Causes of Mid-Pliocene Strengthened Summer and Weakened Winter Monsoons over East Asia

ZHANG Ran,JIANG Dabang,and ZHANG Zhongshi

1Climate Change Research Center,Chinese Academy of Sciences,Beijing 100029

2CAS Center for Excellence in Tibetan Plateau Earth Sciences,Beijing 100101

3Nansen–Zhu International Research Centre,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

4Bjerknes Centre for Climate Research,Uni Research,Bergen 5007,Norway

Causes of Mid-Pliocene Strengthened Summer and Weakened Winter Monsoons over East Asia

ZHANG Ran∗1,JIANG Dabang1,2,3,and ZHANG Zhongshi3,4

1Climate Change Research Center,Chinese Academy of Sciences,Beijing 100029

2CAS Center for Excellence in Tibetan Plateau Earth Sciences,Beijing 100101

3Nansen–Zhu International Research Centre,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

4Bjerknes Centre for Climate Research,Uni Research,Bergen 5007,Norway

The mid-Pliocene warm period was the most recent geological period in Earth’s history that featured long-term warming. Both geological evidence and model results indicate that East Asian summer winds(EASWs)strengthened in monsoonal China,and that East Asian winter winds(EAWWs)weakened in northern monsoonal China during this period,as compared to the pre-industrial period.However,the corresponding mechanisms are still unclear.In this paper,the results of a set of numerical simulations are reported to analyze the effects of changed boundary conditions on the mid-Pliocene East Asian monsoon climate,based on PRISM3(Pliocene Research Interpretation and Synoptic Mapping)palaeoenvironmental reconstruction.The model results showed that the combined changes of sea surface temperatures,atmospheric CO2concentration, and ice sheet extent were necessary to generate an overall warm climate on a large scale,and that these factors exerted the greatest effects on the strengthening of EASWs in monsoonal China.The orographic change produced signif i cant local warming and had the greatest effect on the weakening of EAWWs in northern monsoonal China in the mid-Pliocene.Thus, these two factors both had important but different effects on the monsoon change.In comparison,the effects of vegetational change on the strengthened EASWs and weakened EAWWs were relatively weak.The changed monsoon winds can be explained by a reorganization of the meridional temperature gradient and zonal thermal contrast.Moreover,the effect of orbital parameters cannot be ignored.Results showed that changes in orbital parameters could have markedly affected the EASWs and EAWWs,and caused signif i cant short-term oscillations in the mid-Pliocene monsoon climate in East Asia.

mid-Pliocene,East Asian monsoon,orographic change,orbital parameters

1.Introduction

The mid-Pliocenewarmperiod(mPWP,3.264–3.025Ma, 1 Ma=1 million years)was 1.84°C–3.60°C warmer than the pre-industrial period(Haywood et al.,2013b),with warming both in the ocean and on land.The causes of this warming included the increased atmospheric CO2concentrations,a reduced ice sheet in the polar regions,and changed orography and vegetation(Salzmann et al.,2008;Dowsett et al., 2010).Since the mPWP is the most recent geological period in Earth’s history that featured long-term warming,it has long been a focus for palaeoclimate modeling(e.g.,Chandler et al.,1994;Sloan et al.,1996;Haywood and Valdes,2004; Jiang et al.,2005;Yan et al.,2011;Jiang,2013;Sun et al., 2013;Zhang et al.,2013b;Zhangand Jiang,2014).Recently, the f i rst phase of the Pliocene Model IntercomparisonProject (PlioMIP)was initiated,with standardized designs for simulations(Haywood et al.,2010),to facilitate further model–model intercomparison.

The East Asian monsoon climate in the mPWP has also been widely studied(Jiang et al.,2005;Yan et al.,2012). In particular,since the initiation of PlioMIP,further understanding of the East Asian monsoon climate in the mPWP can be gained through model–model and model–data comparisons.Forexample,Zhanget al.(2013a)usedthePlioMIP simulations to investigate the regional climate in East Asia; and their model results showed that,in the mPWP compared to the pre-industrial period,the multi-model ensemble mean(MMM)indicates that the East Asian summer winds (EASWs)strengthened in monsoonal China,and the East Asian winter winds(EAWWs)weakened in northern monsoonal China.These model results are generally consistent with geological reconstructions(Ding et al.,2001;Xiong et al.,2001;Wan et al.,2007;Sun et al.,2008;Jiang and Ding, 2010).

