A REVIEW OF CLIMATIC CONTROLS ON δ18OIN PRECIPITATION OVER THE TIBETAN PLATEAU: OBSERVATIONS AND SIMULATIONS
Tandong Yao,1,2 Valerie Masson-Delmotte,3 Jing Gao,1 Wusheng Yu,1 Xiaoxin Yang,1 Camille Risi,4 Christophe Sturm,5 Martin Werner,6 Huabiao Zhao,1 You He,1 Wei Ren,1 Lide Tian,1,2 Chunming Shi,3 and Shugui Hou2,7 Received 26 November 2012; revised 20 September 2013; accepted 22 September 2013.
18 [1] The stable oxygen isotope ratio (δ O) in precipitation is attributable to the shifting moisture origin between Bay of an integrated tracer of atmospheric processes worldwide. Bengal (BOB) and southern Indian Ocean. High- resolution Since the 1990s, an intensive effort has been dedicated to atmospheric models capture the spatial and temporal pat- studying precipitation isotopic composition at more than 20 terns of precipitation δ18O and their relationships with mois- stations in the Tibetan Plateau (TP) located at the convergence ture transport from the westerlies and Indian monsoon. Only of air masses between the westerlies and Indian monsoon. In in the westerlies domain are atmospheric models able to this paper, we establish a database of precipitation δ18Oand represent the relationships between climate and precipita- use different models to evaluate the climatic controls of precip- tion δ18O. More significant temperature effect exists when itation δ18O over the TP. The spatial and temporal patterns either the westerlies or Indian monsoon is the sole dominant of precipitation δ18O and their relationships with temperature atmospheric process. The observed and simulated altitude- and precipitation reveal three distinct domains, respectively δ18O relationships strongly depend on the season and the associated with the influence of the westerlies (northern TP), domain (Indian monsoon or westerlies). Our results have Indian monsoon (southern TP), and transition in between. crucial implications for the interpretation of paleoclimate Precipitation δ18O in the monsoon domain experiences an records and for the application of atmospheric simulations abrupt decrease in May and most depletion in August, to quantifying paleoclimate and paleo-elevation changes. Citation: Yao, T., et al. (2013), A review of climatic controls on δ18O in precipitation over the Tibetan Plateau: Observations and simulations, Rev. Geophys., 51, doi:10.1002/rog.20023.
1. INTRODUCTION Plateau (TP) is considered as the “Third Pole” of the Earth. The elevated topography of the TP does not only act as a [2] With an average altitude above 4000 m, a vast geograph- ical coverage, and large-scale climatic influence, the Tibetan barrier to the midlatitude westerlies but also strengthens the Indian monsoon through its dynamical and thermal im- pacts, thus contributing to large-scale atmospheric circulation 1 Key Laboratory of Tibetan Environment Changes and Land Surface [Bothe et al., 2011]. In turn, the influences of the westerlies Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China. and Indian monsoon (Figure 1a) are critical for advection of 2State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions heat and moisture, and climate patterns in the TP region [An Environmental and Engineering Research Institute, Chinese Academy of et al., 2001, 2012]. The TP snow cover and albedo are known Sciences, Lanzhou, China. 3LSCE, UMR 8212, IPSL, CEA/CNRS/UVSQ, Gif-sur-Yvette, France. to have large-scale impacts on atmospheric circulation [Wu 4LMD, IPSL, CNRS/UPMC, Paris, France. and Qian,2003;Ma et al., 2009; Kang et al., 2010; Poulsen 5 Bert Bolin Centre for Climate Research and Department of Geological and Jeffery, 2011]. The TP is particularly sensitive to climate Sciences, Stockholm University, Stockholm, Sweden. 6Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, change and regional anthropogenic forcing [Xu et al., 2009] Germany. and currently experiencing significant warming [Yao et al., 7 Key Laboratory for Coast and Island Development of Ministry of 1996a; Liu and Chen,2000;Tian et al., 2006; Kang et al., Education, School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing, China. 2007; Qin et al., 2009] with important impacts on biodiversity and environment [Klein et al., 2004; Wang et al., 2008]. The Corresponding author: T. Yao and J. Gao, Key Laboratory of Tibetan TP also acts as a water tower for surrounding regions due to Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. monsoon precipitation and glacier melt [Barnettetal., ([email protected]; [email protected]) 2005; Yao et al., 2007; Viviroli et al., 2007; Bates et al.,
