Interdecadal variability of summer precipitation in the Three River Source Region: Influences of SST and zonal shifts of the East Asian subtropical westerly jet

Summer precipitation in the Three Rivers Source Region(TRSR)of China is vital for the headwaters of the Yellow,Yangtze,and Lancang rivers and exhibits significant interdecadal variability.This study investigates the influence of the East Asian westerly jet(EAWJ)on TRSR rainfall.A strong correlation is found between TRSR summer precipitation and the Jet Zonal Position Index(JZPI)of the EAWJ from 1961 to 2019(R=0.619,p<0.01).During periods when a positive JZPI indicates a westward shift in the EAWJ,enhanced water vapor anomalies,warmer air,and low-level convergence anomalies contribute to increased TRSR summer precipitation.Using empirical orthogonal function and regression analyses,this research identifies the influence of large-scale circulation anomalies associated with the Atlantic–Eurasian teleconnection(AEA)from the North Atlantic(NA).The interdecadal variability between the NA and central tropical Pacific(CTP)significantly affects TRSR precipitation.This influence is mediated through the AEA via a Rossby wave train extending eastward along the EAWJ,and another south of 45°N.Moreover,the NA–CTP Opposite Phase Index(OPI),which quantifies the difference between the summer mean sea surface temperatures of the NA and the CTP,is identified as a critical factor in modulating the strength of this teleconnection and influencing the zonal position of the EAWJ.

Persistent Variations in the East Asian Trough from March to April and the Possible Mechanism

The East Asian trough(EAT)profoundly influences the East Asian spring climate.In this study,the relationship of the EATs among the three spring months is investigated.Correlation analysis shows that the variation in March EAT is closely related to that of April EAT.Extended empirical orthogonal function(EEOF)analysis also confirms the co-variation of the March and April EATs.The positive/negative EEOF1 features the persistent strengthened/weakened EAT from March to April.Further investigation indicates that the variations in EEOF1 are related to a dipole sea surface temperature(SST)pattern over the North Atlantic and the SST anomaly over the tropical Indian Ocean.The dipole SST pattern over the North Atlantic,with one center east of Newfoundland Island and another east of Bermuda,could trigger a Rossby wave train to influence the EAT in March−April.The SST anomaly over the tropical Indian Ocean can change the Walker circulation and influence the atmospheric circulation over the tropical western Pacific,subsequently impacting the southern part of the EAT in March−April.Besides the SST factors,the Northeast Asian snow cover could change the regional thermal conditions and lead to persistent EAT anomalies from March to April.These three impact factors are generally independent of each other,jointly explaining large variations in the EAT EEOF1.Moreover,the signals of the three factors could be traced back to February,consequently providing a potential prediction source for the EAT variation in March and April.

Future Changes in the Relationship Between the South and East Asian Summer Monsoons in CMIP6 Models

The future changes in the relationship between the South Asian summer monsoon(SASM)and the East Asian summer monsoon(EASM)are investigated by using the high-emissions Shared Socioeconomic Pathway 5-8.5(SSP5-8.5)experiments from 26 coupled models that participated in the phase 6 of the Coupled Model Intercomparison Project(CMIP6).Six models,selected based on their best performance in simulating the upper-and lower-level pathways related to the SASM-EASM teleconnection in the historical run,can capture the positive relationship between the SASM and the rainfall over northern China.In the future scenario,the upper-level teleconnection wave pattern connecting the SASM and the EASM exhibits a significant weakening trend,due to the rainfall anomalies decrease over the northern Indian Peninsula in the future.At the lower level,the western North Pacific anticyclone is projected to strengthen in the warming climate.The positive(negative)rainfall anomalies associated with positive(negative)SASM rainfall anomalies are anticipated to extend southward from northern China to the Yangtze-Huai River valley,the Korea Peninsula,and southern Japan.The connection in the lower-level pathway may be strengthened in the future.