However,the mechanisms responsible for these changed features of the monsoon climate remain unclear.Previous studies indicate that the uplift of the Tibetan Plateau(TP) and global cooling have both had a signif i cant impact on the change of the monsoon climate in East Asia since the mPWP (An et al.,2001;Zhang et al.,2001;Lu et al.,2010;Ge et al.,2013).Theoretically,the global cooling is closely related to changes in sea surface temperatures(SSTs),atmospheric CO2concentration and ice sheet extent,and affects vegetation.Whereas,the uplift of the TP also favors global cooling. Thus,the changes in these boundary conditions are potentially related to one another.However,we emphasize more the climate effects on a regional scale from the uplift of the TP,versus the effects on a large scale from global cooling. Seen in this way and based on mid-Pliocene palaeoenvironmental reconstructed boundary conditions including SSTs, atmospheric CO2concentrations,ice sheet extent,vegetationandorography,we determinetheboundaryconditionsresponsible for the mid-Pliocene stronger EASWs and weaker EAWWs and investigate whether the crucial factors are the same.It is hoped that the f i ndings will aid understanding of the climate effects of the uplift of the TP and global cooling in the East Asian monsoon climate since the mPWP.

To analyze the climate effects of reconstructed SSTs, an atmosphere-only general circulation model(GCM)was used in this study.The simulated SSTs from a coupled atmosphere–ocean GCM can over-or underestimate the reconstructed SSTs in certain regions(Dowsett et al.,2013; Salzmann et al.,2013),potentially affecting their role in the East Asian monsoon climate.Moreover,considering the changes of boundary conditions on a large scale,we treated the changes of SSTs,atmospheric CO2concentration,and ice sheet together(collectivelyabbreviatedas SCIS)as largescale changes.Previous studies show that the simulated climatic responses arising from the change in ice sheet extent are strong at high latitudes but relatively weak in East Asia (Jiangetal.,2005).Thus,theeffectsofthecombinedchanges of SCIS in East Asia possibly derive more from the changes of SSTs and atmospheric CO2concentration.For comparison,we treated the changes of orographyand vegetation separatelyto emphasizethe effects ona regionalscale in the East Asiancontinent.Althoughthechangesoforographyandvegetation also occur in other regions,the relevant changes in East Asia should have direct impacts in this region.In this way,the climate effects of the boundary conditions between the large and regional scale can be distinguished to a certain degree.

PrevioussimulationsfocusingontheEastAsianmonsoon climatein themid-Pliocenehavenotinvestigatedthe effectof changes in orbital parameters.The question of how changes in orbital parameters affect the East Asian monsoon climate in the mid-Pliocene therefore required further examination, and we address this aspect in the present paper.

The remainder of the paper is organized as follows.In section 2,we describe the model and experimental design.In section 3,the model results,including surface air temperature(SAT)and monsoon circulation,are presented.Section 4 presents the implications for paleoclimate evolution in East Asia and discusses the related uncertainties.Section 5 summarizes the study.

2.Model description and experimental design

2.1.Model description

The GCM used in this study is version 4 of the Community Atmosphere Model(CAM4)(Neale et al.,2013),developed at the National Center for Atmospheric Research (NCAR).The resolution of CAM4 used here is T31 in the horizontal direction,with approximately 3.75°in both latitude and longitude,and 26 layers in the vertical direction. Moreover,version 4 of the Community Land Model(CLM4) is also included(Lawrence et al.,2011),and CLM4 uses the same horizontal resolution as in CAM4.More information and validation results regarding CAM4 and CLM4 can be found in Shields et al.(2012)and Neale et al.(2013).

2.2.Experimental design and boundary conditions

Ten numerical experiments were performed,as listed in Table 1.The f i rst two experiments were the standard experiments for the pre-industrial(PI)and mPWP(MP),respectively.The boundary conditions were set following the PlioMIP experiment guidelines(Haywood et al.,2010),and the boundary conditions used were from the latest version of the PRISM(Pliocene Research Interpretation and Synoptic Mapping)palaeoenvironmental reconstruction,PRISM3 (Dowsett et al.,2010).The PI experiment used the default modern land/sea conf i guration,orography,ice sheet distribution,modern SSTs and sea ice fraction,and the modern vegetation providedby PRISM3.Atmosphericgreenhousegases were set to the pre-industrial values of 280 ppm for CO2, 270 ppb for N2O,760 ppb for CH4,and zero for CFCs.The solar constant was set to 1365 W m−2.Orbital eccentricity, obliquity and precession were set to 0.016724,23.446°and 102.04°(perihelion minus 180°),respectively.By comparison,in the MP experiment,the“alternate”boundary condi-tions from the PlioMIP,with unchanged land/sea conf i guration,were used.In the MP experiment compared to PI,the changed boundary conditions included the topography(Sohl et al.,2009),land cover(Salzmann et al.,2008),an increase of atmospheric CO2concentration(to 405 ppm),SSTs,and sea ice fraction(Dowsett and Robinson,2009)(Fig.1).The sea icefractionintheMP experimentwas basedontherebuilt SSTs,and the sea ice fraction where the SSTs were higher than−1.8°C was set to zero.The other boundary conditions were kept the same as in the PI experiment.

