CHARACTERISTICS OF MIDDLE EAST JET STREAM DURING SEASONAL TRANSITION AND ITS RELATION WITH INDIAN SUMMER MONSOON ONSET?

By using the NCEP/NCAR pentad reanalysis data from 1968 to 2009, the variation characteristics of Middle East jet stream(MEJS) and its thermal mechanism during seasonal transition are studied. Results show that the intensity and south-north location of MEJS center exhibit obvious seasonal variation characteristics. When MEJS is strong, it is at 27.5°N from the 67 th pentad to the 24 th pentad the following year; when MEJS is weak, it is at 45°N from the 38 th pentad to the 44 th pentad. The first Empirical Orthogonal Function(EOF) mode of 200-hPa zonal wind field shows that MEJS is mainly over Egypt and Saudi Arabia in winter and over the eastern Black Sea and the eastern Aral Sea in summer. MEJS intensity markedly weakens in summer in comparison with that in winter. The 26th-31 st pentad is the spring-summer transition of MEJS, and the 54th-61 st pentad the autumn-winter transition. During the two seasonal transitions, the temporal variations of the 500-200 hPa south-north temperature difference(SNTD) well match with 200-hPa zonal wind velocity, indicating that the former leads to the latter following the principle of thermal wind. A case analysis shows that there is a close relation between the onset date of Indian summer monsoon and the transition date of MEJS seasonal transition. When the outbreak date of Indian summer monsoon is earlier than normal, MEJS moves northward earlier because the larger SNTD between 500-200 hPa moves northward earlier, with the westerly jet in the lower troposphere over 40°-90°E appearing earlier than normal, and vice versa.     

Sensible Heating over the Tibetan Plateau Linked to the Onset of Asian Monsoon

Land surface heating of the Tibetan Plateau （TP） plays an important role in the Asian Monsoon system. In this study, the interannual variability of the sensible heat flux over the TP and its connection with the summer monsoon intensity and onset date was examined based on the newly released land ＂reanalysis＂ data, produced by NASA Global Land Data Assimilation System. Although correlation analyses don＇t show a significant relevance to the summer monsoon intensity, the interannual anomalies in sensible heating near the south flank of the TP indicate a weak reversal in the relationship with the onset date of the West Pacific East Asian monsoon. A diagnostic study shows that above （below） normal sensible heating over the south flake of the TP will lead to rapid （slow） warming of the upper air column over the TP and early （later） reversal of （overturn） the meridional temperature gradient and the vertical wind shear. This will further lead to an earlier （later） onset of the Asian Monsoon.     

ON THE PROCESS OF SUMMER MONSOON ONSET OVER EAST ASIA

Using daily observational rainfall data covered 194 stations of China from 1961 to 1995 andNCEP model analyzed pentad precipitation data of global grid point from 1979 to 1997,thedistribution of onset date of rainy season over Asian area from spring to summer is studied in thispaper.The analyzed results show that there exist two stages of rainy season onset over East Asianregion from spring to summer rainy season onset accompanying subtropical monsoon and tropicalmonsoon respectively.The former rain belt is mainly formed by the convergence of cold air and therecurred southwesterly flow from western part of subtropical high and westerly flow from the so-called western trough of subtropical region occurring during winter to spring over South Asia.Thelatter is formed in the process of subtropical monsoon rain belt over inshore regions of South ChinaSea originally coming from south of Changjiang (Yangtze) River Basin advancing with northwardshift of subtropical high after the onset of tropical monsoon over South China Sea.The pre-floodrainy season over South China region then came into mature period and the second peak of rainfallappeared.Meiyu,the rainy season over Changjiang-Huaihe River Basin and North China thenformed consequently.The process of summer tropical monsoon onset over South China Sea in 1998is also discussed in this paper.It indicated that the monsoon during summer tropical monsoononset over South China Sea is the result of the westerly flow over middle part of South China Sea,which is from the new generated cyclone formed in north subtropical high entering into SouthChina Sea,converged with the tropical southwesterly flow recurred by the intensified cross-equatorial flow.     

