Natural and human-induced changes in summer climate over the East Asian monsoon region in the last half century:A review

State Key Laboratory of Severe Weather(LaSW),Chinese Academy of Meteorological Sciences,Beijing 100081,China

Natural and human-induced changes in summer climate over the East Asian monsoon region in the last half century:A review

ZHANG Ren-He

State Key Laboratory of Severe Weather(LaSW),Chinese Academy of Meteorological Sciences,Beijing 100081,China

In the last half century,a signifcant warming trend occurred in summer over eastern China in the East Asian monsoon region.However,there were no consistent trends with respect to the intensity of the East Asian summer monsoon(EASM)or the amount of summer rainfall averaged over eastern China.Both of the EASM and summer rainfall exhibited clear decadal variations.Obvious decadal shifts of EASM occurred around the mid-and late 1970s,the late 1980s and the early 1990s,and the late 1990s and early 2000s,respectively.Summer rainfall over eastern China exhibited a change in spatial distribution in the decadal timescale,in response to the decadal shifts of EASM.From the mid-and late 1970s to the late 1980s and the early 1990s,there was a meridional tri-polar rainfall distribution anomaly with more rainfall over the Yangtze River valley and less rainfall in North and South China;but in the period from the early 1990s to the late 1990s and the early 2000s the tri-polar distribution changed to a dipolar one,with more rainfall appearing over southern China south to the Yangtze River valley and less rainfall in North China. However,from the early 2000s to the late 2000s,the Yangtze River valley received less rainfall.

East Asian summer monsoon;Summer rainfall over eastern China;Natural change;Human-induced change;The last half century

1.Introduction

The eastern region of China is affected by the East Asian monsoon;the area to the east of 100°E is usually referred to as the East Asian monsoon region(Tao and Chen,1987;Zhang et al.,1996;Wang et al.,2004).Over eastern China in the East Asian monsoon region,rainfall mainly occurs in the summer season(June,July and August).Based on the climatological mean of 1951—2014 observed from 160 meteorological stations over eastern China to the east of 100°E, summer rainfall accounts for more than half(52%)of the total annual rainfall(Zhang,2015).The distribution and the amount of summer rainfall over eastern China are mainly controlled by the intensity of the East Asian summer monsoon(EASM). A strong EASM corresponds to more rainfall over northern China,and a weak EASM is associated with more rainfall over the Yangtze—Huaihe River valley(Ding,1994).The temporal variations of EASM and summer rainfall over the East Asianmonsoon region are characterized by clear features of multitimescales ranging from synoptic to interdecadal timescales (Wang et al.,2005;Huang et al.,2012;Qin et al.,2012).

Eastern China is one of the regions in the world with the densest population and fastest economic development in recent decades.Consequently,regional summer climate is being affected by both internal natural factors within the climate system through ocean—land—atmosphere interactions and external forcing arising from anthropogenic factors such as greenhouse gases(GHGs),aerosols,and land usage/cover changes(LUCC).Therefore,changes in EASM and summer rainfall resulting from natural and anthropogenic factors have been a research focus in recent years.Finding out what roles natural and anthropogenic factors play in infuencing EASM and summer rainfall is crucial for understanding East Asian climate change.

The present study provides a comprehensive review on the up-to-date researches on the changes in EASM and summer rainfall over eastern China as well as their causes in the last half century.Section 2 presents the changing features of EASM and summer rainfall over eastern China.The effects of natural and anthropogenic factors are addressed in sections 3 and 4,respectively.Concluding remarks are given in section 5,in which the unsolved issues concerning the changes in EASM and summer rainfall over eastern China are discussed.

2.Changing features of EASM and summer rainfall

Under the global warming background,eastern China(east of 100°E)has also experienced an signifcant increase in summer air temperature,with an increasing trend of 0.1°C per decade between 1951 and 2014,based on observations from 160 meteorological stations in China(Zhang,2015).A lot of studies have been done on the changing features of East Asian summer climate,which have been well documented(Zhou et al.,2009a;Zhang et al.,2013;Hsu et al.,2014;Zhang, 2015).A declining trend in the intensity of EASM was observed since the mid-1960s,with decreased rainfall in northern China and increased rainfall in the Yangtze—Huaihe River basin(Yan et al.,1990;Huang et al.,1999;Wang,2001; Gong and Ho,2002;Hu et al.,2003;Yu et al.,2004;Jiang and Wang,2005).However,a decadal climate shift of EASM occurred in the late 1980s(Zhang et al.,2008).It was reported that a recovery of the declining trend occurred in the early 1990s and,since then,the EASM has become strengthened with enhanced rainfall in the Huaihe River valley(Liu et al., 2012).After the early 1990s,South China experienced more rainfall(Kwon et al.,2005;Zhang et al.,2008;Ding et al., 2008;Wu et al.,2010;Huang et al.,2011;Zhang et al., 2013),and between the late 1990s and the late 2010s,rainfall decreased over the lower—middle reaches of the Yangtze River(Zhang et al.,2013;Xu et al.,2015).

As mentioned above,both EASM and summer rainfall over eastern China exhibited strong decadal variability.By using an index of the western North Pacifc—East Asian summer monsoon(WNP—EASM)(Wang et al.,2001),Zhang et al. (2013)demonstrated that the WNP—EASM index varied with a pronounced decadal variability in the period from 1958 to 2011.Corresponding to the decadal variation of the WNP—EASM index,the summer rainfall averaged over eastern China showed no trend in recent decades,but exhibited a change in its spatial distribution(Li et al.,2010b;Zhang etal.,2013).Compared to 1958—1974,more rainfall appeared around the middle and lower reaches of the Yangtze River valley in 1975—1989(Fig.1a),and moved southward to the south of the Yangtze River valley in 1990—2000(Fig.1b). Relative to the period of 1990—2000,the amount of rainfall increased in both the southernmost area in China and around the north of the Yangtze River valley,and declined in the Yangtze River valley in 2001—2008(Fig.1c).

Fig.1.Averaged summer rainfall differences,(a)1975—1989 minus 1958—1974,(b)1990—2000 minus 1975—1989,and(c)2001—2008 minus 1990—2000.The dotted and solid lines represent the negative and positive anomalies,respectively.The line interval is 30 mm.The thick line is the 0 isoline.Differences larger than 30 mm is shaded(Zhang et al.,2013).