©2013. American Geophysical Union. All Rights Reserved. Reviews of Geophysics, 51 / 2013 1 8755-1209/13/10.1002/rog.20023 2012RG000427 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES
Figure 1. (a) General patterns of moisture transport under the influences of the westerlies and Indian monsoon over the TP. Red triangles depict locations of δ18O monitoring stations: 1-Urumqi, 2-Zhangye, 3-Taxkorgen, 4-Delingha, 5-Hetian, 6-Lanzhou, 7-Kabul, 8-Tuotuohe, 9-Yushu, 10-Shiquanhe, 11-Gaize, 12-Nagqu, 13-Yangcun, 14-Bomi, 15-Lulang, 16-Lhasa, 17-Nuxia, 18-Baidi, 19-Larzi, 20-Wengguo, 21-Dingri, 22-Dui, 23-Nyalam, 24-Zhangmu; open circles show ice core sites: a-Muztagata, b-Dunde, c-Malan, d-Guliya, e-Puruogangri, f-Geladandong, g-Tanggula 1, h-Tanggula 2, i-Zuoqiupu, j-Dasuopu, k-East Rongbuk. Up triangles stand for GNIP stations and down triangles stand for TNIP stations. (b) Schematic representation of the main processes affecting precipitation δ18O over the TP.
2008; Immerzeel et al., 2010; Bolch et al., 2012; Gardelle climate change from natural archives of past precipitation et al., 2012; Jacob et al., 2012]. These features motivate the isotopic composition [Thompson, 2000; Ramirez et al., need for an observation modeling-based understanding of 2003; Yao et al., 2008; Liang et al., 2009; Cai et al., 2010]. 18 the large-scale (moisture transport) and local-scale (evapora- [4] Over the TP, precipitation δ Oreflects integrated infor- tion) atmospheric processes controlling the TP water cycle. mation of the interaction between the westerlies and Indian 18 17 [3] Water stable isotopes (H2 O, H2 O, HDO, usually monsoon, combined with local recycling which is character- expressed in a delta notation as δ18O, δ17O, δD, but restricted ized by evaporation, convection, and droplet reevaporation to δ18O in this study) are integrated tracers of the atmospheric (Figure 1b). Therefore, the TP offers both an exceptional processes [Dansgaard, 1964; Craig and Gordon, 1965] with access to multiple processes affecting present-day and past applications for atmospheric dynamics [Vimeux et al., 2005, precipitation isotopic composition and an exceptional com- 2011; Tian et al., 2007; Risi et al., 2008a, 2010a; Gao plexity for modeling and data interpretation. et al., 2011], hydrology [Gao et al., 2009; Landais et al., [5] Since the 1960s, the documentation of precipitation 2010], cloud processes [Schmidt et al., 2005; Risi et al., isotopic composition has emerged, thanks to monitoring sta- 2012a, 2012b], and quantitative estimations of past regional tions ran at monthly or event bases. In parallel, water stable
2 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES
TABLE 1. Summary Data for 24 Precipitation-Sampling Stations From the TNIP and the GNIPa
JJA DJF Annual Altitude Annual Average Weighted Weighted Weighted No. Station Latitude Longitude (m) n P T δ18O δ18O δ18O Database 1 Zhangmu 27°59′N 85°59′E 2239 71 1318 16.3 13.2 TNIP 2 Nyalam 28°11′N 85°58′E 3810 776 590 3.3 14.7 10.8 12.4 TNIP 3 Dui 28°35′N 90°32′E 5030 152 290 0.9 19.0 6.8 18.6 TNIP 4 Dingri 28°39′N 87°07′E 4330 285 265 7.1 18.2 15.4 18.1 TNIP 5 Wengguo 28°54′N 90°21′E 4500 90 253 4 17.6 16.5 TNIP 6 Larzi 29°05′N 87°41′E 4000 41 216 13.7 20.3 TNIP 7 Baidi 29°07′N 90°26′E 4430 171 313 1.2 17.1 17.2 15.7 TNIP 8 Nuxia 29°28′N 94°34′E 2780 88 347 11.9 13.8 TNIP 9 Lhasa 29°42′N 91°08′E 3658 1041 417 6.3 17.3 11.7 16.2 TNIP 10 Lulang 29°46′N 94°44′E 3327 119 467 5.7 14.7 19.1 14.5 TNIP 11 Bomi 29°52′N 95°46′E 2737 111 431 8.4 14.0 13.3 11.8 TNIP 12 Yangcun 29°53′N 91°53′E 3500 57 296 10.4 18.2 15.9 TNIP 13 Nagqu 31°29′N 92°04′E 4508 1132 500 0.3 17.0 17.8 16.5 TNIP 14 Gaize 32°18′N 84°04′E 4430 322 229 1 10.6 17.4 12.3 TNIP 15 Shiquanhe 32°30′N 80°05′E 4278 96 82 0.5 14.3 18.8 14.4 TNIP 16 Yushu 33°01′N 97°01′E 3682 536 386 3.6 13.2 15.7 13.1 TNIP 17 Tuotuohe 34°13′N 92°26′E 4533 1022 204 1.3 10.9 21.6 11.9 TNIP 18 Kabul 34°40′N 69°05′E 1860 109 330 11.6 0.4 10.5 7.2 GNIP 19 Lanzhou 36°03′N 103°51′E 1517 41 322 10.4 4.6 13.0 5.6 GNIP TNIP 