Youbing Peng

Department of Earth Environmental Science, Xi’an Jiao Tong University, Xi’an, China

Keywords:Drought Hydroclimate Eastern China Data assimilation The last millennium

ABSTRACT Decadal—centennial hydroclimate variability over eastern China during the last millennium is investigated using the product of Paleo Hydrodynamics Data Assimilation (PHYDA). Results reveal that the PHYDA depicts a more homogeneous temporal pattern during the early part of the Little Ice Age with other reconstructions than those during the other periods, and could also identify the droughts of 1352—90 AD, 1445—98 AD, 1580—94 AD, and 1626—65 AD during this period. On centennial time scales, the PHYDA shows that the linkage between the Palmer drought severity index over eastern China and the Atlantic Multidecadal Oscillation (AMO) index is more marked than that with the El Ni?o—Southern Oscillation and the location of the intertropical convergence zone over the Asian—Australian monsoon area during the period after the 1350s. For the decadal droughts, the PHYDA suggests most of the drought events during the last millennium were linked to the El Ni?o—like mean states and the negative AMO states.

1. Introduction

Associated with global warming, the occurrence and durations of drought show an increasing tendency. Its impact is becoming more and more severe in the region of eastern China and its occurrences are of great concern ( Huang et al., 2015 ; Dai, 2012 ; Zou et al., 2005 ;Jiang et al., 2012 ). Because observational datasets are too short to assess the temporal variability of hydroclimate under the “natural ”and“anthropogenic ”condition and to address the decadal—centennial variability and mechanisms for predicting drought change in the future, understanding the decadal—centennial hydroclimate variability and mechanisms during the last millennium has attracted increasing attention.

Estimates for the climate of the last millennium are usually either based on proxy records obtained from natural archives and historical documents or on climate model simulations with a variety of climate forcing factors. Based on historical documents, tree rings, and speleothems, a number of high-resolution hydroclimate reconstructions over eastern China during the last millennium have been carried out in recent years ( Zhang, 2004 ; CNMA, 1981 ; Zheng et al., 2006 ; Cook et al.,2010 ; Yang and Tan, 2009 ; Man, 2012 ). Work to reconstruct the drought status over eastern China has revealed that severe large-scale droughts on decadal—centennial timescales have occurred many times during the last millennium ( Zhang, 2004 ; Zheng et al., 2006 ; Cook et al., 2010 ). Although paleoclimate proxy data provide a critical reference history for hydroclimate over eastern China, interpretation is challenged by considerable errors: different statistical methods used in the reconstructions lead to different results, non-climatic factors influence the proxies, and the poor spatial coverage of the climate proxies lead to errors in large-scale means ( Jansen et al., 2007 ; Jones and Mann, 2003 ;Matsikaris et al., 2015 ). Moreover, a lack of dynamic variables and complete physical constraints does not allow for diagnostic studies of past hydroclimate variability ( Steiger et al., 2018 ). However, global comprehensive datasets that are consistent with atmospheric dynamics can be provided by climate model simulations.

Based on modeling results, considerable efforts have been made to examine precipitation changes over eastern China and possible explanatory mechanisms during the last millennium ( Liu et al., 2011 ; Man et al.,2012 , 2014 ; Ning et al., 2017 , 2019 ; Shi et al., 2016 ; Peng et al., 2010 ,2014 , 2015 ; Peng, 2018 ; Yan et al., 2015 ; Zhou et al., 2020 ). These results show that changes in solar radiation, volcanic activity, the El Nino—Southern Oscillation (ENSO), Interdecadal Pacific Oscillation, North Atlantic Oscillation, and Atlantic Multidecadal Oscillation (AMO) have played important roles in precipitation over eastern China throughout the last millennium. Because of model bias ( Zhou et al., 2017 ), errors in the forcings and the uncertainties caused by the models of chaotic components of internal variability ( Peng et al., 2014 ), models cannot reproduce the exact variability to replicate the information obtained from proxy data, and the interpretation of such simulations is challenged. For example, the modeled linkage between ENSO and severe drought over eastern China during the last millennium remains to be revealed. Our previous work ( Peng et al., 2014 ) showed that a temporally consistent relationship between the drought and sea surface temperature (SST) pattern in the Pacific Ocean could not be found. Meanwhile, another modeling analysis, the ensemble mean of the CESM (Community Earth System Model)—LME (Last Millennium Ensemble) simulations, shows that ENSO plays an important role in precipitation variability over eastern China( Peng et al., 2019 ; Ning et al., 2019 ). It suggests a temporally consistent relationship between drought and negative phases of ENSO can be found in model data ( Peng et al., 2019 ; Ning et al., 2019 ). Thus, it is unclear if the underlying climate dynamics of such events are accurately produced in climate model simulations. This issue is due in part to a lack of constraints from the known proxy record ( Steiger et al., 2018 ).