It seems that the so called south food and north drought summer climate anomaly in eastern China(i.e.,more rainfall in southern China and less rainfall in northern China)(Yu and Zhou,2007),existed only from the mid-1970s to the late 1990s,during which the distribution of rainfall anomalies was also different.From the mid-1970s to the late 1980s,there wasa meridional tri-polar distribution of rainfall anomalies,with more rainfall over the Yangtze River valley and less rainfall in North and South China.However,from the early 1990s to the late 1990s the tri-polar distribution changed to a dipolar one, with more rainfall occurring to the south of the Yangtze River valley and less rainfall to the north.The change in the meridional distribution pattern of summer rainfall anomalies from tri-pole to dipole matches the fndings by Ding et al. (2008)and Huang et al.(2011).However,between the early 2000s and the late 2000s,the south food and north drought phenomenon may have not persisted because less rainfall occurred over the Yangtze River valley.Such change was also reported by Xu et al.(2015),who claimed that another decadal shift of summer rainfall occurred in China in the late 1990s. Here it is clearly shown that accompanying the rising trend of summer air temperature over eastern China,the intensity of EASM and summer rainfall over eastern China did not show a consistent weakening or strengthening trend,but exhibited a decadal variation,with a change in spatial distribution of summer rainfall anomalies.

3.Natural factors affecting EASM and summer rainfall

The changes in EASM and summer rainfall over eastern China in the last half century are signifcantly affected by a range of natural factors related to the underlying land and ocean processes,the coupled air—sea system,and uneven thermal forcing over the Asian continent.The changes in the thermal states of sea surface temperature(SST)and land conditions as well as uneven thermal forcing over the Asian continent can exert signifcant infuence on the atmosphere by altering the general atmospheric circulations,which in turn affect regional changes in EASM and summer rainfall over eastern China.

3.1.Snow cover and Arctic sea ice

The winter snow cover over the Tibetan Plateau showed a remarkable transition from poor to rich snow covers at the end of the 1970s,which corresponded well to the transition from a drought to a rainy period in the Yangtze—Huaihe River valleys (Chen and Wu,2000).The decadal increase of snow depth over the Tibetan Plateau during March—April is consistent with more summer rainfall over the Yangtze River valley,and less in the southeast coast of China(Zhang et al.,2004).Zhao et al.(2010a)also suggested that in winter and spring,the snow cover over the Tibetan Plateau had an increasing trend in the period of 1960—2001,which weakened the EASM and resulted in an increase of the summer rainfall over the Yangtze River valley.The observed EASM and summer rainfall regime shifts in the late 1970s and in the early 1990s were in response to the increases in the snow cover in winter and spring over the Tibetan Plateau,respectively(Ding et al.,2009).Besides the snow cover over the Tibetan Plateau,the decadal variation of the spring(March,April and May)snow cover over the Eurasian continent also had a major impact on the summer rainfall in China.In the late 1980s,there was a decadal shift of spring Eurasian snow cover from more to less,which corresponded well to the increase of summer rainfall in southern China(Zhang et al.,2008;Wu et al.,2009a).

Wu et al.(2009b,2009c)found a coherent decadal variation of Arctic sea ice concentration(SIC)with that of summer rainfall in China,and both of them had the decadal phase shifts around the late 1970s and the early 1990s, respectively.The negative phases of the decadal variation in spring SIC were in 1968—1978 and 1993—2005,with a positive phase in between.In the positive phase,more rainfall appeared in northeastern and central China between the Yangtze River and the Yellow River,and less rainfall over North and South China.In the negative phase,there was an opposite spatial distribution of summer rainfall anomalies to that observed in the positive phase.It showed that in the decadal timescale a decreased(increased)spring SIC in the Arctic Ocean corresponds to an increased(decreased)summer rainfall in Northeast China and central China between the Yangtze River and the Yellow River;and decreased (increased)rainfall in North and South China,which corresponded well with the decadal shifts of summer rainfall anomalies over eastern China in the late 1970s and the early 1990s.

Eurasian snow cover and the Arctic sea ice in spring impact on the summer climate over eastern China in the decadal timescale by infuencing the atmospheric circulation.The above mentioned researches demonstrated that the atmospheric circulation anomalies caused either by snow cover or sea ice in spring can persist to summer,which exert signifcant effect on the summer climate over eastern China.Besides their individual impacts,the combined impacts of both spring Arctic sea ice and Eurasian snow cover may better explain their linkage to the summer climate over eastern China(Wu et al.,2009c).

3.2.Sea surface temperature(SST)

The El Ni~no/Southern Oscillation(ENSO)has a signifcant effect on EASM and summer rainfall over eastern China not only in the interannual timescale(Huang and Wu,1989; Zhang et al.,1996,1999;Wang et al.,2000;Huang et al., 2004;Karori et al.,2013)but also in the decadal timescale (Zhang et al.,1997;Huang et al.,1999).Huang et al.(1999) proposed that the decadal changes of summer rainfall over eastern China are closely associated with the phase of the decadal ENSO cycle that appears over the central and eastern equatorial Pacifc.The SST anomalies(SSTA)in central and eastern equatorial Pacifc were above normal from the middle of the 1950s to the late 1960s,below normal from the early to the late 1970s,and above normal again from the late 1970s to the late 1980s.In the decaying phase of the decadal El Ni~no event from the middle of the 1960s to the mid-1970s,more summer rainfall occurred in northern and southern China and less in the Yangtze and Huaihe River valleys,while an oppositedistribution ofthe summerrainfallanomalies occurred in response to the developing phase of the decadal El Ni~no event from the middle of the 1970s to the late 1980s.

The summer SST in the western North Pacifc(0°—40°N and 100°—180°E)showed a clear decadal shift in the late 1980s,with an increased SST after the late 1980s(Wu and Zhang,2007;Zhang et al.,2008).It was found that the decadal shift of SST in the western North Pacifc is closely related to the increase of summer rainfall over southern China that occurred in the late 1980s(Zhang et al.,2008).Han and Zhang(2009)demonstrated that the dipolar pattern of summer rainfall anomalies over eastern China can be well reproduced by the second leading mode of the empirical orthogonal function(EOF2).They revealed that the dipolar mode change in the meridional pattern of summer rainfall over eastern China in the late 1980s may be infuenced by the increased SST in the tropical Indian Ocean—western Pacifc area(IWP). The dipolar mode of summer rainfall is signifcantly correlated with the SST in IWP(Fig.2a).The increased SST in IWP in the decadal timescale(Fig.2b)can exert an effect on the EASM and summerrainfallby stimulating the East Asian—Pacifc teleconnection(EAP)pattern(Huang and Sun, 1992)(Fig.2c).Zhang et al.(2008)pointed out that the decadal climate shift in the late 1980s was associated with a westward extension of the western Pacifc subtropical high (WPSH),which can be clearly observed in Fig.2c.

