Quaternary Science Reviews 107 (2015) 98e111

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Quaternary Science Reviews

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Hydroclimatic changes in China and surroundings during the Medieval Climate Anomaly and Little Ice Age: spatial patterns and possible mechanisms

* Jianhui Chen a, Fahu Chen a, , Song Feng b, Wei Huang a, Jianbao Liu a, Aifeng Zhou a a MOE Key Laboratory of Western China's Environmental System, Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou, 730000, China b Department of Geosciences, University of Arkansas, Fayetteville, AR, 72701, USA article info abstract

Article history: Investigating hydroclimatic changes during key periods such as the Medieval Climate Anomaly (MCA, Received 17 March 2014 1000e1300 AD) and the Little Ice Age (LIA, 1400e1900 AD) is of fundamental importance for quantifying Received in revised form the responses of precipitation to greenhouse gas-induced warming on regional and global scales. This 9 October 2014 study synthesizes the most up-to-date and comprehensive proxy moisture/precipitation records during Accepted 15 October 2014 the past 1000 years in China and surroundings. The proxy data collected include 34 records from arid Available online 9 November 2014 central Asia (ACA) and 37 records from monsoonal Asia. Our results demonstrate a pattern of generally coherent regional moisture variations during the MCA and LIA. In mid-latitude Asia north of 30N, Keywords: Arid central Asia monsoonal northern China (North China and Northeast China) was generally wetter, while ACA e Monsoonal China (Northwest China and Central Asia) was generally drier during the MCA than in the LIA (a West East Medieval Climate Anomaly mode). The boundary between wetter northern China and drier ACA was roughly consistent with the Little Ice Age modern summer monsoon boundary. In monsoonal China to the east of 105 E, the northern part was Spatial pattern generally wetter, while the southern part was generally drier during the MCA than in the LIA (a North Mechanisms eSouth mode), with a boundary roughly along the Huai River at about 34N. These spatial patterns of moisture/precipitation variations are also identified by instrumental data during the past 50 years. In order to understand the possible mechanisms related to the moisture variations during the MCA and LIA, we investigate the major SST and atmospheric modes (e.g. the El Nino/Southern~ Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO) and the North Atlantic Oscillation (NAO)) which affect the moisture/precipitation variations in the study region using both the instrumental data and the reconstructed time series. It is found that the ENSO may play an important role in determining hydro- climatic variability over China and surroundings on a multi-centennial time-scale; and that the foregoing spatial patterns could be attributed to the La Nina-like~ (El Nino-like)~ condition during the MCA (LIA). In addition, AMO and NAO may also have their own contributions. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction arid central Asia (ACA, including Northwest China, Central Asia and the surrounding region), while a drying trend occurred in Anthropogenic global warming since the mid-19th century has monsoonal northern China including North China and Northeast been widely recognized (Karl et al., 2009; IPCC, 2013). However, in China (Zou et al., 2005; Ma and Fu, 2006; Feng et al., 2007; Shi et al., response to the nearly uniform increasing trend of temperature 2007; Chen et al., 2011; Huang et al., 2013). Fig. 1 illustrates the (although to different extents), the moisture/precipitation changes modern summer monsoon limit marking the boundary between demonstrate significant regional variations (Dai et al., 2004; IPCC, monsoonal Asia and westerly-dominated Asia. Within monsoon- 2013). Based on instrumental data, it was found that during the dominated eastern China (roughly to the east of 105E), there is a last few decades there is a wetting trend in the westerly-dominated di-polar pattern of precipitation anomalies between northern and southern China (Ding et al., 2008; Zhou et al., 2009). However the questions arise, do these patterns during the instrumental period

* Corresponding author. Tel.: þ86 931 8912793; fax: þ86 931 8912330. also exist on longer time scales; and if so what could be the possible E-mail address: [email protected] (F. Chen). mechanisms? Answering these questions may lead to a better http://dx.doi.org/10.1016/j.quascirev.2014.10.012 0277-3791/© 2014 Elsevier Ltd. All rights reserved. J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111 99

Fig. 1. Distribution and types of proxy record in this study. EASM and ISM stands for “East Asian summer monsoon” and “Indian summer monsoon”, respectively. The boundary of modern summer monsoon (thick dashed line) is modified from Chen et al. (2010). understanding of the responses of precipitation to anthropogenic westerlies and the Asian monsoon) are broadly in phase with those warming on regional and global scales. of the Northern Hemisphere during the last millennium (e.g. Yang In a previous study, Chen et al. (2008a) synthesized the trends of et al., 2002a; Ge et al., 2008, 2013). This is supported, on a multi- moisture variation in Asia during the Holocene. They argued that centennial time scale, by a recent temperature reconstruction on there was a “westerly dominated moisture regime” in ACA during the basis of a tree-ring network comprised of 422 chronologies for the Holocene, which exhibited an asynchronous pattern of mois- temperate Asia, including China, India, Mongolia, Korea and Japan ture variations compared to Asian monsoon evolution. Recently, a since 800 AD (Cook et al., 2013). However, our previous work, in proxy-based precipitation reconstruction during the last 500 years which a synthesized moisture time series was reconstructed using further revealed that the contrasting anomaly pattern between ACA 5 paleo-moisture records from ACA, indicated that the framework and monsoonal northern China persisted on inter-annual to of moisture variations in this westerly-dominated region during decadal time scales (Feng et al., 2013a). Here, we focus on the the MCA and LIA was probably asynchronous with variations in pattern of moisture variations on a multi-centennial time scale, monsoonal precipitation (Chen et al., 2010). Here, we focus on the intermediate between the millennial (Holocene) and inter-decadal spatial patterns of hydroclimatic variations in ACA and monsoonal (current warm period and last 500 years) time scales. Asia during the MCA and LIA, based on the most up-to-date and An investigation using a global temperature proxy network has comprehensive multi-proxy moisture/precipitation records for the demonstrated that the Medieval Climate Anomaly (MCA) and Little last millennium. The possible linkages of these moisture patterns to Ice Age (LIA) are multi-centennial-scale climatic events with global major climate variability modes and their underlying physical signatures (Mann et al., 2009). The synthesized temperature mechanisms are also discussed. reconstruction during the last 2000 years from seven continental- scale regions indicated generally colder conditions during the LIA than that during medieval times (PAGES 2k Consortium, 2013). 2. Materials and methods During both intervals, distinct hydroclimatic anomalies have been identified on regional and global scales (e.g. Newton et al., 2006; The following criteria were used for the selection of the proxy Cook et al., 2007; Seager et al., 2007; Feng et al., 2008; Graham records in this study: 1) The proxies should be indicative of mois- et al., 2011). ture or precipitation changes. 2) The records should span at least It has been recognized that the temperature variations in the last 1000 years, and be without depositional hiatuses. 3) The different climatic regions of China (including the westerly- dominant driving mechanism of the variation in proxy records dominated Northwest, monsoonal Northeast, Central East and should be climatic changes, rather than human activities. 4) The Southeast, and the Tibetan Plateau, which is influenced by both the records should have a resolution of better than 50 years. 100 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111

