XUE Feng,ZENG Qingcun,HUANG Ronghui,LI Chongyin,LU Riyu,and ZHOU Tianjun

Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

Recent Advances in Monsoon Studies in China

XUE Feng?,ZENG Qingcun,HUANG Ronghui,LI Chongyin,LU Riyu,and ZHOU Tianjun

Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing100029

This review provides a synopsis of the major progress that has been made in monsoon studies in China and to further bridge the gap between the Chinese and international meteorological community.It consists of seven major sections.After the introduction,the second section begins with the global monsoon systems and their seasonal variation,based on some new methods proposed in recent years.Besides,some major intraseasonal features of East Asian monsoon,including the onset of South China Sea summer monsoon are discussed.In the third section,we review the interactions between ENSO and the East Asian monsoon,focusing in particular on the results of Chinese meteorologists that indicate the inf uence of ENSO on the East Asian summer monsoon(EASM)is obviously different from that on the tropical monsoon.Besides the tropical Pacif c, other ocean basins,such as the Indian Ocean and the Atlantic Ocean,are also important to the East Asian monsoon,and this topic is discussed in the fourth section.In the f fth section,we address the role of land surface processes in East Asian monsoon.For example,we describe work that has shown more snow cover in spring on the Tibetan Plateau is followed by a weakened EASM and more summer rainfall in the Yangtze River valleys.The sixth section focuses on the inf uence of atmospheric circulation in the Southern Hemisphere(SH)on EASM,demonstrating how the signal from the SH is likely to provide new clues for the seasonal forecasting of summer rainfall in China.Finally,in the seventh section,we concentrate on the interdecadal variations of EASM.In particular,we look at a signif cant interdecadal variation that occurred at the end of the 1970s,and how our understanding of this feature could affect forecasting ability.

East Asian monsoon,global monsoon system,ENSO,interannual variability,interdecadal variability

1. Introduction

China is located in the East Asian monsoon region.Climate anomalies in China,especially in the eastern part of the country,are signif cantly affected by the East Asian monsoon.China’ssocio-economicdynamicsareseverelyaffected byclimate disasters,suchas droughts,f oodsandcoldsurges, due to the large variability of the East Asian monsoon.It is estimated that the damages and losses(e.g.the property) caused by such disasters amounts to 3%-6%of the total gross domestic product of China.The advance and retreat of this monsoon,especially the summer monsoon,plays a role in the weather and climate of China.Because it affects agricultural production,the monsoon has been noted since the time of the ancient Chinese.Based on textual research,Zeng (2005)proved that the world’s f rst literature on the monsoon was recorded in the“Grand Stories”written by the great historian SIMA Qian(145 BC-87 BC).King SHUN’s poem“Southerly Wind”was written for singing around the 22nd to 23rd centuries BC.The poem very concisely yet poignantly describes the major features of the East Asian summer mon-soon(EASM)and its socio-economic impacts.The poem tells us that the warm southerly comes in summertime,characterized by hot weather and rich rainfall,thus providing a good harvest and keeping the people from suffering hunger and coldness.The ancient Chinese also knew well the abrupt changes of the summer monsoon,as described in a poem written by SU Dongpo(1037AD-1101 AD)during the Song dynasty.The poem relates that the Mei-yu terminates suddenly in late July,when the southeast wind prevails in the Yangtze River basin.There are more references proving that the ancient Chinese recognizedthat the seasonal variations in East Asia,characterized by wind direction and rainfall,are stepwise and abrupt.Knowledge of the monsoon provided guidance for agricultural productionand for the development of the ancient Chinese civilization.

Descriptions of the weather,climate and the seasonal variations of East Asian monsoon based on modern meteorology originated from the classical works of Zhu(1934)and Tu and Huang(1944).By using limited observational data, Zhu(1934)found that Mei-yu in the Yangtze River basin is causedby theconf uencebetweenthe summermonsoonfrom the south and the cold air mass from the north.When the southeast wind prevails after late July,however,there is less rainfall due to the lack of ascending airf ow.According tothe hot and moist feature of EASM,Tu and Huang(1944) tookthe 20°C isoline ofwet-bulbpotentialtemperatureas the northern boundary of the summer monsoon.They found that there are two abrupt changes during the advance and retreat of EASM.

With the availability of upper atmosphere data after the 1950s,Chinese meteorologists began to explore the relationship of EASM with the variations in the upper atmospheric circulation in East Asia.Tao et al.(1958)noted that the onset of Mei-yu is associated with the northward displacement of the jet stream over East Asia.When the jet stream disappears and the easterly winds begin to appear,the Mei-yu periodterminates.Huang and Yu(1962)and Huang and Tang(1962) found that the variability of the western Pacif c subtropical high(WPSH)plays a role in summer rainfall in China.The northwardjump of the WPSH duringmid-June and mid-July, forinstance,correspondstothe onsetandterminationofMeiyu over the Yangtze River valleys,respectively.Tao and Chu (1964)further revealed that the east-west movement of the WPSH is closely related to that of the South Asian high(the Tibetan high)in the upper troposphere.These studies on EASM were later summarized by Tao and Chen(1987)and Ding(1994).These classical works have laid out a f rm basis for monsoon research and have developed the capability to forecast monsoon activities in China.

Building upon the above classical works,monsoon research in China has advanced remarkably in recent decades. This is largely due to the availability of observational data as well as advances in computing power and climate models. Research in this area now covers all aspects of the monsoon, includingobservationalstudies,climatemodelingandprediction.Due to the advent of the internet and widespread international communication,part of this work is already known to the world’s meteorological community and has also been introduced in a number of summaries and books.In recent years,many Chinese meteorologists have published papers in international journals.However,some very important results have been publishedin Chinese(mostly with an English abstract).Based on the above considerations,we focus in this review on these studies publishedin Chinese journals,althoughthosepublishedininternationaljournalsare alsomentioned.

Several important aspects are highlighted in this article,focusing on seasonal and interannual to interdecadal timescales.In section 2,we begin with an introduction of the global monsoon systems,together with the South China Sea (SCS)summer monsoon and intraseasonal variation of the East Asian monsoon.This is followed by a review of the interannualvariabilityof EASM,as this variabilityis important to summer rainfall prediction in China.Sections 3 and 4 focus on the inf uence of the tropical oceans on EASM,including ENSO,the Indian Ocean and the Atlantic Ocean.Section 5 reviews the inf uence of land surface processes,including snowcoverandsoil moisture.Section6discusses theinteractions of the atmosphericcirculations in both hemispheres and their inf uence on EASM.Finally,the interdecadalvariability of the East Asian monsoon and its inf uence on interannual variability is presented in section 8.

2. Seasonal and intraseasonal variation of the global monsoon system and East Asian monsoon

2.1.Theglobalmonsoonsystem andits seasonalvariation

Classically,monsoon is a climate concept characterized by a large seasonal variation of wind and rainfall.In particular,wind direction exhibits a very large variation during the annual cycle.In the East Asian monsoonregion,for instance, thenortherlywindprevailsinwinterwhile thesoutherlywind prevails in summer,resulting in a cold and dry winter and a hot and humid summer.For this reason,Zeng et al.(1994) proposedthe seasonality of horizontal wind for a quantitative description of monsoon.The seasonality can be expressed by the wind difference between winter and summer divided by the annual mean or the sum of winter and summer.Since the wind direction in winter tends to be opposite to that in summer,it can be expected that seasonality is large in monsoon regions.

Thedistributionofseasonalityallowsthemonsoonregion to be accurately described(Zeng and Zhang,1998;Xue and Zeng,1999).In the lower troposphere(Fig.1a),there are three maxima in both hemispheres.In the tropics,a large seasonality coincides with the tropical monsoon region.The maximum is found in the Asian-Australian monsoon region, with the centers in the western Pacif c warm pool(WPWP) region and the tropical Indian Ocean.In the subtropics,especially in the subtropical Pacif c,there are also maxima of seasonality,with the value less than that in the tropics,which is closely related to the seasonal migration of the subtropical high.In high latitudes,the maxima of seasonality correspond to the seasonal variation of storm tracks of the westerly.Broadly speaking,the above three regions with large seasonality represent the three evident monsoon regions in the lower troposphere,i.e.,the tropical monsoon region,the subtropical monsoon region and temperate-frigid monsoon region.

Seasonalityexhibitsa clearbaroclinicstructureinthevertical direction.In particular,the subtropical monsoon region tends toapproachthe tropicalonewith height;andas a result, the two monsoonregionsmergeinto a whole in the uppertroposphere(Fig.1b).This is called the planetary monsoon due to its large scale.In the stratosphere,seasonality is much larger than that in the troposphere due to the opposite circulation between winter and summer.There is a well-def ned belt between the tropics and subtropics in each hemisphere, and the northern belt is much stronger than the southern one (Fig.1c).It is also noted that there exists a night jet in winter over high latitudes that collapses after late spring,such that the maximum belt in the upper stratosphere tends to be located more poleward than that in the middle stratosphere.

In summary,the def ned seasonality can describe the classical monsoon region as well as other monsoon regions with large seasonal variations.All these monsoon regions with large seasonality can be referred to as the global monsoon system,which includes the tropical,subtropical and temperate-frigidmonsoon in the lower troposphere,the planetarymonsoonin theuppertroposphere,andthestratospheric monsoon.

In order to further reveal the seasonal variation of the global monsoon system,Zeng and Zhang(1992)utilized a spatial correlationin one regionto describe the seasons of the atmospheric circulation.Based on the correlation coeff cient with the winter or summer,it is easy to determine the starting time and duration of each season.The case study by Zhang and Zeng(1998)proved that this method is generally effective.Later,Xue et al.(2002)improved the division of the season to match with the usual division of the seasons.Using the pentad mean data,they identif ed the global distribution of the starting time of winter and summer circulation in the lowertroposphere(Fig.2a).TheAsian wintermonsooninitiates fromthe westerlies in high latitudes of Eurasia at the end of September,followed by development in the south of India and adjacent oceans in the middle October.The winter circulation in East Asia establishes from the end of October to middle November.After that,the SCS,the tropical western Pacif candIndianOceangraduallychangeintothewintercirculation pattern eastward and southward.Finally,the winter circulationin the tropicalmiddlePacif c south to the Aleutian Islands and in the North Atlantic sets up in the second half of December.

