Tibetan Plateau summer monsoon northward extent revealed by measurements of water stable isotopes L. Tian, V. Masson-Delmotte, M Stievenard, T. Yao, J. Jouzel To cite this version: L. Tian, V. Masson-Delmotte, M Stievenard, T. Yao, J. Jouzel. Tibetan Plateau summer mon- soon 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 HAL Id: hal-03100013 https://hal.archives-ouvertes.fr/hal-03100013 Submitted on 9 Feb 2021 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 .t / , /............ ... 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.