Table 1 Proxy palaeo-moisture or palaeo-precipitation records used in this study. Site number is given separately for westerly-dominated Asia (1e34) and monsoonal Asia (35e71), from west to east in each region, as in Fig. 1. For all of the high-resolution records, the moisture classification was based on time series. For all of the other records, it was taken from the literature. The high-resolution records are highlighted in bold.

Site no. Site name Lat. Long. Time period Sample Dating method No. of Proxies used Proxy indication References (N) (E) (cal BP)a resolution datesc (yr)b

1 Caspian Sea 41.62 50.95 Dis.d e 14C 2 Barrier complex Lake level (river Kroonenberg et al., discharge)e 2007 Dis. e 14C 23 Sand bar and bays Lake level Karpychev, 2001 2 Aral Sea 45.00 60.00 1800e015 137Cs,14C 4/9 Reworked Sheet wash Sorrel et al., 2006 dinoflagellate cysts (precipitation) 210 137 14 18 3 Sasikul Lake 37.70 73.18 2540e020 Pb, Cs, C 3/6 d Ocarb, TIC Effective moisture Lei et al., 2014 (precipitation) 4 Northern Pakistan 35.70 74.80 1050e0 1 Ring counting e Treeering d18O Precipitation Treydte et al., 2006 5 Balkhash Lake 46.40 75.60 ca 2500e010 14C 9/17 Pollen Moisture Feng et al., 2013b 6 Issyk-kul Lake 42.50 77.10 Dis. e History e Historical map Lake level Narama et al., 2010 (precipitation) 14 18 7 Bangongcuo Lake 33.67 79.00 ca 11400e070 C 0/11 d Ocarb Moisture Wei and Gasse, 1999; Fontes et al., 1996 8 Cele County 37.00 81.00 ca 4000e040 14C 3/6 Pollen, element Moisture Zhong and Xiong, composition 1999 9 Southern Tarim 38.00 81.00 Dis. e History e Historical River discharge Liu, 1976 rivers documents 10 Keriya River 37.30 81.50 Dis. e 14C 1 Terrace, historical Fluvial Yang et al., 2002b, documents environment 2006 (precipitation) 11 Guliya ice cap 35.20 81.50 1700e10 10 Annual layer e Net accumulation Precipitation Yao et al., 1996; counting, ice flow rate Thompson et al., model 1995 230 12 Kesang Cave 42.87 81.75 3570e50 25 Th 1/4 Speleothem d18O Precipitation Cheng et al., 2012 13 Aibi Lake 45.02 82.86 ca 13870e010 14C 3/6 Pollen Moisture Wang et al., 2013b 14 Bosten Lake 42.00 87.02 ca 1000e0 10 210Pb,137Cs,14C2Carbonate Effective moisture, Chen et al., 2006 content, grain size, precipitation pollen 15 Jili Lake 46.93 87.42 2500e050 210Pb,137Cs,14C 2 Grain size Precipitation/lake Jiang et al., 2010 level 16 Central Tianshan 43.77 87.58 ca 4500e070 14C 1 Pollen Moisture Zhang et al., 2009 Mountains 17 Teletskoye Lake 51.72 87.66 ca 800e0 1 137Cs,14C2Element Precipitation, Kalugin et al., 2007 composition, X moisture eray density 18 Chaiwobu Peatland 43.50 87.90 ca 8500e025 14C 3/19 Cellulose d13C Precipitation Hong et al., 2014 19 Eastern fringe of 39.78 88.39 ca 800e04 14C 5 Plant d13C Moisture Liu et al., 2010 Tarim Basin 20 Luobupo Lake 40.47 90.35 ca 15300e040 14C 1 Grain size, Lacustrine Ma et al., 2008 microbiological and environment geochemical data (precipitation) 21 Achit Nuur 49.42 90.52 ca 22600e050 14C 3/10 Pollen Precipitation Sun et al., 2013 22 Goulucuo Lake 34.60 92.47 ca 1050e010 210Pb,137Cs 1 Carbonate content Effective moisture Li et al., 2004 14 18 23 Balikun Lake 43.70 92.83 ca 9400e075 C 2/7 d Ocarb, TOC Effective moisture Xue and Zhong, 2011 24 Sugan Lake 38.85 93.90 ca 1000e0 10 Varve counting e Chironomid, Salinity (Effective Chen et al., 2009 18 d Ocarb moisture) 25 Dunde ice cap 38.10 96.40 ca 12000e0 100 Annual layer e Pollen Moisture Liu et al., 1998 counting, ice flow model 26 Hurleg Lake 37.28 96.90 1700e015 210Pb,137Cs,14C 5/6 Pollen Effective moisture Zhao et al., 2010b 27 Gahai Lake 37.13 97.52 ca 2600e50 6 14C 1/4 %C37:4 Moisture He et al., 2013b 28 Delingha 37.27 97.53 1010e0 3 Ring counting e Treeering d18O Relative humidity Wang et al., 2013c 29 Zongwulong and 37.08 97.82 1000e0 1 Ring counting e Treeering width Precipitation Shao et al., 2005 Shalike Mts 30 Middle Qilian 38.70 99.69 1225e0 1 Ring counting e Treeering Moisture Zhang et al., 2011b Mountains 31 Khuisiin Lake 46.60 101.80 1240e020 210Pb,137Cs,14C 4/7 Pollen Effective moisture Tian et al., 2013 32 Badain Jaran 39.55 102.37 ca 1200e0 6 Accumulative e Chloride Groundwater Gates et al., 2008; Desert Chloride concentrations in recharge Ma and Edmunds, unsaturated zone 2006 33 Ulann Lake 44.51 103.65 ca 17000e0 50 OSL 0/12 TOC, CN Moisture Lee et al., 2013 34 Gun Nuur 50.25 106.60 ca 10800e020 14C 5/48 Grain size, TOC, Lake level Zhang et al., 2012 carbonate content, (moisture) 13 d Corg, diatom 35 Kahf Defore 17.12 54.08 ca 780e0 1 Annual growth e Speleothem d18O, Precipitation Fleitmann et al., band counting speleothem 2004; Burns et al., annual band 2002 thickness 36 NE Arabian Sea 24.83 65.92 ca 4950e0 1 Varve counting e Varve thickness von Rad et al., 1999 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111 101