TheAsian summermonsooncirculationinitiates fromthe winter circulation in the Southern Hemisphere(SH).In the SH subtropics,an earlier establishment of winter circulation (borealsummer)is foundin the regionsof the Mascarene and Australian highs,which can inf uence Asian summer monsoonthroughthecross-equatorialf ows(CEFs)in East Africa and near Indonesia(Fig.2b).In addition,in agreement with the onset date of SCS summer monsoon(SCSSM),summer circulationin the SCS begins in the second half of May.Over the regions extending from India,eastern China to Japan, summer circulation begins in the f rst half of June,in good agreement with the summer monsoon rain belt.Globally,the boreal or austral winter circulation initiates in high latitudes of Eurasia and Antarctica,both of which are the coldest regions on Earth.Therefore,the seasonal variation of the atmospheric circulation in the lower troposphere initiates from the coldest region in the winter hemisphere.

As the global monsoon system,the establishment of winter and summer circulation also exhibits a clear 3D structure (Fig.3).In the Northern Hemisphere(NH),the winter circulation initiates from the mid-high stratosphere between the tropics and subtropics at the end of October,and then propagates downward.In the SH,the winter circulation initiates fromthestratosphericsubtropicsattheendofAprilandpropagates downward.Similar to the winter circulation,an earlier establishment of the summer circulation is also found in the stratosphere.In general,it takes about two months for the establishment of the winter or summer circulation.The seasonal variation originates from the stratosphere possibly due to thefact that thestratosphericcirculationis verysensitiveto the solar radiation absorbed by the ozone layer;the winter or summer circulation is then established in the polar regions of the lower troposphere.Afterwards,it developsindependently downwards and upwards,f nally resulting in the establishment of the whole circulation system.

2.2.SCSSM

The onset of SCSSM is a key indicator characterizing the abrupt transition from the dry season to the rainy season and subsequent seasonal change in the Asian monsoon region. Unlike the Indian summer monsoon,the onset of SCSSM is achieved during a relatively short period(Tao and Chen, 1987;Ding et al.,2004).It plays a role in connecting EASM with Indian summer monsoon due to the geographical position.Due to the importanceof SCSSM to Asian monsoon,an international experiment about SCSSM called the SCS Monsoon Experiment(SCSMEX)was conducted in 1998;scientists from many countries,including Chinese scientists,participated in this experiment(Ding et al.,2004).

Based on the observational data from SCSMEX and NCEP reanalysis data,Li and Qu(2000)and He et al.(2000) revealed the large-scale features related to the onset of SCSSM.These features can be summarized as follows:the development of the Somali jet and cross-equatorial f ow in the eastern Indian Ocean;the rapid intensif cation of the heat source in the Indochina Peninsula,South China,the Tibetan Plateau and adjacent regions;the development and northwestward movement of the South Asian high at 200 hPa; the acceleration of the westerly in the eastern Indian ocean; and the split of the subtropical high and the formation of the monsoon depression in the Bay of Bengal.Associated with theabovechanges,thesubtropicalhighretreatseastward,and rainfall and southwest wind prevails in the SCS,signalingthe onset of SCSSM.Besides the regional characteristics,the onset of the SCSSM is also connected with the seasonal transition of the global atmospheric circulation from boreal winter to summer.

The onset of SCSSM is directly represented by the abruptness of the wind f eld.After the onset,the northeast windis substitutedbythesouthwestwindintheSCS.Inorder to study the abruptness more objectively,Zeng et al.(2005) proposed a new method called the normalized f nite temporal variation method,which can be obtained from the time series of a functional of the wind f eld.It is shown that the critical dayofabruptnessinoneregioncanbedeterminedobjectively by this method(Zhang et al.,2005).This day is referred to as the presage day,which is just 2-4 days earlier than the onset day of monsoon def ned by the conventional method. As a typical example,Fig.4a shows the daily normalized f nite temporal variation with height in the SCS.During the onset stage,a rapid adjustment is found below 400 hPa,with a maximum between 850 and 700 hPa.Above 700 hPa,the adjustment tends to be longer with smaller amplitudes.The maximumoccurs on 16 May and continues until 28 May.After that,the major adjustment is related to the establishment of summer monsoon in South and East Asia.In particular, a signif cant adjustment occurs in the lower stratosphere and upper troposphere between 20 April and 1 May,and then it extends downwards into the troposphere.Therefore,the adjustment of the stratospheric circulation is an indication of monsoon circulation in the lower troposphere and foreshadows the onset of SCSSM.

The onset of SCSSM undergoes a northward process,as shown in Fig.4b.Between 80°E and 135°E,a major abruptness is found in the west or the east of 100°E.To the east of 100°E,the abruptness occurs on 15 May with a center along 110°E,correspondingto the onset of SCSSM.To the west of 100°E,the abruptness is generally characterized by a stationary mode along 95°E,with a presage day between 26 April and2 May.Therefore,dueto the separationof the Pacif c and Indian Ocean by the Indochina Peninsular,the atmospheric circulation exhibits a different evolution on the two sides,especially in the onset stage of summer monsoon.

Some studies have revealed the physical mechanisms responsible for the onset of SCSSM(Chen et al.,2000b;Ding et al.,2004).Prior to the SCSMEX,four factors were identif ed:(1)the SST anomalies in the tropical ocean,especially ENSO events;(2)the activity of the tropical intraseasonal oscillation;(3)the midlatitude circulation;and(4)the thermal contrastbetweentheAsiancontinentandtheadjacentoceans, especially the heat source in the Tibetan Plateau.Following the SCSMEX,it has been well recognized that the seasonal evolution of the tropical large-scale circulation and the corresponding thermal conditions play a dominant role in the onset process.Under these large-scale circumstances of the seasonal evolution from boreal winter to summer,the propagation of the intraseasonal oscillation triggers the development of deep convection in the SCS and leads to the onset of SCSSM.Among these factors,the thermal contrast between the continentand ocean is a basic factor,the SST and the heat source in the Tibetan Plateau is a direct factor,and the midlatitude disturbance and tropical intraseasonal oscillation is a triggering factor.

2.3.Intraseasonal oscillation of East Asian monsoon

The intraseasonal oscillation(ISO)plays a very important role in modulation of the seasonal variation or the annual march of East Asian monsoon.Zhu and Yang(1989) showed that the EASM exhibits a clear ISO with bi-weekly and quasi-40-day periods.Miao and Lau(1991)found that the 30-60-day oscillation is phase-locked with the seasonal variation of summer monsoon rainfall.Based on power spectrum analysis and a band-pass f lter,Ju et al.(2005)revealed that the ISO of EASM exhibits a wave-train pattern along the East Asian coast that propagates northward.They also noted that the monsoon activity in the tropics is generally opposite to that in the subtropics due to the activity of ISO.

As a dominant component of the EASM system,the WPSH is most evident during boreal summer.In general,the WPSH migrates northward in a stepwise fashion with two distinct northward jumps,as shown by its ridge line in Fig. 5a.In mid-June,it jumps northward for the f rst time,and the Mei-yu season in the Yangtze River valleys starts.The second northward jump usually occurs in late July.The WPSH shifts to its most northern position,signaling the end of the Mei-yu season in the above regions and the start of the rainy season in northern and northeastern China(Fig.5b).Compared with the f rst jump,the second jump of the WPSH is much more evident.Su and Xue(2010)showed that the f rst jump is mainly caused by the enhancement of the convective activities in the SCS,while the second jump is inf uenced by boththe convectiveactivities overthe WPWP andthecircula-tion systems in high latitudes.Through the phase-lock of the northeastwardpropagationof the Rossby wave-trainfromthe WPWP and the downstream propagation of Rossby waves in high latitudes,the convective activities over the warm pool and the circulation systems in high latitudes play a key role in the second northward jump of the WPSH.In addition,the interactions between the WPSH and the release of latent heat on its western edge lead to a low-frequency oscillation of the WPSH.

Some other studies have focused on the intraseasonal variation of the WPSH.Usually,the intraseasonal activity of the WPSH possesses two modes during boreal summer(Tao et al.,2001).While the f rst modeexhibitsa continuouswestward extensionfrom the central Pacif c to west of 120°E with a period of 20-30 days,the second mode is characterized by a stagnant step between 125°E and 155°E during the westward extension.In addition,the activity of the WPSH with a shorter periodof 5-10 days is related to the westerly between 35°Nand45°N.Whenatroughinthewesterlyextendssouthward to 30°N,the WPSH tends to be intensif ed.Tao and Wei(2006)further noted that the stationary Rossby wave in the upper troposphere over Eurasia propagates onto the coast of China(115°-130°E)along the subtropical jet,leading to a northward jump of the WPSH by exciting a longwave ridge there.As a result,hot weather appears in the Yangtze River valleys due to the maintenance of the ridge.

Besides EASM,recent studies have also noted the ISO in East Asianwintermonsoon(EAWM)andairtemperatureand rainfall in China in winter.Liu and Yang(2010)showed that winter rainfallin South China is closelyrelated to the tropical Madden-Julian oscillation(MJO).With the eastward movement of MJO from the western Indian Ocean to the western Pacif c,a wet-dry oscillation with a 20-day period appears in South China.A typical example was the unusually severe storm and freezing rain that occurred in the winter of 2008,when the ISO prevailed in southern China(Tao and Wei,2008).Besides the anomalous circulation in high latitudes of Eurasia,a large amount of moisture was transported into southern China with the ISO of southwest wind ahead of the trough over the Bay of Bengal,exciting severe freezing rain in southern China.

3. Interactions between East Asian monsoon and ENSO

3.1.The inf uence of ENSO on EASM

The severe f ood and droughtevents that occur frequently during the summer monsoon season have motivated many Chinese meteorologists to look for predictors.In the early 1930s,Chinese meteorologists began to explore the possibilities of long-range weather forecasting in China using modern meteorological methods.Based on the limited observational data at that time,Tu(1937)systematically studied the relationships between the climate anomalies in China and the three atmospheric oscillations,i.e.,the North Pacif c Oscillation,the North Atlantic Oscillation(NAO)and the Southern Oscillation(SO),which were f rst identif ed by Sir Gilbert Walker.He found that a cold and dry winter often appears in China when the SO is intensif ed during the preceding fall, and vice versa.However,the correlation between the summer rainfall anomaly in China and the SO in the preceding winter or spring was relatively lower.This fact indicated that there is a higherpredictabilityfor the winter climate anomaly in China.Unfortunately,it seemed that there is diff culty in predicting the summer climate anomaly in China.These results werelater demonstratedtobe true,basedonlonger-term data.It is worth noting that,considering the limited available data and the lack of meteorological knowledge at that time, Tu’s work was a truly remarkable achievement.