ONSET AND RETREAT DATES OF THE SOUTH CHINA SEA SUMMER MONSOON AND THEIR RELATIONSHIPS WITH THE MONSOON INTENSITY IN THE CONTEXT OF CLIMATE WARMING

Global gridded daily mean data from the NCEP/NCAR Reanalysis(1948-2012) are used to obtain the onset date,retreat date and duration time series of the South China Sea summer monsoon(SCSSM) for the past 65 years.The summer monsoon onset(retreat) date is defined as the time when the mean zonal wind at 850 hPa shifts steadily from easterly(westerly) to westerly(easterly) and the pseudo-equivalent potential temperature at the same level remains steady at greater than 335 K(less than 335 K) in the South China Sea area[110-120°E(10-20°N)].The clockwise vortex of the equatorial Indian Ocean region,together with the cross-equatorial flow and the subtropical high,plays a decisive role in the burst of the SCSSM.The onset date of the SCSSM is closely related to its intensity.With late(early) onset of the summer monsoon,its intensity is relatively strong(weak),and the zonal wind undergoes an early(late) abrupt change in the upper troposphere.Climate warming significantly affects the onset and retreat dates of the SCSSM and its intensity.With climate warming,the number of early-onset(-retreat) years of the SCSSM is clearly greater(less),and the SCSSM is clearly weakened.     

Atmospheric circulation characteristics associated with the onset of Asian summer monsoon

The onset of the Asian summer monsoon has been a focus in the monsoon study for many years. In this paper, we study the variability and predictability of the Asian summer monsoon onset and demonstrate that this onset is associated with specific atmospheric circulation characteristics. The outbreak of the Asian summer monsoon is found to occur first over the southwestern part of the South China Sea (SCS) and the Malay Peninsula region, and the monsoon onset is closely related to intra-seasonal oscillations in the lower atmosphere. These intra-seasonal oscillations consist of two low-frequency vortex pairs, one located to the east of the Philippines and the other over the tropical eastern Indian Ocean. Prior to the Asian summer monsoon onset, a strong low-frequency westerly emerges over the equatorial Indian Ocean and the low-frequency vortex pair develops symmetrically along the equator. The formation and evolution of these low-frequency vortices are important and serve as a good indicator for the Asian summer monsoon onset. The relationship between the northward jumps of the westerly jet over East Asia and the Asian summer monsoon onset over SCS is investigated. It is shown that the northward jump of the westerly jet occurs twice during the transition from winter to summer and these jumps are closely related to the summer monsoon development. The first northward jump (from 25–28N to around 30N) occurs on 8 May on average, about 7 days ahead of the summer monsoon onset over the SCS. It is found that the reverse of meridional temperature gradient in the upper-middle troposphere (500–200 hPa) and the enhancement and northward movement of the subtropical jet in the Southern Hemispheric subtropics are responsible for the first northward jump of the westerly jet.     

EVOLUTION OF TROPOSPHERIC TEMPERATURE FIELDS AND CORRESPONDING THERMAL MECHANISMS BEFORE/AFTER ONSET PERIODS OF ASIAN SUMMER MONSOON

The evolution of the tropospheric temperature fields over Indian and South China Sea monsoon areas and their thermal mechanisms are compared and analyzed during the period from March to June, 1996. The results show that the onsets of the Indian and South China Sea summer monsoons are closely associated with the seasonal warming in the troposphere over the zonal belt of 10°N～30°N in these areas, which leads to the inversion of meridional temperature gradient. During the pre-onset period, the warming over the South China Sea monsoon region is mainly due to the warm horizontal advection and diabatic (latent) heating processes. Meanwhile, the warming is suppressed by the vertical adiabatic process (cooling). In spring over the Indian monsoon region, the significant adiabatic heating due to the subsidence motion, which compensates the cooling due to the strong cold advection and diabatic cooling processes, results in a larger warming rate than over the South China Sea monsoon region. However, the meridional temperature gradient over the Indian monsoon region is so large during the late winter and early spring that it takes longer time to warm the troposphere to have the reversion of meridional temperature gradient than it does over the South China Sea monsoon region. It results in the phenomenon that the South China Sea summer monsoon generally breaks out earlier than the Indian summer monsoon.     