The SST in IWP has increased since the late 1970s (Webster et al.,1999).The forcing of the increased SST in the IWP is associated with a westward extension of WPSH,which is a possible reason for the changes in EASM and summer rainfall in the late 1970s when EASM weakened and more rainfall occurred over the lower reach of the Yangtze River valley(Zhou et al.,2009b).Huang et al.(2010)found a strengthened impact of the Indian Ocean basin mode of SSTon the western North Pacifc anticyclone after the late 1970s in summer,indicating an enhanced connection between the Indian Ocean SSTand EASM since then.The intensifed western North Pacifc anticyclone can strengthen the WPSH,favoring more water vapor transport to the lower reach of the Yangtze River valley and thus more rainfall over the region(Zhang, 2001).The warming SST in the Indian Ocean is also associated with the southward shift of the South Asian High(Wei et al.,2012;Qu and Huang,2012),which is closely related to the decreased summer rainfall over North China and increased rainfall in the lower reach of the Yangtze River valley(Wei et al.,2012).Zuo et al.(2013)demonstrated that the relationship between the Indian Ocean SST and Asian summer monsoon has become much stronger after the late 1990s,which is in consistency with the decadal shift of summer rainfall anomalies over eastern China in the early 2000s.

The impacts of SSTA in tropics on the EASM are through altering convections over tropical Indian Ocean(Zhang,2001) and over tropical western Pacifc(Huang and Sun,1992; Zhang et al.,1996;Lu,2001).The convective heating anomalies associated with SSTA exert major infuence on the circulations over East Asia.Some recent researches have revealed that the convective anomalies over Indian Ocean and tropical North Pacifc are not independent,and there exists a cross-basin Indo—Pacifc convection oscillation(IPCO)in summer(Li et al.,2013).The combined effects of convective anomalies over Indian Ocean and tropical North Pacifc through IPCO are closely related to the variations of the circulations over East Asia and summer rainfall in the Yangtze River valley(Zheng et al.,2014).

3.3.Ocean—atmospheric coupled oscillation

There is a close relationship between the PDO(Mantua et al.,1997)and the decadal variation of the distribution pattern of summer rainfall anomalies over eastern China and EASM circulations(Zhang et al.,2007).The decadal variations of dry/wet in North China are well linked with PDO(Ma and Shao,2006;Ma,2007).Deng et al.(2009)found that the decadal shift of the distribution pattern of summer rainfall anomalies over eastern China that occurred in the late 1970s may be infuenced by the transition of the PDO from its negative to positive phase.Qian and Zhou(2014)analyzed the composites of summer rainfall anomalies in China for the negative PDO phase (1946—1976)and positive phase (1977—2002),respectively.It is illustrated that a pattern of more rainfall in the middle and lower reaches of the Yangtze River valley and less rainfall in North China existed in the positive PDO phase,but a reversed pattern was observed in the negative PDO phase.These observations demonstrated that the decadal shift of EASM and summer rainfall over eastern China in the late 1970s was related with the PDO phase transition.Xu et al.(2015)found that the decadal shift of East Asian summer climate around the late 1990s may also be caused by the PDO phase shift.Li et al.(2010a)showed that the observed decadal relationship between PDO and EASM can be well reproduced in numerical models.They suggested that the association of EASM with PDO largely resulted from the decadal variability in tropical oceans,because both PDO and EASM are signifcantly infuenced by the tropical decadal variability.

Zhao and Zhang(2006)found a zonal dipolar pattern of the sea level pressure between Asian continent and North Pacifc, which refects the coupling relationship between the Asian Low and the WPSH in summer.Such dipolar pattern of sea level pressure can also be detected in the summer mean tropospheric eddy temperature,and the zonal teleconnection pattern of the eddy temperature over the extratropical Asian—Pacifc region was referred to as the Asian—Pacifc Oscillation(APO)(Zhao et al.,2007,2010b).The APO represents an out-of-phase relationship in variation of the eddy temperature between Asia and North Pacifc and is associated with the out-of-phase relationship in atmospheric heating.By defning an APO index as the difference between the tropospheric eddy temperatures in subtropics over Asian continent and North Pacifc,Zhao et al.(2007)found that the APO has major impact on the summer climate over East Asia and the APO index is well related with the tri-polar distribution of summer rainfall anomalies.The APO index showed a clear decadal variation.In the period of 1958—2001,the high indexes frequently occurred before the middle 1970s and the low indexes afterward,which demonstrate the possible effect of APO on the decadal shift of summer rainfall anomalies in the mid-1970s.

3.4.Uneven thermal forcing over the Asian continent

Fig.2.(a)Correlation coeffcients between SST and the EOF2 time series of summer rainfall.The dark and light shadings indicate values above 95%and 90% signifcant confdence level,respectively.(b)Time series of the SSTA averaged over the signifcant correlation area shown in(a)where correlation coeffcients are greater than 0.3.(c)Regressed 500 hPa geopotential heights(contours)and 850-hPa winds(vectors)against the time series show in(b).The dark and light shadings indicate the areas at 95%and 90%signifcant confdence level,respectively,for geopotential heights(Han and Zhang,2009).