Table 1 (continued )

Site no. Site name Lat. Long. Time period Sample Dating method No. of Proxies used Proxy indication References (N) (E) (cal BP)a resolution datesc (yr)b

Precipitation, river runoff 37 Arabian Sea 21.80 68.00 ca 1200e020 14C 3/4 TOC, TN Productivity Agnihotri et al., (monsoon 2002 intensity, precipitation) 38 Dharamjali cave 29.52 80.21 ca 1790e010 230Th 2/5 Speleothem d18O Precipitation Sanwal et al., 2013 39 Central and 18.90 81.90 ca 1400e0 1 Annual growth e Speleothem d18O Precipitation Sinha et al., 2011 Northeastern band counting India 40 East Rongbuk 28.03 86.96 1000e0 6 Annual layer e Iceecore dD Precipitation Kaspari et al., 2007 counting, ice flow model 41 Linzhou County 30.30 91.50 920e0 1 Ring counting e Treeering width Precipitation He et al., 2013a 42 Qinghai Lake 37.00 100.00 ca 3500e015 14C 0/2 %C37:4 Salinity Liu et al., 2006 (precipitation) 43 Ximencuo Lake 33.38 101.11 1000e015 210Pb,137Cs 1 TOC, TN Precipitation Pu et al., 2013 44 Longxi Area 35.45 194.78 1040e0 10 History e Historical Precipitation Tan et al., 2008 documents 45 Wanxiang Cave 33.32 105.00 1810e03230Th 11/19 Speleothem d18O Precipitation Zhang et al., 2008 46 Huangye Cave 33.58 105.12 1860e0 5 Annual growth 4/13 Speleothem d18O Precipitation Tan et al., 2011 band counting,230Th 47 Zhijin Cave 26.66 105.68 1120e05230Th 4/5 Speleothem d18O Precipitation, Kuo et al., 2011 summer/winter precipitaion ratio 48 Tianchi Lake 35.26 106.31 ca 6200e05 14C 2/17 Sediment redness Precipitation Zhou et al., 2010 49 Gouchi Lake 37.75 107.52 ca 2400e025 14C 3/9 Pollen Moisture Meng et al., 2009 50 Foyechi Pond 33.95 107.73 1000e040 14C 2 Pollen Precipitation Tong et al., 1996 51 Furong Cave 29.23 107.90 2050e08230Th 2/5 Speleothem d18O Precipitation Li et al., 2011b 52 Dongge Cave 25.28 108.08 ca 9000e04 230Th 9/45 Speleothem d18O Precipitation Wang et al., 2005 53 Foye Cave 33.67 109.08 1270e0 3 Annual growth 1 Speleothem d13C Moisture Paulsen et al., 2003 band counting,230Th 54 Jiuxian Cave 33.57 109.10 ca 8660e03 230Th 3/14 Speleothem d18O Precipitation Cai et al., 2010 55 Dajiuhu Peatland 31.48 110.06 ca 2500e070 14C 2/12 Pollen Precipitation He et al., 2003 56 Huguangyan Maar 21.15 110.28 1360e05137Cs, 14C 3 TOC, biogenic Precipitation Chu et al., 2002 Lake silica, TN 57 Heshang Cave 30.45 110.42 ca 8800e0 100 Annual growth 2/21 Speleothem delta Precipitation Hu et al., 2008 band d18O counting,230Th 58 Tengernur Lake 40.47 110.67 ca 1700e0 40 OSL 1/2 Grain size, pollen Moisture Zhao et al., 2011 59 Gonghai Lake 38.90 112.23 1160e0514C 3/4 Magnetical Precipitation Liu et al., 2011 parameters 60 Daihai Lake 40.55 112.66 ca 10250e035 14C 0/8 Pollen Precipitation Xu et al., 2010 61 North China 36.40 115.12 2000e50 10 History e Historical Precipitation Man, 2009 documents 62 Shihua Cave 39.83 115.67 ca 2200e0 20 Annual growth e Speleothem d18O， Precipitation Hou et al., 2003 band counting d13C 63 Longgan Lake 29.96 116.13 ca 3000e035 14C e Pollen Precipitation Tong et al., 1997 64 Dali Lake 43.26 116.60 ca 12000e010 14C 4/18 TOC, TIC Lake level Xiao et al., 2008 (precipitation) 65 Southern China 27.50 117.00 1500e0 1 History e Historical Precipitation Zheng et al., 2006 documents 66 Lower Huai River 32.36 117.84 2000e50 10 History e Historical Precipitation Man, 2009 and Yangtz River documents Basin 210 14 15 67 Tsuifong Lake 24.50 121.60 1460e010 Pb, C 4/5 Diatom, d Norg, pH, erosion Wang et al., 2013a 13 d Corg, magnetic (precipitation) susceptibility 68 Southern Okinawa 24.80 122.49 1050e015 14C 5/6 Diatom Salinity Li et al., 2011a Trough (precipitation) 69 Maili Pond 42.87 122.88 ca 2000e025 14C 2/6 Pollen Precipitation Ren, 1998 13 70 Xiaolongwan Lake 42.30 126.35 ca 1600e0 15 Varve counting e d Corg Moisture/ Chu et al., 2009 precipitation 71 Korea 38.32 127.00 990e20 10 History e Historical Precipitation Kim and Choi, 1987 documents