Characteristics and Variations of the East Asian Monsoon System and Its Impacts on Climate Disasters in China

Recent advances in studies of the structural characteristics and temporal-spatial variations of the East Asian monsoon （EAM） system and the impact of this system on severe climate disasters in China are reviewed. Previous studies have improved our understanding of the basic characteristics of horizontal and vertical structures and the annual cycle of the EAM system and the water vapor transports in the EAM region. Many studies have shown that the EAM system is a relatively independent subsystem of the Asian- Australian monsoon system, and that there exists an obvious quasi-biennial oscillation with a meridional tripole pattern distribution in the interannual variations of the EAM system. Further analyses of the basic physical processes, both internal and external, that influence the variability of the EAM system indicate that the EAM system may be viewed as an atmosphere-ocean-land coupled system, referred to the EAM climate system in this paper. Further, the paper discusses how the interaction and relationships among various components of this system can be described through the East Asia Pacific （EAP） teleconnection pattern and the teleconnection pattern of meridional upper-tropospheric wind anomalies along the westerly jet over East Asia. Such reasoning suggests that the occurrence of severe floods in the Yangtze and Hualhe River valleys and prolonged droughts in North China are linked, respectively~ to the background interannual and interdecadal variability of the EAM climate system. Besides, outstanding scientific issues related to the EAM system and its impact on climate disasters in China are also discussed.

Characteristics,Processes,and Causes of the Spatio-temporal Variabilities of the East Asian Monsoon System

Recent advances in the study of the characteristics, processes, and causes of spatio-temporal variabilities of the East Asian monsoon （EAM） system are reviewed in this paper. The understanding of the EAM system has improved in many aspects： the basic characteristics of horizontal and vertical structures, the annual cycle of the East Asian summer monsoon （EASM） system and the East Asian winter monsoon （EAWM） system, the characteristics of the spatio-temporal variabilities of the EASM system and the EAWM system, and especially the multiple modes of the EAM system and their spatio-temporal variabilities. Some new results have also been achieved in understanding the atmosphere-ocean interaction and atmosphere-land interaction processes that affect the variability of the EAM system. Based on recent studies, the EAM system can be seen as more than a circulation system, it can be viewed as an atmosphere-ocean-land coupled system, namely, the EAM climate system. In addition, further progress has been made in diagnosing the internal physical mechanisms of EAM climate system variability, especially regarding the characteristics and properties of the East Asia-Pacific （EAP） teleconnection over East Asia and the North Pacific, the ＂Silk Road＂ teleconnection along the westerly jet stream in the upper troposphere over the Asian continent, and the dynamical effects of quasi-stationary planetary wave activity on EAM system variability. At the end of the paper, some scientific problems regarding understanding the EAM system variability are proposed for further study.

Regional-scale Surface Air Temperature and East Asian Summer Monsoon Changes during the Last Millennium Simulated by the FGOALS-gl Climate System Model

The spatial patterns and regional-scale surface air temperature （SAT） changes during the last millennium,as well as the variability of the East Asian summer monsoon （EASM） were simulated with a low-resolution version of Flexible Global Ocean-Atmosphere-Land-Sea-ice （FGOALS-gl） model.The model was driven by both natural and anthropogenic forcing agents.Major features of the simulated past millennial Northern Hemisphere （NH） mean SAT variations,including the Medieval Climate Anomaly （MCA）,the Little Ice Age （LIA） and the 20th Century Warming （20CW）,were generally consistent with the reconstructions.The simulated MCA showed a global cooling pattern with reference to the 1961-90 mean conditions,indicating the 20CW to be unprecedented over the last millennium in the simulation.The LIA was characterized by pronounced coldness over the continental extratropical NH in both the reconstruction and the simulation.The simulated global mean SAT difference between the MCA and LIA was 0.14°C,with enhanced warming over high-latitude NH continental regions.Consistencies between the simulation and the reconstruction on regional scales were lower than those on hemispheric scales.The major features agreed well between the simulated and reconstructed SAT variations over the Chinese domain,despite some inconsistency in details among different reconstructions.The EASM circulation during the MCA was stronger than that during the LIA The corresponding rainfall anomalies exhibited excessive rainfall in the north but deficient rainfall in the south.Both the zonal and meridional thermal contrast were enhanced during the MCA.This temperature anomaly pattern favored a stronger monsoon circulation.