The East Asian summer monsoon is greatly affected by the thermal forcing over the Tibetan Plateau(Ye and Wu,1998; Wu et al.,1997,2012a;Zuo et al.,2011:Wu et al.,2012a, 2012b;Zhang et al.,2012),where a stronger warming has been observed in recent decades(Liu and Chen,2000;Zhou and Zhang,2005;Duan et al.,2006;Zhang and Zhou,2009).Duan et al.(2013)pointed out that both spring sensible heat fux and snow depth over the Tibetan Plateau decreased over the period of 1980—2008,with the trend in sensible heat fux being more signifcant than that in snow depth.They proposed that the sensible heat fux represents the primary and most signifcant change in the Tibetan Plateau thermal forcing and can persist from spring to summer due to the positive feedback between diabatic heating and local circulations.It was found that the decrease of the sensible heat fux in the period of 1980—2008 was closely related with the changes in EASM and summer rainfall over eastern China.Accompanying the weakening of the sensible heat fux during this period,both observations and model results showed that the EASM weakened;the increasing trends in summer rainfall occurred over southern China and between the Yellow River and Huaihe River,and decreasing trends over northern and northeastern China and along the middle and lower reaches of the Yangtze River.

Zuo et al.(2012)found that,although the surface temperature over Asia has increased,a relatively weaker warming in Asia appeared over the region 15°—45°N,60°—120°E, compared to the surrounding regions in the period of 1950—2010.The landmass over this Asian region became a relative heat sink because of the larger warming in other regions.The vertically integrated summer tropospheric temperature in this region decreased after the mid-1990s rather than increasing as showed in other regions.The relative cooling over the Asian region can weaken the Asian Low,resulting in the weakening of East Asian summer monsoon.Consequently, more rainfall occurred in the south of the Yangtze—Huaihe River basin and less in the north.However,Zhu et al.(2012) found that the warming in the northern Asia around the Lake Baikal in the area 45°—65°N,80°—130°E is crucial for the weakening of EASM between 1954 and 2010.The increase of the summer surface air temperature around the Lake Baikal region can induce an anomalous low-level anticyclone,leading to the northeasterly prevailing over northern East Asia and a weakened southwesterly monsoon winds,and drier climate in this region.

The above researches revealed that the warming over the Asian continent is unevenly distributed.The uneven distribution of the warming may cause decadal changes in EASM and summer rainfall over eastern China.However,the physical reasons for the uneven distribution of the warming over the Asian continent are still unknown.

4.Anthropogenic factors affecting EASM and summer rainfall

It has been demonstrated that human activities have infuenced the Earth's climate signifcantly(IPCC,2013).An et al. (2015)comprehensively reviewed the studies on global monsoon dynamics and climate change,and pointed out that natural processes and anthropogenic impacts are of great signifcance to the understanding of monsoon behavior.Since East Asia is one of the most populated regions in the world and has experienced the fastest economic growth in recent decades,it is imperative that the effects of anthropogenic factors be taken into account when changes in EASM and summer rainfall in the last half century are considered.

4.1.Effects of GHGs and aerosols

By using the regional climate model RegCM4.0,and considering only the effect of GHGs on climate change over China in recent decades,Zhang et al.(2015)found that the increase in air temperature over China during 1961—2005 was mainly caused by GHG emissions.They concluded that the south food and north drought pattern of summer rainfall anomalies over eastern China may be largely due to natural climate variability,and that the presence of GHG emissions weakens the strength of this pattern to some extent.

Based on the results of 17 climate models provided by the ffth phase of the Coupled Model Intercomparison Project (CMIP5),Song et al.(2014)investigated the roles played by GHGs and aerosols in summer climate change over East Asia,respectively.Corresponding to increases in GHGs (Fig.3a),the southwesterly winds strengthen over East Asia in the lower troposphere,indicating an intensifcation of EASM.However,as the presence of aerosols increased (Fig.3b),the northeasterly winds prevailed over East Asia, suggesting that enhanced aerosols over East Asia weaken the EASM.It has been proposed that increased aerosol concentrations derived from human activities in East Asia could be a possible cause of the decadal climate shift in the distribution pattern of summer rainfall anomalies across East China, thereby forming the south food and north drought in the late 1970s(Xu,2001;Menon et al.,2002).However,by examining the results of six climate models,Jiang and Wang (2005)showed that the combined effects of anthropogenic GHGs and aerosols may not explain the change in EASM in recent decades,and attributed it to the changes in natural processes.

As shown in Fig.3,the GHGs and aerosols have opposite effects on the EASM.The inverse effects are physically understandable.The land—ocean thermal contrast is a basic reason for monsoon formation.The stronger heating as a result of GHGs on land can enlarge the land—ocean thermal contrast, leading to the EASM being strengthened.On the contrary,the cooling effect of aerosols over the East Asian continent reduces the thermal contrast,resulting in a weakened EASM.It is not surprising that diverse results may be obtained from model results to describe the impacts of anthropogenic factors of GHGs and aerosols.The effects of GHGs and aerosols on EASM should depend on their relative importance.It seems that if GHGs plays a major role,the EASM may strengthen (Kimoto,2005;Song et al.,2014;Zhang et al.,2015),whereas the EASM could weaken if the effect of aerosols are considered to be more important(Menon et al.,2002;Liu et al., 2009;Song et al.,2014).Little impact on EASM due to the combined presence of GHGs and aerosols(Jiang and Wang, 2005;Li et al.,2010a)may result from a comparable contribution from both GHGs and aerosols,which would counteract the effects from each other.

4.2.Effect of LUCC

LUCC in China is considered to be another important anthropogenic factor that infuences climate change(Fu,2003; Li et al.,2006;Gao et al.,2007;Chen et al.,2010,2015;Wang et al.,2014).However,it seems that no consistent results have been obtained from existing research with regards to the LUCC's impact on summer climate over China.Since the regional climate model is able to simulate the East Asian climate better than the global climate model(Yu et al.,2010; Gao et al.,2013),the results that used regional climate models are discussed here.Gao et al.(2007)used a regional climate model(RegCM3)and conducted model simulations over 15 years(1987—2001).They found that land use change in China signifcantly affects the summer climate in southern China, yielding increased precipitation over the region,reduced precipitation over northern China,decreased temperatures along the Yangtze River and increased temperatures in southern China.Yu and Xie(2013)utilized the regional climate model RegCM4.0 and investigated the impacts of LUCC by conducting simulations in a 24-year(1978—2001)period.Their simulation results showed that,in summer,the land cover change lead to decreases in surface air temperature over southern China and in precipitation over the area south of the Yangtze River,a reduction in precipitation and an increase in surface air temperature in the transitional climate zone over northern China.Chen et al.(2015)also used the RegCM4.0 regional climate model and investigated the effects of LUCC in China during 1990—2010.It was found that,in the summer, increases in air temperature occurred in most parts over northern China,as well as over the middle and lower reaches of the Yangtze River valley where the strongest warming occurred,with air temperatures increased by more than 0.5°C. The amount of summer rainfall was reduced by 5%—10%over most parts of eastern China and increased by 10%—15%over central China.