20 Hetian 37°05′N 79°34′E 1375 47 209 9.1 2.2 17.8 5.5 GNIP TNIP 21 Delingha 37°22′N 97°22′E 2981 833 186 2.2 6.3 19.9 7.7 TNIP 22 Taxkorgen 37°46′N 75°16′E 3100 143 115 1.6 3.1 21.0 6.8 TNIP 23 Zhangye 38°56′N 100°26′E 1483 75 154 7.8 4.3 18.7 6.0 GNIP TNIP 24 Urumqi 43°48′N 87°36′E 918 119 304 7.4 6.3 20.6 10.8 GNIP aFor each location, the database has undergone systematic quality control considering the following factors: (i) precise δ18O measurements (analytical uncer- tainty of 0.2‰ or better), (ii) duration of the sampling period, and (iii) obvious aberrant values (δ18O data which are larger than 5‰ and strongly deviate from surrounding δ18O data are removed). Stations with event-based δ18O observations are highlighted in bold. Data at Yangcun, Nuxia, Larzi, Dingri, and Zhangmu are shorter than 1 year and the annual δ18O for those stations are amount-weighted δ18O averages of available months. isotope modeling has evolved from simple Rayleigh distilla- regions [Rozanski et al., 1992; Araguás-Araguás et al., tion models [Dansgaard, 1964; Craig and Gordon, 1965] to 1998]. The monitoring efforts were initiated with ice coring the introduction of water δ18O in complex atmospheric in the 1980s [Thompson et al., 1989; Yao et al., 1991] and general circulation models [Joussaume et al., 1984; Jouzel followed by station sampling [Yao et al., 1991, 1996a, et al., 1987; Hoffmann et al., 1998; Mathieu et al., 2002; 1999; Zhang et al., 1995; Tian et al., 2003; Yu et al., 2008; Noone and Simmonds,2002;Lee et al., 2007; Schmidt et al., Yang et al., 2012]. The station sampling has been developed 2007; Yoshimura et al., 2008; Tindall et al., 2009; Noone into a network of more than 30 stations with 19 stations and Sturm,2010;Risi et al., 2010a], and more recently in running continuously and systematically (Figure 1a and regional atmospheric models [Sturm et al., 2005, 2007a, Table 1). Later on, event-based sampling of precipitation 2007b; Yoshimura et al., 2010; Pfahl et al., 2012] and con- δ18O has been carried out within the China Network of vection models [Risi et al., 2008b]. Progress in the process- Isotope in River and Precipitation (CNIRP) and the Tibetan based understanding of the large-scale and local drivers of Observation and Research Platform (TORP) at more than precipitation δ18O arises from the new data and the synergy 40 stations on the TP and surrounding regions. Within the between data and models for present or past climates. Global Network of Isotopes in Precipitation (GNIP) ran by [6] An important progress in the model-data comparison the International Atomic Energy Agency (IAEA), monthly has been achieved in performing δ18O simulations at suffi- precipitation data have been collected since the 1980s at cient spatial resolution to capture regional water cycle pro- Lhasa. In the following discussion, we refer to precipitation cesses and nudged to atmospheric reanalyses providing sampling as station δ18O, all these data embedded in precip- realistic synoptic atmospheric patterns [Yoshimura et al., itation δ18O, and to those from firn and ice core measure- 2008]. The comparison of nudged simulation outputs with ments as ice core δ18O. 18 station δ O provides a quantitative and physically based [8] Pioneering studies have revealed obvious positive rela- understanding of the processes controlling precipitation tionship between water δ18O and temperature in the northern δ18O. It also provides a tool that is complementary to the TP and Tianshan Mountains [Yao and Thompson, 1992; Yao classical meteorological information to evaluate the key et al., 1996a, 1999; Zhang et al., 1995; Tian et al., 2001b]. processes of the water cycle [Yoshimura et al., 2003; Risi Data in the northern TP evidenced a strong seasonal cycle, et al., 2010a]. with maximum values in summer and minimum values in 18 [7] Systematic monitoring efforts for TP water δ O winter, which are similar to results from other nonmonsoon (Table 1) started in the 1980s, later than those for other regions. However, the southern TP precipitation δ18O was
3 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES
and temporal differences of precipitation δ18O with consider- ation of the effect of large-scale atmospheric processes (the westerlies versus Indian monsoon) and local factors, such as geography (latitude and altitude) and local climate param- eters (temperature and precipitation amount). We compare the observations with simulations to understand the atmo- spheric controls on precipitation δ18O and to assess the abil- ity of models to capture the driving mechanisms of δ18O variations. Although deuterium measurements allow to test the quality of the sample preservation and to investigate deu- terium excess in relationship with water cycle processes [Tian et al., 2005; Hren et al., 2009], few long deuterium records on high resolution are available for the TP and we decided to focus this paper on precipitation δ18O data. We do not use measurements of δ18O in TP river waters either, which are available from sporadic sampling only. [11] Section 2 describes the databases studied in this paper, and the atmospheric model simulations, while section 3 inves- tigates the relationships between large-scale atmospheric circu- lation and δ18O based on observations and three atmospheric models equipped with stable isotopes, including moisture ori- Figure 2. Time span of the database referred to in this paper gin diagnostics. The relationships between precipitation δ18O available for the monthly GNIP stations (grey) and event- with temperature and precipitation amount (section 4) are based TNIP stations (black). Stations are ordered from south investigated through observations and simulations. Section 5 (bottom) to north (top). is dedicated to the spatial and temporal characteristics of the relationship between δ18O and altitude through observations revealed to be progressively depleted over the course of the and models. In section 6, we summarize our findings and pro- monsoon season, from June to September [Yao et al., 1991]. pose suggestions for future research. Several studies related the spatial variation of the TP precipita- tion δ18O to the complexity of moisture sources and transports 2. DATABASES AND MODELS [Aizen et al., 1996; Araguás-Araguás et al., 1998; Aggarwal [12] To comprehensively review the processes and mecha- δ18 et al., 2004]. Different seasonalities of precipitation Owere nism of precipitation δ18O, we establish a database, including fl fl used to delimitate the in uence of monsoon moisture ow into monitoring stations and ice cores in the TP (section 2.1). We the TP [Tian et al., 2005, 2007; Yu et al., 2008, 2009]. then describe the three models, LMDZiso, ECHAM5-wiso, [9] Different speculations have been proposed on the and REMOiso, used to investigate the mechanisms driving mechanisms relating monsoon circulation to the TP precipi- the variability of precipitation δ18O. tation δ18O. The uplift of monsoon moisture flow due to high TP altitude is found to impact δ18O because of the well- 2.1. Station δ18O Database 18 18 known altitude effect on precipitation δ O[Molnar et al., [13] The precipitation δ O monitoring was initiated in the 1993; Liu et al., 2003]. Recent modeling studies have 1980s and has gradually completed as a network of more highlighted the importance of upstream distillation linked than 30 observation stations. We have selected event-based with convection and the relationship between water δ18O precipitation data from 19 observation stations (Figure 1a) and large-scale Asian monsoon circulation at the interannual of the Tibetan Network for Isotopes in Precipitation (TNIP) [Vuille et al., 2005], millennial [Pausata et al., 2011] and and monthly data from 5 observation stations of the GNIP da- orbital [LeGrande and Schmidt, 2009] time scales. Atmos- tabase (GNIP data accessible at http://www.iaea.org/water) pheric general circulation models (GCMs) equipped with which have the most continuous sampling and provide a water δ18O can therefore help interpret δ18O observations spatially distributed information over and around the TP and further understand the processes relating to moisture (Figure 2). The longest of these records is at Kabul, with advection and precipitation δ18O[Lee et al., 2011]. monthly based data starting in 1960s. The precipitation data Modeling results also demonstrate that the climate effect on with highest temporal resolution were collected every 3 h near precipitation δ18O does influence paleo-elevation estimation Nagqu from 13 to 27 August 2004. The meteorological [Rowley et al., 2001; Bershaw et al., 2012]. Comparing parameters from the nearest meteorological stations have also GCMs with δ18O observations helps to evaluate the deficien- been compiled for comparison with δ18Odata(seesection4). cies in the simulated hydrological cycle and to diagnose arti- The database was established up to 2007 due to the delays facts in atmospheric model physics [Schmidt et al., 2007; Risi between sampling, measurements, and quality controls. 