It is now known that the SO and El Ni?no are essentially the same phenomenon,which are a result of the interactions between the atmosphere and ocean in the tropical Pacif c, later referred to as ENSO.Since the 1980s,it has been recognized that ENSO is the strongest interannual signal in the tropical ocean-atmosphere system,and that it plays a crucial role in the interannual variability of the global atmospheric circulation.The interaction between ENSO and EASM is obviouslyofprimaryimportancein understandingthe EASM variability and the prediction of summer rainfall in China.Of particular interest is the response of the monsoonal f ows to SST in the equatorial eastern Pacif c.Numerous studies havebeen devoted to the inf uence of ENSO on EASM and summer rainfall in China.Previous studies have noted the possible association of the tropical Hadley cell anomaly caused by the SST anomaly in the eastern Pacif c with the WPSH and the rainfall anomaly in China during El Ni?no years(Chen, 1977;Xu,1986).Later,Fu and Teng(1988)found that in the different stages of ENSO,the response of the EASM circulation to the ENSO signal is totally different.In the onset year, the WPSH tends to be weakened and retreats eastward.As a result,there is less rainfall over the Yangtze River valleys. However,in the succeedingyear after the onset of ENSO(the decaying stage of ENSO),the WPSH tends to be intensif ed with a more westward extension,and there is more rainfall overthe Yangtze River valleys.They also noted that the summer monsoon anomaly in China during the decaying stage of ENSO is muchmore signif cant,indicatingthat ENSO shows a more lagged effect on EASM than other regions,such as the Indiansummermonsoon.The phase-dependentimpactof ENSO onthe East Asian climateis also evidentin the seasonally evolving,dominant interannual variability modes(Wu et al.,2009a).

At the same time,Huang and Li(1988)showed that the inf uenceofENSO on EASM is moredirectlycorrelatedwith theconvectionactivitylargelydrivenbytheWPWP SST than the SST anomalyin the equatorialeastern Pacif c.Huang and Sun(1992,1994)further found that the WPSH exhibits an obviously northward shift in early summer with an enhanced convection around the Philippines.This results in less rainfall in the Yangtze River valleys and more rainfall outside of this region.In contrast,there appears to be an almost opposite rainfall distribution in China along with a more southward WPSH when the convection activity around the Philippines is weaker than normal.Based on theoretical considerations and model experiments,they proposed an East-Asia-Pacif c(EAP)teleconnection pattern caused by the convection anomaly mentioned above through the propagation of the planetary wave(Fig.6).The EAP pattern is also evident in the atmospheric circulation f elds that dominate the water vapor transport associated with the typical interannual variability modes of summer precipitation(Zhou and Yu,2005). The EAP pattern provides a physical basis for summer rainfall predictionin China.Thesignif cantimpactofthewestern Pacif c SST on the interannual variability of the WPSH,in conjunction with the remote eastern Pacif c forcing,has been demonstrated by numerical modeling(Wu and Zhou,2008).

On the other hand,a recent study by Lin and Lu(2009) suggests that subtropical precipitation anomalies in East Asia,in turn,can signif cantly affect large-scale circulations and may be crucial for the maintenance of the EAP pattern. Diagnosis by using observational and reanalysis data indicated that the patterns are clearer in summers when the subtropical rainfall anomalies are greater.The simulated results using a linear baroclinic model indicated that a subtropical heat source,which is equivalent to the diagnosed positive subtropical precipitation anomaly,induces zonally elongated zonal wind anomalies that resemble the diagnosed ones in both the upper and lower troposphere over extratropical EastAsia.The results showed that there is a more complex relationship between subtropical rainfall variability in East Asia and the western Pacif c SST anomaly.

Based on the concept of phase-dependent inf uence,it has recently been found that,while the overall summerrainfall-SST relationship has a negative correlation over the western North Pacif c(WNP),this relationship experiences a signif cant interannual variation(Wu et al.,2009b).During ENSO-developing(decaying)summers,the rainfall-SST correlation is signif cantly positive(negative).The positive correlation is attributed to the interplay between the anomalous Walker circulation and the CEFs associated with the enhancedWNP summermonsoon.The formerleads to negative rainfall anomalies in the western Pacif c,whereas the latter leads to a cold SST anomaly resulting from enhanced surface latent heat f uxes.The negative correlation is attributed to the maintenance of an anomalous Philippine Sea anticyclone from the El Ni?no winter peak to the subsequent summer.The anomalous anticyclone,on the one hand,suppresses the local rainfall,and on the other hand,induces a warm in situ SST anomaly through both enhanced solar radiation(resulting from a decrease in cloud amount)and the reduced surface latent heat f ux(resulting from the decrease of the monsoon westerly).The rainfall-SST correlation is insignif cant in the remaining summers.Thus,the overall weak,negative rainfall-SST correlation is attributed to the signif cant negative correlation of the ENSO-decaying summers.

The subtropical westerly jet over East Asia is an important componentof the EASM system(Li et al.,2005a;Zhang et al.,2006).The meridional displacement of the jet streamlikely plays a role in connecting El Ni?no events in the preceding winter and the eastern China rainfall anomaly in the subsequent early summer.Li(1992)and Lu(2005)found that there appears to be a clear signal of the SST anomaly in May and June associated with the summer rainfall anomaly in northern China.Recently,using station precipitation data, Lu and Lin(2009)found that after the El Ni?no peak in June, the precipitation is signif cantly suppressed in the Huaihe-Yellow River regimes,while it is enhanced in the Yangtze River valleys.They indicated that this relationship between ENSO and the eastern China rainfall anomaly is established possibly through the meridional displacement of the Asian jet stream in the upper troposphere,which results in suppressed precipitation in the Huaihe-Yellow River regimes. The enhanced precipitation in the Yangtze River valleys results from the WNP anticyclonic anomaly in the lower troposphere.They further suggested that this meridional displacement of the Asian jet is due to the effects of ENSO on the zonal f ow in the NH.After the El Ni?no peak,the ENSO-related warming in the tropical troposphere persists into the following early summer.This increases the meridional gradient of temperature and,by thermal wind balance,leads to the enhancement of westerlies in the subtropics south of the westerly jet stream and the resultant southward displacement of the westerly jet stream.

Perhaps the best example showingthe inf uenceof ENSO on EASM and summer rainfall in China is the catastrophic f ood in the Yangtze River valleys in the summer of 1998.It was reported that the f ood caused the deaths of 3000 people and economic damage of 250 billion Yuan.This extreme disaster prompted Chinese meteorologists to carry out a series of studies(Tao et al.,1998;Huang et al.,1998;Lu,2000).A very strong El Ni?no event occurred in 1997 and decayed in 1998.In the decaying stage of the ENSO cycle,the convectionactivities aroundthe Philippinestendedto be suppressed, and the WPSH tended to extend more southwestward.The southerly wind in the south f ank of the WPSH brought a large amount of moisture from the tropics,which converged in the Yangtze River valleys.As a result,there were a series of heavy rainfall events in the summer of 1998.Therefore, the strong El Ni?no event played a crucial role in this severe f ood,although other factors cannot be excluded.

A unique feature of the interannual variability of EASM is its pronounced biennial oscillation,which is often referred to as tropospheric biennial oscillation(TBO).Strong TBO signals are exhibitedin many componentsof EASM systems, such as the WPSH and summer rainfall in China(Huang, 1988).TheWPWP is believedto beakeyareaconnectingthe TBO and ENSO(Xu and Zhu,1998;Huang et al.,2006a).If the warm pool is warmer in a particular year,the WPSH and the summer rain belt over China tends to extend more northward.This is due to a stronger convection around the PhilippinesandtheassociatedEAPpattern;thus,thereismorerainfall in northern China and less rainfall in the Yangtze River valleys.In the meantime,seawater upwelling is intensif ed due to the stronger convergence in the warm pool.As a result,the SST in the warm pool tends to be decreased along with a suppressed convection in the succeeding year;an opposite EAP pattern and summer rainfall distribution appears in China.

3.2.Nonlinear response of EASM to ENSO

The aforementioned studies have revealed the relationship between ENSO and EASM and the associated ENSO-type rainfall distribution in China.This classical theory provides us with an important basis for summer rainfall prediction in China.In fact,the present prediction system in China depends,to a certain degree,on the prediction of the ENSO signal in the model,and it does result in success in summer rainfall prediction in China(Zeng et al.,2003).

Observationalanalysis has shown that most ENSO events reach moderate intensity,and that strong ENSO events occur infrequently.When the ENSO signal is not so strong,it is possible that some other factors can,to some degree,modulate the response of the EASM circulation to the ENSO signal,thereby inducing a different rainfall distribution.In fact, if we examine summer rainfall patterns in China with ENSO year by year,it is easy to see that the rainfall anomaly distribution in some years is not coincident with,and is sometimes opposite to,the aforementioned relationship between ENSO and EASM,such as in 1995(Zhao,1999).Since the anomaly of EASM circulation and summer rainfall in China induced by a strong ENSO is much more signif cant than that induced by a moderate ENSO,only the strong ENSO signal is ref ected in the previous composite or correlation analysis.For this reason,it is necessary to divide the ENSO events based on their intensities if the nonlinear response of EASM to ENSO is to be depicted precisely.

Based on the above considerations,Xue and Liu(2008) divided ENSO events into strong and moderate categories. They then analyzed the inf uence of ENSO with different intensities on EASM and summer rainfall in China based on a compositeanalysis.While the rainfall distributionin all composite ENSO years agrees well with the aforementionedrelationshipbetweenENSOandEASM,therainfallpatterninthe moderate ENSO years generally corresponds to the northern pattern,with more rainfall along the Yellow-Huaihe River regimes and the coastal areas of southernChina and less rainfall in the Yangtze River valleys and southward.This rainfall pattern is very much different from the classical ENSO-type rainfall pattern,with more rainfall in the Yangtze River valleys.Since the rainfall anomaly induced by the strong ENSO is much largerthan that inducedby the moderateENSO(e.g., the devastating f ood in 1998 in the Yangtze River valleys), only the strong ENSO signal is represented in the previous result.

