multi-decadal temperature discrepancies along the eastern margin of the Tibetan Plateau

Quaternary Science Reviews 89 (2014) 85e93

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Decadal/multi-decadal temperature discrepancies along the eastern margin of the Tibetan Plateau Hai Xu a, *, Enguo Sheng a, b, Jianghu Lan a, Bin Liu a, b, Keke Yu a, b, Shuai Che a a b

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Graduate University of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 July 2013 Received in revised form 28 January 2014 Accepted 9 February 2014 Available online 5 March 2014

Knowledge of the synchronicity and discrepancy of temperature variations along the Eastern margin of the Tibetan Plateau (ETP) is critical in understanding the driving forcing of regional temperature variations. In this study, we established d15N timeseries in organic matter and d13C timeseries in ostracod shells from sediments of Lake Lugu and attributed their variations to decadal/multi-decadal temperature variations. We compared temperature variations along the ETP transect during the past four centuries based on our presently developed and previously developed temperature proxy indices, as well as temperature variations reconstructed by other researchers. We found that: (1) Over the north ETP area (N-ETP), the decadal/multi-decadal variations in temperature correlate well with each other. (2) Over the south ETP area (S-ETP), temperature variations correlate not so well with each other; while those at south to west portion of the Tibetan Plateau are rather local. (3) The decadal variations in temperature are generally synchronous with those in precipitation over the N-ETP area, and they are broadly antiphase/out-of-phase with the corresponding ones over the S-ETP area. (4) The long term temperature and precipitation trends are coupling over the N-ETP but decoupling over the S-ETP. We speculate that because the N-ETP is located at the frontier of the Asian summer monsoon (ASM) region, temperature variations there are not as strongly influenced by the ASM; they are most likely dominated by changes in solar activities, and show general similarity to the average of the Northern Hemisphere. Over the S-ETP area, decadal temperature variations are obviously influenced by precipitation. Because the decadal/ multi-decadal precipitation variations are anti-phase and/or out-of-phase between the N-ETP and SETP, the decadal/multi-decadal temperature variations between these two regions are also anti-phase and/or out-of-phase. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Temperature discrepancy Lake Lugu Tibetan Plateau Asian summer monsoon Solar activity

1. Introduction Temperature variation, especially during the past millennium, is one of the most concerned issues worldwide. Its general trend during the last millennium has been reconstructed by numerous studies (e.g., Briffa et al., 2004; Jones and Mann, 2004; Mann, 2007), which have greatly improved our understanding of the climatic changes and the role of various natural and anthropogenic forcings. It is well known that temperature varies on different timescales and the corresponding forcings are also variable (e.g., Jones and Mann, 2004). This would reasonably result in variable temperature patterns in different regions on different timescales. Because most of

* Corresponding author. Fenghui South Road, #10, Xi’an 710075, Shaanxi Province, China. Tel./fax: þ86 29 8832 5139. E-mail addresses: [email protected], [email protected] (H. Xu). http://dx.doi.org/10.1016/j.quascirev.2014.02.011 0277-3791/Ó 2014 Elsevier Ltd. All rights reserved.

the large scale temperature curves were generated from averages over wide geographic areas, differences in regional temperature variations could possibly be masked. This may limit our understanding of regional temperature variations and weaken the reliability of regional climatic predictions. Therefore, it is crucial to master the details in temperature variations for different regions and shed light on the underlying dynamics. The Tibetan Plateau (TP) plays an important role in modulating the large-scale atmospheric circulation over Asia. Along the ETP transect, the East Asian summer monsoon (EASM), the Indian summer monsoon (ISM), the East Asian winter monsoon, and the westerly jet stream prevail (Liu and Chen, 2000; Yu and Kelts, 2002; Ji et al., 2005; Shen et al., 2005; Xu et al., 2007; An et al., 2012). Variations in precipitation over the S-ETP area are dominated by the ISM (as can be inferred from the streamlines in Fig. 1), while those over the N-ETP area are likely to be influenced both by the EASM and by the westerly (e.g., An et al., 2012). Therefore, the ETP

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Fig. 1. Locations of the comparison sites along the Eastern margin of the Tibetan Plateau (ETP). Data of Lake Lugu are from this study. Data of tree rings in Hongyuan and those of sediments in Lake Qinghai are from the major author’s previous work; other data are collected from literatures (see Table 1). Numbers in the legend (left lower corner) are corresponding to the site numbers listed in Table 1. The three dotted circles represent three typical regions (N-ETP, S-ETP, and south to southwest Tibetan Plateau) discussed in the text. Upper left corner shows the streamlines averaged from June to August at 850 hPa during 1968e1996 based on the NCEP/NCAR reanalysis data (Kalnay et al., 1996). The satellite images are from the basemaps in ArcGIS (ESRI data & maps).

