Chemical Geology 268 (2009) 147–154

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Chemical Geology

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Climatic significance of tree-ring δ18O in the , northwestern China and its relationship to atmospheric circulation patterns

Xiaohong Liu a,⁎, Xuemei Shao b, Eryuan Liang c, Tuo Chen a, Dahe Qin a, Wenling An a, Guobao Xu a, Weizhen Sun a, Yu Wang a a State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Lanzhou 730000, China b Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Laboratory of Tibetan Environment Changes and Land Surface Processes (TEL), Institute of Research, Chinese Academy of Sciences, Beijing 100085, China article info abstract

Article history: It is important to understand the history and dynamics of climate in a transitional region between areas with Received 28 April 2009 different atmospheric circulation patterns, where the vegetation and ecosystems are vulnerable to Received in revised form 26 July 2009 environmental change. We investigated variations in the long-term oxygen isotope composition (δ18O) in Accepted 8 August 2009 tree rings of Qinghai spruce (Picea crassifolia) and their relationships to climatic parameters in the arid Qilian Editor: R.L. Rudnick Mountains of northwestern China from 1870 to 2006. We found that the mean temperature from the previous November to the current February was significantly and positively correlated with the tree-ring δ18 Keywords: O values. The temperature effect, (the positive relationship between the temperature and the 18 18 Oxygen isotopes precipitation δ O value) can explain the connection between temperature and the tree-ring δ O values. Tree rings Due to pooling of the earlywood and latewood into yearly tree-ring samples, it appears that the cellulose Temperature effects δ18O may be influenced by isotopically nonhomogeneous water sources and climatic conditions during the Atmospheric circulation patterns previous and current growing seasons. Subtle shifts and amplitude deviations in cellulose δ18O, which abruptly became more positive around 1977–1978, may be attributed to the shifting climatic regime in China and to temperature variations, respectively. Our results illustrated the potential for investigating climatic or atmospheric circulation patterns based on oxygen isotope records in tree rings in regions near the interface between different large-scale synoptic circulations. © 2009 Elsevier B.V. All rights reserved.

1. Introduction and Stuiver, 1981; Saurer et al., 1997; Rebetez et al., 2003; Liu et al., 2004; Danis et al., 2006). Tree-ring isotopic data are a powerful tool for reconstructing The East Asian monsoon determines the key environmental climatic conditions where temperature and precipitation are primary conditions in China. The amount of precipitation in is mainly driving factors (see McCarroll and Loader, 2004, and the references controlled by the strength of the summer monsoons, which bring moist therein). Oxygen isotope ratios (δ18O) in rainfall are positively air from the Pacific and Indian Oceans over the continent, yielding correlated with atmospheric temperature (e.g., Dansgaard, 1964; precipitation (An, 2000). In northwestern China, the climate in most Jouzel et al., 1997). Although other parameters such as continentality areas is also influenced by the westerly flow (Wang et al., 2003, Tian or atmospheric circulation patterns also play a role in determining the et al., 2003; Qian et al., 2007). The oscillations between periods of dry δ18O value (Rozanski et al., 1993; Jouzel et al., 1997), the temperature and wet climate in northern China, as well as the transitions between at the location of the precipitation is widely recognized to be the most these states, are linked to interactions between the westerly flow and important parameter that determines the relative proportions of 18O the East Asian monsoon flow (Qian et al., 2007). The transitional region and 16O at middle and high latitudes (Rozanski et al., 1993; Jouzel between two monsoonal flows is very vulnerable to changes in these et al., 1997). Such climatic information is therefore contained in the flows. The climate in this region is influenced by the position of the δ18O value of precipitation and may therefore be recorded in the δ18O convergence interface between the East Asian monsoon and the values in tree annual growth rings after trees take up the water (Burk westerly flow, which changes from year to year and from decade to decade; as a result, climate anomalies will occur if the interface position moves northward or southward as a result of this flow interaction. ⁎ Corresponding author. State Key Laboratory of Cryospheric Sciences, Cold and Arid The activities of the atmospheric circulations in China determine the Regions Environmental and Engineering Research Institute, Chinese Academy of isotopic composition of the local precipitation (Wei and Lin, 1994; Tian Sciences, Donggang West Road No. 320, Lanzhou 730000, China. Tel.: +86 931 496 7342; fax: +86 931 827 1124. et al., 2001), and therefore affect the oxygen isotope composition of E-mail address: [email protected] (X. Liu). plant cellulose (Liu et al., 2004). Recent studies have also suggested the