There is growing interest in applying data assimilation to estimate pre-instrumental climate. Data assimilation provides a mathematical framework that combines the empirical information from proxy data with the representation of the dynamical processes that govern the climate system given by climate models ( Matsikaris et al., 2015 ). Therefore, data assimilation is both consistent with the model physics and with the empirical knowledge. It provides clear hypotheses that can be tested with additional model simulation and independent proxy data and checks the physical consistency of the proxy-based reconstructions.In this study, the Paleo Hydrodynamics Data Assimilation (PHYDA)product is used to elucidate more clearly the mechanisms of the decadal—centennial hydroclimate variability over eastern China during the last millennium.

2. Data and methods

The PHYDA includes three global variables gridded at a resolution of ～2°and eight climate indices at annual, summer (June—August, JJA),and winter (December—February) temporal resolutions over the past 2000 years ( Steiger et al., 2018 ). The oラine data assimilation approach was used in the PHYDA by optimally fusing proxy information with the dynamical constraints of climate models. The proxy database includes 2978 annually resolved proxies ( PAGES 2K Consortium, 2017 ;Breitenmoser et al., 2014 ; Steiger et al., 2018 ) and the simulations were run number 10 from the full-forcing ensemble simulations of CESM—LME (CESM—LME10) ( Otto—Bliesner and Brady, 2015 ). By using two skill metrics, the PHYDA data were evaluated against various 20th century instrumental data. The results indicated that the PHYDA data represent well the climate variables of temperature, Palmer drought severity index (PDSI), climate indices, and the location of the ITCZ (Intertropical Convergence Zone). Readers are referred to the supporting information and Steiger et al. (2018) for additional details about the PHYDA. Because few proxy data are available in the early part of the Common Era in the PHYDA, we focus on the hydroclimate variability over eastern China during the last millennium. To investigate its dynamics, we analyze the reconstructed PDSI, near-surface air temperature, the Ni?o3.4(5°S—5°N, 120°—170°W) SST index, the AMO index, and the location of the ITCZ over the Asian—Australian monsoon area (longitudinal range:95°—130°E) in JJA. The area-averaged PDSI and near-surface temperature were calculated using the PHYDA data for the region east of 105°E between 25°and 45°N.

Four independent reconstructions of hydroclimate over eastern China are also analyzed. Two are datasets of the dryness—wetness index (DWI) derived from Chinese historical documents and instrumental measurements, one being a proxy dataset of rainy season precipitation over eastern China from 1470 to 2000 AD ( CNMA, 1981 ; Zhang, 1988 )and the other comprising proxy data of 1500-year time series of regional DWI over eastern China (east of approximately 105°E; 25°—40°N) reconstructed by Zheng et al. (2006) . The third reconstruction is the Monsoon Asia Drought Atlas (MADA; Cook et al., 2010 ), which provides annual tree-ring drought reconstructions on a 2.5°×2.5° gridded network of summer PDSI data, and is also used to provide an investigation of hydroclimate changes over eastern China. Lastly, a composite 2000-year East Asian summer monsoon (EASM) index time series is used, synthesized based on the speleothem δO variations from Dongge, Heshang,and Huangye, which can reasonably describe the summer monsoon circulation ( Man, 2012 ).