transect is one of the most sensitive regions for studying global climatic changes. Mastering the similarities and differences in temperature variations along the ETP transect is thus vital to understand the mechanisms of temperature and precipitation variations. Meteorological stations over the Tibetan Plateau are relatively scarce and the meteorological records are relatively short. Fortunately, various bio-geological paleao-climatic archives are widely spread along the ETP transect and scientists have made great efforts to reconstruct the paleaoclimates through different approaches. For example, at the N-ETP area, Yao et al. (2006) studied temperatures during the last millennium based on d18O in ice cores. Kang et al. (2000) and Liu et al. (2005) reconstructed temperature variations during the past one to two millennia from tree ring widths at Dulan and Qilian Mt., respectively. Climatic changes over the N-ETP area have also been investigated from lake sediments by numerous previous studies (e.g., Liu et al., 2006; Xu et al., 2006a,b; He et al., 2013). Around the Mid-ETP area, Wu et al. (2002) studied climatic changes from proxy indices in gastropod remains (e.g., d18O and d13C) of sediments in Xingcuo Lake, Zoige Plateau. Gou et al. (2006) reconstructed temperature variations at Animaqin Mt. from tree ring widths. Xu et al. (2010, 2012) reconstructed temperature and precipitation during the past 270 years at Hongyuan, Zoige Plateau, based on d13C and d18O in tree rings, respectively. At/near the S-ETP area, Shao and Fan (1999) reestablished temperature variations at the western Sichuan Plateau based on tree ring widths. Liang et al. (2008) reconstructed temperature variations based on tree ring width at the source region of the Yangtze River, south TP. Fan et al. (2008), Wang et al. (2010), and Li et al. (2012) reconstructed temperature variations in Hengduan Mt. inferred from different tree

ring data (see location in Fig. 1). Although common features are seen between these previous reconstructions, notable discrepancies also exist both in timing and magnitude, which have not been well addressed previously. It is necessary to focus on these differences and examine the underlying physics. In this study, we extracted temperature proxy indices from sediments in Lake Lugu. We compared the decadal/multi-decadal temperature variations along the ETP transect based on our newly and previously developed temperature proxy indices, as well as those from others. We focused on the phase relationship of decadal/multi-decadal climatic variations during the past about four hundred years between the N-ETP and S-ETP and the possible physics involved. 2. Methods 2.1. Climatic proxy indices extracted from sediments in Lake Lugu Lake Lugu is a deep pull-apart basin located in northwestern Yunnanesouthwestern Sichuan Province, the S-ETP. Climatic changes around Lake Lugu are mainly controlled by changes in Indian summer monsoon intensity. Mean annual air temperature is about 12.8  C; while mean annual precipitation is about 1000 mm, with approximately 80%e90% concentrated during May to October (Zhang et al., 2013). The lake is a hydrologically semi-closed system, and the lake level therefore keeps relatively steady on short term timescales (e.g., decadal scale). Three surface sediment cores (27.70972 N, 100.78475 E; alt. 2694 m; water depth at the sampling site: w45 m) were collected using a self-designed heavy corer, in September 2007. The lithologies of these three short cores

H. Xu et al. / Quaternary Science Reviews 89 (2014) 85e93

correlate well with one another. Core Lugu07-A & -B (w27 cm long) were cut into slices at every 1 cm interval in situ. Samples from Lugu07-B were selected for 137Cs and 210Pb dating. A piece of buried plant debris (likely from terrestrial sources) was found at 27 cm in core Lugu07-B, and was used to carry out AMS 14C dating. We selected ostracod shells from core Lugu07-A, and measured the d13Costracod on a continuous-flow isotope ratio mass spectrometry (GV MultiFloweIsoPrime), with an analytical error less than 0.1&. Carbonates of samples in core Lugu-B were removed by reaction with dilute HCl, and C/N ratios of the organic matter were determined on an elemental analyzer (vario EL III), with errors less than 0.2%. Another core (Lugu07-C; also w27 cm long) was cut into slices at every 0.5 cm interval. The nitrogen isotopic ratios (d15N; against air) of organic matter were measured by an EA e continuous flow isotope ratio mass spectrometry (Flash EA 1112 Series e Finnigan Delta Plus XP), with an analytical error less than 0.2&. The d13Costracod and d15N were used as indicators of temperature variations; while the C/N ratio was used to indicate the long term precipitation trend (see climatic significance of the indices in Section 4.1). We carried out drilling again in November, 2012, at Lake Lugu (27.70885 N; 100.79004 E; water depth: w60 m). The 2012sampling site is approximately 500 m closer to the shoreline compared with the 2007-sampling site. Core Lugu12-1-3 was 127 cm long and was cut into slices at every 1 cm interval. We measured 137Cs radioactivities of the uppermost twelve samples and performed radiocarbon dating based on buried plant debris. Bulk carbonate contents were determined by HCl titration for core Lugu07-C and core Lugu12-1-3 to double-check the dating models (see below). 2.2. Other data collection

d13Ccarb and a stacked temperature proxy index from Lake Qinghai, N-ETP (see Xu et al., 2006a, 2008 for details) and d13C in tree rings at Zoige Plateau, Mid-ETP (Xu et al., 2010) by our previous studies were used for comparison. All of these indices are suggested to reflect temperature variations. To better understand the climatic variations along the ETP transect, we collected climatic proxy indices within a much wider geographic area, and extend the comparisons to south-southwest Tibetan Plateau (see comparison sites in Fig. 1 and Table 1).

