YAN Qing, ZHANG Zhong-Shi,2, and WEI Ting

1Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China2Bjerknes Centre for Climate Research, UniResearch, Bergen 5007, Norway

3Chinese Academy of Meteorological Sciences, Beijing 100081, China

Simulated Spatiotemporal Characteristics of Climate Change in China during the Han Dynasty (1–200 A.D.)

YAN Qing1, ZHANG Zhong-Shi1,2, and WEI Ting3

1Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2Bjerknes Centre for Climate Research, UniResearch, Bergen 5007, Norway

3Chinese Academy of Meteorological Sciences, Beijing 100081, China

In this study, a 2000-year simulation forced by transient, external forcings is carried out with the Community Earth System Model. The authors investigate the spatiotemporal features of climate change in the Han Dynasty (1-200 A.D.) using the empirical orthogonal function (EOF) method. The leading EOF mode of the annual mean temperature anomalies shows a uniform variation of temperature over the whole of China, while the second EOF mode indicates opposite variations of temperature between western and eastern China. For the annual mean precipitation anomalies, the first EOF mode indicates a meridional dipole pattern over eastern China, with increased (decreased) precipitation to the south of the Yangtze River and decreased (increased) precipitation to the north. The leading mode of the 850 hPa winds and sea level pressure in summer exhibits a southwesterly (northeasterly) anomaly over South China, which is associated with a strengthened (reduced) meridional sea level pressure gradient. Compared to reconstructions, the model can capture the majority of features of climate changes in the Han Dynasty, though it underestimates the magnitude.

Han Dynasty, last two millennia, climate modeling, climate change

1　Introduction

Climate in China, as reconstructed from geological proxy and historical documents, has featured several warm and cold intervals during the last two millennia (Ge et al., 2013b; Yang et al., 2002). The warm intervals mainly include the western and eastern Han Dynasties (200 B.C.-180 A.D.), the Sui and Tang Dynasties (541-810 A.D.), the Song and Yuan Dynasties (931-1320 A.D.), and the 20th century (Ge et al., 2013b). Understanding climate changes during past warm intervals may shed light on the role of natural forcings on recent global warming. Though paleo-proxies offer the potential to extend temperature records to millennia and even longer, they are sporadically dispersed and provide limited information on the possible mechanisms behind temperaturevariations.

Climate modeling, as a complementary method, provides a means of studying the spatial pattern of climate change over China in the last two millennia, and exploring the possible mechanisms. Using the climate model ECHO-G—ECHO-G is a coupled climate model consisting of the 4th generation atmospheric general circulation model (ECHAM4) and the ocean model HOPE (Hamburg Ocean Primitive Equation)—Liu et al. (2005) found that the simulated higher temperature in the Medieval Warm Period (MWP; 1000-1300 A.D.) can be largely attributed to the increased solar radiation. Yan et al. (2015) argued that the warming in the Sui and Tang Dynasties was constrained to eastern China and may have been a regional phenomenon. Peng et al. (2009) indicated that dry conditions were dominant in eastern China during the Little Ice Age. Man et al. (2012) suggested that precipitation exhibited a meridional tripole pattern over eastern China during the MWP, with enhanced precipitation over northern China and reduced precipitation over the Yangtze River valley.

However, previous modeling studies have largely focused on the climate changes of the warm periods in China during the past 1000-1500 years. Little attention has been paid to climate change in the Han Dynasty (1-200 A.D.), during which temperatures are believed to have been comparable to the temperatures of the 20th century (Yang et al., 2002). The spatial pattern of temperature changes in the Han Dynasty is largely unclear owing to limited proxy data, as well as knowledge on the precipitation and monsoon circulations. Therefore, we perform a 2000-year simulation with a coupled global climate model, and investigate the spatiotemporal structures of climate changes in China during the Han Dynasty.

2　Methods

2.1 Model description

The climate model used in this study is the Community Earth System Model (CESM) developed at the National Center for Atmospheric Research. CESM consists of four components: an atmospheric component, Community Atmosphere Model version 4 (CAM4); a land component, Community Land Model version 4 (CLM4); an oceanic component, Parallel Ocean Program version 2 (POP2); and a sea ice component, Los Alamos Sea Ice Model ver-sion 4 (CICE4). In this study, CAM4 has a horizontal resolution of ～3.75° × 3.75° and 26 vertical levels. POP2 has a nominal 3° horizontal resolution and 32 vertical levels. CLM4 and CICE4 adopt the same horizontal resolution as CAM4 and POP2, respectively. CESM has been proven skillful in depicting the majority of the features of present-day climate (e.g., Tian and Jiang, 2013a; Yan et al., 2014) and has been widely used in modeling both past and future climates (e.g., Wei et al., 2012; Tian and Jiang, 2013b).