18 18 et al., 2010b]. [14] The station δ O database at 24 stations provides δ O [10] In this review, we compile a database including sta- values at the scale of precipitation events, in addition to tion δ18O and ice core δ18O in the TP region to address spatial monthly and annual values (Figure 2 and Table 1). The data
4 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES
TABLE 2. Characteristics of the 11 TP Ice Coresa
Ice Core °N °E Altitude (m) Annual Average δ18O (Period A.D.) Reference of Ice Core δ18O East Rongbu 27.15 86.08 6500 17.5 (1844–1998) Hou et al. [2003a] Dasuopu 28.38 85.72 7200 19.88 (1900–1996) Yao et al. [2006b] Zuoqiupu 29.20 96.92 5500 13.29 (1956–2006) Xu et al. [2009] Tanggula 1 33.07 92.08 5800 15.31 (1940–1990) Yao et al. [1997] Tanggula 2 33.12 92.08 5743 13.3 (1935–2004) Joswiak et al. [2010] Geladandong 33.58 91.18 5720 12.34 (1935–2004) Kang et al. [2007] Puruogangri 33.90 89.10 6000 14.67 (1900–1998) Yao et al. [2006a] Guliya 35.28 81.48 6200 14.25 (1900–1992) Yao et al. [1997, 2006b] Malan 35.83 90.67 5680 12.58 (1887–1998) Wang et al. [2006] Dunde 38.10 96.42 5325 9.93 (1900–1985) Yao et al. [2006b] Muztagata 38.28 75.10 7010 16.71 (1955–2003) Tian et al. [2006] aThe quality control method for ice core data follows that of Masson-Delmotte et al. [2008]. Based on their study, for each location, the database was screened for quality control and a score was attributed, ranging from 0 (minimum quality control) to 4 (maximum quality control). In this study, each of the following four tests contributes one point to the total score: (i) precise δ18O measurements (analytical uncertainty of 0.2‰ or better), (ii) number of measurements (at least 10 measurements combined to provide interannual variability statistics), (iii) age control on the sampling period, and (iv) seasonal resolution of the measurements. of Yangcun, Nuxia, Larzi, Dingri, and Zhangmu are only the longest records obtained in the western TP and the shortest analyzed at the monthly scale due to their short observation ones in the Tanggula Mountains. Here we use 11 ice core period (less than 1 year). Among the 24 stations, 19 TNIP records (Table 2) as reference values for the very high eleva- observation stations have been sampled at each precipitation tion where the other data are not available. The δ18O records event. Those stations include Taxkorgen, Delingha, Tuotuohe, from ice cores were measured in different laboratories with Yushu, Shiquanhe, Gaize, Nagqu, Yangcun, Bomi, Lulang, varying precisions, better than ±0.2‰, and all the δ18Ore- Lhasa, Nuxia, Baidi, Larzi, Wengguo, Dingri, Dui, Nyalam, cords listed in Table 2 have at least annual resolution. 18 and Zhangmu. The precipitation samples were collected im- [17] Records of past δ O and accumulation rates in TP ice mediately after each precipitation event at each station. cores have been used to investigate past regional climate Four TNIP stations (including Tuotuohe, Delingha, Lhasa, variability [Thompson et al., 1989, 2000; Yao et al., 1996a, and Nyalam) and four GNIP stations (Urumqi, Zhangye, 1996b, 1999, 2000; Hou et al., 2000, 2002]. Postdepositional Lanzhou, and Kabul) have been sampled for more than 10 processes, which are temporally and spatially variable, can years and data are available for interannual variability stud- dampen and alter the original precipitation δ18O signal through ies (Figure 2). The TNIP samples collected prior to 2004 melting, sublimation, diffusion, and wind erosion [Stichler were measured using the MAT-252 mass spectrometer from et al., 2001; Schotterer et al., 2004]. Melting effect on isotopic the State Key Laboratory of Cryosphere and Environment, signal has been traditionally minimized by drilling ice cores Chinese Academy of Sciences, Lanzhou, with a precision from the regions that experience little or no snowmelt. Ten