Reappraisal of Asian Summer Monsoon Indices and the Long-Term Variation of Monsoon

The Webster and Yang monsoon index (WYI)-the zonal wind shear between 850 and 200 hPa was calculated and modified on the basis of NCEP/NCAR reanalysis data. After analyzing the circulation and divergence fields of 150-100 and 200 hPa, however, we found that the 200-hPa level could not reflect the real change of the upper-tropospheric circulation of Asian summer monsoon, especially the characteristics and variation of the tropical easterly jet which is the most important feature of the upper-tropospheric circulation. The zonal wind shear U850-U(150 100) is much larger than U850-U200, and thus it can reflect the strength of monsoon more appropriately. In addition, divergence is the largest at 150 hPa rather than 200 hPa, so 150 hPa in the upper-troposphere can reflect the coupling of the monsoon system. Therefore, WYI is redefined as DHI, i.e., IDH=U850* - U(150 100)*, which is able to characterize the variability of not only the intensity of the center of zonal wind shear in Asia, but also the monsoon system in the upper and lower troposphere. DHI is superior to WYI in featuring the long-term variation of Asian summer monsoon as it indicates there is obvious interdecadal variation in the Asian summer monsoon and the climate abrupt change occurred in 1980. The Asian summer monsoon was stronger before 1980 and it weakened after then due to the weakening of the easterly in the layer of 150-100 hPa, while easterly at 200 hPa did not weaken significantly. After the climate jump year in general, easterly in the upper troposphere weakened in Asia, indicating the weakening of summer monsoon; the land-sea pressure difference and thermal difference reduced, resulting in the weakening of monsoon; the corresponding upper divergence as well as the water vapor transport decreased in Indian Peninsula, central Indo-China Peninsula, North China, and Northeast China, indicating the weakening of summer monsoon as well. The difference between NCEP/NCAR and ERA-40 reanalysis data in studying the intensity and long-term variation of Asian summer monsoon is also compared in the end for reference.     

Current Progresses in Study of Impacts of the Tibetan Plateau on Asian Summer Climate

The current progresses in the study of impacts of the Tibetan Plateau on Asian summer climate in the last decade are reviewed. By analyzing evolution of the transitional zone between westerly to the north and easterly to the south (WEB), it is shown that due to the strong heating over the Tibetan Plateau in spring, the overturning in the prevailing wind direction from easterly in winter to westerly in summer occurs firstly over the eastern Bay of Bengal (BOB), accompanied with vigorous convective precipitation to its east. The area between eastern BOB and western Indo-China Peninsula thus becomes the area with the earliest onset of Asian monsoon, which may be referred as BOB monsoon in short. It is shown that the summertime circulations triggered by the thermal forcing of the Iranian Plateau and the Tibetan Plateau are embedded in phase with the continental-scale circulation forced by the diabatic heating over the Eurasian Continent. As a result, the East Asian summer monsoon is intensified and the drought climate over the western and central Asian areas is enhanced. Together with perturbations triggered by the Tibetan Plateau, the above scenarios and the associated heating have important influences on the climate patterns over Asia. Furthermore, the characteristics of the Tibetan mode of the summertime South Asian high are compared with those of Iranian mode. Results demonstrate that corresponding to each of the bimodality of the South Asian high, the rainfall anomaly distributions over Asia exhibit different patterns.     

DETERMINATION OF SOUTH CHINA SEA MONSOON ONSET AND EAST ASIAN SUMMER MONSOON INDEX

Results of the definition of South China Sea summer monsoon onset date and East Asian summer monsoon index in recent years are summarized in this paper. And more questions to be resolved are introduced later.     