87

3. Results 3.1. Geochronology Both the 137Cs and 210Pb (Fig. 2a) radionuclides of core Lugu07-B show classical patterns. We assigned the 137Cs peak as the 1963-yr time marker and got an average mass accumulation rate of 0.01 g cm2 a1. The 137Cs geochronologies of these three short cores (core Lugu07-A, B, &C) were then calculated from this mass accumulation rate (see the supplementary data). To validate the 137 Cs dating model, we also calculated the dates based on the 210Pb radioactivities. As shown in Fig. 2b, the 210Pb dates calculated independently from a Constant Input Concentration model (CIC) and those calculated from a Constant Flux and Constant Sedimentation rate (CFCS) model match well, and both correlate well with the corresponding 137Cs dates. The AMS 14C age of the buried plant debris in sample Lugu07-B-27 was 605  20 cal. years BP (Fig. 2b), which is also broadly consistent with the corresponding extrapolated 137Cs-age (523.4 years BP). The 137Cs curve of core Lugu12-1-3 (Fig. 2c) shows similar pattern with that of core Lugu07-B (Fig. 2a). The sedimentation rate of core Lugu12-1-3 is much higher than that of core Lugu07-B, which is possibly related to the complex lake bottom topography. The chronology of core Lugu12-1-3 was finally generated from a combination of the 137Cs time-marker and the AMS 14C dating results (details not shown). 3.2. Proxy indices The d18O values of the ostracod shells of core Lugu07-A range between 5.58& and 2.09&, with an average of 4.16&. The atomic C/N ratios of the organic matter in core Lugu07-B vary between 8.65 and 11.57, with an average of 10.29. The d15N values of core Lugu07-C range between 2.00& and 4.88&, with an average of 3.38&. The bulk carbonate contents (carb%) of core Lugu07-C vary between 2.93% and 33.33%, with an average of 19.05%; while those in core Lugu12-1-3 range from 3.87% to 35.20%, with an average of 23.95% (Fig. 2d). The carb%, with a large range but small variability, can be ideally used to double check the dating models of different cores by curve comparison. As shown in Fig. 2d, the carb% curve of core Lugu07-C synchronizes well with that of core Lugu12-1-3, suggesting that

Table 1 Comparison sites. Sites/site numbers N-ETP

Dulan area

1

Qilian Delingha Dunde ice core Lake Qinghai

2 3 4 5

Mid-ETP

Anemaqin Xingcuo Lake Hongyuan

6 7 8

S-ETP

West Sichuan Plateau Lugu Lake Source Yangtze region Hengduan Mt. Hengduan Mt. Hengduan Mt. Western TP Nepal Himalayas Southeast TP Southwest TP

South to southwest TP

9 10 11 12 13 14 15 16 17 18

Archives/indices

Climatic significance

References

Tree ring width Tree ring width Tree ring width Tree ring width Ice core d18O Sediment d13Ccarb Stacked temp. curve Tree ring width Sediment d18Oshells Tree ring d13C Tree ring d18O Tree ring width d13Costrcod, d15Norg Tree ring width Tree ring width Tree ring width Tree ring density Tree ring width Tree ring width Tree ring width Tree ring width

Temperature Temperature Temperature Precipitation Temperature Temperature Temperature Temperature Temperature Temperature Precipitation Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature Temperature

Kang et al., 2000 Liu et al., 2009 Liu et al., 2005 Shao et al., 2005 Yao et al., 2001 Xu et al., 2006a, 2008 Gou et al., 2006 Wu et al., 2002 Xu et al., 2010 Xu et al., 2012 Shao and Fan, 1999 This study Liang et al., 2008 Li et al., 2012 Fan et al., 2008 Wang et al., 2010 Yadav and Singh, 2002 Cook et al., 2003 Yang et al., 2010a Yang et al., 2010b

H. Xu et al. / Quaternary Science Reviews 89 (2014) 85e93

137 Cs

1.2

0.06 0.05 0.04

Cs137(Bq/g

1.0

below 137Cs limits

0.8

210Pb Bq/g)

0.6

0.03

0.4

0.02 0.2

0.01 0

0.0 0

1

2 3 4 Mass depth (g/cm 2 )