It is obvious that different results were obtained from different studies on the impacts of LUCC.The inconsistency in model results indicates that no conclusive answer may be given on the climatic effect of LUCC over China.How the summer climate over China is affected by LUCC is still unclear.The differences among model results may result from the differences in experiment designs,observational data used in the models and models'physical schemes in different studies.Moreover,although a lot of studies have been conducted on the LUCC's impacts on the climate over China, almost all of them were based on numerical modeling and seldom validated with the observational data.Therefore,it is necessary to further examine the physical linkage between LUCC and climate change over China with observational data, to verify modeled results and provide an observational basis for numerical modeling.

5.Concluding remarks

In the last half century,there was a signifcant increasing trend for the summer air temperature,while there was no increasing or decreasing trend with respect to EASM intensity or the amount of summer rainfall averaged over eastern China. Both EASM intensity and summer rainfall over eastern China exhibited clear decadal variability,and the spatial distribution pattern of summer rainfall anomalies varied in the decadal timescale.

The investigation of the impact of anthropogenic factors has usually been based on the use of climate models.However, the effect of anthropogenic factors on summer climate over China remains uncertain.It has been demonstrated that the increased presence of GHG strengthens,but increased aerosols weaken the EASM.The simulated changes of East Asian climate in models may depend largely on the sensitivity of the models to GHGs and aerosols.As for the impacts of LUCC, signifcant uncertainty still exists in the model results.Therefore,a careful investigation is needed to check if anthropogenic factors including GHGs,aerosols,and LUCC can be objectively refected in the climate model.The realistic refection of the roles of these factors in the model is crucial for understanding their impacts on the climate over eastern China in the EASM region.

Fig.3.Linear trends of sea level pressure(SLP)(shaded;units:hPa(44year)-1)and 850 hPa winds(vectors;units:ms-1(44year)-1)in summer during 1958—2001.(a)GHG forcing run,and(b)aerosol forcing run from a multi-model ensemble(MME).The dotted areas indicate that the rainfall trends are statistically signifcant at the 90%confdence level.The multi-model ensemble is constructed by using 35 realizations from 17 CMIP5 models(Song et al.,2014).

A lot of research has revealed that the changes in EASM and summer rainfall over eastern China in recent half century are closely related to the internal forcing from natural factors.These factors include the Eurasian snow cover,Arctic sea ice,SSTs in tropical Pacifc and Indian Ocean,uneven thermal forcing over the Asian continent,and the coupled ocean—atmospheric system of PDO and APO.It is still unclear how these natural factors interact in the decadal timescale and their relative importance in infuencing the East Asian summer climate.Moreover,relative to the infuences of internal forcing from natural factors,the extent caused by the infuence of external forcing due to anthropogenic factors on the East Asian climate remains an open question.In fact, the global warming induced by anthropogenic factors may infuence the underlying land and ocean thermal conditions and the coupled ocean-atmospheric system which then affects the climate over China.However,it is still unclear how to detect and attribute the contributions of external forcing to internal forcing.In addition,there are direct and indirect effects related to the presence of aerosols(IPCC,2007). Aerosol processes,especially indirect effects,are still poorly treated in climate models.Furthermore,it should be noted that although aerosols may have important infuence on the East Asian summer climate,their intensity and distribution may also be affected by summer monsoon circulations (Zhang et al.,2010;Yan et al.,2011).Therefore,the interaction between aerosols and EASM should be taken into account to fully understand the role played by aerosols and their subsequent effects on the East Asian summer climate.

Acknowledgements

The author would like to express his sincere gratitude for the constructive comments of anonymous reviewers.This work was supported by the National Natural Science Foundation of China(41221064).

An,Z.-S.,Wu,G.-X.,Li,J.-P.,et al.,2015.Global monsoon dynamics and climate change.Annu.Rev.Earth Planet.Sci.43,29—77.

Chen,H.-S.,Li,X.,Hua,W.-J.,2015.Numerical simulation of the impact of land use/land cover change over China on regional climates during the last 20 years.Chin.J.Atmos.Sci.39,357—369(in Chinese).

Chen,J.-M.,Zhao,P.,Guo,X.-Y.,2010.Numerical simulation of the impact of changes in the vegetation in the western China on the summer climate over the northern China.Acta Meteor.Sinica 68,173—181(in Chinese).

Chen,L.-T.,Wu,R.-G.,2000.Interannual and decadal variations of snow cover over Qinghai-Xizang Plateau and their relationships to summer monsoon rainfall in China.Adv.Atmos.Sci.17,18—30.

Ding,Y.-H.,1994.Monsoons over China.Kluwer Academic Publishers,New York.

Ding,Y.-H.,Wang,Z.-Y.,Sun,Y.,2008.Inter-decadal variation of the summer precipitation in East China and its association with decreasing Asian summer monsoon.Part I:observed evidences.Int.J.Climatol.28, 1139—1161.

Ding,Y.-H.,Sun,Y.,Wang,Z.-Y.,et al.,2009.Inter-decadal variation of the summer precipitation in China and its association with decreasing Asian summer monsoon.Part II:possible causes.Int.J.Climatol.29, 1926—1944.

Deng,W.-T.,Sun,Z.-B.,Zeng,G.,et al.,2009.Interdecadal variation of summer precipitation pattern over eastern China and its relationship with the North Pacifc SST.Chin.J.Atmos.Sci.33,835—846(in Chinese).

Duan,A.-M.,Wu,G.-X.,Zhang,Q.,et al.,2006.New proofs of the recent climate warming over the Tibetan Plateau as a result of the increasing greenhouse gases emissions.Chin.Sci.Bull.51,1396—1400.

Duan,A.-M.,Wang,M.-R.,Lei,Y.-H.,et al.,2013.Trends in summer rainfall over China associated with the Tibetan Plateau sensible heat source during 1980—2008.J.Clim.26,261—275.

Fu,C.-B.,2003.Potential impacts of human-induced land cover change on East Asia monsoon.Glob.Planet Change 37,219—229.