a Calendar year before 2000 AD. b Average resolution during the last 1000 years. If there were multiple proxies in one site, the resolution was calculated using the most highly-resolved proxy. c No. of dates within the last 1000 years/total dates in the sequence,210Pb and/or137Cs dates are counted as one. d Discontinuous record. e This means the proxy indication is lake level and was further used to reflect river discharge variations in the original reference (similarly hereinafter). 102 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111

Four records not quite spanning the whole of the last millen- The high-resolution proxy records were first linearly interpo- nium; four low-resolution (but with a resolution still better than lated to their average resolution, to avoid the influence of an un- 100 years) records; and four discontinuous records with a clear even distribution of the original data on the data treatment steps. hydroclimatic significance were also included and used as sup- Then, time series which reflect dryness (e.g. oxygen isotope data porting evidence, especially for areas for which high-resolution from speleothems) were multiplied by 1 so that higher values records are scarce. In total, 71 proxy moisture/precipitation re- indicated wetter climatic conditions. Finally, the interpolated cords (34 from westerly-dominated Asia and 37 from monsoonal values during the MCA (or LIA) were compared to the median (Mi) Asia) during the last millennium (Fig. 1, and see Table 1 for details of the entire time series during the last millennium. If 2/3 of the of those records) were compiled. Amongst these, 37 records with interpolated values during the MCA (or LIA) were higher than Mi for an average resolution better than 10a (14 from westerly- a given location, the wetness for that location was classified as dominated Asia and 23 from monsoonal Asia) were labeled as “wet”; and if 2/3 of the values during the MCA (or LIA) were lower high-resolution records (highlighted in bold in Table 1), and about than Mi, the wetness was classified as “dry”. Otherwise, the wetness 80% of which had at least three age control points for the past was classified as “moderate”. As mentioned above, in order to make 1000 years. In cases where more than one record has been pub- full use of the detailed climatic information, the “moderate” grade lished for the same site, two conditions existed: 1) If these records was further classified. If more than 1/2 but less than 2/3 values were indicate generally consistent moisture/precipitation changes, the higher (lower) than Mi, the wetness was classified as “moderately one with highest resolution was included (e.g. Huguangyan Maar wet (moderately dry)”. Lake: two records (Chu et al., 2002; Zeng et al., 2012)). 2) If these The wetness classification of the Sugan Lake salinity record records indicate generally inconsistent moisture/precipitation (Chen et al., 2009) was used to demonstrate this procedure changes, the most recent record was included (e.g. Qinghai Lake: (Fig. 2). Firstly, the original time series (green dashed line) was more than three records (Ji et al., 2005; Shen et al., 2005; Liu et al., linearly interpolated to its average resolution (red dashed line). 2006)). Given that higher salinities indicate a drier climate, the interpo- To investigate the overall moisture/precipitation variations, the lated time series was then multiplied by 1 (blue solid line) so temporal ranges of the MCA and LIA are defined as 1000e1300 AD that higher values indicate a wetter climate. The calculated results and 1400e1900 AD, respectively (IPCC, 2007). Although there are suggested that 71.4% (20 out of 28) data values during the MCA some differences on decadal scales, nearly all of the synthesized were lower than Mi (grey solid line), while 66.7% (32 out of 48) temperature reconstructions from China and surroundings data during the LIA were higher than Mi. The wetness grade demonstrated generally higher temperatures from 1000 to during the MCA and LIA was thus finally determined as “dry” and 1300 AD, and lower temperatures from 1400 to 1900 AD (Yang “wet”, respectively. et al., 2002a; Ge et al., 2008, 2013; Cook et al., 2013). To capture For the low-resolution records, the wetness grades were the general spatial pattern, the wetness during the MCA and LIA in determined based on the original descriptions of moisture condi- individual records was classified into 3 grades (dry, moderate, wet). tions in the literature. Specifically, there are 3 situations: For high-resolution records, the “moderate” grade was further (1) If the definitions of the MCA and LIA in the original classified into “moderately dry” and “moderately wet”, in order to publications were similar to those of the present study, the take full advantage of the detailed climatic information they could moisture grades were assigned based on the original de- provide. During this grading process, the following procedure and scriptions. For example, the pollen record from Maili (site 69 in criteria were used: Table 1) suggest that that the region was wet from 950 to

Fig. 2. The MCA and LIA wetness classification procedure for the Sugan Lake (site 24 in Table 1) salinity record (Chen et al., 2009). J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111 103

Fig. 3. Wetness map based on multi-proxy records during the MCA (a) and LIA (b). Wetness was classified into dry, moderate and wet. The “moderately dry” and “moderately wet” classes for the high-resolution records are indicated by “minus” and “plus” symbols overlaid on top of the grey solid circles, respectively. 104 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111