An Introduction to the Integrated Climate Model of the Center for Monsoon System Research and Its Simulated Influence of El Ni?no on East Asian–Western North Pacific Climate

This study introduces a new global climate model--the Integrated Climate Model （ICM）--developed for the seasonal prediction of East Asian-western North Pacific （EA-WNP） climate by the Center for Monsoon System Research at the Institute of Atmospheric Physics （CMSR, IAP）, Chinese Academy of Sciences. ICM integrates ECHAM5 and NEMO2.3 as its atmospheric and oceanic components, respectively, using OASIS3 as the coupler. The simulation skill of ICM is evaluated here, including the simulated climatology, interannual variation, and the influence of E1 Nifio as one of the most important factors on EA-WNP climate. ICM successfully reproduces the distribution of sea surface temperature （SST） and precipitation without climate shift, the seasonal cycle of equatorial Pacific SST, and the precipitation and circulation of East Asian summer monsoon. The most prominent biases of ICM are the excessive cold tongue and unrealistic westward phase propagation of equatorial Pacific SST. The main interannual variation of the tropical Pacific SST and EA-WNP climate E1 Nifio and the East Asia-Pacific Pattern--are also well simulated in ICM, with realistic spatial pattern and period. The simulated E1 Nifio has significant impact on EA-WNP climate, as in other models. The assessment shows ICM should be a reliable model for the seasonal prediction of EA-WNP climate.

Climate change impacts the distribution of Quercus section Cyclobalanopsis(Fagaceae),a keystone lineage in East Asian evergreen broadleaved forests

East Asian evergreen broadleaved forests(EBFLs) harbor high species richness,but these ecosystems are severely impacted by global climate change and deforestation.Conserving and managing EBLFs requires understanding dominant tree distribution dynamics.In this study,we used 29 species in Quercus section Cyclobalanopsis-a keystone lineage in East Asian EBLFs-as proxies to predict EBLF distribution dynamics using species distribution models(SDMs).We examined climatic niche overlap,similarity,and equivalency among seven biogeographical regions’ species using’ecospat’.We also estimated the effectiveness of protected areas in the predicted range to elucidate priority conservation regions.Our results showed that the climatic niches of most geographical groups differ.The western species under the Indian summer monsoon regime were mainly impacted by temperature factors,whereas precipitation impacted the eastern species under the East Asian summer monsoon regime.Our simulation predicted a northward range expansion of section Cyclobalanopsis between 2081 and 2100,except for the ranges of the three Himalayan species analyzed,which might shrink significantly.The greatest shift of highly suitable areas was predicted for the species in the South Pacific,with a centroid shift of over 300 km.Remarkably,only 7.56% of suitable habitat is currently inside protected areas,and the percentage is predicted to continue declining in the future.To better conserve Asian EBLFs,establishing nature reserves in their northern distribution ranges,and transplanting the populations with predicted decreasing numbers and degraded habitats to their future highly suitable areas,should be high-priority objectives.

Differences and Links between the East Asian and South Asian Summer Monsoon Systems:Characteristics and Variability

This paper analyzes the differences in the characteristics and spatio–temporal variabilities of summertime rainfall and water vapor transport between the East Asian summer monsoon（EASM） and South Asian summer monsoon（SASM） systems. The results show obvious differences in summertime rainfall characteristics between these two monsoon systems. The summertime rainfall cloud systems of the EASM show a mixed stratiform and cumulus cloud system, while cumulus cloud dominates the SASM. These differences may be caused by differences in the vertical shear of zonal and meridional circulations and the convergence of water vapor transport fluxes. Moreover, the leading modes of the two systems＇ summertime rainfall anomalies also differ in terms of their spatiotemporal features on the interannual and interdecadal timescales. Nevertheless, several close links with respect to the spatiotemporal variabilities of summertime rainfall and water vapor transport exist between the two monsoon systems. The first modes of summertime rainfall in the SASM and EASM regions reveal a significant negative correlation on the interannual and the interdecadal timescales. This close relationship may be linked by a meridional teleconnection in the regressed summertime rainfall anomalies from India to North China through the southeastern part over the Tibetan Plateau, which we refer to as the South Asia/East Asia teleconnection pattern of Asian summer monsoon rainfall. The authors wish to dedicate this paper to Prof. Duzheng YE, and commemorate his 100 thanniversary and his great contributions to the development of atmospheric dynamics.