3. Results

Fig. 1 (a) compares the hydroclimate variation over eastern China between the PHYDA and other proxy records during the last millennium. According to the PDSI series of the PHYDA, the characteristics of the centennial hydroclimate change over eastern China during the last millennium can be highlighted as follows: wet conditions prevail in the Medieval Climate Anomaly (MCA, 1000—1350 AD), followed by a drying trend during the Little Ice Age (LIA, ～1350—1850 AD), and then it was wet again after the 1930s. The comparison shows that similar features are generally observed in the PHYDA and other reconstructions —namely, the drying conditions over the early part of the LIA (1350s—1680s) and the industrial-era wetting. Differences also exist among the reconstructions —most noticeably, the drier period during the 18th—19th centuries (the latter part of the LIA) in the PHYDA is absent in most of the other reconstructions. Another period where the various reconstructions exhibit significant disagreement is during the MCA.

On decadal time scales, we focus on the droughts (see the supporting information for the definition of drought). As shown in Fig. 1 (a),all of the proxy data, including the PHYDA, show more drought events occurred in the early part of the LIA. The PHYDA shows that four severe decadal droughts occurred during this period, and most of them overlapped with the other proxy data —that is, the droughts of 1352—90 AD, 1445—98 AD, 1580—94 AD, and 1626—65 AD. Similar to the centennial hydroclimate change, droughts with the largest differences occurred during two periods, corresponding to the MCA and the 18th—19th centuries. The DWI from Zheng et al. (2006) identified four drought events as having occurred during the MCA, all which except the 1123—47 AD drought could not be observed in the PHYDA and other records. The PHYDA supports the drought of 1123—47 AD, albeit the drought’s duration was much longer. Interestingly, evidence from stalagmite records suggests that most severe flooding occurred over eastern China during this period. In the 18th and 19th centuries, the PHYDA and the treering records reveal several drought events occurred during this period,which could not be identified by speleothem-based paleoclimate record and the DWI.

To assess the possible causes of hydroclimate variability over eastern China in the PHYDA during the last millennium, we examine the Ni?o3.4 SST index, the AMO index, and the location of the ITCZ in summer ( Fig. 1 (b)). The comparison shows that the centennial dry—wet variation in the PHYDA after the 1350s is generally in better agreement with the AMO than the Ni?o3.4 SST index and the location of the ITCZ, with drier (wetter) conditions associated with the negative (positive) phase of the AMO. After the 1350s, the Pearson’s correlation coefficients are? 0.483 between the PDSI and the Ni?o3.4 SST index, 0.74 (significant at the 0.01 level) between the PDSI and AMO index, and ? 0.305 between the PDSI and the location of the ITCZ, indicating the linkage between the PDSI and AMO index on centennial time scales is more marked than that with ENSO and the location of the ITCZ. Previous works ( Man et al.,2012 ; Ottera et al., 2010 ) have revealed that both the North Atlantic SST and EASM precipitation show a strong low-frequency response to natural forcing, such as the varying solar activity. Conversely, the PHYDA shows that the solar forcing ( Schmidt and Jungclaus, 2012 ) probably has a minor agreement on the centennial scale with the AMO variability and PDSI variability over eastern China, with correlation coefficients of 0.294 between the solar forcing and the AMO and 0.288 between the solar forcing and the PDSI during the last millennium. This reflects a more coherent relationship between the AMO index and centennial variability of hydroclimate over eastern China. Meanwhile, during the MCA, the centennial hydroclimate changes in the PHYDA show more consistency with the location of the ITCZ.

