rich glaciersnow pit at Dasuopu (7000 m), ranging between 14%0 precipitationin the south part of theTibetan Plateau [Gao et and20%0, with an averageof 17.3%0.In glaciermeltwater (at al., 1985;Lin et al., 1990].The moisturebeing transported 5900m) which might represent an accumulation over several directlyfrom a nearbysea leads to low d in theprecipitation thousandyears, d (13.5%0)is stillmuch higher than at Lhasa. in southTibet. This studyalso indicates that the southeast The obviousdifference of d betweenthe two sidesof the monsooncannot pass the orographic barrier of theTanggula Tanggulamountains is due to thelimit between tropical and mountains,at leastin 1996. temperateair masses. North of theTanggula mountains, high The differencein d betweenthe two sidesof the Himalayas d and/5•80 in the precipitationare consistentwith local alsoindicates different moisture origins. Two kindsof water summerconvective precipitation occurring under dry climatic trajectoriesreaching the Tibetan Plateau from the south have conditionsand resulting from continental moisture recycling beenevidenced [Lin et al., 1990]:(1) to the east,humid [Tianet al., 1996],with no directinflow of oceanicair. marineair massfrom the Bay of Bengal,moving along the Meteorologicaldata (e.g., Figure4) indeedlocate the Brahmaputra-YalongzangboRiversand reaching in the south maximumnorthward extent of the IntertropicalConvergence of the TibetanPlateau. Along this valleytrajectory, large Zone in summer time close to the Tanggula mountains precipitationamounts are released due to the uplift of humid [Araguas-Araguaset al, 1998]. airmasses, resulting in low d (nosignificant land recycling) In the area betweenthe Tanggulaand the Himalayas, andlow/5•80 (depletion due to intenseconvection). (2) To precipitationis directly controlled by the southwestIndian the west,another trajectory can bring moisture from the IndianOcean, over the Indianpeninsula towards the southern TanggulaMts. Himalayas slopeof the TibetanPlateau. In thiscase, the evaporation • 800 fromthe surface of thepeninsula (with low relative humidity) canmodify the initial marine air mass and result in highd in the followingprecipitation (Figure 2). Anotherpossible •600 reasonfor the high d in the Himalayasis the high-altitude '•- 400 precipitationassociated with low cloud temperature. Simultaneousprecipitation sampling at differentlevels may help betterunderstand a possiblealtitude-deuterium excess '• 200 relationship. Atmosphericgeneral circulation models fitted with the o H , explicitmodeling of waterisotopes [Jouzel et al., 1987; Delingha Tuotuohe Lhasa Nyalam Hoffmannet al., 1998]simulate unusually high annual mean Figure4. Annualprecipitation at the four stations on the d valuesin centralEurasia (including the Tibetan Plateau), at TibetanPlateau. Annual precipitation is much higher in the placeswhere in summermore than 50% of the simulated south of the Tibetan Plateau due to the summer monsoon precipitationprovides from continental recycling [Koster et precipitation. al., 1993; Joussaumeet al., 1986]. Althoughtheir spatial 28,086 TIAN ET AL.: TIBETAN PLATEAU WATER STABLE ISOTOPES

ß i Tuotuol•

x )Cudaban{•na Po{ynomia{(Xixiab•{•aa) Polynomial(Tuotuohe)

Polynomial(Lhasa)

-45 -40 -35 -30 -25 -20 -15 -10 -5 0 5

oxygen18 Figure5. Temporalvariations ofd withrespect to •5•80 in thefour precipitation sampling sites.