2.2 Experimental design

To simulate climate change over China in the last two millennia, we first perform a pre-industrial 300-year spin-up run. Starting from the end of this run, CESM is then integrated for 2000 years with continuous changes in solar radiation, volcanic eruptions, greenhouse gas (GHG) concentrations, and land cover. The external forcings used are the same as in Yan et al. (2015). In brief, the total solar irradiance (TSI) forcing is obtained from Vieira et al. (2011) and Lean (2009). The TSI shows obvious decadal oscillations during the Han Dynasty (1-200 A.D.) and is approximately 0.2 W m-2higher relative to the long-term mean. The volcanic forcing is obtained from the ice core-based index of Gao et al. (2008). Because this dataset only covers the last 1500 years, the volcanic forcing is set to zero during the Han Dynasty. With respect to land cover forcing, the reconstructed land-use data by Klein Goldewijk et al. (2011) and Hurtt et al. (2009) are used for the periods of 1-1850 A.D. and 1851-2000 A.D., respectively. The GHGs (i.e., CO2, CH4, and N2O) are derived from high-resolution ice cores in Antarctica (available at https://pmip3.lsce.ipsl.fr). Note that there are few changes in land cover and GHGs during the Han Dynasty.

3　Results

The simulation indicates that the regionally averaged annual mean temperature and precipitation increase by～0.014°C and 8.2‰ in the Han Dynasty (1-200 A.D.) relative to the climate mean of 1-2000 A.D., respectively. It should be noted that the Han Dynasty is not a stable warm period, but with clear decadal fluctuations (Ge et al., 2013a). The standard deviation of the annual mean temperature anomalies during the Han Dynasty reaches～0.3°C in our simulation. Therefore, we employ the empirical orthogonal function (EOF) method to study the spatiotemporal characteristics of climate changes during the Han Dynasty in the following subsections.

3.1 Temperature

The leading EOF mode of the annual mean temperature anomalies in China accounts for 34.2% of the variance. It indicates a uniform warming (cooling) pattern in China during the Han Dynasty (Fig. 1a). The maximum warming (cooling) appears in north-central and northwest China. The corresponding principal component shows that significant decadal fluctuations exist, and warm conditions broadly dominate the period of 60-140 A.D., while cold conditions prevail during the period 140-200 A.D. (Fig. 1c).

The second EOF mode explains 20.2% of the variance. This mode indicates opposite variations of annual mean temperature between eastern and western China duringthe Han Dynasty (Fig. 1b). The most intense temperature changes are located in northeast China and the Tibetan Plateau. The principal component of the second EOF mode indicates that temperature is higher in eastern China and lower in western China for the period 1-100 A.D. (Fig. 1d). The negative phase (i.e., higher temperature in western China) mainly occurs after 100 A.D. on the decadal time scale.

3.2 Precipitation

The leading EOF mode of the annual mean precipitation anomalies, explaining 26.1% of the variance, indicates a meridional dipole pattern over eastern China in the Han Dynasty (Fig. 2a). Enhanced (decreased) precipitation to the south of the Yangtze River is generally accompanied by decreased (enhanced) precipitation to the north. The corresponding principal component shows obvious decadal oscillations of this precipitation pattern during the Han Dynasty (Fig. 2c). The positive phase of the leading EOF mode occurs mainly in the periods 50-110 A.D. and 150-190 A.D.

The second EOF mode of the annual mean precipitation anomalies accounts for 17.6% of the variance. This mode indicates a meridional tripole-like pattern over East China (Fig. 2b); that is, enhanced (suppressed) precipitation in the region ～25-38°N with suppressed (enhanced) precipitation along its two sides. The corresponding principal component shows decadal and multidecadal fluctuations during the Han Dynasty. The positive phase of the second EOF mode broadly dominates the period 50-130 A.D., and the negative phase mainly occurs afterwards (Fig. 2d).

3.3 Monsoon circulations

Figure 3 shows the spatial pattern of combined EOFs of summer 850 hPa winds and sea level pressure anomalies and the corresponding principal components. The first EOF mode, explaining 20.2% of the variance, indicates a southwesterly anomaly over South China (south of 30°N) with a westerly anomaly over the northwestern Pacific (along 30°N) for the positive phase (Fig. 3a). The anomalous southwesterly is favorable for enhanced precipitation in the Yangtze River valley. The westerly anomaly limits the amount of water vapor being transported from the western North Pacific to North China and hence contributes to the suppressed precipitation over that region. These wind anomalies are attributed to changes of sea level pressure. For the positive phase of the leading EOF mode, the meridional land-sea gradient of sea level pressure is enhanced, which can result in an anomalous southwesterly over South China. Meanwhile, the zonal land-sea gradient of sea level pressure is weakened in the subtropics, leading to a westerly anomaly over the western North Pacific. The principal component of the leading EOF mode shows that the summer monsoon is broadly strong (i.e., positive phase) during the Han Dynasty, except during 145-180 A.D. when the summer monsoon is greatly weakened (Fig. 3c).

The second EOF mode accounts for 14.3% of the total variance. For the positive phase (Fig. 3c), there is an easterly anomaly at low latitudes (15-25°N), which is unfavorable for water vapor transport from the Indian Ocean and Bay of Bengal and hence leads to decreased precipitation over southern China. Meanwhile, a southwesterlyanomaly appears over the Yangtze River and Jiang-Huai area, which may partially contribute to the enhanced precipitation over that region. The southwesterly anomaly is attributed to the intensified meridional sea level pressure gradient over eastern China.