0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.08.005 148 X. Liu et al. / Chemical Geology 268 (2009) 147–154 great potential of using the oxygen isotopic composition in tree-ring site (38°31.4′N, 100°14.8′E) is at Dayekou in the middle of the Qilian cellulose to infer changes in North American atmospheric circulation Mountains (Fig. 1), which is about 15 km from Zhangye city. The patterns (Feng et al. 2007) and variability in the intensity of the North climate in the middle of the Qilian Mountains is dominated by the American monsoon (Roden and Ehleringer, 2007). westerly flow and partly affected by the East Asian monsoon. The tree- The Qilian Mountains (~99°E to 103°E, 37°N to 39°N) are located at ring cores were collected from the dominant Qinghai spruce (Picea the northeastern edge of the Tibetan Plateau, at the convergence of crassifolia), a tree species that is sensitive to environmental change the Qinghai–Xizang (Tibet) plateaus, and Xinjiang (Gou et al., 2005; Liang et al., 2006; Liu, et al., 2007, 2008b). Qinghai plateaus, and the Loess Plateau (Fig. 1). Annual precipitation in the spruce generally grows on thin soils. The sampled trees ranged in the Qilian Mountains ranges from 84.7 to 410.3 mm based on the data elevations from 2920 to 3040 m. For cross-dating, at least 20 from local meteorological stations (Eastern station: at Wushaoling; dominant trees were selected and two cores were taken per tree at western station: at Jiuquan). The annual pan evaporation varies breast height using 5-mm increment borers (Haglöf, Mora, Sweden). between 1041.2 and 1234.2 mm, and the annual relative humidity An additional 15 cores were obtained using 10-mm increment borers averages about 57% (Fu and Che, 1990). The climates of the eastern, from different trees for isotopic analyses. western, and middle regions of the Qilian Mountains are controlled by Many rings were narrow or had indistinct latewood, so to avoid the East Asian monsoon, the westerly flow, and the interaction of the interpretation errors, we used all the wood from each year for the two flows, respectively (Tang et al., 2007). In this ecologically isotopic analyses. After cross-dating, we selected eight cores without vulnerable semi-arid region, interdecadal climate shifts have impor- obvious damage from different trees and cut them into sub-samples tant effects on the regional ecosystems. Non-exchangeable hydrogen with a 1-year resolution under a binocular microscope. We then or oxygen isotopes in tree-ring cellulose can potentially allow us to pooled the samples from the same year. The pooled annual samples reconstruct these climatic changes (Liu et al., 2008a) and shifts in the were first milled, and then α-cellulose was extracted using a method region's climatic system (Feng et al., 2007). based on those of Green (1963) and Loader et al. (1997). To measure In the present paper, we report the results of a study of the δ18O the oxygen isotope compositions, we loaded 0.14 to 0.16 mg of α- values in tree rings from forests in the middle of the Qilian Mountains of cellulose into silver capsules, and determined the ratio using a High arid northwestern China, and discuss the climatic significance of this Temperature Conversion Elemental Analyzer coupled to a Finnigan data and the potential correlations between δ18O in tree rings and shifts MAT-253 mass spectrometer (Thermo Electron Corporation, Bremen, in the atmospheric circulation patterns. We hypothesized that the tree- Germany) at the State Key Laboratory of Cryospheric Sciences, ring δ18O values would record climatic information related to the water Chinese Academy of Sciences. The oxygen isotope analyses were sources (precipitation and soil water) from the previous winter, spring, repeated four times for each annual cellulose sample, from which we and summer from which the trees absorbed their water (Saurer et al., calculated the mean values. The 18O/16O ratios were expressed as 1997; Robertson et al., 2001; Treydte et al., 2006; Danis et al., 2006). δ18O, which represents the per mil deviation relative to the Vienna Standard Mean Ocean Water standard. We measured the ratio for a 2. Methods benzoic acid working standard with a known δ18O value (IAEA-601, 23.3‰) every seven measurements to monitor the analytical precision The study area is located in the Qilian Mountains on the and to calibrate the samples for analytical accuracy. The analytical northeastern margin of Tibetan Plateau. The location of the sampling uncertainty was less than 0.3‰ (1σ).