2000 1800 1700 1500 1400 1300 0

5

5

10

15

20

25

30

Depth/cm 40

c

30

0.03 0.02

d

Lugu12-1-3 Lugu07-C

35 Carb (%)

radioactivity (Bq/g) 137 Cs

137Cs-Dates AM S14C CIC-dates CFCS-dates

1600

0.05 0.04

b

1900

Dates (a AD)

a

0.07

radioactivity (Bq/g)

radioactivity (Bq/g)

0.08

210 Pb

88

25 20 15 10

0.01

5

0.00 0

1

2

3

4

5

2

Mass depth (g/cm )

0 1500

1600

1700

1800

1900

2000

Date (a AD)

Fig. 2. Age model of Lake Lugu surface sediment cores. a. Upper left panel shows 137Cs and 210Pb radioactivities of core Lugu07-B. b. Upper right panel shows different dates for core Lugu07-B plotted against depth. c. Lower left panel shows 137Cs radioactivity of core Lugu12-1-3. d. Lower right panel shows comparison of bulk carbonate contents between core Lugu07-C and core Lugu12-1-3. The AMS 14C age (see supplementary materials) is the median probability calibrated by the software Calib 6.02.

both dating models are reliable. We acknowledge there are still certain dating uncertainties existing in our records. However, considering the coincidence between our d15N records and the western Sichuan tree-ring records (Shao and Fan, 1999), we are confident that our dating results support the major findings in this study (see below). 4. Discussions 4.1. Climatic significance of the proxy indices The d15N values of organic matter in lake sediments can be influenced by many factors, like the concentration of dissolved nitrate, N2-fixing processes, diagenetic processes, microbial nitrogen fixers, bacterial decomposition, and kinetic isotopic effect, etc (Hodell and Schelske, 1998; Xu et al., 2006a; Li et al., 2008). What dominate the d15N variations depends on the local physical/ chemical processes. In general, N2-fixing algae prefer to use 14N to synthesize organic matter. However, when primary productivity increases greatly, more 15N will be involved beyond the scope of isotopic fractionation, leading to an increased d15Norg in the sediments (e.g., Hodell and Schelske, 1998; Xu et al., 2006a). For Lake Lugu, although we cannot absolutely exclude the influence of other processes, the lake productivity is likely the most important/significant one. Sufficient nutrients are supplied from the surrounding catchments and from rich waterfowl excrement within the lake. This means that the growth-limiting factor for the primary productivity within the lake should most likely be summer temperature. Higher temperature favors higher primary productivity, and thus leads to higher d15Norg. As a result, the variations of d15N in organic matter can be used to reflect regional temperature variations. This “d15Neclimate” response pattern is very similar to that in Lake Qinghai (Xu et al., 2006a). Ostracods live at the bottom of the lake. Because the water column is deep, the bottom temperature likely remains relatively

constant. Therefore, the isotopic fractionation caused by temperature variations can be neglected, and the d13Costracod should be linearly correlated with the d13C of the dissolved inorganic carbon (DIC). We are not clear about the supply of DIC from the ground water to the lake; however, this contribution should be relatively constant and therefore has minor influence on the variations in d13CDIC in the lake water DIC pool. High temperature favors higher primary productivity in the lake and will lead to higher d13C values of the DIC left in the lake water DIC pool due to the selective use of 12 C during in-lake primary productive processes (Leng and Marshall, 2004; Xu et al., 2006a). As a result, d13Costracod is expected to be broadly and positively correlated with local temperature variations. This climatic response pattern is also similar to that of Lake Qinghai (Xu et al., 2006a). Vascular plants are rich in fiber but low in proteins and the atomic C/N ratios of organic matter are therefore high (generally greater than 20; Meyers, 2003). In contrast, algae and/or plankton contain less fiber but much more proteins and hence have low atomic C/N ratios (between 5 and 12 and generally less than 10; Meyers, 2003). When precipitation is higher, the input of terrestrial organic matter increases and the C/N ratio of the bulk organic matter of the lake sediment will increase, resulting in a positive “C/ Neprecipitation” relationship. 4.2. Temperature variations over N-ETP to Mid-ETP Fig. 3 shows comparisons of temperature proxy indices (and/or reconstructed temperature variations) over the N-ETP area, namely at Dulan (a; Kang et al., 2000), Qilian (b; Liu et al., 2005), Lake Xingcuo (c; Wu et al., 2002), Hongyuan (d; Xu et al., 2010), and Lake Qinghai (e; Xu et al., 2006a, 2008). The decadal/multi-decadal temperature variations are broadly consistent and most of the curves show generally increasing trends. Temperature variations recorded in the Dunde ice core also show obvious increasing trends during the past several hundred years (Yao et al., 2001, 2006). The