CHARACTERISTICS OF 10-DAY MEAN SURFACE SENSIBLE HEAT FLUX VARIATIONS IN THE SCS MONSOON REGION AND POSSIBLE CONNECTIONS WITH THE SCS SUMMER MONSOON ONSET

The time and space variations of the ten-day mean surface sensible heat flux have beenanalyzed in this paper based on the data of NCEP/NCAR from January of 1979 to December of1995 in the South China Sea(SCS)monsoon region.It is found that large variations of the surfacesensible heat flux standard deviations exist in the northwestern Indochina Peninsula and the IndianPeninsula regions,and their locations and strength change significantly during the onset period ofSCS monsoon.The negative deviations appear evidently earlier in the Indocbina Peninsula than inthe Indian Peninsula but the deviation strength in the Indian Peninsula is stronger than that in theIndochina Peninsula.The appearance of the zonal negative mean deviations in the southern part ofthe Indochina Peninsula corresponds to the date of the SCS summer monsoon onset,while theoccurrence of the deviation decrease corresponds to the date of the South Asian monsoon onset.The sensible heat flux increases dekad by dekad before the onset of the summer monsoon in theIndian Peninsula and the Indochina Peninsula and decreases after the monsoon onset.Therefore,the surface sensible heat flux changes in the Indochina and the Indian Peninsula regions maybe havesome connections with the SCS monsoon onset and the Indian monsoon onset,and the IndochinaPeninsula maybe becomes the sensitive or key region to the SCS monsoon onset and the land maybeplays an important role in triggering summer monsoon onset.     

The Role of Land-sea Distribution and Orography in the Asian Monsoon. Part Ⅱ:Orography

The role of various mountains in the Asian monsoon system is investigated by AGCM simulations with different mountains.The comparison of the simulation with Asian mountains(MAsia run)with the simulation without mountains(NM run)reveals that the presence of the Asian mountains results in a stronger South Asian summer monsoon(SASM),characterized by enhanced lower-tropospheric westerly winds,uppertropospheric easterly winds,and stronger water vapor convergence.In East Asia,the southerly winds and water vapor convergence are significantly strengthened in association with the intensified zonal pressure gradient between the East Asian continent and the Pacific Ocean.Both the dynamical and thermodynamic forcing of the Tibetan Plateau play important role in strengthening the Asian summer monsoon.In winter,the presence of Asian mountains significantly strengthens the continental high,which leads to a stronger Asian winter monsoon.The presence of African-Arabian mountains helps to intensify the exchange of mass between the Southern Hemisphere and Northern Hemisphere by strengthening the cross equatorial flows in the lower and upper troposphere over East Africa. Asian mountains also play a crucial role in the seasonal evolution of Asian monsoons.In comparison with the NM run,the earlier onset and later withdrawal of lower-tropospheric westerly winds can be found over South Asia in the MAsia run,indicating a longer SASM period.The African-Arabian mountains also moderately contribute to the seasonal variation of the South Asian monsoon.In East Asia,the clear southto-north march of the southerly winds and subtropical rainfall starts to occur in early summer when the effects of Asian mountains are considered.     

Seasonal Variation of the Meridional Wind in the Temperate Jet Stream and Its Relationship to the Asian Monsoon

The features of the temperate jet stream including its location, intensity, structure, seasonal evolution and the relationship with the Asian monsoon are examined by using NCEP/NCAR reanalysis data. It is indicated that the temperate jet stream is prominent and active at 300 hPa in winter over the region from 45°-60°N and west of 120°E. The temperate jet stream is represented by a ridge area of high wind speed and dense stream lines in the monthly or seasonal mean wind field, but it .corresponds to an area frequented by a large number of jet cores in the daily wind field and exhibits a distinct boundary that separates itself with the subtropical jet. A comparison of the meridional wind component of the temperate jet stream with that of the subtropical jet shows that the northerly wind in the temperate jet stream is stronger than the southerly component of the subtropical jet, which plays an important role in the temperate jet stream formation and seasonal evolution, and thus the intensity change of the meridional wind component can be used to represent the temperate jet stream＇s seasonal variation. Analysis of the temperature gradient in the upper troposphere indicates that the temperate jet stream is accompanied by a maximum zonal temperature gradient and a large meridional temperature gradient, leading to a unique jet stream structure and particular seasonal evolution features, which are different from the subtropical jet. The zonal temperature gradient related to the land-sea thermal contrast along the East China coastal lines is responsible for the seasonal evolution of the temperate jet. In addition, there exists a coordinated synchronous change between the movement of the temperate jet and that of the subtropical jet. The seasonal evolution of the meridional wind intensity is closely related to the seasonal shift of the atmospheric circulation in East Asia, the onset of the Asian summer monsoon and the start of Meiyu in the Yangtze and Huaihe River Valleys, and it correlates well with summer and wint     