Fig. 1. Map of the study area, the location of the sampling site and nearby meteorological stations. The shaded region and the solid line running through its centre represent the range and mean position of the East Asian summer monsoon (Tang et al., 2007). X. Liu et al. / Chemical Geology 268 (2009) 147–154 149

Fig. 2. (a) Monthly mean temperatures, total precipitation, and mean relative humidity near the study site of the period 1951–2006 (data from the Zhangye meteorological station). (b) Trends in the mean annual climatic variables from 1951 to 2006.

We examined the relationships between δ18O in the tree rings and precipitation averages 129.0 mm. The following variables were monthly climate variables using bootstrapped correlation analysis included in the correlation analysis: mean temperature (°C), total (Guiot, 1991) with the DendroClim 2002 program (Biondi and monthly precipitation (mm), and mean monthly relative humidity Waikul, 2004). Climate data were obtained from the Zhangye (%), all from the previous September to the current October. The meteorological station (38°56′N, 100°26′E, 1483 m elevation, records monthly climate data indicated that June, July, and August were the from 1951 to 2006), which is about 15 km from our tree-ring site; this warmest months, and that the highest amount of precipitation is the nearest climate station with long-term records. The Zhangye occurred during the same period (Fig. 2). The relative humidity was meteorological records in show that the mean monthly temperatures relatively low from March to May, and then increased until it reached ranges from 21.5 °C (July) to − 9.8 °C (January). The annual a plateau between September and December. We tested for significant

Fig. 3. The annual resolved δ18O series for the middle of the Qilian Mountains from 1870 to 2006 along with the sample depth used for the isotopic analysis (dotted line). The average δ18O values for two sub-periods (1940 to 1977 and 1978 to 2006) are also shown (short-dashed lines). 150 X. Liu et al. / Chemical Geology 268 (2009) 147–154