of ±0.2‰, while the samples collected since 2004 have been of our 11 ice cores were recovered from high elevation sites measured using the MAT-253 mass spectrometer from the on cold glaciers, without meltwater percolation, while the Key Laboratory of Tibetan Environment Changes and Zuoqiupu ice core was recovered from a temperate glacier. Land Surface Processes, Chinese Academy of Sciences, Among these ice cores, minimal postdepositional modification Beijing, with an accuracy of ±0.1‰. All the precipitation was estimated for the Dasuopu ice core, considering its high δ18O data are reported with respect to the Vienna standard accumulation and the distinct seasonal cycles in chemical spe- mean ocean water (VSMOW) and represent precipitation cies [Thompson, 2000]. The consistency of annual δ18Ore- amount-weighted values. cords from the two Muztagata ice cores [Tian et al., 2006; [15] The monthly samples collected at TNIP stations were Zhao et al., 2008] also points to negligible local biases. embedded in GNIP (Urumqi, Zhangye, Hetian, Lanzhou, and However, it is impossible to quantify the impact of Kabul) and analyzed at the IAEA Laboratory in Vienna, with postdepositional processes on δ18O and accumulation. We an accuracy of ±0.1‰. From the event-based data, monthly, therefore only use their annual averages (see Table 2) as refer- annual, and multiannual δ18O were calculated, taking into ence values at the ice core sites to extend the vertical coverage account precipitation weighting. for mapping the spatial δ18O distribution and for discussing al- titude effects. The annual average δ18O value for each ice core 2.2. Ice Core δ18O Database in Table 2 is arithmetically calculated based on all the annual 18 [16] Few meteorological stations exist at very high δ O values for the period listed in the brackets. elevation in the TP. Ice cores allow to complement precip- itation sampling for very high elevations. Several ice 2.3. Meteorological Observations and Analyses cores have been recovered in the past 30 years [Yao and [18] The meteorological data (temperature and precipita- Thompson, 1992; Yao et al., 1995, 1997, 2006a, 2006b, tion amount) from TNIP/GNIP stations are used to analyze 2008; Thompson et al., 1997, Thompson et al., 2000; Hou the factors impacting on precipitation δ18O variations in et al., 2003a; Tian et al., 2006; Wang et al., 2006; Kang different domains. Temperature and precipitation amount et al., 2007; Joswiak et al., 2010] over the TP (Figure 1a) with are recorded immediately after each precipitation event
5 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES from the TNIP stations. Temperature averages are calcu- addition, for numerical stability reasons, the temperature lated for monthly/annual analyses, and the corresponding was slightly nudged in the zoomed simulation with time precipitation amounts are calculated as monthly/annual data scales of 1 h and 10 days outside and inside the zoomed re- for each TNIP station. The meteorological data from GNIP gion, respectively. The nudging is weaker inside the zoomed stations (downloaded together with corresponding δ18O region to allow for the fine resolution of topographic features data from the website http://www-naweb.iaea.org/napc/ih/ to benefit the simulation of the large-scale atmospheric circu- GNIP/IHS_GNIP.html) are calculated by the same methods lation and temperature. as for the TNIP stations. [22] The German Weather Service (DWD-Deutscher [19] For meteorological verification, we use the reanalysis Wetterdienst) originally developed the regional circulation data from the National Center for Environmental Prediction model REMO. It was later used to explore climate change (NCEP)–National Center for Atmospheric Research (NCAR) by incorporating the physics scheme of the ECHAM4 gen- [Kalnay et al., 1996], together with the monthly precipitation eral circulation model. Then, based on the stable water iso- data set from the Global Precipitation Climatology Project tope module of ECHAM4, Sturm et al. [2005] embedded (GPCP) (http://www.esrl.noaa.gov/psd/). We also refer to the water stable isotopes into REMO. REMOiso proves to monthly values for zonal and meridional winds (at 500 hPa reproduce reasonably well climatic and isotopic features and 850 hPa), as well as sea level