Table 1.Experimental design.“MP”stands for mPWP conditions, and“PI”stands for pre-industrial or modern conditions.

To distinguishthe individualeffects of differentboundary conditions on the mid-Pliocene East Asian monsoon climate (Table 1),six more experiments were designed to perform a full factor separation(Lunt et al.,2012).We use subscripts to indicate the different boundary conditions(“c”for combined changes of SCIS,“o”for orography,and“v”for vegetation). Thus,the factorization is as follows,using Taylor expansions (Lunt et al.,2012):

Moreover,based on the MP experiment,two further experiments were designed to explore the sensitivity of the East Asian monsoon climate to orbital extremes during the mPWP(Dolan et al.,2011).The orbital parameters in the MP NHmax experiment represented the point in time when the Northern Hemisphere’s summer(July)insolation at 65°N was at a maximum(at 3.037 Ma),and the orbital parameters in the MP NHmin representedthe point in time when the Northern Hemisphere’s summer insolation at 65°N was at a minimum(at 3.049 Ma).All experiments were integrated for 50 years,and all reached a quasi-equilibrium state within the f i rst 30 years.The climatological means from the last 20 years in each experiment were analyzed below.

3.Model results

3.1.Annual mean SAT

The climate was generally warmer and wetter in the mPWP in East Asia.In the MP experiment,compared to PI,the annual mean SAT generally increased in East Asia, with more warming in central-western China.In contrast,little cooling appeared at the southern margin of the Tibetan Plateau(Fig.2a).These results agree well with the MMM of model results from the PlioMIP(Zhang et al.,2013a).

After comparison,the combined changes of SCIS increased the annual mean SAT substantially,but the change of orography and vegetation only increased the annual mean SAT locally.The contribution of the combined changes of SCIS increased the SAT signif i cantly across East Asia,with an almost zonal distribution and larger warming occurring in the north(Fig.2b).In comparison,because of the nonuniform change of orography(Fig.1a)and the effect of the temperature lapse rate,the SAT change was not uniform(Fig. 2c).More warming appeared in central-western China,and cooling existed around this region(Fig.2c),due to the decreasedorographyin central-westernChinaandthe increased orography around this region in the mPWP compared to the pre-industrial period(Fig.1a).Furthermore,for the contribution of vegetational change,due to the large increase of surface albedo around Northeast China(Fig.1b),the SAT markedly decreased in this region(Fig.2d).On the whole, the combined changes of SCIS had substantial effects on the warming in East Asia,while the change of orography and vegetation only exerted local warming effects.

The changes in orbital parameters signif i cantly modulated the climate in East Asia.Due to the changed insolation(f i gurenotshown),thechangeoforbitalparametersfrom modern values to the values during 3.037 Ma and 3.049 Ma resulted in markedly different climate effects on the annual mean SAT in East Asia(Figs.2e and f,Table 2).Although the SAT in boreal summer(June,July and August;JJA)generallyincreasedinthe MP NHmaxexperimentanddecreased in the MP NHmin experiment,compared to PI in East Asia (Table 2),the changes in annual mean SAT manifested in a different way(Fig.2e vs.Fig.2f).

3.2.Boreal summer and winter monsoon circulation

The monsoon circulation was also greatly affected by the changes in boundary conditions.In the MP experiment compared to the PI experiment,in JJA,southwesterly winds largely strengthened in monsoonal China(Fig.3a);while in boreal winter(December,January and February;DJF),the conditions were more complex.Northeasterly wind anomalies appeared in southern monsoonal China,while southeasterly wind anomalies occurred in northern monsoonal China, and northwesterly wind anomalies appeared in Northeast

China(Fig.4a).These results indicate that the mid-Pliocene EASWs largely strengthened in monsoonal China,and the EAWWs slightly weakened in northern monsoonal China. Like the changes in SAT,these characteristics also generally agree with the MMM of the PlioMIP models(Zhang et al., 2013a).