Table 1 China (Liu et al., 2008a). As shown in Fig. 3 and Table 1, the entire δ18O 18 The statistical characteristics of the δ O time series during different periods. series can be divided into three distinct periods. From 1977 to 2006, 18 Time period tree-ring δ O was higher, averaging around 30.8‰ and with a small standard deviation (1.7‰), indicating a greater reliance on water 1870–2006 1870–1939 1940–1976 1977–2006 sources enriched in 18O. However, during the period from 1940 to Maximum (‰) 33.9 33.9 32.8 33.2 δ18 ‰ ‰ 1976, the cellulose O had a higher standard deviation (3.0 ) and a Minimum ( ) 22.9 22.9 23.8 27.3 ‰ δ18 Mean (‰) 28.7 28.5 27.2 30.8 lower mean (27.2 ). A slow decreasing trend for O occurred from Median (‰) 28.9 28.3 26.1 31.4 1870 to 1940, with values ranging from 22.9‰ to 33.9‰ (Table 1; Variance 8.6 7.8 9.0 3.0 Fig. 3). Range (‰) 11.0 11.0 9.0 5.9 Standard deviation (‰) 2.9 2.8 3.0 1.7 3.2. Relationships with the environmental variables Standard error 0.3 0.3 0.5 0.3 Skewness −0.06 0.10 0.70 −0.66 Kurtosis −1.32 −1.04 −1.09 −0.65 We used forward evolutionary correlation functions (FERF) and First-order autocorrelation 0.49 ––– backward evolutionary correlation functions (BERF) (Biondi and Waikul, 2004) between δ18O and the monthly temperature and precipitation to investigate the temporal stability of the main climatic signals (Fig. 4). relationships between cellulose δ18O and the climate variables using Both analyses revealed an increasing positive correlation between Pearson's correlation coefficient (Blasing et al., 1984). cellulose δ18O and the mean temperature from the previous November to the current February from 1951 to 2006 (Fig. 4a, b). After 2000, the 3. Results June temperature had a positive and significant effect on cellulose δ18O, which was caused by the gradually warming climate (Fig. 2). From 1951 3.1. The δ18O series to 1977 and from 1995 to 2006, we found significant and positive correlations between precipitation in the previous October and cellulose We developed an annually resolved mean δ18O record for the δ18OinboththeFERFandtheBERFanalyses(Fig. 4c, d). From 1978 to middle of the Qilian Mountains from 1870 to 2006 based on samples 1986, precipitation from April to May was negatively correlated with of either seven cores (from 1870 to 1873) or eight cores (from 1874 to cellulose δ18O. 2006) from different trees (Fig. 3). Table 1 summarizes the statistical Overall, temperature fluctuations are the main climatic informa- properties of this data. The δ18O values ranged from 22.9 to 33.9‰, tion recorded in our oxygen isotope series for the tree rings. Thus, we with an overall average of 28.7‰, which is about 1.0‰ greater than compared the mean temperature from the previous October to the previous results from a study in the Helan Mountains in northern current February with the δ18O series (Fig. 5a). About 29.2% of the

Fig. 4. The backward and forward evolutionary correlation functions (BERF and FERF, respectively) calculated between the temperature and precipitation for selected months and the δ18O in tree rings. (a, b) BERF and FERF correlations, respectively, between temperature and δ18O; (c, d) BERF and FERF correlations, respectively, between of precipitation and δ18O. The same legend is used for graphs a and b. Only the periods during which correlation coefficients had P<0.05 are shown. X. Liu et al. / Chemical Geology 268 (2009) 147–154 151

The additional 1.9‰ increase in cellulose δ18O may have been caused by increased water stress or a change in water sources.

4. Discussion

Most studies have shown that the δ18O values in tree rings can be used to reconstruct δ18O variations in precipitation during the growing season (Saurer et al., 1997; Danis et al., 2006; Roden and Ehleringer, 2007) or the climatic conditions before tree growth begins (Robertson et al., 2001; Treydte et al., 2004). In northwestern China, the δ18O value of precipitation is mainly controlled by atmospheric temperature (Yao et al., 1996; Tian et al., 2003; Yu et al., 2008). Based on isotopic datasets from Zhangye (IAEA/WMO, 2000), the variations in the δ18O value of precipitation were strongly correlated with temperature, and accounted for about 64.0% of the variation in rainfall δ18O(Fig. 6). In our study, tree-ring δ18O appears to be responding indirectly to rainfall via the effects of temperature on the δ18O signature of rain. When rainfall enters the soil and becomes available for use by trees, the temperature information is recorded in the soil water. Therefore, the influence of temperature can be recorded in the cellulose δ18O(Saurer et al., 1997). Recently, Rebetez et al. (2003) suggested that the δ18O value measured in tree rings can be used to reconstruct past air temperatures in different seasons, and that the oxygen isotope composition is therefore a reasonable tool for reconstructing past temperatures in a region. Treydte et al. (2004) demonstrated that the δ18O values of winter precipitation strongly influenced cellulose δ18O values, which also supports our results. The interesting stages of tree-ring δ18O can be explained by the temperature variations (Fig. 5b, c). The different deviations of δ18O in 1951–1977 and 1978–2005 corresponded well with the deviations in temperature. Compared to the abrupt increases in tree-ring δ18O after 1977–1978, the warming trend in temperature was obvious, although temperature cannot fully explain the variations in tree-ring δ18O. In contrast to our results, tree-ring δ18O in the Helan Mountains was correlated with the amount of summer (May to August) precipitation (Liu et al., 2008a), and the “amount effect” (i.e., the negative relationship between the amount of precipitation and the precipitation δ18O) could explain this response of tree-ring δ18Oto precipitation (Liu et al., 2004). Unfortunately, we could not calculate a robust correlation between the temporal δ18O series for cellulose and precipitation owing to missing precipitation isotope values at the Zhangye station. Tree-ring δ18O depends on the relative amounts of ground water and summer rain taken up by the roots of trees, on the leaf water 18O enrichment, and on biochemical processes. Tree-ring δ18O data may record environmental information from the previous growing season