To further illustrate the inf uence of ENSO on EASM, Xue and Liu(2008)compared the composite WPSH at 500 hPa during the strong and moderate ENSO years with the climatological mean.Similar to the previous result,the WPSH tendsto be enhancedwith a moresouthwestwardextensionin the ENSO years.Also,the inf uence of ENSO on the WPSH tends to be enhanced from June to August,with the smallest difference from the climatological mean in June and thelargest difference in August.This indicates a long lagging effect of ENSO on the EASM circulation.On the other hand, there is a signif cant difference between the strong and moderate ENSO years.While the inf uence of the strong ENSO starts earlier,with a large anomaly,the inf uence of the moderate ENSO is relatively weak.There is a small difference from the climatological mean in June.This difference tends to be enhanced in July and reaches its maximum in August. As a result,the inf uence of the moderate ENSO on the Meiyu along the Yangtze-HuaiheRiver valleys during June-July is not so signif cant.A furthercomparisonbetween two moderate ENSO years(1995 and 2003)shows that,due to the different inf uences of the circulation in high latitudes and the SH circulation in June and July,the WPSH may also exhibit a different change,thereby inducing a different rainfall distribution in China.In other words,the response of the EASM circulation to moderate ENSO can be,to a large degree,modulated by the internal atmospheric dynamics.Furthermore,comparison with the strong ENSO in 1983 shows that strong ENSO not onlyplays a direct role in summer rainfall in China,but also controls the inf uence of other factors, such as the SH circulation.Strong ENSO is essentially different from moderate ENSO,and therefore the intensity of ENSO must be dividedin practical prediction.By comparing the EASM activities in the strong La Ni?na years of 1988 and 1989,Xue(2008)also showed that,due to the modulation of some other factors on La Ni?na signals,summer precipitation in China may exhibit a different pattern even in the background of two similar La Ni?na events.The results indicate that the response of EASM and summer rainfall in China to ENSO is essentially nonlinear.

Besides theEASM,theESWM circulationexhibitsasimilar nonlinear response to the ENSO cycle.Based on a composite analysis,He et al.(2008)compared the EAWM circulation response to strong and weak ENSO with the tropical one.In the tropics,the response to an El Ni?no is generally opposite to a La Ni?na,and the anomalous intensity is also proportional to the ENSO intensity,i.e.,the atmospheric response in the tropics exhibits a quasi-linear feature.By comparison,the responses of the EAWM circulation in mid-high latitudes are complex.The phase reversal of the circulation pattern is not evident for the warm and cold events as it is in the tropics.Moreover,the most signif cant signal is found when a strong El Ni?no occurs,characterized by the enhancement of the westerly jet in East Asia and a weaker EAWM. As a result,eastern China experiences a warmer winter due to the anomalous southerly wind along the East Asian coast. Instead,the inf uence of strong La Ni?na events is relatively weaker with a large uncertainty.

Besides the intensity of ENSO,recent studies have emphasized the inf uence of the spatial distribution of the SST anomaly in the tropical Pacif c on EASM and summer rainfall in China.Different from canonical El Ni?no events,there is another type of El Ni?no with the most signif cant SST anomaly being in the central Pacif c(CP-El Ni?no).Wang et al.(2009a)found that when the central Pacif c is warmer (colder)in spring,the Mei-yu onset in the Yangtze River valleys tends to be later(earlier).Qian et al.(2009)further indicated that there is more Mei-yu rainfall in the Huaihe River valleys when a CP-El Ni?no occurs in spring.By contrast,a canonical El Ni?no in the eastern Pacif c corresponds to more Mei-yu rainfall south of the Yangtze River valleys.More recently,Su and Xue(2011)showed that the two northward jumps of the WPSH are related with the SST anomalies in the different regions of the tropical Pacif c.The f rst jump is positively correlated with the SST anomalies in the tropical central Pacif c from the preceding winter to June.By contrast,the second jump is positively related to ENSO in the preceding winter,but this correlation tends to be weakened with the decay of ENSO and disappears in July.Due to the EASM activity being inf uenced by the two northward jumps of the WPSH,it is suggested that simply forecasting the intensity of ENSO without regard to the spatial structure of the SST anomalies will not adequately predict the associated rainfall anomalies in China.

3.3.TheroleofEAWMandextratropicalatmosphericdisturbances in the ENSO cycle

Since the seminal work of Bjerknes,ENSO has been considered to be a product of the interactions between the atmosphere and the ocean.Besides the inf uence of ENSO on EASM,the East Asian monsoon also plays a role in ENSO, especially in the onset of ENSO.Li(1989)found that,prior to the occurrence of an El Ni?no event,cold waves frequently appear in East Asia.A strong winter monsoon in East Asia leads toan enhancedconvectionandweakenedtradewindsin the western Pacif c,which induces the 30-60-day oscillation and anomalous Kelvin wave.Li(1990)further indicated that there is an obvious interaction between ENSO and EAWM. While EAWM can excite an El Ni?no event,it can also be reduced by the El Ni?no.By using a coupled general circulation model,Li and Mu(1998)performeda series of numericalexperiments and proved that a stronger winter monsoon in East Asia results in anobviousSST anomalyin theeasternPacif c. It is thereforesuggestedthat a strongerwinter monsoonplays a role in triggering the onset of an El Ni?no event.

Theroleofwesterlybursts(orthewesterlyanomaly)over the western Pacif c in triggering the onset of ENSO has been emphasized(Li,1995;Huang et al.,1996).Fu and Huang (1997)found that these westerly bursts are closely related to the southward propagationof the westerly in East Asia along with a Eurasian teleconnectionpattern in high latitudes.With the eastward propagation of the westerly,the SST in the central and eastern Pacif c tends to increase.Li and Mu(2002) indicated that the westerly anomalies can induce the subsurface ocean temperatureanomalyin the Pacif c warm pooland its eastward propagation;they concluded that the ENSO cycle is essentially related to the subsurface ocean temperature anomalies in the tropical Pacif c driven by the zonal wind anomalies over the western and central Pacif c.

On the other hand,it should be noted that not all westerly anomalies associated with strong EAWM trigger the onset of ENSO.Zhanget al.(2000)showed that the westerly anomaly in the western Pacif c undergoes two remarkable enhance-ments in the spring and summer of the onset year of an El Ni?no.Besides the eastward propagation of the westerly from the equatorial Indian Ocean,the strong convergence of the meridionalwind from extratropicalatmospheric disturbances in both hemispheres is more important to the enhancementof thewesterlyanomaly.ComparedwiththeNH,themeridional wind to the east of Australia is more stable and makes a more signif cant contribution to the enhancement of the westerly anomaly.

In addition to triggering the onset of ENSO,extratropical atmospheric disturbances also play a role in the development and decay of ENSO(Xue and He,2007).In the development phase of ENSO,the westerly anomaly in the equatorial central and eastern Pacif c is further enhanced by the cyclonic anomaly in the southeastern Pacif c and the associated southerly anomaly.It is also noted that the decay of ENSO is much faster than the development;other factors besides the classical Kelvin wave are therefore involved in the decay of ENSO.In fact,the rapid eastward movement of the anticyclonic anomaly in Australia results in a weakening of the cyclonic anomaly in the subtropical South Pacif c;the Southern Oscillation pattern tends to disappear.Moreover,when ENSO reaches its mature phase,the deepened Aleutian low and the intensif ed anticyclonic anomaly(in particular,the wind anomaly in the subtropical North Pacif c)enhances the subtropical cell in the ocean,inducing upwelling of surface water in the equatorial eastern Pacif c,thereby accelerating the decay of ENSO.

The persistence of ENSO is also inf uenced by extratropical atmospheric disturbances(Yang and Li,2005a;Li et al.,2008a).It has been found that the intensity of longerduration ENSO is generally weaker than that of shorterduration ENSO.For longer-duration ENSO,the decay of the anomalouscyclonein the easternNorth Pacif c andthe development of the anomalous anticyclone in the WNP tend to be slower.In addition,the stronger winter monsoon and weaker summermonsoonin Australia lead to a longerdurationof the westerly anomaly in the equatorial western Pacif c.Therefore,the westerly anomaly in the equatorial western Pacif c is favorable for the persistence of ENSO,while the easterly anomaly foreshadows the decay of ENSO.

4. Inf uence of the Indian Ocean and Atlantic Ocean on East Asian monsoon

4.1.Inf uence of the Indian Ocean

Chinese meteorologists have been aware since the early 1980s of the inf uence of the Indian Ocean SST on EASM and summer rainfall in China.These earlier studies noted that there is a signif cant correlation between summer rainfall in the Yangtze River valleys and the Indian Ocean’s SST anomalies(Luo et al.,1985;Chen et al.,1985).Chen(1988) revealed that there is an opposite correlation of SST anomalies between the east and west of the tropical Indian Ocean, which is often related to ENSO.He further showed that the different types of zonal SST anomalies in the tropical Indian Ocean play a role in the position and intensity of the WPSH and EASM through the Walker circulation(Chen,1991).

Since the discovery of the Indian Ocean dipole(IOD,or Indian Ocean zonal mode),more attention has been paid to the inf uence of the tropical Indian Ocean on EASM.Many studies have focused on the inf uence of the IOD on summer rainfall in China(e.g.,Li and Mu,2001;Yan et al.,2001; Xiao et al.,2002;Qian and Guan,2007,among many others).In general,the results further demonstrate that there is a signif cant correlation between the spring IOD and summer rainfall in China,and that there is more rainfall in southern China associated with a positive phase of the IOD.When the ENSO signal is relatively weak,the IOD seems to be a plausible predictor.The inf uence of the IOD can be seen more clearly in 2003 and 2004,when the SST anomalies in the tropical Pacif c were weak.Xiao and Liang(2006)revealed that the different summer rainfall distributions in these two years were to a large degree caused by the different phases of the IOD.The signif cant impact of the IOD on East Asian climate has also been demonstrated by a series of numerical experiments(Yan and Zhang,2004;Yan et al.,2007).In a positive phase of the IOD,for instance,there appears to be an anomalous easterly in South Asia and an anomalous anticyclone near the Bay of Bengal,resulting in less rainfall in southern India and more rainfall in southern China.

Recent studies have revealed that the IOD is closely related with ENSO through the Walker circulation(Li et al., 2002;Mu and Li,2002);therefore,the combined effects of the IOD and ENSO and their modulation on the response of EASM to the ENSO signal are also noted(Yang and Li, 2005b;Yang et al.,2006).Xiao et al.(2000)found that during El Ni?no years,SST anomalies with an east-warmer and west-colder distribution in the Indian Ocean enhance the inf uence of ENSO on EASM.This exaggerates the southernf ood and northern-droughtrainfall pattern in China.Furthermore,Liu et al.(2008b)pointed out that the lesser amount of rainfall in northern China during El Ni?no years can be mitigated to a certain degree if the IOD is in a positive phase, while the co-occurrence of La Ni?na events and a negative phaseoftheIODisnotconducivetosummerrainfallinnorthern China.They noted that there appear to be some regional features of summer rainfall in China under the combined effects of ENSO and the IOD during the different phases. Therefore,the effects of the IOD on EASM cannot be neglected,especially when the ENSO signal is weak.