1290 AD, indicating a strengthening of the East Asian summer 3. Results monsoon (EASM) at that time (Ren, 1998). This period is similar to the standard definition of the MCA (1000e1300 AD), and the Based on the 71 proxy records and methods described in the wetness grade of this site during the MCA was therefore classi- previous section, wetness grades for individual sites during the fied as “wet”. MCA and LIA are shown in Fig. 3. The figure clearly demonstrates (2) If the definitions of the MCA and LIA in the original publi- that on a multi-centennial time scale during the last millennium, cations differed from the present, the moisture grades needed to be distinctly different responses of moisture/precipitation to overall reclassified. For example, Agnihotri et al. (2002) present a 1200 rather similar patterns of temperature change (Yang et al., 2002a; year record of the total organic carbon content of a sediment core Ge et al., 2008, 2013; Cook et al., 2013) occurred not only in mid- from the northeastern Arabian Sea (site 37 in Table 1). They argued latitude westerly-dominated Asia and monsoonal Asia north of that the Indian summer monsoon (ISM) was weak from 1450 to 30N, but also within monsoon-dominated eastern China east of 1750 AD and that the region experienced a dry LIA. However, if the 105E. Specifically, during the MCA (Fig. 3a), most sites from moisture conditions during the entire LIA (1400e1900 AD) were westerly-dominated ACA, including Central Asia, arid Northwest considered, the wetness grade of this site was re-classified as China, the northern Tibetan Plateau and part of Mongolia, docu- “moderate” due to the strengthening of the monsoon since the ment a relatively dry (dry or moderately dry) climate. In contrast, mid-18th century. most sites from northern China record a strong or moderately (c) If the proxy records were discontinuous, the wetness grades strong summer monsoon. The proxy records from southern China, were assigned only if there was solid evidence of the moisture including the lower Yangtze River Catchment, South China, Taiwan conditions. For example, the high level of the Caspian Sea (Site 1 in and its adjacent sea area, all indicate relatively low monsoonal Table 1) during the LIA was indicated by geomorphological evi- precipitation. During the LIA (Fig. 3b), ACA is generally relatively dence (Karpychev, 2001; Kroonenberg et al., 2007), while a low wet, while a weak or moderately weak monsoon dominated level during the MCA was indicated only by inference. Thus the northern China, and southern China experienced pluvial condi- wetness grade of this site during the LIA is classified as “wet”, while tions. In addition, no clear spatial patterns of hydroclimatic change no grade was assigned for the MCA. are observed in regions mainly dominated by the Indian summer It is necessary to point out that the “standard” definition of MCA monsoon during the MCA and LIA, and thus no further discussion is (1000e1300 AD) and LIA (1400e1900 AD) used in this study could presented in the following sections regarding this region. inevitably overshadow the local characteristics of moisture changes In brief, during the broadly warm medieval times, there existed in some records. However, given that the focus of our study is to a “dry western part and wet eastern part” spatial pattern in mid- investigate the general spatial pattern of moisture/precipitation latitude Asia to north of 30N. The boundary was roughly close to variability on a multi-centennial scale, this standard-period the modern summer monsoon limit. In addition, there existed a method is believed to have no significant influence on the reli- “wet northern part and dry southern part” pattern within ability of the results. Furthermore, this fixed “standard” definition monsoonal eastern China with the Huai River, located between the method is actually a strict one, especially in the case of the high- Yellow River and the Yangtze River at about 34N, roughly marking resolution records. As far as the LIA is concerned (and the same the boundary. During the LIA generally low temperatures occurred, situation obtains for the MCA), in most previous publications any with broadly inverse moisture conditions to those which obtained centennial-scale climatic event from the 15th to the 19th century during the MCA: a “wet western part and dry eastern part” and a was labeled as LIA (or LIA-related); however, only if the moisture “dry northern part and wet southern part”. anomaly was demonstrated for more than 2/3 of the entire 500 a time-span, was it designated as a “wet” or “dry” LIA using our 4. Discussion method. For instance, the Guliya (site 11 in Table 1) record clearly features a high accumulation rate interval from 1530 to 1820 AD 4.1. Spatial modes of hydroclimatic variability in China and within the LIA (Thompson et al., 1995; Yao et al., 1996). However, surroundings on a multi-centennial time scale only 64.7% (no more than 2/3) of the data were higher than Mi during the entire 1400e1900 AD interval, and the LIA at the Guliya 4.1.1. “WesteEast” mode of hydroclimatic variability site was thus designated as “moderately wet” by our method. In It has been clearly revealed by instrumental data using various other words, if any spatial pattern is identified using these strict summer monsoon indices that the EASM has weakened since the criteria, it is reasonable to consider that the underlying climatic 1970s (Li and Zeng, 2002; Ding et al., 2008). Specifically, since the signal could be significant. 1980s decreased precipitation and increased temperature have To understand the possible mechanisms responsible for the together resulted in aridification in monsoonal northern China, multi-centennial scale hydroclimatic variations during the MCA while arid Northwest China has become wetter (Ma and Fu, 2006). and LIA, the variations in several 1000-year reconstructions of The latter phenomenon is consistent with the finding that climate large-scale climate forcing modes, including the El Nino/Southern~ in Northwest China, notably in the Xinjiang region, changed from a Oscillation (ENSO) (Conroy et al., 2009; Oppo et al., 2009), the warm and dry condition to a warm and wet regime in the late Atlantic Multidecadal Oscillation (AMO) (Mann et al., 2009) and the 1980s, according to a synthesis of climatic, hydrologic and botanical North Atlantic Oscillation (NAO) (Trouet et al., 2009), were evidence (Shi et al., 2002, 2007). Furthermore, a tree-ring-based explored. Additionally, the variations of observed precipitation in PDSI (Palmer Drought Severity Index) reconstruction in the cen- our study region during 1900e2008 and their relationship with tral Tianshan Mountains (westerly-dominated Asia) (Li et al., 2006) global sea surface temperature (SST) were also analyzed and and Helan Mountains (monsoonal margin) (Li et al., 2007) compared to the hydroclimatic patterns during the MCA and LIA. demonstrated that during the last 30 years there has been an in- The gridded precipitation data during 1900e2008 were obtained verse trend of moisture change between these two regions, sup- from the Center for Climate Research at the University of Delaware porting the existence of a “WesteEast” mode of hydroclimatic (Legates and Willmott, 1990) and the gridded SST were obtained variability in mid-latitude China on a decadal time scale. Chen et al. from Hurrell et al. (2008). The spatial patterns of the observed (2011) investigated the spatiotemporal characteristics of precipi- annual precipitation were analyzed using empirical orthogonal tation in Central Asia during 1930e2009, and found that annual function (EOF) analysis (Lorenz, 1956). precipitation generally increased during the last 80 years in this J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111 105 region, especially in winter (the main