Fig. 5. (a) Scatter plot for δ18O as a function of the mean temperature from the previous November to the current February with the linear regressive line. (b) Comparison between high-frequency variations indicated by the first-order difference between δ18O and temperature from the previous November to the current February which showing the correspondence of both time series. (c) Comparison of differences in δ18O and temperature from the previous November to the current February during two sub- periods. Horizontal lines represent mean values for δ18O and temperature before and after 1977.

variation in cellulose δ18O can be explained by these temperature variations. For the high-frequency variations (first-order difference fluctuations), setting 1978 as a dividing year between two periods with different ranges in temperature and δ18O suggested that the wide fluctuations in cellulose δ18O from 1940 to 1978 were caused by larger fluctuations in temperature during this period (Fig. 5b). The increase in mean cellulose δ18O that occurred after 1977 was ‰ about 3.9 (Fig. 5c). Based on the temperature effects on cellulose δ18 δ18 Fig. 6. Temperature effects on the O value in precipitation at the Zhangye O(Fig. 5a), the mean warming of 1.4 °C that occurred after the meteorological station. The δ18O values in precipitation were obtained from IAEA/ 18 1952 to 1977 period (Fig. 5c) would produce a 2.0‰ increase in δ O. WMO (2000). 152 X. Liu et al. / Chemical Geology 268 (2009) 147–154

(Treydte et al. 2006). Thus, the selection of cellulose from one growth cellulose δ18O and the temperatures during the previous growing season (latewood) should avoid adding supplementary sources of season can therefore be explained by the strong isotopic contribution uncertainty related to the use of the previous season's photosynthetic of earlywood to the growth rings. products, which are mostly used in the early-wood. However, in arid The negative influence of relative humidity on cellulose δ18Oin regions such as northwestern China, tree growth in each year is May (r=−0.32; P<0.05) reflects the effects of atmospheric humidity relatively small, therefore narrow rings form. Given that the tree rings on stomatal conductance and transpiration rates (Saurer et al., 1997; of Qinghai spruce contain about 90% earlywood vessels that form Shu et al., 2005). In July, precipitation had significant and negative during the spring and early summer, a large fraction of the effects on δ18O(r=−0.24; P<0.05), which affects the leaf water 18O incorporated water may originate from frozen winter precipitation enrichment by controlling stomatal conductance and biochemical and soil water, whereas latewood formation starts in late August and processes, and therefore modifies the fluctuations in cellulose δ18O uses the current growing season's water. The response of tree-ring (Saurer et al., 1997). However, the climatic conditions during the δ18O (strong correlations with climatic conditions in the previous current growing season only have a minor influence on tree-ring year) would be reflected by analyzing the whole annual ring rather cellulose δ18O values, as evaporative losses are high during the than just the latewood (Cullen and Grierson, 2007). Moreover, the summer. The lack of a response to summer precipitation has also been relationship between the soil water available to trees and precipita- reported for woody perennial plants growing in semi-arid environ- tion δ18O is not straightforward, as it is expected to depend on the ments (Phillips and Ehleringer, 1995). time when soil water is renewed at the rooting depths of the trees. In Previous studies showed that a shift in the summer rainfall regime , the precipitation δ18O(snowinwinter)is (i.e., decreased summer rainfall) in China occurred during the late determined by air temperature (Fig. 6; Yu et al., 2008). The maximum 1970s (Hu 1997; Weng et al., 1999; Gong and Ho, 2002; Qian et al., snow-depth in winter will supply the winter recharge of soil water. 2007). Our study site is situated in the area where the East Asian Due to the pooling of early-wood and latewood of each ring as a single summer monsoon converges with the westerly flow (Tang et al., sample, the early growth of trees mainly used soil water input by the 2007). In general, climate in the Qilian Mountains is influenced by the previous winter's precipitation (snow). The correlation between interactions of these two synoptic weather systems. If climatic