There have been studies that have focused on the role of Indian Ocean SST anomalies in the onset of the SCSSM, which signals the seasonal transition from the winter circulation to summer circulation in the Asian monsoon region (Liang et al.,2006;Wen et al.,2006;Yuan and Li,2009). TherearetwotypesofleadingmodesofSSTvariabilityinthe Indian Ocean:the basin-scale warm/cold mode(IOBM),and the IOD.Both of these play signif cant roles in the onset of SCSSM.The IOBM warming induces an anomalous Walker circulationinthetropicalIndo-Pacif cregiontogetherwithan anomalous descending motion and suppressed convection in the tropical western Pacif c,thereby leading to a delayed on-set.It is also suggested that the IOBM signif cantly prolongs the effect of ENSO on the onset of SCSSM in the succeeding year.A positive IOD in the preceding period usually corresponds to a delayed onset,such as in 1994,when there was a strong IOD event(Yuan and Li,2009).In a similar way,the Indian Ocean SST anomaly has an impact on the intensity of SCSSM(Li et al.,2006).

Several physical mechanisms have been proposed to explain the inf uence of the Indian Ocean’s SST anomaly on the EASM.Zhou and Wang(2006)found that the spring Hadley cell plays a role in linking the SST anomalies in the Indian Ocean and EASM.Wen et al.(2006)indicated that the inf uence can be realized through changing the intensity of the Indian summer monsoon.Yang et al.(2007)suggested that the IndianOcean warming associated with ENSO induces robust climatic anomalies in summer in the Indo-west Pacif c region,prolonging ENSO’s inf uence after the tropical eastern Pacif c SST has returned to a normal state.A numerical experiment indicated that,in response to the Indian Ocean warming,precipitationincreases overmost of the basin,forcing a Matsuno-Gill pattern in the upper troposphere with a strengthened South Asian high(Li et al.,2008c).Zhou et al. (2009b)demonstrated that a similar mechanism also works at an interdecadal scale;that is,the Indian Ocean warming results in an intensif ed South Asian high and a westward extension of the WPSH.Analysis of CLIVAR C20C models(i.e.,the AGCMs forced by historical SSTs)revealed the dominance of the tropical Pacif c’s and Indian Ocean’s SST anomalies in forcing the interannual variability of immense Asian monsoon circulation components(Zhou et al.,2009a). Liu et al.(2008a)found that there appears to be a series of Rossby wave trains in the NH troposphere associated with the IOD,initiating from India and extending northeastward. It has been further revealed that the inf uence of the IOD on the climate anomaly in the NH is dynamically linked by the energy propagation of planetary waves.Analysis of the output of Atmospheric Model Intercomparison Project(AMIP) models also shows that both the tropical Pacif c and tropical Indian Ocean regions are importantin forcingthe interannual variability modes of the Asian-Australian monsoon(Zhou et al.,2009a).

In addition to the summer season,Wu et al.(2009a)revealedtheseasonally evolvingdominantinterannualvariability modes of the East Asian climate.The f rst two dominant modes account for 44%of the total interannual variance,corresponding to the post-ENSO and ENSO turnabout years,respectively.A diagnosis of the upper-level velocity potential and mid-level vertical motion f elds revealed a season-dependentIndian Ocean forcing scenario.The Indian Ocean’s basin-wide warming during El Ni?no’s mature winter and the subsequent spring does not have a signif cant impact on the anomalous circulation in the WNP.This is because convection over the tropical Indian Ocean is suppressed by theremoteforcingfromtheequatorialcentral-easternPacif c. Thebasin-widewarmingplaysan activerole in impactingthe WNP anomalous anticyclone during ENSO’s decaying summer through the atmospheric Kelvin wave or Hadley circulation,as suggested in many previous studies.

4.2.The inf uence of the Atlantic Ocean

Compared with the Pacif c and Indian oceans,the inf uences of the tropical Atlantic Ocean on the interannual variability of EASM and the summer climate anomaly in China are less frequently mentioned in the literature.This is because of its great distance from the Asian monsoon region and the relatively weak signal of the SST anomaly.However, the catastrophic f ood in the Yangtze River valleys in summer of 1998 prompted scientists to study the role of the Atlantic Ocean’s SSTs in EASM.Lu and Dong(2005)showedthat,in this case,the Atlantic SST anomalieshad a signif cant impact on EASM in addition to the anomalous SSTs in the Pacif c and Indian oceans.They used an atmospheric model to examine the impact of Atlantic SST anomalies on the climate anomalies of the WNP and East Asia during the summer of 1998.They did this by performing a series of simulations using global SSTs and subsets of SSTs,which were carried out to isolate the impact of the Atlantic Ocean.They further suggested that the impact from the Atlantic occurs as a result of a Rossby-Kelvinwave respondingto the Atlantic SST anomalies.Actually,the main features of the precipitation anomalies in Asia during the summer of 1998 can be well captured by other models,possibly due to the strong impact of tropical SST anomalies in this particular case.

Besides the tropical Atlantic Ocean,many studies have emphasized the role of the NAO or Arctic Oscillation(AO) in climate anomalies in China.Wu and Huang(1999)found that with a high index of NAO the Siberian high tends to be weakened along with a weaker EAWM and above-normal temperatures in northern China.Xu et al.(2001)showed that the onset of Mei-yu in the Yangtze River valleys is positively correlated with NAO in the previous winter.Moreover,the earlier onset of Mei-yu often correspondsto a warmer SST in the North Atlantic,and vice versa.Using longer-term data, Gong et al.(2002)further indicated that there is a signif cant negative correlation between summer rainfall in the Yangtze River valleys and AO in May.When AO in springis stronger, the jet stream in East Asia tends to move poleward,resulting in drier conditions.It is suggested that the signal of the NAO or AO is helpful for summer rainfall prediction in China.It has also been revealed that a positive phase of the cold season NAO is associated with springtime cooling downstream of the Tibetan Plateau and def cient spring rainfall in regions south of the Yangtze River valleys(Yu et al.,2004;Yu and Zhou,2004).

In addition to short-term f uctuations,North Atlantic SSTs also exhibit a low-frequency f uctuation with a period of about 65-80 years.This is referred to as the Atlantic Multidecadal Oscillation(AMO).Observational analysis has indicated that duringthe positive(negative)phase of the AMO, China experiences warmer(cooler)summers,and eastern China experiences wetter(drier)conditions(Lu et al.,2006). This observational relationship between the AMO and summer climate in China can be conf rmed by model simulations (Lu et al.,2008).Furthermore,it has been found that theAMO can affect winter climate in China.Both observational analyses and simulated results by several AGCMs indicate that the AMO has an impact on the EAWM(Li and Bates, 2007).A recent study suggests that a positive phase of the AMO can lead to warmer air temperatures in all four seasons(Wang et al.,2009b).This implies that a transition in the AMO from its current positive phase to a future negative phase will alleviate the warming trend in China caused by greenhouse gases.

Other patterns of Atlantic SST f uctuationmay also affect decadal variations of climate in China.Gu et al.(2009)indicated that both Mei-yu rainfall amounts and Mei-yu lengths exhibit a decadal f uctuation with a period of about 12 years. Theysuggestedthatsuchaf uctuationisrelatedtothetripolelike SST anomalies in the preceding winter.

Various paleoclimate records have shown that the Asian monsoon has been punctuated by numerous sub-orbital timescale events,andthat these eventswere coevalwith those that happened in the North Atlantic.Based on numerical experiments,Wang et al.(2004)showed that a warmer North Atlantic,representinganintensif edthermohalinecirculation, may lead to a wetter climate over China.This is consistent with evidence derived from climate reconstructions.To illustrate a dynamical explanation for this climatic link between these two remote areas,Lu and Dong(2008)investigated the Asian summer monsoon’s responses to the Atlantic Ocean forcing by applying an additional freshwater f ux into the North Atlantic.Their simulated results showed that the weakenedAtlantic thermohalinecirculation,due to the freshwater f ux,led to a signif cantly suppressed Asian summer monsoon.Furthermore,the weakened Atlantic thermohaline circulation can also modulate the ENSO-South Asian monsoon interaction,which suggests the existence of non-local mechanisms for the decadal-multidecadal modulation of the ENSO-monsoon relationship(Lu et al.,2008).

5. Role of land surface processes in East Asian monsoon

It is well-known that the intensity of monsoon is related to land surface processes as well as oceanic state.Early studies noted the inf uence of snow over the Tibetan Plateau duringwinterandspringonEASM.ZhangandTao(2001)found that when there is morespring snow overthe TibetanPlateau, the local sensible heat from spring to summer becomes weak. As a result,more rainfall is found over the Yangtze River valleys due to a later onset and weaker EASM.Chen et al. (2000a)investigatedtheroleofsnowoverthe TibetanPlateau based on the viewpoint of air-sea interaction,and suggested that the winter snowfall over the plateau can affect the activity of EAWM.The zonal wind and SST over the western Pacif c are further changed through the Hadley cell over East Asia and convective activity in the SCS,thus exerting an inf uence on the WPSH and summer rainfall distribution in China.Chen(2001)pointed out that,besides the strong El Ni?no,theanomaloussnowfallduringthepreviouswinterover the plateau also played a role in the serious f ooding of 1998 in the Yangtze River valleys.The evolution of summer circulation in 1998 was similar to that in years with more snow, leading to a delayed northward movement of the WPSH and maintenance of the rain belt over the Yangtze River valleys. Accordingly,the plateau’s snow cover is now successfully used to predict summer rainfall in China.

The inf uences of snow cover over Eurasia on East Asian climate have also been noted in recent years,particularly since the improvedavailability of satellite observationaldata. Chen and Sun(2003)found that a Eurasian teleconnection pattern can be excited by anomalous snow cover through a radiation effect,thus inf uencing the activity of EAWM.Due to a lagged effect on the atmospheric circulation,the winter snow can further play a role in EASM and summer climate in China.A recent study by Mu and Zhou(2010)showed that the total fresh snow extent(TFSE)in winter over northern Eurasia is closely related with summer climate anomalies in China.With more TFSE in winter,there is an anomalous cold low to the east of Lake Baikal and a lower surface air temperature in northeastern China during summertime.With an enhanced subtropical westerly jet in East Asia and a more northward location of the WPSH,a hot and dry summer occurs south of the Yangtze River valleys(Fig.7).More importantly,this relationship is independent of ENSO and stable during the last 40 years.Furthermore,Mu and Zhou(2012) revealed the related physical mechanism,showing that the consistent seasonal evolution of land surface anomalies acts as a bridge for the lagged correlation.A larger TFSE in winter is followed by a slower northeastward melting progress of snow and frozen soil over northern Eurasia in spring.Subsequently,in summer,a lower soil temperature is found over East Asian middle and high latitudes due to more intensive melting of snow and frozen soil.The local cooling,through the anomalous meridional wind,is connected to the WPSH and climate anomaly in China by enhancing the East Asian jet.In particular,the signal of winter TFSE tends to be strengthened during the melting process in spring and becomes more signif cant in summer.Hopefully,the TFSE can be used as anotherindicator for predictingsummer climate in China.