pressure from the NCEP/ across South America, Europe, and Greenland [Sturm et al., NCAR reanalysis data with a resolution of 2.5° × 2.5°, 2007a, 2007b; Sjolte et al., 2011]. The regional REMOiso extracted during the period corresponding to station observa- simulation used here is driven by observed large-scale wind tions (1958–2007 for the whole study region, 1992–2007 for fields using a spectral nudging technique. Winds are nudged Delingha, 1991–2005 for Tuotuohe, and 1994–2007 for toward those simulated by ECHAM, which is itself nudged Lhasa). The GPCP data are from 1979 to 2009 with a resolu- by ER15 reanalysis. The resolution is 0.5° latitude × 0.5° lon- tion of 2.5° × 2.5° for the whole study region. gitude (~53 km) with 19 vertical hybrid σ-pressure levels, over a 1979–1983 (5 year) time slice. 2.4. Atmospheric Model Simulations [23] ECHAM5 is the fifth generation of an atmospheric [20] We have selected three isotopic atmospheric circulation general circulation model developed at the Max Planck models (LMDZiso, REMOiso, and ECHAM5-wiso) that have Institute in Hamburg (Germany). ECHAM5 is substantially sufficient spatial resolution to resolve the issues relating to the improved in both the numeric and physics compared to TP topography. Two of them (LMDZiso and REMOiso) are its predecessor ECHAM4, including a flux-form semi- nudged to atmospheric reanalyses over part of the observation Lagrangian transport scheme, separate prognostic equations period, to provide a realistic synoptic weather framework. The for cloud liquid water and cloud ice, a prognostic-statistical model outputs (precipitation δ18O, altitude, temperature, and cloud cover parameterization, a new cloud microphysical precipitation amount) from the nearest grid points are com- scheme, a mass flux scheme for tropical convection pro- pared with corresponding observations. cesses with modifications for deep convection, and a new [21] LMDZ is the atmospheric component of the IPSL data set of land surface parameters. ECHAM5 successfully model and developed at the Laboratoire de Meteorogie simulates the global patterns of surface air temperature Dynamique (LMD). Risi et al. [2010b] described the repre- and the precipitation amount but overestimates precipitation sentation of water stable isotopes in LMDZiso and its perfor- amount during the Asian summer monsoon season. The mances on different time scales for present and past climates. water stable isotopes simulations from ECHAM5 (hereafter Gao et al. [2011] analyzed LMDZiso results by comparison ECHAM5-wiso) have been run over 10 years with fixed with three station data on the southern TP. They highlighted present-day boundary conditions and the horizontal model the importance of both realistic large-scale atmospheric resolution is 0.75° latitude × 0.75° longitude in T159 spec- dynamics ensured by nudging to re-analyses, and high spatial tral mode. The global performance of ECHAM5-wiso was resolution for comparison with station data. In this paper, we previously assessed with respect to spatial and seasonal use two different simulations conducted with LMDZiso: (1) a GNIP data. Note that this ECHAM5-wiso simulation was nudged simulation run from 1979 to 2007 with a resolution run without nudging to reanalyses and therefore can only of 2.5° latitude × 3.75° longitude (i.e., 277.5 km × 416.25 be compared with the mean seasonal cycle of observations. km), with the boundary conditions similar to those described in Risi et al. [2010b]; and (2) a zoomed LMDZiso simulation 3. IMPACT OF THE WESTERLIES AND INDIAN with the resolution of 50–60 km in a domain extending MONSOON ON SPATIAL AND TEMPORAL PATTERNS between 0°N and 55°N in latitude and 60°E and 130°E in OF PRECIPITATION δ18O longitude, which was run from 2005 to 2007. The three- dimensional fields of the zonal and meridional winds of both 3.1. Observed Spatial and Temporal Characteristics of simulations were nudged toward those of the ERA-40 Precipitation δ18O reanalyses [Uppala et al., 2005] until 2002 and toward those [24] Three different seasonal patterns from the observed of the operational ECMWF reanalyses thereafter [Risi et al., TP precipitation δ18O in different TP regions are shown in 2010b]. The relaxation time scales are 1 h for the standard Figure 3: (1) minimum δ18O in winter and maximum δ18O simulation, and 1 h (respectively 5 h) for the zoomed simula- in summer, associated with high precipitation and temperature tion outside (respectively inside) the zoomed region. In in summer (Figures 3a–3c); (2) maximum δ18Oinlate