Fig. 7. Variations in (a) the annual cellulose δ18O of tree rings, (b) the East Asian summer monsoon (EASM) index, (c) the East Asian winter monsoon (EAWM) index (Guo et al., 2003), (d) Zonal index (ZI79P), and (e) the North Atlantic oscillation (NAO) index from June to August (Li and Wang, 2003). The thick line represents smoothing of the data using an 11-year fast-Fourier-transform filter to emphasize long-term fluctuations. X. Liu et al. / Chemical Geology 268 (2009) 147–154 153 changes alter the seasonality of the precipitation or the source of the tree rings is mostly influenced by large-scale synoptic circulation water, the oxygen isotope composition in tree rings may also change. (Reynolds-Henne et al., 2007). The intensity of the East Asian summer monsoon also decreased Based on the observed climatic data, the yearly mean precipitation significantly after 1977 (Guo et al., 2003; Qian et al., 2007), resulting remained relatively stable over a long time scale, although with great in less precipitation in northwestern China. When the East Asian variability within different years or decades. However, the temper- summer monsoon weakens, the position of the peak interaction ature increased at a rate of 0.3 °C/decade from 1951 to 2007, resulting between the two air mass flows would move eastward. Then the in higher evaporation and greater plant water stress. This would cause impacts of the westerly flow will strengthen, bringing moist air from a relatively positive δ18O value (i.e., 18O enrichment) in the soil water the North Atlantic Ocean to northern China (Fig. 7b, d). Recently, Qian and cellulose. The increased cellulose δ18O (about 2.0‰ of the total et al. (2007) noted that in northern China, interdecadal transitions increase of ca. 3.9‰; Fig. 3) can be partially explained by a drier could be observed in 1979, with a decrease of 30.8 mm in total climate caused by warming air temperature (Fig. 2b). In the arid precipitation between the means for the 1959 to 1978 period region of northwestern China, evaporated moisture plays an impor- (336.1 mm) and the 1979 to 1998 period (305.3 mm). tant role in determining the atmospheric water balance, and leads to These inter-decadal changes in atmospheric circulation may alter higher δ18O values in precipitation (Liu et al., 2008c). Further, the the δ18O in precipitation and therefore alter the isotopic composition increased plant water stress that has been observed in recent decades of the soil water used by trees (Jouzel et al., 1997). Thus, we examined (Zou et al., 2005) as a result of climatic warming has complicated local the correlations between cellulose δ18O and the East Asian summer water cycles. However, in a study in the Helan Mountains (Liu et al., and winter monsoons. The cellulose δ18O value changed abruptly 2008a), the δ18O values in tree rings of pine did not record this shift from a mean (±SD) of 27.2±3.0‰ from 1940 to 1976 to a mean of and showed only a slight increase (Liu et al., 2004). These differences 30.8±1.7‰ from 1977 to 2006 (Table 1), suggesting a shift in water may be related to (1) dissimilarity in the values of factors that sources or climate conditions in the study area. From 1873 to 2000, we determine the δ18O of local precipitation (Liu et al., 2004; Tian et al., found significant negative correlations (r=−0.25 and P=0.004 and 2003); and (2) the ability of tree species to absorb soil water from r=−0.42 and P<0.001, respectively; Fig. 7) between cellulose δ18O different depths in the soil profile (Marshall and Monserud, 2006). and the East Asian summer and winter monsoon indices (Guo et al., Species-dependent differences in rooting depth will influence 2003). When we calculated the correlations for two sub-periods moisture availability, as will the residence time of water in the soil (1873 to 1977 and 1978 to 2006), we found distinct response