resolutionstrongly limits their ability to capturesynoptic accountsfor depletedisotopic values and increasingd levels, climate processes,such broad-scalemodel resultsseem to which remain, however, significantly lower than at confirm the role of continentalmoisture recycling in Xixiabangma(see previoussection). A significantproportion producinghigh excesslevels in northemTibet. of the precipitationevents are characterized by high values(above -10%o) and a wide rangeof d levels.This large 2.3. Summer1996 Temporal Relationships Between d and dispersionresults at least partly from local stormsin the •'O Lhasavalley, and the high/5•80 values probably result from The day-to-day8'80-d fluctuations undergo large and two differentprocesses: (1) dropletreevaporation under very complexfluctuations which are difficultto interpretwithout dry conditions(very low d values,associated with very little many simultaneouslocal and large-scalemeteorological rainfall) or (2) moisture provided by inland continental observations.Therefore we only discuss here the relationships recycling (very high d values). betweend and/5'80 observed forthe full monitoring period, Tuotuohehas less depleted/5•80 precipitation and higher d startingfrom south to north(Figure 5). valuesthan the southernsites. Here d increaseswith Xixiabangrnasnowfall shows the mostnegative /5•80 probably reflecting an increasing contribution from values,due to boththe strongprecipitation amount effect in continentalmoisture. Even if the local climate is quite dry, themonsoon region and the very low air temperaturein high only very few precipitationevents have low d values.Apart altitude(Table 1); at thislocation, d is verystable with high from these specificevents, the cold local temperaturemay values(typically 20%o). When/5•80 decreases below -22%o, as preventfrequent droplet reevaporation. a resultfrom either very strong precipitation or long-distance In summer1996, Delingha(Figure 5) precipitationmainly moisturetransport, d is anticorrelatedand increases. This may reflectsthe summertemperatures with relatively high be due to a warmer oceanic moisture source or to a behavior values comparedto the other sites studiedhere. Note that specificto closedclouds. Indeed, depleted isotopic values are Delinghais locatednorth of the Kunlun Range,rather open associatedwith strong monsoon activity and deep convection, towardthe Tarim. When precipitationoccurs there in the early bringingwater vapor to veryhigh altitudes. On theopposite, springat coldtemperatures, low/5•80 values are associated when/5'80 increases above -22%o (weak precipitation with low excesslevels, maybedue to dropletreevaporation intensity),d alsoincreases slightly, which may result from a underdry conditions.This is alsothe casefor severalshallow larger proportionof moistureprovided by continental rainevents associated with very high/5'80 values (between-5 recycling. Interestingly,atmospheric general circulation and+5%o) and low d values. modelsindeed simulate high d in summerover Tibet and centralAsia, at placeswhere they evaluate the proportion of 2.4. Seasonald Cyclesat Delinghaand Lhasa ' recycledmoisture to accountfor more than 50% of the total ForDelingha, 3 yearsof observationsare available, which precipitation[Hoffmann et at., 1998;Koster et at., 1993]. enables to obtain a first estimate of the mean seasonal At Lhasa,the rainfall exhibits a similarisotopic behavior to isotopiccycle (Figure 6a). The/5•80 cycle clearly shows that Xixiabangrna,with a clearminimum in d corresp6ndingto Delinghais outsideof the monsooninfluence and mainly {5•80values around -15%o. Again, the strong monsoon activity reflects the seasonalair temperaturecycle with minimum TIAN ET AL.' TIBETAN PLATEAU WATER STABLE ISOTOPES 28,087

evidencedby recentstudies [Tian et al., 1996; Yao et al., 1999]. continentalrecycling continentalrecycling 30 • • Forcomparison, d and b•SO in Lhasaprecipitation show quitedifferent seasonal variations (Figure 6). Beforeand after themonsoon season, d andb•80 are quite high due to the continentalmoisture origin, as in Delingha. However, the strongIndian monsoon precipitation from Juneto September resultsin very low d in precipitation,modifying completely the seasonalcycle. Similar characteristicshave also been observedduring the Pacificmonsoon in southeastChina [Wei and Lin, 1994].