4　Discussion

A large amount of geological proxies and historical documents offer the potential to reconstruct past climate changes. Here, we summarize the available proxies to produce a general picture of the mean climate changes in the Han Dynasty. It is shown in Fig. 4 that multiple proxies indicate warmer conditions in eastern China (east of～100°E) in the Han Dynasty, while several records broadly suggest cold conditions in the Tibetan Plateau (Yang et al., 2002; Tan et al., 2003; Thompson et al., 2006; Ge et al., 2013b). The second EOF mode of temperature is broadly in accordance with the reconstructions for most of the Han Dynasty. However, the simulated uniform warming (cooling) pattern over China (i.e., the first EOF mode) is not supported by the available proxies. Based on historical documents records, Ge et al. (2013b) pointed out that precipitation generally exhibits a ‘south flood/north drought' pattern over eastern China in the Han Dynasty, with increased precipitation to the south of the Yangtze River and decreased precipitation to the north (Fig. 4). The leading EOF mode of precipitation is roughly consistent with the reconstructions, mainly for the periods 50-110 A.D. and 150-190 A.D. The decreased precipitation over the northeastern Tibetan Plateau in our simulation is consistent with the tree ring data (Yang et al., 2014) during the aforementioned periods. According to stalagmite δ18O records and humification of peat (Wang et al., 2005; Yu et al., 2006; Cai et al., 2010; Dong et al., 2010; Li et al., 2011; Jiang et al., 2012), the East Asian summer monsoon was generally stronger in the Han Dynasty. The intensified summer monsoon is broadly reproduced in our simulation for the period 1-140 A.D.

Regarding the external forcings used, the TSI shows a slight increase of ～0.2 W m-2in the Han Dynasty with few changes in volcanic eruptions, land cover, and GHGs. The time series of TSI show a high correlation (r = 0.51) with the simulated global mean temperature during the Han Dynasty. These results indicate that variations of TSI may contribute largely to the simulated global temperature variability. On the regional scale (e.g., China), inter-nal variability and other factors (e.g., sea surface temperature) may also play a certain role in the simulated climate changes. Therefore, it is meaningful to further isolate the role of internal variability and external forcings by performing additional sensitivity experiments.

The model can capture the majority of the features of the reconstructed climate changes in the Han Dynasty; there are, however, uncertainties to be considered. The CESM predicts a slight warming (～0.014°C) in the Han Dynasty, while the reconstructions show a warming of～0.19 ± 0.28°C (Ge et al., 2013a). This indicates that the model underestimates the magnitude of the warming. However, the simulated warming magnitude may depend on the selected time period and the TSI forcing used. As the Han Dynasty was not a stable warm period, the simulated temperature increases by 0.07°C if it is averaged over the period 100-150 AD. In this study, we use the reconstructed TSI with weak variability in the last two millennia. Using a TSI forcing with high variability, the simulated temperature should be much higher in the Han Dynasty (e.g., Wang and Liu, 2014). Additionally, the model shows limited skill in reproducing the spatial pattern of precipitation in China due to the coarse horizontal resolution. Dynamical downscaling methods could be used to improve the simulation of precipitation in China and provide more information on local precipitation variation (Ju et al., 2007; Yu et al., 2010; Gao et al., 2013).

5　Summary

The Han Dynasty is one of the key warm intervals in China during the last two millennia. However, the spatiotemporal features of climate change in this warm interval remain largely unclear. In this study, we carry out a 2000-year simulation with the CESM forced by continuous external forcings. We then investigate the characteristics of climate change in the Han Dynasty using the EOF method. The main conclusions can be summarized as follows:

The leading EOF mode of the annual mean temperature anomalies shows a uniform variation over the whole of China. The warming pattern mainly occurs during 60-140 A.D., according to the corresponding principal component. The second EOF mode indicates opposite variations of temperature between western and eastern China. For the annual mean precipitation anomalies, the first EOF mode indicates a meridional dipole pattern over eastern China, with increased (decreased) precipitation to the south of the Yangtze River and decreased (increased) precipitation to the north. The leading EOF mode of the 850 hPa winds and sea level pressure anomalies in summer exhibits a southwesterly (northeasterly) anomaly over South China, which is caused by the strengthened (reduced) meridional sea level pressure gradient. The corresponding principal component indicates decadal fluctuations of the summer monsoon, with an enhanced summer monsoon dominating most of the Han Dynasty.

Acknowledgements. We thank the editor and the three anonymous reviewers for their constructive comments, which helped to improve the manuscript considerably. This work was jointly supported by the Strategic and Special Frontier Project of Science and Technology of the Chinese Academy of Sciences (Grant No. XDA05080803) and the National Natural Science Foundation of China (Grant Nos. 41402158, 41472160, and 41305073).

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10.3878/ AOSL20150036.

27 April 2015; revised 24 June 2015; accepted 26 June 2015; published 16 November 2015

YAN Qing, yanqing@mail.iap.ac.cn