that is accessed, and will therefore influence the isotopic composition patterns. The correlation between cellulose δ18O and the east Asian of water source. To date, we cannot separate the effects and relative winter monsoon (EAWM) index remained significant (r=−0.46; contributions of changes in water sources and evaporated moisture to P<0.001) from 1873 to 1977, but the correlation between cellulose changes in plant δ18O. Although cellulose δ18O values in our study δ18O and the East Asian summer monsoon index was no longer reflected the observed temperature and climatic variations, further significant (r=0.003; P=0.97). For the period from 1978 to 2000, calibration work will be required to improve the interpretation of there was no significant correlation between cellulose δ18O and either palaeoclimatic data using this parameter. of the two monsoon indices. fl The intensity of the westerly ow is represented by the Zonal 5. Conclusions Index, calculated using the SLP data and from the NCEP/NCAR fi reanalysis data set based on the de nition by Rossby et al. (1939). We studied the variations in oxygen isotopic compositions in tree- fi We rst calculated the difference in sea level pressure between 35°N ring cellulose collected from an arid area of northwestern China. The and 55°N, and then averaged the difference values from 80°E to 110°E, δ18O values showed a significant increase (i.e., enrichment of 18O) in just west of the study region, for each month from 1948 to 2004. response to a shifting climatic regime. Tree-ring δ18O values were Corresponding to the weaker EASM index after 1977/1978, the most strongly correlated with the mean temperature from the fl 18 westerly air ow became stronger over the study region (Fig. 7d). previous November to the current February. The response of δ Oto fi From 1948 to 2004, the most signi cant July to September zonal index temperature changes could be explained by the temperature effect on for the previous year (ZI79P) explained about 30.0% of the variance of precipitation δ18O. Owing to the climatic and atmospheric circulation δ18 tree-ring O. shifts that have occurred in China, the correlations between tree-ring Climatic effects of the North Atlantic Oscillation (NAO) on the δ18O and indices for the North Atlantic oscillation and the east Asian central Tibetan Plateau have been also reported (Wang et al., 2003), summer and winter monsoons varied over time, as might be expected fi fl δ18 with a signi cant in uence on the O value in ice cores from May to based on changes in the strength of the westerly flow and the East fl October. Monthly indices that re ect meridional shifts in the subpolar Asian monsoon during the past century. low-pressure systems and subtropical high-pressure systems in the NAO index provide a measure of the strength of the westerly flow. The Acknowledgments significant and negative correlation between our cellulose δ18O values and the NAO index show that variations in the annual cellulose δ18O This research was supported by the Major State Basic Research were related to variations in the summer NAO index from June to Development Program of China (973 Program, 2007CB411506), the August (r=−0.29 and P=0.001; 1873 to 2006; Fig. 7e). Over the National Natural Science Foundation of China (40871002), and the entire study period, variations in the cellulose δ18O value associated foundation of the State Key Laboratory of Cryospheric Sciences with the summer NAO index accounted for ~27.9% of the total (058143). We thank two anonymous reviewers and the editor, variance (based on a 5-year running mean). Before 1977, the whose comments and suggestions were very helpful in improving the correlation between cellulose δ18O and the NAO index from June to quality of this paper. August was also significant (r=−0.36, P<0.001). 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