Table 2.The differences of regionally averaged SAT(units:°C)and meridional 850 hPa winds(units:m s−1)between experiments and the contribution of the combined changes of SCIS,orographic change,and vegetational change in the MP experiment compared to the PI experiment.

The combined changes of SCIS exerted the greatest effect on strengthening the EASWs in monsoonal China,but the changeof orographyhad the greatest effect on weakening of the EAWWs in northern monsoonal China in the mPWP. In detail,the contributionsof the orographicand vegetational changes were relatively weak for the mid-Pliocene strengthened EASWs in monsoonal China(Figs.3c and d;Table 2), while the combined changes of SCIS exerted the greatest effect(Fig.3b;Table 2).Whereas for EAWWs,the combined changes of SCIS weakened EAWWs mainly in and around Northeast China(Fig.4b),and the vegetational change had a large effect on the strengthened EAWWs in Northeast China (Fig.4d).In contrast,the change of orographyhad the greatest effect on weakening of EAWWs in northern monsoonal China(Fig.4c;Table 2).Thus,the combined changes of SCIS and orographic change both played important roles in the change of East Asian monsoon winds.

Moreover,the changes in orbital parameters also further modulated the change of monsoon winds.Stronger southwesterlywindanomaliesoccurredin monsoonalChinainJJA (Fig.3e vs.Fig.3a),and weaker southeasterly wind anomalies appeared in northern monsoonal China in DJF(Fig.4e vs.Fig.4a).By comparison,the changes in orbital parameters represented in Figs.3f and 4f had an opposite effect,due to the opposite change in insolation,with weaker southwesterly wind anomalies in monsoonal China in JJA(Fig.3f vs. Fig.3a)but stronger southeasterly wind anomalies in northern monsoonal China in DJF(Fig.4f vs.Fig.4a).Thus,changes in orbital parameters could have markedly affected the monsoon winds in the mid-Pliocene.

Further investigations showed that the changed monsoon winds could be explained by a reorganization of the meridional temperature gradient and the zonal land–sea thermal contrast(Figs.5 and 6).In JJA in the MP and MP NHmax experiments,compared to PI,and also the contribution of the combined changes of SCIS(Figs.5a,b and e),the SAT generally increased more in the middle latitudes than the low latitudes(Fig.7a).This induced a decreased meridional temperature gradient,and also the anomalous southerly winds in East Asia(Figs.3a,b and e),to compensate for the warming-induced loss of atmospheric mass.Meanwhile,the SAT changes were not evenly distributed between the land and ocean,and the SAT increased more over the East Asian continent than the adjacent oceans(Fig.7b),especially in the MP NHmaxexperimentcomparedto the PI experiment.As a result,the zonal land–sea thermal contrast was enhanced and the correspondingland–sea air pressure gradient was also increased(f i gurenotshown),strengtheningthesoutherlywinds in East Asia.By comparison,the increased meridional temperature gradient and weakenedland–sea thermal contrast,in particular in the MP NHmin experiment compared to the PI experiment(Figs.7a and b;Table 2),hindered the strengthening of EASWs.

As for JJA,the changeof monsoonwinds in DJF was also related to the change of the meridional temperature gradient and the zonal thermal contrast.In the MP and MP NHmin experiments,compared to the PI experiment,the decreased monsoon winds in northern monsoonal China were more closely related to the increased SAT(Figs.6a and f)and the corresponding weakened surface air pressure in northwestern China(f i gure not shown).This led to a decrease in the meridional temperature gradient and also a weakening of the zonal thermal contrast in East Asia(Figs.7c and d).For the contribution of orographic change,the change of meridional temperature gradient was weak(Fig.7c),and thus the southeasterly wind anomalies in northern monsoonal China weremore related to the increased SAT in northwestern China and the resultant weakened zonal thermal contrast(Figs.7d and 6c).In addition,this warming was further enhanced in the MP and MP NHmin experiments(Fig.7d).