Like snow cover,prior soil moisture can also play a role in summer rainfall anomalies.It is generally believed that wetter soil moisture in spring may reduce land surface temperature and the associated thermal contrast between land and sea in summer,resulting in a weak EASM.Early studies were generally conf ned to numerical simulations due to limitations of observational data.Wang(1991)showed that the monthly climate anomaly in China is affected by the initial anomaly of soil moisture,with the affected area conf ned to theanomalouscenterandits south.Zenget al.(1998)demonstrated that an atmospheric model coupled with an advanced land surface model including soil moisture can provide a more reasonable simulation of the intraseasonal features of the Asian summer monsoon.By introducing a soil moisture retrieval scheme into an operationalpredictionmodel,Guo et al.(2007)provedthatabetterpredictionofsummerrainfallinChina can be expected with an improvement in the representation of the initial soil moisture.Based on a regional climate model,it was also shown that the simulation of surface air temperature and precipitation in China can be improved to a large degree by including the assimilation of soil moisture data(Hu et al.,2010;Zhang et al.,2012).

More recently,some studies have revealed a relationship between soil moisture anomalies in spring and summer rainfall in China using reanalysis data.Zuo and Zhang(2007) found that,corresponding to wetter soil moisture in spring from the Yangtze River valleys to northern China,there is more summer rainfall over the Yangtze River valleys.Liang and Chen(2010)showed that there is less summer rainfall in southernChinaifthelocalsoil moisturein springis drierthan normal.Le et al.(2007)found that the EASM is positively correlatedwithsoil moistureinspringinsouthwesternChina. Zhan and Lin(2011)indicated that there is a positive correlationbetweenrainfallin June andsoil moisturein springin the Yangtze River valleys.However,Dai and Zuo(2010)denied there is a signif cant connection between spring soil moisture and summer rainfall in China,except in some small areas. Clearly,there is great uncertainty in this area of study due to limited observational data;further work is needed to reveal a def nite relationship between spring soil moisture and summer rainfall in China.

6. Interactions of the atmospheric circulation between the two hemispheres and East Asian monsoon

6.1.Cross-equatorial f ows and the essence of monsoon

In the early 1930s,Li(1936a)pointed out that there is an interaction in atmospheric circulation between the two hemispheres.Based on the limited data available at the time,Li (1936a)found that a violent cold surge in East Asia invaded the SH across the Equator,resulting in rainfall in Darwin, northern Australia.It was similarly found that a cold surge in austral winter from Australia can induce the occurrence of typhoons in the tropical western Pacif c(Li,1936b).It should be noted that given the sparse distribution of weather stations in the tropics in the 1930s,Li’s(1936a,b)work was a truly noteworthy advance.With the increase in the number of weather stations and the advent of satellite observations, the notion of interactions in the atmospheric circulations between the two hemispheres became increasingly clear.In general,a cold surge occurs at the polar region of the winter hemisphere,and the strong,cold air moves rapidly toward the Equator,resulting in precipitation in the summer hemisphere.The winter hemisphere circulation,therefore,plays an active role in the interactions.In fact,it is clear that the summer monsoon circulation in South Asia,and even part of the EASM circulation,initiates from the South Indian Ocean. Another branch of the EASM circulation initiates from the CEF between Sumatra and Sulawesi over Indonesia.

As a channel connecting the two hemispheres,the CEF plays an important role in the exchange of mass,momentum and energy between the two hemispheres.After the 1980s, Chinese meteorologists found other CEFs from Australia in addition to the Somali jet(Wang and Li,1982;Tang et al., 1985).Based on more reliable data,Gao and Xue(2006)further proved that,besides the Somali jet,the CEF at 125°E is of secondary importance to EASM.Many studies have emphasized the role of CEF in EASM and summer rainfall in China.Huang et al.(1989)found that the Mei-yu rainfall in the Yangtze River valleys is positively correlated with crossequatorial water vapor transport.Li et al.(1998)noted that thereis anevidentdisplacementofCEF duringf oodingyears in China.Wang and Xue(2003)found that the Somali jet plays a role in inter-hemispheric water vapor transport,resulting in the summer rainfall anomaly in China.They also showed that the Somali jet usually foreshadows the variation of EASM,and thus the signal of the Somali jet is useful for summer rainfall prediction in China.

Recent studies have revealed that CEF is important to the onset of SCSSM(Li and Wu,2002).The onset is directly inf uenced by the intensif cation and eastward extension of the equatorial westerly over the tropical Indian Ocean,whichis associated with the intensif cation of the Somali jet.Gao and Xue(2006)indicated that two pentads prior to the onset of SCSSM there was a rapid enhancement of the Somali jet.This accelerated the eastward extension of the westerly over the Bay of Bengal and the eastward retreat of the WPSH over the SCS.In the meantime,the rapid enhancement of the CEF in the SCS was favorable for the northward march of the subtropical high.The SCSSM started,f nally,under the combined actions of the two CEFs.Furthermore,the SCSSM onset tended to occur earlier when the two CEFs were stronger and established earlier,and vice versa.

The interactions of the atmospheric circulation between the two hemispheres can be indicated by mass transport across the Equator.Zeng and Li(2002)noted that two factors are important for this transport.One is the annual cycle of solar radiation,approximatelyrepresentedby the zonal mean,and the other is the differences in the surface characteristics,including sea-land distribution and topography.On average,the former is about two times more important than the latter,and hence they are referred to as the f rst and second force,respectively.The two forces are in-phase over the Asian-Australian region,forming the most pronouncedmonsoon system overthe globe.Conversely,the two forces counteract each other in the tropical eastern Pacif c and Atlantic, whichiswhythetradewindprevailsthere.Theresultsproved that the annual cycle of solar radiation plays a primaryrole in the formation of the tropical monsoon,while the difference in surface characteristics is of secondary importance.

6.2.The role of the SH circulation in EASM

An early study by Tao et al.(1962)showed that the alternating changes between the zonal and meridional circulations in East Asia are closely related to those in Australia during boreal summer.The prevailing meridional circulation in Australia often leads to that in the tropical latitudes in East Asia with an intense mass transport from the SH to the NH.The effects of cold air activity in Australia on EASM were also demonstrated in a numerical experiment(He et al., 1991).With the intensif cation of the Australian high(AH), the meridional perturbation propagates northward from Australia,resulting in a northward movement of the wind f eld and rainfall over East Asia.

Huang and Tang(1988)later noted the importance of the Mascarene high(MH)to EASM.Usually,after the intensifcation of the MH,the AH tends to be intensif ed through the energy dispersion of a Rossby wave in the SH westerly.Subsequently,the intensif cation of CEF results in the changes in EASM.Xue and He(2005)further found that both highs play a role in the east-west oscillation of the WPSH.Taking 1980 as a typical example,they showed that the two highs exhibit a biweekly oscillation,and that the AH downstream of the MH tends to be intensif ed with the intensif cation of the MH.Afterwards,the CEFs to the north of the two highs are affected,and the WPSH is further inf uenced through advection.The WPSH tends to be enhanced and extends westward with the intensif cation of the MH,while it tends to be weakenedand retreats eastwardwith theintensif cationof the AH.As a result,the WPSH exhibits an obvious east-west oscillation.Moreover,the WPSH tends to extend westward continuously in boreal summer because the warm advection associated with the MH is muchstrongerthan the cold advection associated with the AH.It is worth noting,in particular, that the low-frequency oscillation of the WPSH lags behind that of the MH or AH by 10-25 days.The two highs,therefore,are useful for mid-range forecasting of the WPSH and the associated Mei-yu rainfall.

The two highs also play a role in the interannual variation of summer rainfall in China.Based on correlation analysis and a case study,Xue et al.(2004)found that with a stronger MH in boreal spring,there is more rainfall from the Yangtze River valleys to Japan,while there is less rainfall outside of this region.Compared with the MH,the inf uence of the AH is conf nedto southernChina.WhentheAH is stronger,there is more rainfall in southern China.Moreover,the relationship between the two highs and summer rainfall in China is generally reproduced by numerical experiments(Xue et al., 2003a).This result is of importance to summer rainfall prediction in China.

The SH circulation in the midlatitudes plays a role in EASM,especially in the seasonal transition from boreal spring to summer.He and Yang(1981)showed that there is a close relationship between the intensity of the southwest summer monsoon and the SH circulation pattern.When a meridionalpattern appearsin the SH,the southwest monsoon tends to be intensif ed.Tao et al.(1983)found that two pentads prior to the onset of the Asian summer monsoon,the westerly jet over 40°-160°E in the SH midlatitudes tends to be intensif ed along with the intensif cation of CEF,which plays a triggering role in the establishment of the EASM circulation.Zhu and Lu(1984)also noticed the role of SH cold airactivityintheNHseasonaltransition.HeandChen(1989) analyzed the inf uence of a quasi-40-day oscillation in the midlatitudeSH onthe Asian summermonsoon.Theyshowed that after the outbreak of the SH cold air,the westerly to the south of the MH and AH and the easterly to the north tend to be intensif ed through the meridional propagation of the oscillation,resulting in the intensif cation of CEF and an active Asian summer monsoon.Based on the similarity theory proposed by Zeng and Zhang(1992),Xue et al.(2002)further showed that the earliest seasonal transition in the lower troposphere occurs in the Antarctic region at the end of March. The subsequent transition takes place in the SH subtropics in mid-April,f nally resulting in the onset of SCSSM from middle to late May.All of these studies indicate that the SH circulation and EASM are closely connected through the CEFs, and that the SH circulation plays a leading role.Hence,the SH circulation is valuable for EASM prediction.