The climatology and interannual variability of the East Asian summer monsoonsimulated by a weakly coupled data assimilation system

With the motivation to improve the simulation of the East Asian summer monsoon(EASM) in coupled climate models, oceanic data assimilation(DA) was used in CAS-ESM-C(Chinese Academy of Sciences–Earth System Model–Climate Component) in this study. Observed sea surface temperature was assimilated into CAS-ESM-C. The climatology and interannual variability of the EASM simulated in CAS-ESM-C with DA were compared with a traditional AMIP-type run.Results showed that the climatological spatial pattern and annual cycle of precipitation in the western North Paci?c, and the ENSO-related and EASM-related EASM circulation and precipitation, were largely improved. As shown in this study, air–sea coupling is important for EASM simulation. In addition, oceanic DA synchronizes the coupled model with the real world without breaking the air–sea coupling process. These two successful factors make the assimilation experiment a more reasonable experimental design than traditional AMIP-type simulations.

Variation of East Asian Summer Monsoon and Its Relationship with Precipitation of China in Recent 111 Years

Based on the monthly average SLP data in the northern hemisphere from 1899 to 2009, East Asian summer monsoon intensity index in recent 111 years was calculated, and the interdecadal and interannual variation characteristics of East Asian summer monsoon were analyzed. The results showed that East Asian summer monsoon in the 1920s was the strongest. The intensity of East Asian summer monsoon after the middle period of the 1980s presented weakened trend. It was the weakest in the early 21st century. Morlet wavelet analysis found that the interdecadal and interannual variations of East Asian summer monsoon had quasi-10-year and quasi-2-year significance periods. The interannual variation of precipitation in the east of China closely related to intensity variation of East Asian summer monsoon. In strong （weak） East Asian summer monsoon year, the rainfall in the middle and low reaches of Yangtze River was less （more） than that in common year, while the rainfall in North China was more （less） than that in common year. The weakening of East Asian summer monsoon was an important reason for that it was rainless （drought） in North China and rainy （flood） in the middle and low reaches of the Yangtze River after the middle period of the 1980s.

The Interannual Variability of East Asian Winter Monsoon and Its Relation to the Summer Monsoon

Based on the NCEP/ NCAR reanalysis data the interannual variability of the East Asian winter mon-soon (EAWM) is studied with a newly defined EAWM intensity index. The marked features for a strong (weak) winter monsoon include strong (weak) northerly winds along coastal East Asia, cold (warm) East Asian continent and surrounding sea and warm (cold) ocean from the subtropical central Pacific to the trop-ical western Pacific, high (low) pressure in East Asian continent and low (high) pressure in the adjacent ocean and deep (weak) East Asian trough at 500 hPa. These interannual variations are shown to be closely connected to the SST anomaly in the tropical Pacific, both in the western and eastern Pacific. The results suggest that the strength of the EAWM is mainly influenced by the processes associated with the SST anom-aly over the tropical Pacific. The EAWM generally becomes weak when there is a positive SST anomaly in the tropical eastern Pacific (El Ni?o), and it becomes strong when there is a negative SST anomaly (La Ni?a). Moreover, the SST anomaly in the South China Sea is found to be closely related to the EAWM and may persist to the following summer. Both the circulation at 850 hPa and the rainfall in China confirm the connection between the EAWM and the following East Asian summer monsoon. The possible reason for the recent 1998 summer flood in China is briefly discussed too. Key words East Asian winter monsoon - Interannual variability - SST - Summer monsoon This study was supported by “ National Key Programme for Developing Basic Sciences” G1998040900 part 1, and by key project (KZ 952-S1-404) of Chinese Academy of Sciences.

Interannual Meridional Displacement of the East Asian Upper-tropospheric Jet Stream in Summer

On the interannual timescale, the meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) is significantly associated with the rainfall anomalies in East Asia in summer. In this study, using the data from the National Centers for Environmental Prediction-Department of Energy (NCEP/DOE) reanalysis-2 from 1979 to 2002, the authors investigate the interannual variations of the EAJS's meridional displacement in summer and their associations with the variations of the South Asian high (SAH) and the western North Pacific subtropical high (WNPSH), which are dominant circulation features in the upper and lower troposhere, respectively. The result from an EOF analysis shows that the meridional displacement is the most remarkable feature of the interannual variations of the EAJS in each month of summer and in summer as a whole. A composite analysis indicates that the summer (June-July-August, JJA) EAJS index, which is intended to depict the interannual meridional displacement of the EAJS, is not appropriate because the anomalies of the zonal wind at 200 hPa (U200) in July and August only, rather than in June, significantly contribute to the summer EAJS index. Thus, the index for each month in summer is defined according to the location of the EAJS core in each month. Composite analyses based on the monthly indexes show that corresponding to the monthly equatorward displacement of the EAJS, the South Asian high (SAH) extends southeastward clearly in July and August, and the western North Pacific subtropical high (WNPSH) withdraws southward in June and August.