rainy season). Their study precipitation in the middle and lower Yangtze River Catchment and extends the region where moisture/precipitation is increasing southeastern coastal region is increasing (Hu and Feng, 2001; Zhou within the context of global warming to the main body of westerly- et al., 2009), resulting in the so-called “northern China drought and dominated Asia. Recently, the “WesteEast” dipolar mode has southern China flood” pattern. Such a pattern was also revealed by further been identified on inter-annual to decadal time-scales EOF analysis using instrumental data from 740 weather stations during the past half millennium by a gridded warm season pre- (Ding et al., 2008). cipitation data set for Asia, reconstructed on the basis of 284 On the centennial time scale, Wang et al. (2001) classified the annual-resolution proxy records and several long-term instru- spatial patterns of precipitation in eastern China into six types mental data series (Feng et al., 2013a). based on historical documents (950e1999 AD). Their results sug- On a multi-centennial time scale we proposed the possibility gested that during the MCA, there were two prominent types: that, based on limited evidence, a relatively wet climate dominated “floods in the south of the Yangtze River and droughts in the north arid Northwest China during the LIA (Chen et al., 2008b). Based on of it”, and “floods along the middle and lower Yangtze River, the up-to-date and comprehensive set of proxy records presented droughts to the north and the south of it”. During the LIA, the here, the spatial structures evident in Fig. 3 clearly demonstrate that opposite conditions existed. On the basis of a reconstruction of the (1) the “WesteEast” mode not only existed in mid-latitude China Asian-Pacific Oscillation (APO), reflecting the thermal contrast be- during the LIA, but also during the MCA; and (2) not only Northwest tween the Asian continent and northern Pacific, Zhou et al. (2011) China experienced relatively wetter conditions in the LIA than in the analyzed the variations of summer monsoon circulation and pre- MCA, but other areas dominated by the westerlies, including Central cipitation in eastern China. They suggested that the late MCA (early Asia, the northern Tibetan Plateau and even part of Mongolia, also LIA) was the interval characterized by the strongest (weakest) Asian experienced coherent hydroclimatic conditions. monsoon during the last millennium, and the spatial pattern of The asynchronous variations in moisture between the westerly- precipitation change was that of “Yellow River Catchment flood dominated ACA and monsoonal Asia have already been identified (drought) and Yangtze River Catchment drought (flood)”. From a on the millennial time scale during the Holocene (e.g., Chen et al., paleoclimatological perspective, this synthesis study using multiple 2008a; Zhao et al., 2009; Zhang et al., 2011a). Generally, a strong archives and diverse proxies clearly indicated that: (1) Not only on monsoon during the early Holocene may not have advanced into the centennial scale, even regarding the MCA and LIA as a multi- ACA because most lakes in this region exhibited a relatively low centennial climatic event, there exists a distinct “NortheSouth” stand or even dried up at that time. The wettest climate dominated mode of precipitation change in eastern China. (2) On a multi- ACA during the middle Holocene, while the summer monsoon did centennial scale, the pattern of “flood (drought)” during the MCA not experience a significant change compared to the early Holo- (LIA) not only dominated the Yellow River Catchment, but also cene. During the late Holocene, the monsoon was at its weakest, dominated in Northeast China; while the opposite conditions not and the climate became drier in ACA (but still wetter than during only dominated the Yangtze River Catchment, but also dominated the early Holocene). in South China, Taiwan and its adjacent sea area. Therefore, the “WesteEast” mode of hydroclimatic variability on It is difficult to discuss fully the asynchronous variations in a multi-centennial scale revealed in the present study also occurred precipitation between the northern and southern parts of eastern on a millennial scale during the Holocene, and on a decadal scale China on a millennial time scale based on current proxy records. during the last 500 years (including the instrumental period). This Evidence exists for a dry middle Holocene in southern China (Zhou indicates significantly different responses of moisture/precipitation et al., 2004; Xie et al., 2013), which is different from the humid variation to a similar pattern of temperature change in regions climate during the middle Holocene in northern China (e.g. Sun dominated by different circulation systems and on different time et al., 2010; Lu et al., 2013; Yang et al., 2013; Wang et al., 2014). scales. However, other proxy records from southern China have indicated a In addition, it is noteworthy that several sites recorded moisture strong EASM during that period (e.g. Wang et al., 2007b; Hu et al., conditions which differed from those of adjacent sites in westerly- 2008). Therefore, it is essential that more high-quality hydro- dominated Asia. Among them, Hurleg Lake (site 26 in Table 1), climatic records are recovered in order to shed light on the spatial located in the transitional zone between the monsoon and the pattern of precipitation variability on the multi-millennial time westerlies, may have exhibited a complex moisture response to scale in eastern monsoonal China. Nevertheless, the stable exis- temperature change because of the impact of both climate systems. tence of the “NortheSouth” mode on time scales from multi- However, a recent salinity reconstruction from nearby Gahai Lake centennial to decadal during the last millennium suggest that (site 27 in Table 1) revealed a generally dry MCA and a generally they are controlled by a common set of forcings. wet LIA (He et al., 2013b), consistent with most records from the westerly-dominated region. The other records (Tianshan Moun- 4.2. Possible mechanisms tains (site 16 in Table 1), Luobupo Lake (site 20 in Table 1) and the Chaiwobu peatland (site 18 in Table 1)) are either low-resolution or What mechanisms were involved in producing the “WesteEast” possess few age control points. For example, the depth-age re- and “NortheSouth” modes of spatial hydroclimatic changes in lationships in Luobupo Lake were mainly inferred using chronolo- China and surroundings on a multi-centennial time scale? To gies from other cores in that area. However, the possibility cannot answer this question, we focused on the major climate variability be ruled out that these records genuinely reflect local moisture modes affecting moisture/precipitation in the study region (e.g., variability. ENSO, AMO and NAO). On the basis of the relationships between these climate variability modes and regional moisture/precipita- 4.1.2. “NortheSouth” mode of hydroclimatic variability tion using both the instrumental data and the reconstructed time In monsoon-dominated eastern China, the spatial pattern of series (Fig. 4), the role of each climate variability mode on the precipitation changes on a decadal scale has long been of interest to moisture changes is discussed in the following sub-sections. climatologists (Ren et al., 2000; Wang, 2001; Zhao and Zhou, 2006; Ding et al., 2008, 2009; Zhou et al., 2009; Zhao et al., 2010a). 4.2.1. ENSO and spatial patterns of precipitation in Asia Associated with the weakening EASM in recent decades, the pre- ENSO is one of the most important coupled ocean-atmosphere cipitation in monsoonal northern China is decreasing, while the modes for the globe and has a significant impact on precipitation 106 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