H. Xu et al. / Quaternary Science Reviews 89 (2014) 85e93

89

a b c

d

e

Fig. 3. Temperature variations over the N-ETP to Mid-ETP areas. The Dulan (a) and Qilian (b) tree ring widths (temperature indicators) are redrawn from Kang et al. (2000) and Liu et al. (2005), respectively. Temperature curve at Xingcuo Lake (c) is redrawn from Wu et al. (2002). Hongyuan tree ring d13C (temperature indicator; d; Xu et al., 2010) and the Lake Qinghai temperature index (e; Xu et al., 2006a) are from the major author’s previous works. See locations in Fig. 1 and Table 1.

relatively horizontal long term trend in Lake Qinghai record can possibly be ascribed to the lake hydrodynamics (Fig. 3) (Xu et al., 2006a, 2008). Gou et al. (2006) found similar temperature variations between Animaqin Mt. and those over northeastern Tibetan Plateau. The long term temperature trends over the Zoige Plateau (Wu et al., 2002; Xu et al., 2010), Mid-ETP, are also broadly similar to those over the N-ETP area, e.g., Dulan (Kang et al., 2000) and Qilian (Liu et al., 2005). Previous works show that both the temperature trends and variations over the N-ETP inferred from tree ring widths (Liu et al., 2005, 2009) are broadly consistent with the averaged Northern Hemisphere temperature. These similarities

suggest a possible common major forcing of temperature variations over the N-ETP to Mid-ETP areas (see below). 4.3. Temperature variations over S-ETP The d13Costracod (Fig. 4a) and d15Norg (Fig. 4b) in Lake Lugu show general increasing trends, which seem to be accelerated during approximately the past century and are possibly relevant to the observed global warming trend. There are no obvious temperature trends in the chronologies derived from tree ring widths in western Sichuan (Shao and Fan, 1999; Fig. 4d) and central Hengduan Mt.

a

b

f

c d e

Fig. 4. Temperature variations over the S-ETP area. The d13Costracod (a) and d15Norg (b) curves are from this study. Curve c denotes a stack of the 15.6-yr and 53.8-yr components in d15Norg of core Lugu07-C. The western Sichuan temp. curve (31-yr running mean; d) is redrawn from Shao and Fan (1999). Curve e denotes the 11-yr running mean of Hengduan Mt. temp (Fan et al., 2008). Curve f denotes the Kenya coral d18O series redrawn from Cole et al. (2000).

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(Fan et al., 2008; Fig. 4e), which can be possibly ascribed to the removal of the growth trend when compositing the chronologies. However, the multi-decadal variations extracted from tree rings (Shao and Fan, 1999; Fan et al., 2008) are roughly in-phase with those extracted from Lake Lugu sediments (Fig. 4). The correlation coefficient between Lugu d15N and western Sichuan temperature trend is w0.299 (a < 0.01; Table 2). We extracted three major components (15.6-yr, 53.8-yr, and 77.0-yr) in the d15Norg of core Lugu07-C and found that the stack of 15.6-yr and 53.8-yr cycles (Fig. 4c) generally resemble the long term average of temperature variations at western Sichuan (Fig. 4d). Temperature variations reconstructed from tree rings in the Yangtze river source region (Liang et al., 2008) and Hengduan Mt. (Wang et al., 2010; Li et al., 2012) are also broadly consistent with those at Lake Lugu and western Sichuan regardless of the dating differences between different studies (figures not shown). It is interesting that both the temperature trend and the decadal variations at Lake Lugu also show a general similarity with the sea surface temperatures in Malindi in the western Indian Ocean (Cole et al., 2000; Fig. 4f). Presently, the decadal/multi-decadal temperature records over the typical ISM region are still limited. The tele-connection between the S-ETP and the tropical Indian Ocean suggests a possible common driving forcing on decadal/multi-decadal temperature variations over the typical ISM region.

a

b 15 N

c

d

Fig. 5. Temperature differentiation between the N-ETP and S-ETP. The temperature curves are similar with those in Figs. 3 and 4 correspondingly except that the long term cubic trends of Qilian (a), Lake Lugu (c) and Dulan (d) were removed. Curve b denotes the reconstructed western Sichuan temperature variations (31-yr running means).