Recent Advances in Studies of the Interaction between the East Asian Winter and Summer Monsoons and ENSO Cycle

Recent advances in studies on the interaction between the East Asian monsoon and the ENSO cycle are reviewed in this paper. Through the recent studies, not only have the responding features and processes of the East Asian winter and summer monsoon circulation anomalies and summer rainfall anomalies in East Asia to the ENSO cycle during its different stages been understood further, but also have the thermal and dynamic effects of the tropical \vestern Pacific on the ENSO cycle been deeply analyzed from the observational facts and dynamic theories. The results of observational and theoretical studies showed that the dynamical effect of the atmospheric circulation and zonai wind anomalies in the lower troposphere over the tropical western Pacific on the ENSO cycle may be through the excitation of the equatorial oceanic Kelvin wave and Rossby waves in the equatorial Pacific. These studies demonstrated further that the ENSO cycle originates from the tropical western Pacific. Moreover, these recent studies also showed that the atmospheric circulation and zonai wind anomalies over the tropical western Pacific not only result from the air-sea interaction over the tropical western Pacific, but are also greatly influenced by the East Asian winter and summer monsoons. Additionally, the scientific problems in the interaction between the Asian monsoon and the ENSO cycle which should be studied further in the near future are also pointed out in this paper.

Evaluation of East Asian Climatology as Simulated by Seven Coupled Models

Using observation and reanalysis data throughout 1961-1990, the East Asian surface air temperature, precipitation and sea level pressure climatology as simulated by seven fully coupled atmosphere-ocean models, namely CCSR/NIES, CGCM2, CSIRO-Mk2, ECHAM4/OPYC3, GFDL-R30, HadCM3, and NCAR-PCM, are systematically evaluated in this study. It is indicated that the above models can successfully reproduce the annual and seasonal surface air temperature and precipitation climatology in East Asia, with relatively good performance for boreal autumn and annual mean. The models' ability to simulate surface air temperature is more reliable than precipitation. In addition, the models can dependably capture the geographical distribution pattern of annual, boreal winter, spring and autumn sea level pressure in East Asia. In contrast, relatively large simulation errors are displayed when simulated boreal summer sea level pressure is compared with reanalysis data in East Asia. It is revealed that the simulation errors for surface air temperature, precipitation and sea level pressure are generally large over and around the Tibetan Plateau. No individual model is best in every aspect. As a whole, the ECHAM4/OPYC3 and HadCM3 performances are much better, whereas the CGCM2 is relatively poorer in East Asia. Additionally, the seven-model ensemble mean usually shows a relatively high reliability.

Possible Impacts of Winter Arctic Oscillation on Siberian High, the East Asian Winter Monsoon and Sea-Ice Extent

Using the NCEP/ NCAR reanalysis dataset covering a 40-year period from January 1958 to December 1997, sea surface temperature (1950-1992), and monthly sea-ice concentration dataset for the period from 1953 to 1995, we investigate connections between winter Arctic Oscillation (AO) and Siberian high (SH), the East Asian winter monsoon (EAWM), and winter sea-ice extent in the Barents Sea. The results indicate that winter AO not only influences climate variations in the Arctic and the North Atlantic sector, but also shows possible effects on winter SH, and further influences EAWM, When winter AO is in its positive phase, both of winter SH and the EAWM are weaker than normal, and air temperature from near the surface to the middle troposphere is about 0.5-2degreesC higher than normal in the southeastern Siberia and the East Asian coast, including eastern China, Korea, and Japan. When AO reaches its negative phase, an opposite scenario can be observed. The results also indicate that winter SH has no significant effects on climate variations in Arctic and the North Atlantic sector. Its influence intensity and extent are obviously weaker than AO, exhibiting a 'local, feature in contrast to AO. This study further reveals the possible mechanism of how the winter AO is related to winter SH. It is found that winter SH variation is closely related to both dynamic processes and air temperature variations from the surface to the middle troposphere. The western SH variation mainly depends on dynamic processes, while its eastern part is more closely related to air temperature variation. The maintaining of winter SH mainly depends on downward motion of airflow of the nearly entire troposphere. The airflow originates from the North Atlantic sector, whose variation is influenced by the AO. When AO is in its positive (negative) phase, downward motion remarkably weakened (strengthened), which further influences winter SH. In addition, winter AO exhibits significant influences on the simultaneous sea-ice extent in the Barents Sea.