6.3.The role of the Antarctic oscillation and sea-ice coverage in EASM

As a dominant mode in the SH,the Antarctic oscillation (AAO)represents the out-of-phase relationship of sea level pressure between the subtropics and high latitudes.With a barotropic structure,the AAO is also evident in temperatureand zonal winds(Gong and Wang,1998).As a strong interannual signal,the AAO plays a role in the SH circulation, includingthe MH and AH.Correlationanalysis indicates that the MH is mainly determined by the AAO,while the AH is inf uenced by both the AAO and ENSO(Xue et al.,2004). Composite analysis further shows that with the intensif cation of the AAO and MH,the Somali jet and the Indian monsoon westerly tend to be strengthened.Accordingly,the AH and the associated CEF become stronger,whereas the trade winds over the tropical western and central Pacif c become weaker.In association with the above changes,convective activities near the Philippine Sea are largely suppressed.As a consequence,a Rossby wave train appears from East Asia via the North Pacif c to the western coast of North America.In particular,when the AAO tends to be stronger during boreal spring through summer,there is more summer rainfall overthe Yangtze River valleysand less rainfall outside of this region.The inf uence of the AAO has been further demonstrated based on longer data(Wang and Fan,2005;Sun et al., 2008).Therefore,the AAO signal is of importance to summer rainfall prediction in China.It should be emphasized, however,that such an anomalous circulation pattern and associated rainfall distribution appear under a small difference in SST;hence,it is essentially different from the EAP pattern proposed by Huang and Li(1988).

Besides the AAO,Antarctic sea-ice coverage is also important to the prediction of EASM and summer rainfall in China.Fu(1981)foundthat Mei-yuin the YangtzeRiver valleys is negatively correlated with the Antarctic sea-ice in the preceding year.Peng and Wang(1989)showed that there is a negative relationship between Antarctic sea-ice and the intensity of the WPSH,with an eight-month lag in correlation. Theridge-lineofthe WPSH andthe tropicalcycloneslanding in China are also inf uenced by Antarctic sea-ice(Zhao and Ji,1989).Xue et al.(2003b)further indicated that when the Antarctic sea-ice tends to increase fromboreal springto summer,there is more rainfall in northern China and less rainfall in southern China.The effects of Antarctic sea-ice on the SH circulation as well as EASM have also been demonstrated in numerical experiments(Yang and Huang,1992;Wang and Huang,1994).Furthermore,there is a six-month lagged correlation of the AAO with Antarctic sea-ice,possibly due to the inf uence of the semi-annual oscillation in the SH high latitudes(Gao et al.,2003).

In summary,during the seasonal transition from boreal spring to summer,the SH circulation,including the AAO and the associated MH and AH,maintains seasonal persistence due to the effects of Antarctic sea-ice coverage.The EASM and summer rainfall in China are inf uenced by the SH circulation through the CEFs,as shown in Fig.8(Xue,2005). Because of the predictabilitybarrierin borealspring,summer monsoonpredictionbased on the ENSO signal is largelylimited.As stated above,the SH circulation,especially the AAO and Antarctic sea-ice coverage,may provide some valuable information for EASM prediction due to the seasonal persistence during boreal spring through summer.In other words, the predictability barrier can be overcome to a certain degree if the signal of the AAO is taken into account.

7. Interdecadal variations of the East Asian monsoon and climate anomalies in China

7.1.Theweakeningofthe East Asian monsoonduring the late 1970s and climate shift in China

In the last two decades,northern and northeastern China has suffered from severe and persistent droughts,while the Yangtze River basin and southern China have been dominated by much more signif cant heavy rainfall/f ood events. Many studies have indicated that this is related to a notable interdecadal variation of EASM during the late 1970s together with an overall climate shift in China(Peng et al., 1999;Wang,2001;Xue,2001;Huang et al.,2006b;Ding et al.,2007;Zhou et al.,2008a).Following the weakening of EASM circulation,the WPSH has extended more southwestward.Accordingly,the summer rainfall regime has experienced an obvious abrupt shift in eastern China,with def cient rainfall and associated droughts in northernChina and excessive rainfall and increased f ooding conditions in the Yangtze River valleys,staring from the late 1970s.The weakening of EASM and the rainfall regime shift coincides well with other climate change signals that have been observed in other regions as well as in other variables(Yu and Zhou,2007;Zhou et al.,2008b,2008c).

Until now,different physical mechanisms have been proposed to explain the late 1970s climate shift.Xue(2001)noted that the atmosphere-ocean interaction in the western Pacif c and Indian Ocean plays a role in the weakening of EASM.GongandHo(2002)foundthatchangesintheWPSH are strongly associated with the variation of SSTs in the eastern tropical Pacif c and tropical Indian Ocean,which is responsibleprimarilyfortherainfallregimeshiftinChina.This hypothesiswas demonstratedby Zhouet al.(2009b),who examined the responses of fve AGCMs to specif ed,identical IndianOcean-WesternPacif c(IWP)warming.Thespecif ed IWP warmingled toa westward extensionoftheWPSH in all fve AGCMs.

Other studies have emphasized the role of the Pacif c decadal oscillation(PDO),which shows a clear phase transition on a decadal timescale(Zhu and Yang,2003;Yang et al.,2005;Zhang et al.,2007;Gu et al.,2007).During the warm phase of the PDO,there is higher SST in the central and eastern Pacif c but lower SST in the North Pacif c. In this case,the northward moisture transport in East Asia is greatly weakened and cannot reach northern China,thus causing a decrease in precipitation or droughts.In contrast, the Yangtze River basin and southern China receive a large amount of moisture and have strong upward motion,creating favorable conditions for the frequent occurrence of heavy rainfall.

Since the monsoon circulation is driven by the thermal contrast between land and sea,the weakening of the EASM circulation is also related to land-surface conditions,in additiontotheoceanicstate.Xuetal.(2007)investigatedtheconsistency of interdecadal variation in EASM with change in the sea-land springtime surface air temperature over eastern China and the adjacent oceans.They found that the springtime sea-land surface air temperature distribution after 1978 showed a shift in interdecadal trends.It is therefore suggested that the regional springtime sea-land surface air temperature in East Asia might have,in part,led to a weakening of the effect of sea-land thermal drive on EASM.Ding et al. (2009)noted that there was an abrupt increase in winter and spring snow over the Tibetan Plateau(TP)after 1977.Subsequently,the atmospheric heating f elds over the TP assumed a signif cant weakening after the late 1970s.This weakening is closely related to the signif cantly reduced surface sensible heat f ux into the atmosphere and the subsequent cooling over the TP and its surrounding atmosphere.The above interdecadal variability of heating f elds over the land area in the Asian region has consistently reduced the land-sea thermal contrast in summer in the Asian monsoon region,thus leading to the weakening of EASM.

Congruentwiththesummerclimatetransition,thespringtime climate over East Asia also exhibits a similar interdecadalscaletransition.Thisis evidentinboththespringtime rainfall and the cooling tendency located downstream of the TP.It has beensuggestedthatteleconnectionsassociatedwith the interdecadal change of the NAO are one mechanism(Yu and Zhou,2004;Li et al.,2005b;Xin et al.,2006).Yu et al. (2008)attempted to clarify the interdecadal climate change over East Asia within the context of 3D coherent structures. The results indicated that the decadal-scale shift of the East Asian climate exhibits a distinct 3D structure.The surface climate change is coherently connected with the temperature change over the middle and upper troposphere,with clear seasonal features.A prominent cooling trend is found over East Asia in the upper troposphere around 300 hPa.Accompanying this summer cooling,the upper level westerly jet stream over East Asia shifted southward and the EASM tended to be weakened,resulting in the tendency toward increased droughts in northern China and more f ooding along the Yangtze River valleys.Yu and Zhou(2007)also regarded the spring and summer interdecadal scale climate changes as the seasonal manifestation of one phenomenon.The impact of springtime troposphericcoolingover East Asia on the surface climate has been demonstrated by numerical experiments(Xin et al.,2006).In addition,an extended analysis by Zhou and Zhang(2009)revealed an interdecadal variability of July-August tropospheric temperatures across the entire subtropical NH.A major mode was identif ed,with one signif cant cooling center over East Asia and two warming centers over the North Atlantic and North Pacif c,respectively. Hence,the interdecadal climate transition of the East Asian climate may be a local manifestation of the NH climate shift.

Various numerical experiments have been performed to reveal the physical mechanisms involved in the weakening of the EASM.The weakening can usually be simulated by an atmospheric model forced by historical SST(Zeng et al., 2007;Fu et al.,2009).Li et al.(2008b)analyzed the ensemble runs from 1950-2000by using two different AGCMs that were forced separately by observedtropical and global SSTs. Theyfoundthat theobservedSST forcing,primarilyfromthe tropics and including both the Pacif c and the Indian Ocean, was able to reproduce most of the observed atmospheric circulationchangesassociatedwiththeweakeningoftheEASM since the 1970s.However,most atmospheric models fail to capture the major features of the interdecadal climate shift (Han and Wang,2007),especially with respect to the monsoon’s rainfall change.Despite reasonable simulation of the observed circulation changes,they fail to reproduce the relativelysmall-scalerainfallchangepatternsoverEast China(Li et al.,2008b).Recently,using the fourth generation AGCM with a relatively high resolution developed at the Institute of Atmospheric Physics,Chinese Academy of Sciences,Chen and Xue(2013)showed that the model simulates the decadal weakening of EASM circulation as well as the anomalous summer rainfall pattern in eastern China,i.e.,the increase over the Yangtze River basin and the decrease over northern and southern China.Further analysis indicated that the decadal weakening of EASM is mainly driven by the warming in the tropical oceans,which is related with the phase transition of the PDO in the late 1970s.Hence,simulated results are model-dependent.

Although coupled atmosphere-ocean models forced by greenhouse gases can simulate the global warming trend during the latter half of the 20th century,they do not reproduce the weakening of EASM(Jiang and Wang,2005;Yu et al.,2008).Therefore,although it can be argued that the weakening is caused by anthropogenic global warming,it ismore likely attributable to the natural variability of the global atmosphere-oceansystem(Chen and Xue,2013).

7.2.Interannual variability of the East Asian monsoon during different decades

Many studies have shown that the interannual variability of EASM is modulated by the decadal climate background. In particular,the relationship between ENSO and EASM is variable during different decades(Wang,2002).There is a high correlation and a low correlation period,and a larger interannual variability is found during the higher correlation period.This variable relationship also appears in a coupled ocean-atmospheremodel(Jiang et al.,2004).