(a) 29 Indo Pacific Warm Pool 28.5 C) o 28

25 SST ( 27.5 (b) 24.5 27 C) o 24 SST ( SST 23.5

Eastern equatorial Pacific 23 0.5 C)

(c) o 0.3

0.1

-0.1

AMO -0.3

3 -0.5 Temperature Anomaly ( (d)

1

Z-Scores -1 NAO

-3

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Year A.D.

Fig. 4. (a) SST reconstruction for the western equatorial Pacific(Oppo et al., 2009); (b) SST reconstruction form the eastern equatorial Pacific(Conroy et al., 2009); (c) Reconstruction of temperature anomalies in the AMO region (Mann et al., 2009); (d) Winter NAO reconstruction (Trouet et al., 2009). Red and blue rectangle indicates MCA and LIA, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

in the study region (e.g., Wang et al., 2000; Bronnimann,€ 2007). To condition), arid central Asia and southern China receive less pre- reveal the spatial pattern of annual precipitation in Asia and its cipitation, whereas monsoonal northern China as well as the South/ possible association with ENSO and other variability modes, the Southeast Asia receive excessive precipitation. When the eastern EOF method was applied to the observed annual precipitation data tropical Pacific is warm (El Nino~ condition), the above results are during 1900e2008 (Legates and Willmott, 1990). The first EOF reversed. The results in Fig. 5 are also consistent with previous mode is characterized by in-phase variations over North China, studies of the relationship between ENSO and precipitation in arid southern Northeast China, South Asia and Southeast Asia; and by central Asia (e.g., Zhu and Li, 1992; Wei and Chen, 2002; Li and Li, out-of-phase variations over arid central Asia and southern China 2004; Wang et al., 2007a) and eastern China (e.g. Qian et al., (Fig. 5a). This pattern resembles the “WesteEast” mode between 2007; Su and Xue, 2011). the westerly-dominated ACA and monsoonal northern China, and The intensity and position of the Western Pacific Subtropical the “NortheSouth” mode in eastern China in instrumental obser- High (WPSH) likely played a key role in facilitating the influence of vations and proxy data (Ding et al., 2008; Feng et al., 2013a, and our ENSO on precipitation variations in eastern China. In El Nino~ years, Fig. 3). The temporal variations associated with the first EOF mode the WPSH intensified and extended westwards and shifted south- (PC1) exhibit distinct inter-annual and decadal variations, which wards (Qian et al., 2007; Su and Xue, 2011). The rain-belt of the are closely linked to the Nino~ 3.4 SST (Hurrell et al., 2008. Fig. 5b). summer monsoon cannot advance to North China as in normal The correlation between the two time series is 0.40 (significant at years, but it persists in the middle and lower Yangtze River the 99.9% confidence level after accounting the autocorrelation in Catchment and South China, leading to excessive precipitation in both time series following the approach of Wilks, 2006), suggesting southern China and less precipitation in northern China. In La Nina~ that the first EOF mode is significantly related to ENSO. This notion years, the WPSH weakened and retreated northeastwards, causing is further supported by correlating the PC1 of the first EOF mode a smooth transport of moisture by low-level jet along the north- and the SST over the global ocean, which exhibits a typical SST western edge of the WPSH to northern China (Zhou and Yu, 2005). pattern associated with La Nina~ (Fig. 5c). The results presented in Correspondingly, the precipitation in southern China decreased. On Fig. 5 suggest that, when the eastern tropical Pacific is cold (La Nina~ the other hand, the circulation anomalies associated with ENSO can J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111 107

(a) 3 R = -0.40 (P≤0.001) PC1 -1.5 (b) Nino 3.4 SSTa

2 -1 C) 1 -0.5 o

0 0

-1 0.5 PC1 (sigma unit) Nino 3.4 SSTA ( SSTA 3.4 Nino

-2 1

-3 1.5 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 (c)