6 YAO ET AL.: TP PRECIPITATION STABLE ISOTOPES
Figure 3. Seasonal patterns of observed precipitation δ18O, precipitation amount (P), and temperature (T) in different TP domains. (a) Monthly weighted δ18O(‰), (b) precipitation amount (mm/month), and (c) temperature averages (°C) in the westerlies domain (seven stations). (d–f) Same as Figures 3a–3c but for the transition domain (four stations). (g–i) Same as Figures 3a–3c but for the monsoon domain (13 sta- tions). For each station, data are averaged over observation periods (see Table 1 and Figure 2 for details). winter/early spring and minimum δ18O in late summer [Yang et al., 2012]. After the BOB monsoon establishment, (more than 10‰ of seasonal amplitude for multiyear aver- the Indian monsoon from the southern Indian Ocean de- aged data; Figure 3g), associated with a maximum tempera- velops [Joseph et al., 2006; Wang et al., 2009], which might ture in summer (Figure 3i) and a precipitation pattern with be another major moisture source leading to most depleted distinctive two peaks with one in early spring and one in precipitation δ18O around August. 18 summer (Figure 3h); and (3) seasonal pattern of δ Ois [26]Wenowbriefly describe the large spatial gradients aris- complex and no clear extrema either for winter or summer ing from these distinct patterns. Summer precipitation is max- (Figure 3d), with precipitation and temperature showing imum in the Bay of Bengal (BOB) and decreases northward in small maxima in summer (Figures 3e and 3f). We examine the TP (Figure 4b) with an annual gradient of 23.28 mm/°N these patterns using reanalysis data (section 3.3.1) and (R = 0.41, p > 0.1), and strong differences between the JJA models (section 3.3.2). The stations with enriched summer gradient, 11.21 mm/°N (R = 0.60, p > 0.1), and the δ18O (Figure 3a) correspond to the region, north of 35°N, December-January-February (DJF) gradient, 0.25 mm/°N dominated by the westerlies (hereafter “the westerlies do- (R =0.03,p > 0.1). Annual average temperatures at 24 precip- main”), and depict a close link between δ18O and local tem- itation stations (Figure 4d) show a linear decrease with latitude perature and weak relationships with precipitation amount by 0.34°C/°N (R =0.34, p > 0.1). Temperature gradient is (see section 4). The stations with clear summer depletion larger (0.73°C/°N, R =0.57, p > 0.1) in JJA and smaller (Figure 3g) correspond to the monsoon region, south of ( 0.25°C/°N, R =0.28,p > 0.1) in DJF. In summer, the south- 30°N, dominated by the Indian monsoon (hereafter “mon- ern TP is characterized by strong southerlies and southwest- soon domain”), showing an antiphase between δ18Oand erlies at 500 hPa, which gradually weakens from 30°N to precipitation amount (Figures 3g and 3h). The regions 35°N before turning into the prevailing westerlies to the north where seasonal cycles show more complicated δ18Ovaria- of 35°N (Figure 4k). In winter, the westerlies prevail over the tions (Figure 3d, located between 30°N and 35°N) are whole TP (Figure 4l), without obvious variation of precipita- defined as the transition domain, suggesting shifting influ- tion from south to north (Figures 4c and 4l). Precipitation ences between the westerlies and Indian monsoon. This δ18O generally increases northward (Figure 4g) at a rate of regional classification based on seasonal patterns of precip- 0.69‰/°N (R =0.64, p < 0.01, n = 35). Spatial gradients itation δ18O (Figures 3a, 3d, and 3g) is consistent with cli- appear more pronounced in JJA (1.2‰/°N, R = 0.82) than in matic zones defined by the spatial distribution of annual DJF ( 0.52‰/°N, R = 0.54). The spatial changes of pre- and June-July-August (JJA) temperature, precipitation, cipitation δ18O in different seasons, particularly that of the and wind fields (Figure 4). JJA δ18O (Figure 4h), clearly depict the spatial pattern of [25] Figure 3g shows a unique feature characterized by summer precipitation (Figures 4b and 4k) associated with abrupt depletion of precipitation δ18O around May and by depleted annual δ18O(<