3. Conclusions -? ...... '0 d excess Our data have significantlyincreased the density of -30 precipitationisotopic composition data in the TibetanPlateau region.We have shownthe interestof the deuteriumexcess IAtlasa parameteras a tool to delimitthe northwardextent of the Indian monsoon. The characteristicsof stableisotopes in precipitationto the 30 ....•...... ,.... • O...."'.... '"'"...... •..... • O northofstable oftheisotopes Tibetan inprecipitationPlateau represent inthe theinland common ofAsia condition where

2010 OO 0....::...... ";00•i ';?..O :i..60 O...... O .-!.....O"':!':" ...... O...... predominant.thegeneral climate circulation isOurdrydata andmodels,confirm the previouslocalsuggesting hydrologicalresults thatobtained the cycle highwith is .::....::..... '...... O 0 continentalityinlandAsia (more than 50% of the precipitation • 0 , , ::•...... beingrecycled overthe continent) resultsin high summer

-10 area beyond the monsooninfluence, stable isotopesare • 12i • • 6 7'•8 9 11 121'3....1'4• • • 18 19 2021 23 24 mainlyb180 and influenceddinprecipitation. by temperature, Ourstudywhichalso providesreveals thatabasis inthis for dexcess coreretrieved in theAsian interior. -20-30 • •oxygen 18 • theThereconstructionhigh-altitude ofprecipitationpalcoclimate datainformation obtained fromhere futureseem iceto indicatethat the excessincrease at cold temperaturesis not Figureisotopic6. composition (a) Mean atseasonal Delingha variations for 1993-1996. in precipitationMonthly limited tothe polar regions. Highd values inthe high averagesandstandard deviations arecalculated asthe mean Himalayas canalso be partly ascribed tothe closed cloud ofdaily data (months withonly one precipitation eventare isotopicbehavior of deeply convective clouds at low notdisplayed). (b)Same asFigure 6abut for Lhasa (1986 to temperatureandhigh altitude (Xixiabangma glacier). This 1991)IAEA/WMO monthly measurements. resultsupports theneed for further precipitation andwater vaporsampling at variousaltitudes along a commonmoisture trajectory. valuesin December-Januaryandhigh values between June Ourdata provide afirst spatial-temporal framework, which andOctober. Because of the very dry climate (about 200 willbe useful to interpret past d fluctuationsmeasured along mm/yr,Table 1), somemonths undergo only very few Tibetanice cores. Pursuing the collection network over precipitatingevents even over 3 yearsof observationsseveral years will help relate the isotopic composition ofthe (FebruarytoApril and October toDecember). Thed seasonal precipitation tothe moisture origin under different monsoon cycleistherefore somehow biased toward individual rainfall configurations (El Nifio or La Nifiasituations). Past values,with negative levels in February andApril as a result fluctuationsofd have recently been measured along ice cores ofdroplet reevaporation underdry conditions. Nevertheless, it drilled inthe mid-Himalayas [Thompson etal., 2000]. For the isobvious that Delingha ischaracterized bya maximum din interpretationofsuch fluctuations, a better understanding of summerand autumn, unlike most Northern Hemisphere the relationship between d in precipitationand monsoon stations[Rozanski etal, 1993]. The extreme continentality of activity isrequired and can be achieved only by means of Delinghalocation may be responsible forsuch high d valuesmodem precipitation monitoring. andalmost positive b•80 in late summer,due to inland moisturerecycling toward Delingha. Thisresult issupported Acknowledgments. Thiswork is supported bythe Chinese byIAEA data obtained atHetian, located atthe same latitude National Basic Research Program (G1998040802), Chinese as De!inghabut at the extremewest of China.A similar Academyof Sciences(KZCX2-301), Innovation project of Coldand seasonalityis also observedin waterisotope records Arid Regions Environmental andEngineering Research Institute, measuredalong an east Mongolian icecore [Schotterer etal., CAS(CACX210506 andCACX 210046). Ourthanks aregiven toall thepersons involved in the precipitationsampling. We especially 1997],suggesting common processes incentral Asia. Such a thankengineer O.Cattani forhelping inthe measurement offid and continentaleffect could account for thehigh dependency of engineerSun Weizhen for helping inthe measurement ofb•80. This precipitationb•80 on localtemperature in centralAsia isLSCEcontribufion0533. 28,088 TIAN ET AL.: TIBETAN PLATEAU WATER STABLE ISOTOPES

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