4.4. Temperature over the southwestern TP Over the southwestern TP, temperature variations seem to be rather local (e.g., Yadav and Singh, 2002; Cook et al., 2003; Yadav et al., 2004; Yang et al., 2010a,b) and they show at least two distinct features. One is that on decadal/multi-decadal scales these curves do not match well with each other, which is quite different from the general synchronies of temperature variations over the NETP area on this timescale. Temperature variations on decadal/ multi-decadal timescales over the southern to western TP are also not so consistent with those of the averaged Northern Hemisphere (Fig. S1). Xu et al. (2010) has suggested a possible linkage between ENSO events and temperature variations over the Zoige Plateau, Mid-ETP. Annual/multi-annual temperature variations in Yunnan also show possible linkages with ENSO events (e.g., Zhang et al., 2004). However, the reconstructed temperatures over the southern to western TP do not show obvious connections with ENSO events, suggesting considerable local influences. Another notable feature is that most of the reconstructions lack obvious long term trends, no matter those derived from tree ring width, tree ring density, or from glacial records. This relative steady temperature trend during the past several hundred years seems to be quite unique, as it defies the observed global warming trend (Yadav et al., 2004; Ganjoo, 2009). We suppose that such a steady temperature trend is possibly linked to the wide spread snow covers over southern to southwestern TP because the melting of the snow covers absorbs much heat and damps the increasing regional temperature trend (Xu et al., 2012). Table 2 Correlation coefficients between timeseries of Lugu d15N (this study), Western Sichuan temperature trend (Shao and Fan, 1999), Qilian temperature (Liu et al., 2005), and Dulan tree ring width (Kang et al., 2000).

Lugu West Sichuan Dulan

West Sichuan

Dulan

Qilian

0.299**

0.274** 0.289**

0.151** 0.003 0.591**

**Significant level <0.01. The Lugu, Dulan, and Qilian timeseries were detrended and all were 1-yr interpolated before correlation analysis.

4.5. Temperature differentiation between the N-ETP and S-ETP Although the meteorological records show general synchronicities in temperature variations over the Tibetan Plateau on annual/multi-annual scales (e.g., Yang, 2012), the decadal/multidecadal temperature variations between the N-ETP and S-ETP areas are rather different. We selected Qilian and Dulan temperature series in the N-ETP area, and western Sichuan and Lugu temperature series in the S-ETP area and compared their decadal/ multi-decadal variations after removing the long term cubic trends. As shown in Fig. 5, the multi-decadal temperature variations in Qilian Mt. (Fig. 5a) and western Sichuan (Fig. 5b), both derived from tree ring widths, are generally out of phase and/or even anti-phase (r ¼ 0.003; not significant; Table 2). Temperature variations at Lake Lugu (Fig. 5c) show a much clearer antiphase relationship with those in Dulan (Fig. 5d; r ¼ 0.274, a < 0.01). The anti-phase relationship of decadal temperature variations between western Sichuan and Dulan is also clear (Fig. 5; Table 2; r ¼ 0.289, a < 0.01). 4.6. Precipitation differences between the N-ETP and S-ETP Xu et al. (2007) previously showed an inverse relationship between decadal precipitation variations at Southern Oman (a typical ISM region; Burns et al., 2002; Fig. 6a) and those over the N-ETP area (namely at Lake Qinghai, Fig. 6c and at Delingha, Fig. 6d). Our recent work showed that the multi-decadal (nearly quasicentennial) precipitation variations at Hongyuan, Mid-ETP, are synchronous with those in Southern Oman (Xu et al., 2012). This means that the multi-decadal variations in precipitation between the Mid-ETP and N-ETP should also be generally anti-phase. As shown in Fig. 6, the 110-yr component in precipitations at Hongyuan inferred from tree ring d18O (Fig. 6b) does show general anticorrelations with those over the N-ETP, again suggesting an inverse relationship between multi-decadal (or quasi-centennial) precipitation over the typical ISM region and the N-ETP during the past several hundred years.

O (‰) 18

Hongyuan

b

a 20 22 24 26 18

O 18O

13

C 13C

28 1700

c

1750

1800

1850

1900

1950

-24.0 -23.5 -23.0 -22.5 -22.0 -21.5 -21.0 -20.5 -20.0 -19.5 -19.0 2000

Hongyuan

18

a

C (‰)

91

13

H. Xu et al. / Quaternary Science Reviews 89 (2014) 85e93

Date (a AD) 11

d

5

9

3

8 Fig. 6. Precipitation differences between the N-ETP and S-ETP. a. The thin and thick curves show the d18O (precipitation indicator) of a stalagmite (S3) in southern Oman and its 31-year running mean (Burns et al., 2002). b shows the 110-yr component in Hongyuan tree ring d18O series (precipitation indicator; Xu et al., 2012). c. The thin and thick curves represent C/N ratios (precipitation indicator) in Lake Qinghai and the polynomial fits (Xu et al., 2006a). d denotes the 31-yr running mean of precipitation at Delingha reconstructed from tree ring widths (Shao et al., 2005).