Fig. 5. (a) EOF1 pattern computed based on the annual total precipitation over the study region (1900e2008 AD). The numbers in the plot are the eigenvectors of the first EOF mode. Precipitation dataset (0.5*0.5) was obtained from the Center for Climate Research of University of Delaware. (b) PC1 and annual SST anomaly (SSTA) in Nino~ 3.4 (1900e2008 AD), R ¼0.40, P 0.001. SSTA dataset was obtained from Koninklijk Nederlands Meteorologisch Instituut. (c) Correlation between PC1 and annual SST over the global ocean; light blue (brown) shadings indicate the negative (positive) correlations are significant at the 95% confidence level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

be quasi-horizontally propagated to middle and high latitudes precipitation in Northeast China during 1948e2007. They further through planetary waves in the troposphere to influence arid demonstrated that, based on tree ring chronologies, such connec- central Asia (Huang, 1986; Luo, 2005). In typical El Nino~ years, tions were persistent since 1568 AD. The AMO could lead to vari- when the WPSH extends westwards, the east South Asia High ations in troposphere temperature over the Eurasian continent (SAH) pattern usually dominates the high layer of the troposphere through atmospheric circulation, and to changes in the thermal over the Tibetan Plateau and its northern adjacent area, and a contrast between land and ocean, and finally result in strength- plateau vortex or trough dominates the middle layer, together ening (positive AMO) or weakening (negative AMO) of the Asian forming a “high upper layer and low lower layer” pressure field and summer monsoon (Lu et al., 2006; Feng and Hu, 2008; Wang et al., finally causing wetter climatic conditions in arid Northwest China 2009). (Qian et al., 2001). In typical La Nina~ years, the reverse processes The North Atlantic Ocean was generally warm (cold) during the generally result in drier conditions. MCA (LIA) (Feng and Hu, 2008; Mann et al., 2009. Fig. 4c). Ac- The spatial dry/wet patterns during the MCA (LIA) are similar to cording to the relationship between AMO and precipitation in the EOF mode of the observed precipitation (Figs. 3 and 5), implying China (Li and Bates, 2007; Wang et al., 2011), the AMO (or AMO- that the substantial variability which occurred in the mean state of like SST in the North Atlantic) may also have played some role in the Pacific ocean-atmosphere system over the recent 100 years may producing the NortheSouth precipitation dipole pattern during also have operated during the last millennium. This result is the MCA and LIA. consistent with an SST reconstruction for the tropical Pacific (Conroy et al., 2009; Oppo et al., 2009). As shown in Fig. 4, the 4.2.3. NAO and precipitation in westerly-dominated Asia eastern (western) tropical Pacific Ocean was cooler (warmer) in the The NAO is one of the most prominent teleconnection patterns ~ MCA. According to the observations (Fig. 5), this La Nina-like con- over the middle and high latitudes of the Northern Hemisphere dition would lead to a “northern China flood and southern China (Hurrell, 2005). The observational study by Aizen et al. (2001) drought” pattern as well as to a drier arid central Asia during the showed that there was an inverse association between NAO and ~ MCA. In contrast, the warmer eastern tropical Pacific (El Nino-like precipitation in Central Asia, which has also been supported by condition) in the LIA (Fig. 4) would lead to “northern China drought regressing the observed precipitation on land with the low-pass and southern China flood” as well as to a wetter arid central Asia filtered time series of the NAO (Seager et al., 2007). during the LIA. The NAO can influence the location of jet stream and storm tracks over the North Atlantic and Eurasia. When the NAO is in a 4.2.2. AMO and precipitation in monsoonal China negative phase, the axis of maximum moisture transport and the An increasing amount of instrumental and modeling data indi- cyclonic storm tracks shifted southwards in the North Atlantic cate that the AMO, basin-wide SST variability mode on a multi- (Hurrell, 1995), together with moist air from the Mediterranean Sea decadal time scale in the North Atlantic (Kerr, 2000; Enfield et al., (Lioubimtseva et al., 2005), and resulting in increased precipitation 2001) plays a significant role in climatic variations over the globe in westerly-dominated Asia. When the NAO is in a positive phase, (Knight et al., 2006; Feng et al., 2011; Wyatt et al., 2011). Li and the maximum moisture transport and the cyclonic storm tracks Bates (2007) showed that the AMO warm phases are associated shift northeastwards (IPCC, 2007), and the moisture sources and with excessive cool season precipitation in monsoonal northern the synoptic weather conditions lead to a drier climate. A tree-ring- China and less precipitation in southern China. Wang et al. (2011) based reconstruction of the NAO index for the last millennium suggested that the AMO is positively correlated to the annual demonstrated generally positive (negative) NAO indices during the 108 J. Chen et al. / Quaternary Science Reviews 107 (2015) 98e111

MCA (LIA) (Trouet et al., 2009. Fig. 4d), providing an explanation for availability of more high-resolution proxy records, synthesis the relatively dry (wet) climatic conditions over westerly- studies and numeric modeling efforts. dominated Asia in the MCA (LIA). Though the centennial variations of ENSO, AMO and NAO Acknowledgments during the last millennium may all influence patterns of mois- ture/precipitation in westerly-dominated ACA and monsoon- We would like to thank the anonymous reviewers and the editor dominated Asia, the resemblance between Figs. 3 and 5 sug- for their constructive comments and suggestions, which substan- gests that the ENSO probably played a more important role in tially improved the manuscript. We also thank Dr. Jan Bloemendal shaping the pattern of dry/wet variations in our study region. for considerably improving the English. This research was sup- Fig. 5 also shows that the relationship between precipitation in ported by the National Basic Research Program of China Asia and SST in the Atlantic is much weaker compared to the (2010CB950202, 2012CB955301), the Natural Science Foundation Pacific Ocean, again implying that the AMO and NAO played a of China project (41471162, 41130102), and the Doctoral Program of secondary role in driving centennial-scale dry/wet fluctuations in Higher Education Project (20120211130001). SF is partially sup- Asia. However, how the ENSO, AMO and NAO interact, and the ported by the US NSF grant (AGS-1439964). relative role each mode plays in influencing centennial-scale hydroclimate changes in Asia, is still unclear and deserves further investigation. References This study has determined the occurrence of similar spatial Agnihotri, R., Dutta, K., Bhushan, R., Somayajulu, B.L.K., 2002. 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