Gao,X.-J.,Zhang,D.-F.,Chen,Z.-X.,et al.,2007.Land use effects on climate in China as simulated by a regional climate model.Sci.China Ser.D.Earth Sci.50,620—628.

Gao,X.-J.,Wang,M.-L.,Giorgi,F.,2013.Climate change over China in the 21st century as simulated by BCC_CSM1.1-RegCM4.0.Atmos.Ocean. Sci.Lett.6,381—386.

Gong,D.-Y.,Ho,C.-H.,2002.Shift in the summer rainfall over the Yangtze River valley in the late 1970s.Geophys.Res.Lett.29,1436.

Han,J.-P.,Zhang,R.-H.,2009.The dipole mode of the summer rainfall over East China during 1958—2001.Adv.Atmos.Sci.26,727—735.

Hsu,H.H.,Zhou,T.-J.,Matsumoto,J.,2014.East Asian,Indochina and western North Pacifc summer monsoon:an update.Asia-Pac.J.Atmos. Sci.50,45—68.

Hu,Z.-Z.,Yang,S.,Wu,R.-G.,2003.Long-term climate variations in China and global warming signals.J.Geophys.Res.108(D19),4614.

Huang,G.,Hu,K.-M.,Xie,S.-P.,2010.Strengthening of tropical Indian Ocean teleconnection to the northwest Pacifc since the mid-1970s:an atmospheric GCM study.J.Clim.23,5294—5304.

Huang,R.-H.,Wu,Y.-F.,1989.The infuence of ENSO on the summer climate change in China and its mechanisms.Adv.Atmos.Sci.6,21—32.

Huang,R.-H.,Sun,F.-Y.,1992.Impacts of the tropical western Pacifc on the East Asian summer monsoon.J.Meteor.Soc.Jpn.70,243—256.

Huang,R.-H.,Xu,Y.-H.,Zhou,L.-T.,1999.The inter-decadal variation of summer precipitations in China and the drought trend in North China. Plateau Meteor.18,465—476(in Chinese).

Huang,R.-H.,Chen,J.-L.,Liu,Y.,2011.Interdecadal variation of the leading modes of summertime precipitation anomalies over eastern China and its association with water vapor transport over East Asia.Chin.J.Atmos.Sci. 35,589—606(in Chinese).

Huang,R.-H.,Chen,W.,Yan,B.-L.,et al.,2004.Recent advances in studies of the interaction between the East Asian winter and summer monsoons and ENSO cycle.Adv.Atmos.Sci.21,407—424.

Huang,R.-H.,Chen,J.-L.,Wang,L.,et al.,2012.Characteristics,processes, and causes of the spatio-temporal variabilities of the East Asian monsoon system.Adv.Atmos.Sci.29,910—942.

IPCC,2007.Climate Change 2007:The Physical Science Basis.Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.Cambridge University Press,Cambridge and New York.

IPCC,2013.Climate Change 2013:The Physical Science Basis.the Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.Cambridge University Press, Cambridge and New York.

Jiang,D.-B.,Wang,H.-J.,2005.Natural interdecadal weakening of East Asian summer monsoon in the late 20th century.Chin.Sci.Bull.50,1923—1929.

Karori,M.A.,Li,J.-P.,Jin,F.-F.,2013.The asymmetric infuence of the two types of El Ni~no and La Ni~na on summer rainfall over Southeast China.J. Clim.26,4567—4582.

Kimoto,M.,2005.Simulated change of the East Asian circulation under global warming scenario.Geophy.Res.Lett.32,L16701.

Kwon,M.-H.,Jhun,J.-G.,Wang,B.,et al.,2005.Decadal change in relationship between East Asian and WNP summer monsoons.Geophys.Res. Let.32,L16709.

Li,H.-M.,Dai,A.-G.,Zhou,T.-J.,et al.,2010a.Responses of East Asian summer monsoon to historical SST and atmospheric forcing during 1950—2000.Clim.Dyn.34,501—514.

Li,J.-P.,Wu,Z.-W.,Jiang,Z.-H.,et al.,2010b.Can global warming strengthen the East Asian summer monsoon?J.Clim.23,6696—6705.

Li,Q.-P.,Ding,Y.-H.,Dong,W.-J.,2006.A numerical simulation on impact of historical land-use changes on regional climate in China since 1700.Acta Meteor.Sinica 64,257—270(in Chinese).

Li,Y.-J.,Li,J.-P.,Feng,J.,2013.Boreal summer convection oscillation over the Indo-Western Pacifc and its relationship with the East Asian summer monsoon.Atmos.Sci.Lett.14,66—71.

Liu,H.-W.,Zhou,T.-J.,Zhu,Y.-X.,et al.,2012.The strengthening East Asian summer monsoon since the early 1990s.Chin.Sci.Bull.57,1553—1558.

Liu,Y.,Sun,J.-R.,Yang,B.,2009.The effect of black carbon and sulphate aerosols in China region on East Asian monsoons.Tellus B 61,642—656.

Liu,X.-D.,Chen,B.-D.,2000.Climatic warming in the Tibetan Plateau during recent decades.Int.J.Climatol.20,1729—1742.

Lu,R.-Y.,2001.Interannual variability of the summertime North Pacifc Subtropical High and its relation to atmospheric convection over the warm pool.J.Meteor.Soc.Jpn.79,771—783.

Ma,Z.-G.,2007.The interdecadal trend and shift of dry/wet over the central part of North China and their relationship to the Pacifc Decadal Oscillation(PDO).Chin.Sci.Bull.52,2130—2139.

Ma,Z.-G.,Shao,L.-J.,2006.Relationship between dry/wet variation and the Pacifc Decade Oscillation(PDO)in northern China during the last 100 years.Chin.J.Atmos.Sci.30,464—474(in Chinese).

Mantua,N.J.,Hare,S.R.,Zhang,Y.,et al.,1997.A Pacifc interdecadal climate oscillation with impacts on salmon production.Bull.Amer.Meteor.Soc. 78,1069—1079.

Menon,S.,Hasen,J.,Nazarenko,L.,et al.,2002.Climate effects of black carbon aerosols in China and India.Science 297,2250—2253.

Qian,C.,Zhou,T.-J.,2014.Multidecadal variability of North China aridity and its relationship to PDO during 1900—2010.J.Clim.27,1210—1222.