4.7. Coupling and decoupling between temperature and precipitation 4.7.1. Coupling over the N-ETP area Our previous works show that temperature and precipitation at Lake Qinghai are generally synchronous on multi-decadal/quasicentennial scales (Xu et al., 2006a, 2007). In fact, variations in the tree ring widths in Dulan can be reasonably interpreted as changes in both temperature (Kang et al., 2000) and precipitation (Zhang et al., 2003). The temperature curve established independently by Liu et al. (2009) around this area is also very similar to the precipitation curve developed by Zhang et al. (2003). In addition, the precipitation at Delingha reconstructed from tree rings (Shao et al., 2005) also shows a very similar trend to the temperature trends both in Dulan (Kang et al., 2000) and Qilian Mt. (Liu et al., 2005). All these lines of evidence suggest that temperature and precipitation are generally coupling on multi-decadal/quasi-centennial scales over the N-ETP area. 4.7.2. Decoupling over the Mid/South-ETP Notably, our previous work showed that the precipitation has a decreasing long term trend over the typical ISM regions during the past one to two hundred years, which is inversely related to the coevally increasing temperature trends (Fig. 7a; Xu et al., 2012). This long term “decreasing precipitationeincreasing temperature” pattern can also be extracted from Lake Lugu (Fig. 7b). Such a long term “precipitationetemperature” decoupling pattern is significantly different from the general coupling between temperature and precipitation over the N-ETP area. 4.8. Possible forcing on temperature variations along the ETP transect The sun is the energy source of the earth and many studies have shown close linkages between solar activity and earth temperature

7 1600

1700

2

15

15N N

C/N

1800

1900

N (‰)

4

15

10

Lugu

Lugu C/N

b

1 2000

Date (a AD) Fig. 7. Different long term temperature and precipitation trends in Hongyuan (upper) and Lake Lugu (lower). Panel a. the thin and thick pink curves denote d13C (temperature indicator; Xu et al., 2010) and its trend at Hongyuan; while the thin and thick blue curves represent d18O (precipitation indicator; Xu et al., 2012) and the trend. Panel b. d15N and C/N ratio curves indicate variations in temperature and precipitation at Lake Lugu, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

variations (e.g., Eddy, 1976; Beer et al., 2000; Reid, 2000; Gray et al., 2010). Meanwhile, changes in atmospheric circulations may also influence regional temperature variations. For example, a large number of studies have documented that the “El NiñoeSouthern Oscillations” (ENSO) plays an important role in modulating the global climatic changes (Angell, 1981; Graham, 1995; Privalsky and Jensen, 1995; Cane, 2005). Changes of regional hydrological cycles induced by ENSO are expected to change the latent heating, and would consequently influence the regional temperatures (Graham, 1995; Xu et al., 2010). What plays the dominant role in controlling regional temperature variations depends on different situation. 4.8.1. Driving force over the N-ETP Because the N-ETP area is located in the Asian summer monsoon frontier zone, the direct influence of monsoon intensity on regional temperature should be relatively weak. Temperature variations over this region may be mainly influenced directly/indirectly by a common driving forcing, like solar activities. In fact, a large number of studies revealed the “temperatureesolar activity” relationship over the N-ETP area. For example, our previous work showed that the multi-decadal temperature variations at Dulan (Fig. 8a) and Lake Qinghai (Fig. 8b) are obviously related to solar activity (Fig. 8c). During the past several hundred years, the low temperatures at Lake Qinghai and Dulan are broadly synchronized with the classical solar minima, e.g., the Spörer minimum (1402e1516 a AD), the Maunder minimum (1645e1715 a AD), the Dalton minimum, and a decrease in solar activity during 1920e1940 a AD (Xu et al., 2008). The quasi-100-year temperature variations over the Zoige Plateau, Mid-ETP, are also suggested to be linked to solar activity (Xu et al., 2006b). The common solar forcing is possibly responsible for the above mentioned similarities between temperature over the N-ETP to Mid-ETP areas and that of the average Northern Hemisphere.

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b

a

c

d

e

Fig. 8. Comparisons between solar activities and temperature variations along the ETP. a. Detrended Dulan tree ring width (temperature indicator). b. d13Ccarb curve of the carbonate of Lake Qinghai (temperature indicator; Xu et al., 2006a). c. Total Solar Irradiance (TSI) reconstructed from Antarctica ice core 10Be by Delaygue and Bard (2011). d. The reconstructed western Sichuan temperature variations. e. The detrended Lugu d15N.