Gao and Wang(2007)examined the correlation between summer rainfall in China and SST anomalies in the tropical Pacif c in the preceding winter.They found that the signif cance of ENSO on summer rainfall prediction in China decreased remarkably after the late 1970s.Before the late 1970s,when above-normal SSTs occurred over the central and eastern Pacif c during the preceding winter,more summer rainfall could be found in northern China and south of the Yangtze River valleys and less summer rainfall over the Huaihe River regime,and vice versa.However,all of these relationships were found to obviously weaken after the late 1970s.For example,during 1951-1974 there were 43 stations in total whose summer rainfall anomalies could be predicted by the Ni?no3 SST anomaly of the preceding winter, with an accuracy of anomaly sign of greater than 67%,while the total number of such stations reduced to 15 during 1980-2003.Zhu et al.(2007)found that the signif cant differences in the ENSO-related atmospheric circulation anomalies in East Asia during different decades are probably responsible for the interdecadal variation between ENSO and EASM.Ye and Lu(2011)further indicated that the ENSO-related rainfall anomalies tended to be similar between early and late summer before the late 1970s;that is,the period characterized by a stronger ENSO-summermean rainfall relationship. After the late 1970s,however,the anomalous rainfall pattern in eastern China was found to be almost reversed between early and late summer,resulting accordingly in a weakened relationship between ENSO and the total summer rainfall in eastern China.Therefore,the background of the interdecadal variation of the predictor must be fully considered when the effect of ENSO is taken into account in operational predictions of summer precipitation in China.

It is interesting to note that,with the decadal variation in thelate1970s,theSHcirculationandits inf uenceonsummer rainfallineasternChina alsochanged(Sunet al.,2012).With a positive anomaly of the AAO in boreal spring,there was more rainfall oversouth of the Yangtze River and less rainfall over the Yangtze-HuaiheRiver valleys before the late 1970s. On the other hand,there has been more rainfall from south ChinatotheYangtze-HuaiheRivervalleysandless rainfallin northernand northeasternChina after the late 1970s.Clearly, the inf uence of the AAO tends to intensify and extend more northward.Itis thereforesuggestedthat theAAO canbeused as an important predictor of summer rainfall in China when the effect of ENSO as a predictor is weak,as is presently the case.

Wang and Huang(2006)indicated that the relationship of summer rainfall between northern China and India is also variable during different decades,with a higher correlation appearing during strong EASM periods.In general,a lower SST in the eastern Pacif c is associated with more La Ni?na events and a higher correlation.After the late 1970s,the SST in the eastern Pacif c tended to be enhanced due to the phase transitionofthePDO.Therelationshipofsummerrainfallbetween the two regions became weak,as found in the ENSOEASM relationship mentioned above.

As well as the EASM,Xu et al.(1997)noted that the relationship between the EAWM and ENSO has also changed during different decades.This relationship is modulated by both the winter monsoon and the oceanic decadal background.When they are in-phase,a strong winter monsoon is favorable for the onset of an El Ni?no event.On the other hand,a strong winter monsoon corresponds to a La Ni?na event.

8. Concluding remarks

Because of the importance of the East Asian monsoon in controlling climate anomalies in China,Chinese scientists have invested great effort in monsoon studies since the 1930s.In recent years,the availability of a complete dataset and powerful computers have allowed Chinese scientists to explore many aspects of the East Asian monsoon in a more comprehensive fashion than was previously the case.These major advances have been reviewed in this paper,including those related to seasonal variation,interannual variation and interdecadal variation.The major conclusions can be summarized as follows:

First,as the monsoon is characterized by large-scale seasonal wind reversal,seasonality can therefore be used to describe the monsoon regions.In addition to the traditional tropical monsoon,there are the subtropical and temperatefrigid monsoonsin the lower troposphere,the planetarymonsoon in the upper troposphere,and the stratospheric monsoon.These regions with their large seasonality make up the global monsoon system,and exhibit a signif cant baroclinic structure.Similarities of atmospheric circulation have been proposedto describe the seasonal variation of monsoons,and it has been shown that the seasonal variation originates from the upperstratosphere and the coldest regionof the lower troposphere.

The Asian monsoon exhibits clear patterns of seasonal variation,the most signif cant of which is the onset of SCSSM around mid-May,signaling the seasonal transition from winter to summer over the entire Asian summer monsoon system.Recent studies have shown that,instead of a regional phenomenon,the onset of SCSSM is connected with the seasonal variation of the global atmospheric circulation.The thermal contrast between the Asian continent and adjacent oceans plays a dominant role in the onset.The CEFs associated with the SH circulation and the stratospheric circulationmay foreshadowthe onset.Also evidentis the ISO duringthe summer monsoon period.Different from the ISO in the tropical monsoon region,as represented by the active or break monsoon rain,the circulation system and rain belt in the East Asian monsoon region exhibit a northward movement in a stepwise fashion with two distinct jumps.In particular,the f rst and second jump of the WPSH are closely connected with the start and end of the Mei-yu period in the Yangtze River basin.

Due to the importance of summer rainfall prediction in China,the relationship between EASM and ENSO has been studied extensively.As the strongest signal in the coupled ocean-atmosphere system,ENSO plays a fundamental role in EASM and summer rainfall in China.It has been shown that,compared with tropical summer monsoons,such as Indian summer monsoon,ENSO has a long-delayed effect on EASM.In the succeeding year after the onset of El Ni?no events,a suppressed convection in the warm pool of the tropical western Pacif c excites a Rossby wave train and the associated East-Asia-Pacif c teleconnection pattern,and the WPSH is located more southwestward.As a result,there is more rainfall along the Yangtze River basin and less rainfall in southern and northern China.A typical example occurred in the summer of 1998,when the Yangtze River valleys experienced a devastating f ood due to the strong ENSO event of 1997-98.Recent studies also revealed that there is a nonlinear relationship between the magnitude of ENSO and the EASM circulation and the associated summer rainfall in China;the composite summer rainfall in China in moderate ENSO years exhibits a northern rainfall pattern,which is totally different from the classical ENSO-type rainfall pattern. In addition,the summer rainfall anomaly in China is sensitive to the spatial distribution of SST anomalies in the tropical Pacif c.These results show a more complex relationship between ENSO and EASM than previously thought.Furthermore,extratropicalatmosphericdisturbances,includinga stronger EAWM,play a triggeringrole in the onset of ENSO, indicatingthat the monsoonand ENSO are an interactivesystem.

The Indian Ocean and Atlantic Ocean also play a role in EASM.Chinese scientists have noted the inf uence of the Indian Ocean on EASM since the early 1980s,and recent studies have revealed the inf uence of the IOD on EASM,especially when the ENSO signal is relatively weak.Moreover, the Indian Ocean acts as a charger,largely prolonging the effect of ENSO on EASM in the succeeding year.The inf uence of the Atlantic Ocean,especially the NAO,has also been noted in recent years.These results provide additional information that is useful for summer rainfall prediction in China.

Besides the oceanic state,there is now ample evidence that land surface processes play a role in the EASM interannual variability.When there is more spring snow on the TP,for instance,the local sensible heat during spring through summer tends to be weakened,leading to a weaker EASM and more rainfall in the Yangtze River basin.A recent study also noted that the total fresh snow extent in winter over Eurasia is closely related with summer climate anomalies in China,providing a new predictor for summer climate prediction in China.Soil moisture is believed to be another factor that may inf uence the EASM.Compared with snow cover,however,thereis uncertaintyregardingtherelationship between spring soil moisture and summer rainfall in China, largely due to insuff cient observational data.

There is interaction of the atmospheric circulation between the two hemispheres,and the SH circulation exerts an inf uence on EASM through the CEFs.With the intensif cation of the AAO,the MH and AH in the southern subtropics, along with the CEFs,tend to be enhanced.Correspondingly, the WPSH tends to be intensif ed and located more southwestward,thereby inducing an ENSO-like rainfall anomaly in East Asia.It is also noted that the SH circulation plays a leading role duringthe seasonal transition from boreal winter to summer.In the meantime,the SH circulation maintains a seasonal persistence due to Antarctic sea-ice coverage.It is therefore suggested that the signal of the SH circulation,especially the AAO,is valuable for the seasonal forecasting of summer rainfall in China.

In addition to interannualvariability,the East Asian monsoon also exhibits an interdecadal variability,the most signif cant feature of which is the weakening of EASM since the late 1970s.Following this weakening,summer rainfall patterns in China have changed,with more rainfall over the Yangtze River valleys and less rainfall over northern China. Different physical mechanisms have been proposed to explain this unique climate shift,including the phase transition of the PDO and global warming.Also noteworthy is that the interannual variability of EASM has changed after this climate shift.For instance,the relationship between summer rainfall in China and ENSO has tended to weaken since the 1970s,making seasonal forecasts in China based on ENSO more unreliable.Further studies are needed to reveal the nature of this weakening relationship between EASM and ENSO.

Recent studies have shown that the East Asian monsoon is a complex,nonlinear phenomenon involving atmospheric, oceanic and land-based processes,and it exhibits variability on a variety of time scales,ranging from intraseasonal and interannual to interdecadal time scales.It is inf uenced by many factors,including external factors(e.g.,ENSO)and internal atmospheric dynamics(e.g.,the AAO).It is now recognized that the prediction of the monsoon is limited by the high-frequencyvariabilityassociatedwith its internaldynamics.To further enhance monsoon predictability,it is important to determine in what ways the distribution of these highfrequency signals interact with the large-scale stationary patterns induced by the slowly changing boundary conditions.

Although marked progress has been made in monsoon studies in China over the years,the physical mechanisms for the occurrence of droughts and f oods in China and the associated East Asian monsoon variability are still not fully understood.While it is generally accepted that the mean monsoon system is highly stable,state-of-the-art climate models are relatively poor in simulating the East Asian monsooncompared to other monsoon regions.Although the prediction of monsoon rainfall in China using dynamical models has made great advances,it is far from satisfactory(Wang et al.,2015).It is unclear at this stage whether the inability of current models to simulate and predict monsoon rainfall is due to model def ciencies or the intrinsic lack of predictability of the monsoon.In addition,understanding,modelingand predictingmonsoonsareofgreatimportanceto theprojection of future climate change due to the increase in the concentration of greenhouse gases.The East Asian monsoon region is one of a few places for which nearly all climate models project an enhanced interannual variability of summer rainfall in association with global warming(Lu and Fu,2010). Therefore,our ability to predict the interannual changes in monsoon circulation and rainfall is a critical requirement for the sustainable development of China.There is a vital need in the near future to obtain a complete picture of the spatial and temporal aspects of the East Asian monsoon in order to advance our understanding and awareness of the monsoon to a new level and to provide guidance for the improvement of seasonal prediction systems in China.

Acknowledgements.The authors would like to thank Prof. WANG Huijun for his help in organizing the writing of this paper. The authors have benef ted from the insightful comments and suggestions of three anonymous reviewers.They also thank Dr.SU Tonghua for preparing some of f gures.This work was supported by the Chinese Academy of Sciences(Grant No.XDA05110201) and the National Natural Science Foundation of China(Grant No. 41475052).

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(Received 2 June 2014;revised 3 August 2014;accepted 23 August 2014)

?Corresponding author:XUE Feng

Email:fxue@lasg.iap.ac.cn