Fig. 1. (a) Hydroclimatic changes over eastern China in the summer (JJA) PDSI index based on the PHYDA from Steiger et al. (2018) during the last millennium,in the composite EASM index based on the speleothem δ18 O variations from Man (2012) during the last millennium, in the PDSI index based on the availability of the tree-ring based MADA from Cook et al. (2010) during the last 700 years, and in the DWI based on Chinese historical documents and instrumental measurements from Zheng et al. (2006) during the last millennium and from CNMA (1981) during the last 530 year. The anomaly of summer near-surface air temperature in the PHYDA of Steiger et al. (2018) during the last millennium is shown by the cyan curve. The severe decadal droughts identified in each record are shown by the yellow bars. The gray band shows the 5th to 95th percentile range. (b) Climate records from the PHYDA of hydroclimatic changes in the PDSI index over eastern China,climate indices of Ni?o3.4, AMO, and the location of the ITCZ over the Asian—Australian monsoon area during the last millennium, and the difference in total solar irradiance (TSI) from the value of 1360 W m?2 since 1000 AD. The severe decadal droughts identified in the summer PDSI index of the PHYDA are shown as yellow bars. The 10-yr resolved standardized anomalies (black curve) are shown along with smoothed versions using a 100-yr FFT (red curve).

For the drought events, as shown by Fig. 1 (b), the PHYDA suggests El Ni?o—like mean states were sustained through most of the events —for example, the droughts of 1105—55 AD, 1352—90 AD, 1580—94 AD, and 1626—65 AD, which overlap with the droughts identified in the DWI,and the droughts that occurred during the 18th and 19th centuries. The PHYDA reveals that these droughts also coincide with the negative AMO state and the northward displacement of the ITCZ, except that there is an AMO neutral state during the drought of 1105—55 AD and there is little change in the ITCZ location during the drought of 1580—94 AD. The relationship between the drought of 1445—98 AD and SST anomalies in the tropical Pacific is complex. During the early phase of this drought, sustained La Ni?a—like conditions can be seen. We note that, during this period, two large Kuwae volcanic eruptions occurred, and both eruptions injected sulfur gases into the stratosphere with probable impacts on the global climate ( Cole-Dai and Ferris, 2013 ). The Kuwae eruption that took place in late 1452 AD or early 1453 AD was the largest stratospheric sulfate event during the last 700 years ( Gao et al., 2006 ). According to research by Ning et al. (2017) , significantly lower EASM precipitation and a robust La Ni?a response to large tropical eruptions on the interdecadal time scale could be observed during the last 110 years because of amplification through the negative AMO state caused by the large volcanic eruptions. Fig. 1 (b) shows that a negative AMO state coinciding with a robust La Ni?a and a northward displacement of the ITCZ can be observed during this period, indicating volcanic eruption may have played a role in the early phase of this event in the PHYDA. The influence of these two volcanic eruptions on hydroclimate over eastern China can also be observed in the tree-ring and stalagmite records ( Fig. 1 (a)).However, it could not be observed in the DWI from Zheng et al. (2006) .A shift toward El Ni?o—like conditions and a northward ITCZ shift since the 1480s coincide with more severe drought conditions since the 1480s in the drought of 1445—98 AD. The states of El Ni?o—like conditions and a northward ITCZ in the drought of 1445—98 AD after 1484 AD are similar to the conditions shown in the droughts of 1352—90 AD and 1626—65 AD, as well as the droughts that occurred during the 18th and 19th centuries. This indicates that the drought conditions from 1484 AD to 1498 AD were related to the El Ni?o—like conditions during this period. These drought conditions in the PHYDA linked to the El Ni?o overlap with the drought of 1479—96 AD identified in the DWI. The drought of 1580—94 AD, which also overlaps with the droughts identified in the DWI, occurred in active periods of La Ni?a—like mean states and negative AMO states. The states of these two climate indices in the drought of 1580—94 AD are similar to the conditions shown in the early phase of the drought from 1445 AD to 1498 AD. However, there is little change in the ITCZ location during this drought and no large eruptions have been detected in Antarctic ice-core records during the period of this drought ( Gao et al.,2008 ).