4.8.2. Driving force over the S-ETP In summer, the Indian summer monsoon carries large amount of water vapor to China through the “Vapor Channel” at southeastern Tibetan Plateau (e.g., Yang et al., 1987). Therefore, it is possible that the decadal/multi-decadal temperature variations over the S-ETP area are influenced both by decadal changes in solar activities and by the decadal variations in ISM precipitation. Our interest is then extended to the driving forcings on the ISM intensity. Lines of evidence suggested that the millennial/centennial variations in Asian summer monsoon intensities are clearly related to solar activity (e.g., Neff et al., 2001; Fleitmann et al., 2003; Hong et al., 2003; Gupta et al., 2005; Wang et al., 2005). However, influences on the decadal/multi-decadal ISM intensity are rather complex. Many factors are expected to modify the decadal ISM intensity, such as the SST of the Indian Ocean (e.g., Clark et al., 2000; Giannini et al., 2003; Chung and Ramanathan, 2006), the snow cover over the Tibetan Plateau (e.g., Bamzai and Shukla, 1999; Peings and Douville, 2010), and the ENSO occurrences (Kumar et al., 1999; Krishnamurthy and Goswami, 2000). These would reasonably and strongly complicate the “solaremonsoon” relationship. In fact, although decadal/multi-decadal variations in precipitation have been shown to be linked to solar activities by some studies (e.g., Agnihotri et al., 2002; Xu et al., 2012 and references therein), the corresponding patterns are quite variable and some are even controversial. As a result, the relationship between decadal temperature variations and solar activities would also be complex. As shown in Fig. 8, the relationship between multi-decadal temperature variations over the S-ETP and solar activities is weak. During the classical solar minima, temperatures at western Sichuan (Fig. 8d) and Lake Lugu (Fig. 8e) are not low (and sometimes even at high levels). For example, during the Maunder minimum (1645e 1715 a AD) and the Dalton minimum (1770e1820 a AD) temperatures at Lake Lugu are obviously higher (see Fig. 8). As mentioned above, because the decadal variations in precipitation over the typical ISM areas (including the S-ETP area) are inversely related to those over the N-ETP area (see Fig. 6, and also refer to Xu et al., 2007) and because the decadal precipitation may

largely modify temperature variations over the S-ETP area by release of latent heating (e.g., Xu et al., 2010), an anti-phase/out-ofphase relationship of decadal temperature variations between the N-ETP and the S-ETP areas can be expected. As mentioned above, variations in temperature over the Zoige Plateau are more similar to those over the N-ETP, while variations in precipitation over the Zoige Plateau coincide better with those in the typical ISM regions (Xu et al., 2012). This decoupling temperature and precipitation trends possibly implies that the Zoige Plateau and/or the adjacent regions are a climatic dynamic division zone between the N-ETP and S-ETP areas. 5. Conclusions The decadal/multi-decadal temperature and precipitation are generally synchronous over the N-ETP area, and they are broadly anti-phase/out-of-phase with their corresponding counterparts over the S-ETP area. The long term temperature and precipitation trends are coupling over the N-ETP but decoupling over the S-ETP. Since the N-ETP area locates in the summer monsoon frontier zone, regional temperature variations should be relatively weakly influenced by changes in summer monsoon intensity and they generally follow the average of north hemisphere. While over the S-ETP area, both temperature and precipitation are obviously influenced by changes in monsoon intensity. Since the decadal/multi-decadal precipitation variations are anti-phase and/or out-of-phase between the N-ETP and S-ETP, the decadal/multi-decadal temperature variations between these two regions are also anti-phase and/ or out-of-phase. Acknowledgments We thank Prof. Steven C. Clemens, Prof. Xiaodong Liu, and the anonymous reviewers for helpful comments and suggestions on this manuscript. This work is supported by the projects (No.: 41173122; 41073103) funded by the Natural Science Foundation of China, and supported by the National Basic Research Program of China (No.: 2013CB955903; 2010CB833405). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2014.02.011. References Agnihotri, R., Dutta, K., Bhushan, R., Somayajulu, B.L.K., 2002. Evidence for solar forcing on the Indian monsoon during the last millennium. Earth Planet. Sci. Lett. 198, 521e527. An, Z., Colman, S.M., Zhou, W., et al., 2012. Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka. Sci. Rep.. http://dx.doi.org/10.1038/srep00619. Angell, J.K., 1981. Comparison of variations in atmospheric quantities with sea surface temperature variations in the equatorial eastern Pacific. Mon. Weather Rev. 109 (2), 230e241. Bamzai, A.S., Shukla, J., 1999. Relation between Eurasian snow cover, snow depth, and the Indian summer monsoon: an observational study. J. Clim. 12, 3117e 3132. Beer, J., Mende, W., Stellmacher, R., 2000. The role of the sun in climate forcing. Quat. Sci. Rev. 19, 403e415. Briffa, K.R., Osborn, T.J., Schweingruber, F.H., 2004. Large-scale temperature inferences from tree rings: a review. Glob. Planet. Change 40, 11e26. Burns, S.J., Fleitmann, D., Mudelsee, M., Neff, U., Matter, A., Mangini, A., 2002. A 780year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman. J. Geophys. Res. 107 (D20), 4434. http:// dx.doi.org/10.1029/2001JD001281. Cane, M.A., 2005. The evolution of El Niño, past and future. Earth Planet. Sci. Lett. 230, 227e240. Chung, C.E., Ramanathan, V., 2006. Weakening of North Indian SST gradients and the monsoon rainfall in India and the Sahel. J. Clim. 19, 2036e2045.

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