4.Implications for paleoclimate evolution in East Asia and related uncertainties

The combined changes of SCIS and orographic change from the mPWP to the present have clearly had different effects on the evolution of East Asian monsoon climate.The model results show that,in the mPWP compared to the preindustrial,the EASWs strengthened in monsoonal China and the EAWWs weakened in northern monsoonal China(Zhang et al.,2013a).These results are consistent with existing geological evidence(Ding et al.,2001;Xiong et al.,2001; Wan et al.,2007;Sun et al.,2008;Jiang and Ding,2010). Our analysis indicates that the reason for the strengthened EASWs and weakened EAWWs was the changes in different boundary conditions.That is,the combined changes of SCIS largely contributed to the strengthened EASWs in the mid-Pliocene through a decreased meridional temperature gradient and a strengthenedzonal land–seathermal contrast;while the orographicchange had an important role in the decreased EAWWs in northern monsoonal China,which derived more from the increased SAT in northwestern China and the resultant weakened zonal thermal contrast.It should be noted that the simulated increased SAT in northwestern China was likely attributable more to the lowered orography of the TP in the mPWP compared to the pre-industrialperiod(Fig.1a). The lowered orography of the TP is consistent with geological evidence,which indicates that several accelerated rises of the northernTP haveoccurredsince the mid-Pliocene(Zheng et al.,2000;Fang et al.,2005;Li et al.,2014).Thus,in turn, from the mPWP to the present,we can see that the EASWs have weakened in monsoonal China and the EAWWs have strengthened in northern monsoonal China.Furthermore,the combined changes of SCIS,mainly representing global cool-ing on a large scale,are more likely to have been responsible for the weakened EASWs,while regional tectonic activity(including the uplift of the northern TP)has played an important role in the strengthening of the EAWWs(Zhang et al.,2015).Our results indicate that the two factors could both haveimportantbutdifferentclimateeffectsforEastAsianclimate evolution on the geologicaltime scale,in contrast to the notion of excessive emphasis on one aspect in this process.

The mPWP was verylong(～0.24Ma)andcontainedseveral orbital-scale cycles(Willeit et al.,2013).The model results in the present study show that changes in orbitalscale cycles may signif i cantly modulate the evolution of East Asian monsoonclimate.Changes in orbital parameters could markedly affect SAT,precipitation,and also East Asian monsoon winds.Thus,it is understandable that different proxy data,indicating different climate information,coexist together.Moreover,orbital-scale cycles could further modulate the impact of orographic change and the combined changes of SCIS on the evolution of East Asian monsoon climate.

The uncertainties in boundary conditions could have affected the model results.For example,a large discrepancy was found in the simulated East Asian monsoon climate between the model results based on reconstructed SSTs and those based on simulated SSTs(Zhang et al.,2013a).The reasons might derive both from the simulated and reconstructed SSTs.Even so,the inconsistency between the simulatedandreconstructedSSTs(Dowsettet al.,2013;Salzmann et al.,2013)is more likely related to the time slab nature of the proxydata,representativeof an average of multiple warm climates,while the model results only ref l ect a climate state based on constant external forcing(Haywood et al.,2013a). Further effort is needed to reduce the uncertainties in boundary conditions.

More experiments are needed.All the simulations in this study were performed using an atmosphere-only model,and hence f i xed SSTs.Even so,along with changes in orbital parameters,if using a coupled atmosphere–ocean GCM,the simulated SSTs may also change,further affecting the land–sea thermal contrast,and in turn,the East Asian monsoon climate.Thus,more experiments are required to further understand the climate effects of changed orbital parameters. Besides,because of the different physical processes and pa-rameterizations(Zhang et al.,2013a),the relative effects of orographic change compared to other forcings may be different in other GCMs,and thus more experiments with other GCMs are needed.

5.Summary

Based on the PRISM3 palaeoenvironmental reconstruction,the causesofthechangesin themid-PlioceneEast Asian monsoon climate are analyzed in this paper.The model results show that the combined changes of SCIS exerted the greatest effect on the strengthening of EASWs in monsoonal China,and the changes of orography had the greatest effect on the weakening of EAWWs in northern monsoonal China, in the mPWP,as compared to the pre-industrial period.The reorganization of the meridional temperature gradient and zonal thermal contrast are revealed as the underlying mechanisms for the changedmonsoonwinds.Moreover,changes in orbital parameters could further markedly modulate the East Asian monsoon climate.

Acknowledgements.This study was supported by the Strategic Priority Research Program(Grant No.XDB03020602)of the Chinese Academy of Sciences,and by the National Natural Science Foundation of China(Grant Nos.41175072 and 41305073).

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(Received 23 August 2014;revised 8 November 2014;accepted 21 November 2014)

∗Corresponding author:ZHANG Ran Email:zhangran@mail.iap.ac.cn

©Institute of Atmospheric Physics/Chinese Academy of Sciences,and Science Press and Springer-Verlag Berlin Heidelberg 2015