Qin,D.-H.,Ding,Y.-H.,Mu,M.,2012.Evolution of Climate and Environment in China:2012.China Meteorological Press,Beijing.

Qu,X.,Huang,G.,2012.An enhanced infuence of tropical Indian Ocean on the South Asia High after the late 1970s.J.Clim.25,6930—6941.

Song,F.-F.,Zhou,T.-J.,Qian,Y.,2014.Responses of East Asian summer monsoon to natural and anthropogenic forcings in the latest 17 CMIP5 models.Geophys.Res.Lett.41,596—603.

Tao,S.-Y.,Chen,L.-X.,1987.Review of recent research on the East Asian summer monsoon in China.In:Chang,C.-P.,Krishnamurti,T.N.(Eds.), Monsoon Meteorology.Oxford University Press,Oxford,pp.60—92.

Wang,B.,Wu,R.,Fu,X.,2000.Pacifc-East Asian teleconnection:how does ENSO affect East Asian climate?J.Clim.13,1517—1536.

Wang,B.,Wu,R.,Lau,K.M.,2001.Interannual variability of the Asian summer monsoon:contrasts between the Indian and the western North Pacifc-East Asian monsoon.J.Clim.14,4073—4090.

Wang,B.,Lin,Ho,Zhang,Y.,et al.,2004.Defnition of South China Sea monsoon onset and commencement of the East Asia summer monsoon.J. Clim.17,699—710.

Wang,B.,Li,T.,Ding,Y.,et al.,2005.Eastern Asian-Western North Pacifc monsoon:a distinctive component of the Asian-Australian monsoon system.In:Chang,C.-P.,Wang,B.,Lau,N.-C.(Eds.),The Global Monsoon System:Research and Forecast,pp.72—94.WMO/TD No.1266(TMRP Report No.70).

Wang,H.-J.,2001.The weakening of the Asian monsoon circulation after the end of 1970s.Adv.Atmos.Sci.18,376—386.

Wang,Y.-L.,Feng,J.-M.,Gao,H.,2014.Numerical simulation of the impact of land cover change on regional climate in China.Theor.Appl.Climatol. 115,141—152.

Webster,P.J.,Moore,A.,Loschnigg,J.P.,et al.,1999.Coupled ocean-—atmosphere dynamics in the Indian Ocean during 1997—98.Nature 401, 356—360.

Wei,W.,Zhang,R.-H.,Wen,M.,2012.Meridional variation of South Asian High and its relationship with the summer precipitation over China.J. Appl.Meteor.Sci.23,650—659(in Chinese).

Wu,B.-Y.,Zhang,R.-H.,2007.Interdecadal shift in the western North Pacifc summer SST anomaly in the late 1980s.Chin.Sci.Bull.52, 2559—2564.

Wu,B.-Y.,Yang,K.,Zhang,R.-H.,2009a.Eurasian snow cover variability and its association with summer rainfall in China.Adv.Atmos.Sci.26,31—44.

Wu,B.-Y.,Zhang,R.-H.,Wang,B.,2009b.On the association between spring arctic sea ice concentration and Chinese summer rainfall:a further study. Adv.Atmos.Sci.26,666—678.

Wu,B.-Y.,Zhang,R.-H.,Wang,B.,et al.,2009c.On the association between spring arctic sea ice concentration and Chinese summer rainfall.Geophys. Res.Lett.36,L09501.

Wu,G.-X.,Li,W.-P.,Guo,H.,et al.,1997.Tibetan Plateau sensible heat pump and Asia summer monsoon.In:Ye,D.-Z.(Ed.),Memorial Corpus for Zhao Jiuzhang.Science Press,Beijing,pp.116—126(in Chinese).

Wu,G.-X.,Liu,Y.-M.,Dong,B.,et al.,2012a.Revisiting Asian monsoon formation and change associated with Tibetan Plateau forcing.I:Formation.Clim.Dyn.39,1169—1181.

Wu,G.-X.,Liu,Y.-M.,He,B.,et al.,2012b.Thermal controls on the Asian summer monsoon.Sci.Rep.2,404.

Wu,R.,Wen,Z.-P.,Yang,S.,et al.,2010.An interdecadal change in southern China summer rainfall around 1992—93.J.Clim.23,2389—2403.

Xu,Q.,2001.Abrupt change of the mid-summer climate in central east China by the infuence of atmospheric pollution.Atmos.Envir 35,5029—5040.

Xu,Z.-Q.,Fan,K.,Wang,H.-J.,2015.Decadal variation of summer precipitation over China and associated atmospheric circulation after the late 1990s.J.Clim.28,4089—4106.

Yan,L.-B.,Liu,X.-D.,Yang,P.,et al.,2011.Study of the impact of summer monsoon circulation on spatial distribution of aerosol in East Asia based on numerical simulations.J.Appl.Meteor.Climatol.50,2270—2282.

Yan,Z.-W.,Ji,J.-J.,Ye,D.-Z.,1990.Northern hemispheric summer climatic jump in the 1960s.Part I:precipitation and temperature.Sci.China Ser.B 33,97—103(in Chinese).

Ye,D.-Z.,Wu,G.-X.,1998.The role of the heat source of the Tibetan Plateau in the general circulation.Meteor.Atmos.Phys.67,181—191.

Yu,E.-T.,Wang,H.-J.,Sun,J.-Q.,2010.A quick report on a dynamical downscaling simulation over china using the nested model.Atmos.Ocean. Sci.Lett.3,325—329.

Yu,R.-C.,Zhou,T.-J.,2007.Seasonality and three-dimensional structure of the interdecadal change in East Asian monsoon.J.Clim.20,5344—5355. Yu,R.-C.,Wang,B.,Zhou,T.-J.,2004.Tropospheric cooling and summer monsoon weakening trend over East Asia.Geophys.Res.Lett.31,L22212. Yu,Y.,Xie,Z.-H.,2013.A simulation study on climatic effects of land cover change in China.Adv.Clim.Change Res.4,117—126.

Zhang,D.-F.,Gao,X.-J.,Luo,Y.,et al.,2015.Downscaling a 20th century climate change of a global model for China from RegCM4.0:attributable contributions of greenhouse gas emissions and natural climate variability. Chin.Sci.Bull.60,1631—1642(in Chinese).