An Index Measuring the Interannual Variation of the East Asian Summer Monsoon-The EAP Index

Based on the EAP (East Asia/Pacific) teleconnection in the summer circulation anomalies over the Northern Hemisphere, an index measuring the strength of the East Asian summer monsoon, i.e., the so-called EAP index, is defined in this paper. From the analyses of observed data, it is clearly shown that the EAP index defined in this study can well describe the interannual variability of summer rainfall and surface air temperature in East Asia, especially in the Yangtze River valley and the Huaihe River valley, Korea, and Japan. Moreover, this index can also reflect the interannual variability of the East Asian summer monsoon system including the monsoon horizontal circulation and the vertical-meridional circulation cell over East Asia. From the composite analyses of climate and monsoon circulation anomalies for high EAP index and for low EAP index, respectively, it is well demonstrated that the EAP index proposed in this study can well measure the strength of the East Asian summer monsoon.

How Well do Existing Indices Measure the Strength of the East Asian Winter Monsoon?

Defining the intensity of the East Asian winter monsoon （EAWM） with a simple index has been a difficult task. This paper elaborates on the meanings of 18 existing EAWM strength indices and classifies them into four categories： low level wind indices, upper zonal wind shear indices, east-west pressure contrast indices, and East Asian trough indices. The temporal/spatial performance and prediction potential of these indices are then analyzed for the 1957-2001 period. It reveals that on the decadal timescale, most indices except the east-west pressure contrast indices can well capture the continuous weakening of the EAWM around 1986. On the interannual timescale, the low level wind indices and East Asian trough indices have the best predictability based on knowledge of the El Nio-Southern Oscillation and Arctic Oscillation, respectively. All the 18 existing indices can well describe the EAWM-related circulation, precipitation, and lower tropospheric air temperature anomalies. However, the variations of surface air temperature over large areas of central China cannot be well captured by most indices, which is possibly related to topographic effects. The results of this study may provide a possible reference for future studies of the EAWM.

Modulation of the Arctic Oscillation and the East Asian Winter Climate Relationships by the 11-year Solar Cycle

The modulation of the relationship between the Arctic Oscillation （AO） and the East Asian winter climate by the 11-year solar cycle was investigated. During winters with high solar activity （HS）, robust warming appeared in northern Asia in a positive AO phase. This result corresponded to an enhanced anticyclonic flow at 850 hPa over northeastern Asia and a weakened East Asian trough （EAT） at 500 hPa. However, during winters with low solar activity （LS）, both the surface warming and the intensities of the anticyclonic flow and the EAT were much less in the presence of a positive AO phase. The possible atmospheric processes for this 11-year solar-cycle modulation may be attributed to the indirect influence that solar activity induces in the structural changes of AO. During HS winters, the sea level pressure oscillation associated with the AO became stronger, with the significant influence of AO extending to East Asia. In the meantime, the AO-related zonal-mean zonal winds tended to extend more into the stratosphere during HS winters, which implies a stronger coupling to the stratosphere. These trends may have led to an enhanced AO phase difference; thus the associated East Asian climate anomalies became larger and more significant. The situation tended to reverse during LS winters. Further analyses revealed that the relationship between the winter AO and surface-climate anomalies in the following spring is also modulated by the 11-year solar cycle, with significant signals appearing only during HS phases. Solar-cycle variation should be taken into consideration when the AO is used to predict winter and spring climate anomalies over East Asia.

The Progresses of Recent Studies on the Variabilities of the East Asian Monsoon and Their Causes

The variabilities of the East Asian summer monsoon arc an important research issue in China, Japan, and Korea. in this paper, progresses of recent studies on the intrascasonal, interannual, and interdecadal variations of the East Asian monsoon, especially the East Asian summer monsoon, and their causes are reviewed. Particularly, studies on the effects of the ENSO cycle, the western Pacific warm pool, the Tibetan Plateau and land surface processes on the variations of the East Asian summer monsoon are systematically reviewed.