To check if the relationships between the hydroclimate change over eastern China and the climate indices are robust in the PHYDA, the regression patterns of PDSI anomalies against these three climate indices on centennial (Fig. S1) and decadal (Fig. S2) time scales are examined.According to these regression patterns, the negative phase of the AMO could have caused the drier conditions over eastern China and the location of the ITCZ was not significantly associated with the hydroclimate change over eastern China both on centennial and decadal time scales.Meanwhile, the regression patterns between the PDSI and the Ni?o3.4 SST index are different on these two different time scales. El Ni?o—like mean states could have caused the drier conditions over eastern China on decadal time scales, whereas the regression pattern on centennial timescales is characterized by an insignificant north—south dipole pattern. Thus, we conclude that the AMO variation may play an important role in the centennial hydroclimate change over eastern China and the negative phase of the AMO and the El Ni?o—like mean states may be the main contributors to the decadal droughts. To test the possible mechanisms for these connections between the hydroclimate over eastern China and these climate indices —for example, previous works have suggested the influence of the AMO on precipitation over eastern China acts chiefly via a Eurasian wave train emanating from the North Atlantic to China or the AMO—Northern Hemisphere teleconnection wave train ( Qian et al., 2014 ) —the related atmospheric circulation should be analyzed. Unfortunately, due to the lack of reconstructions of dynamic variables such as geopotential height and others, the specific processes and mechanisms in action during the AMO and ENSO phases remain to be revealed.

4. Discussion

The hydroclimatic variation on the centennial time scale and the decadal droughts identified in the PHYDA and other reconstructions show that the reconstructions depict a more homogeneous temporal pattern during the early part of the LIA. The decadal droughts that occurred during this period were linked to the El Ni?o—like mean states and the negative AMO state. These relationships resemble results based on instrumental data ( Zhou et al., 2009 ; Qian et al., 2014 ), indicating the relationships between the hydroclimatic change over eastern China and the AMO and ENSO are robust during the last millennium, at least during the early part of the LIA. However, the physical mechanism involved in this phenomenon remains unclear because of the lack of reconstructions of dynamic variables such as winds, sea level pressure, geopotential height etc., in the PHYDA. We note that the amplitudes of ENSO and the AMO are remarkably smaller during 1000—1500 AD than 1500—2000 AD. According to the general data assimilation equation as shown in the supporting information, the oラine data assimilation approach leads to a reconstruction that reverts to the prior ensemble, which is estimated by CESM—LME10 ( Tardif et al., 2019 ) in the limit of no proxy information. Therefore, it is possible that the larger amplitudes of ENSO and the AMO after 1500 AD can be attributed to improved skill over the model climatology because of the greater availability of proxy data during this period. The temporal evolution of influences from proxies should be further investigated in the future.

Differences also exist, particularly during the two periods of the MCA and the 18th—19th centuries. For the first period, due to the existence of many missing records in the DWI before 1470 AD, fewer proxy data being available in the early part of the last millennium in the PHYDA,and both drought events and the centennial hydroclimatic change over this period before the 1350s being inconsistent among reconstructions,there is large uncertainty in the hydroclimatic change in the MCA. For the period of the 18th—19th centuries, we hypothesize three reasons to explain the differences. Firstly, it is possible that the high temperatures during these two centuries may have played some role in the drying trend and drought events in the PHYDA and the tree-ring chronology reconstructions ( Fig. 1 (a)). Because the PDSI in the PHYDA and the treering chronology reconstructions are affected by a combination of temperature and precipitation, the PDSI may represent local combinations of dry/wet and cold/warm conditions, resulting from abnormally low precipitation and high temperature ( Yang and Shi, 2013 ). We also hypothesize that the droughts in the 18th—19th century over eastern China in the PHYDA may be influenced by the tree-ring records, because proxies over eastern China in the PHYDA include the tree-ring records. As discussed before, such influences should be further investigated. The third possibility is that the PHYDA overestimates the influence of the AMO and ENSO on the hydroclimate over eastern China during this period, as discussed before. It suggests that further improvements in the reconstructed hydroclimate over eastern China may be possible with a greater availability of proxy data and a careful selection of proxies over the region.

Funding

This work was supported by the National Natural Science Foundation of China [grant number 41605046 ].

Acknowledgments

We are grateful to all of the authors of the cited reconstructions for making their data available to the community. We would also like to thank Jingyun Zheng and Wenmin Man for providing their proxy data.We acknowledge the critical comments from the anonymous reviewers and editor. Thank Dr. Yongjie Huang (IAP/CAS) for providing map database ( https://github.com/PoloMela/NCL-Chinamap ).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.aosl.2021.100038 .