Zhang,L.,Liao,H.,Li,J.-P.,2010.Impacts of Asian summer monsoon on seasonal and interannual variations of aerosols over eastern China.J. Geophys.Res.115,D00K05.

Zhang,Q.-Y.,Lu¨,J.-M.,Yang,L.-M.,et al.,2007.The interdecadal variation of precipitation pattern over China during summer and its relationship with the atmospheric internal dynamic processes and extra-forcing factors. Chin.J.Atmos.Sci.31,1290—1300(in Chinese).

Zhang,R.-H.,2015.Changes in East Asian summer monsoon and summer rainfall over eastern China during recent decades.Sci.Bull.60, 1222—1224.

Zhang,R.-H.,Zhou,S.-W.,2009.Air temperature changes over the Tibetan Plateau and other regions in the same latitudes and the role of ozone depletion.Acta Meteor.Sinica 23,290—299.

Zhang,R.-H.,Sumi,A.,Kimoto,M.,1996.Impact of El Ni~no on the East Asian monsoon:a diagnostic study of the’86/87 and’91/92 events.J. Meteor.Soc.Jpn 74,49—62.

Zhang,R.-H.,Sumi,A.,kimoto,M.,1999.A diagnostic study of the impact of El Ni~no on the precipitation in China.Adv.Atmos.Sci.16,229—241.

Zhang,R.-H.,2001.Relations of water vapor transports from Indian monsoon with those over East Asia and the summer rainfall in China.Adv.Atmos. Sci.18,1005—1017.

Zhang,R.-H.,Wu,B.-Y.,Zhao,P.,et al.,2008.The decadal shift of the summer climate in the late 1980s over eastern China and its possible causes.Acta Meteor.Sinica 22,435—445.

Zhang,R.-H.,Koike,T.,Xu,X.-D.,et al.,2012.A China—Japan cooperative JICA atmospheric observing network over the Tibetan Plateau(JICA/Tibet Project):an overview.J.Meteor.Soc.Jpn 90C,1—16.

Zhang,R.-H.,Wu,B.-Y.,Han,J.-P.,et al.,2013.Effects on summer monsoon and rainfall change over China due to Eurasian snow cover and ocean thermal conditions.In:Singh,B.R.(Ed.),Climate Change—Realities, Impacts over Ice Cap,Sea Level and Risks.InTech,Rijeka,pp.227—250.

Zhang,Y.,Wallace,J.M.,Battisti,D.S.,1997.ENSO-like interdecadal variability:1900—93.J.Clim.10,1004—1020.

Zhang,Y.,Li,T.,Wang,B.,2004.Decadal change of the spring snow depth over the Tibetan Plateau:the associated circulation and infuence on the East Asian summer monsoon.J.Clim.17,2780—2793.

Zhao,P.,Zhang,R.-H.,2006.Relationship of interannual variation between an eastern Asian—Pacifc dipole pressure pattern and East Asian monsoon. Chin.J.Atmos.Sci.30,307—316(in Chinese).

Zhao,P.,Zhu,Y.-N.,Zhang,R.-H.,2007.An Asian—Pacifc teleconnection in summer tropospheric temperature and associated Asian climate variability. Clim.Dyn.29,293—303.

Zhao,P.,Yang,S.,Yu,R.-C.,2010a.Long-term changes in rainfall over eastern China and large-scale atmospheric circulation associated with recent global warming.J.Clim.23,1544—1562.

Zhao,P.,Cao,Z.-H.,Chen,J.-M.,2010b.A summer teleconnection pattern over the extratropical Northern Hemisphere and associated mechanisms. Clim.Dyn.35,523—534.

Zheng,J.-Y.,Li,J.-P.,Feng,J.,2014.A dipole pattern in the Indian and Pacifc oceans and its relationship with the East Asian summer monsoon.Environ. Res.Lett.9,074006.

Zhou,S.-W.,Zhang,R.-H.,2005.Decadal variations of temperature and geopotential height over the Tibetan Plateau and their relations with Tibet ozone depletion.Geophys.Res.Lett.32,L18705.

Zhou,T.-J.,Gong,D.-Y.,Li,J.,et al.,2009a.Detecting and understanding the multi-decadal variability of the East Asian Summer Monsoon:recent progress and state of affairs.Meteor.Z.18,455—467.

Zhou,T.-J.,Yu,R.-C.,Zhang,J.,et al.,2009b.Why the western Pacifc subtropical high has extended westward since the late 1970s?J.Clim.22, 2199—2215.

Zhu,C.-W.,Wang,B.,Qian,W.-H.,et al.,2012.Recent weakening of northern East Asian summer monsoon:a possible response to global warming. Geophys.Res.Lett.39,L09701.

Zuo,Z.-Y.,Zhang,R.-H.,Zhao,P.,2011.The relation of vegetation over the Tibetan Plateau to rainfall in China during the boreal summer.Clim.Dyn. 36,1207—1219.

Zuo,Z.-Y.,Yang,S.,Kumar,A.,et al.,2012.Role of thermal condition over Asia in the weakening Asian summer monsoon under global warming background.J.Clim.25,3431—3436.

Zuo,Z.-Y.,Yang,S.,Zhang,R.-H.,et al.,2013.Long-term variations of broadscale Asian summer monsoon circulation and possible causes.J.Clim.15, 8947—8961.

Received 3 August 2015;revised 8 September 2015;accepted 24 September 2015

Available online 8 October 2015

E-mail address:renhe@cams.cma.gov.cn.

Peer review under responsibility of National Climate Center(China Meteorological Administration).

Production and Hosting by Elsevier on behalf of KeAi

http://dx.doi.org/10.1016/j.accre.2015.09.009

1674-9278/Copyright?2015,National Climate Center(China Meteorological Administration).Production and hosting by Elsevier B.V.on behalf of KeAi. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

The decadal changes in EASM and summer rainfall over eastern China in the last half century are closely related to natural internal forcing factors such as Eurasian snow cover,Arctic sea ice,sea surface temperatures in tropical Pacifc and Indian Ocean,ocean—atmospheric coupled systems of the Pacifc Decadal Oscillation(PDO)and Asian—Pacifc Oscillation(APO),and uneven thermal forcing over the Asian continent. Up to now,the roles of anthropogenic factors,such as greenhouse gases,aerosols,and land usage/cover changes,on existing decadal variations of EASM and summer rainfall in this region remain uncertain.