THE TIMING OF SOUTH-ASIAN HIGH ESTABLISHMENT AND ITS RELATION TO TROPICAL ASIAN SUMMER MONSOON AND PRECIPITATION OVER EAST-CENTRAL CHINA IN SUMMER

The timing of the South Asian High (SAH) establishment over the Indochina Peninsula (IP) from April to May and its relations to the setup of the subsequent tropical Asian summer monsoon and precipitation over eastern-central China in summer are investigated by using NCEP/NCAR daily reanalysis data, outgoing longwave radiation (OLR) data and the daily precipitation data from 753 weather stations in China. It is found that the transitions of the zonal wind vertical shear and convection establishment over tropical Asia are earlier (later) in the years of early (late) establishment of SAH. In the lower troposphere, anti-cyclonic (cyclonic) anomaly circulation dominates the equatorial Indian Ocean. Correspondingly, the tropical Asian summer monsoon establishes earlier (later). Furthermore, the atmospheric circulation and the water vapor transport in the years of advanced SAH establishment are significantly different from the delayed years in Asia in summer. Out-of-phase distribution of precipitation in eastern-central China will appear with a weak (strong) SAH and western Pacific subtropical high, strong (weak) ascending motion in the area south of Yangtze River but weak (strong) ascending motion in the area north of it, and cyclonic (anti-cyclonic) water vapor flux anomaly circulation from the eastern-central China to western Pacific. Accordingly, the timing of the SAH establishment at the upper levels of IP is indicative of the subsequent onset of the tropical Asian summer monsoon and the flood-drought pattern over eastern-central China in summer.     

Determination of onset date of the South China Sea summer monsoon in 2006 using large-scale circulations

Since the South China Sea (SCS) summer monsoon (SCSSM) is pronouncedly featured by abruptly intensified southwesterly and obviously increased precipitation over the SCS,the lower-tropospheric winds and/or convection intensities are widely used to determine the SCSSM onset.The methods can be used successfully in most of the years but not in 2006.Due to the intrusion of Typhoon Chanchu(0601)that year,the usual method of determining SCSSM onset date by utilizing the SCS regional indices is less capable of pinpointing the real onset date.In order to solve the problem,larger-scale situations have to be taken into account.Zonal and meridional circulations would be better to determine the break-out date of SCSSM in 2006.The result indicates that its onset date is May 16.Moreover,similar onset dates for other years can be obtained using various methods,implying that large-scale zonal and meridional circulations can be used as an alternative method for determining the SCSSM onset date.     

THE EFFECTS OF SEA SURFACE TEMPERATURE OVER THE EQUATORIAL WESTERN PACIFIC AND THE INDIAN OCEAN ON THE ASIAN SUMMER MONSOON

In this paper,the p-σ five layer primitive equation model segmented by mountains and physical parameterizations including short wave radiation; long wave radiation; large-scale and convective condensation;heat and moisture transport from surface to the first model level is used.The horizonial resolution is 5° lat.×5° long.with the integration region from 25°S to 55°N and from 5°W eastward to 175°W.The model was spun up with perpetual June boundary conditions and forcing starting with June zonal mean heights and geostrophic wind field.In order to investigate the effects of SST (sea surface tempefuture) over the equatorial Western Pacific and the Indian Ocean on the Asian summer monsoon,four sets of numerical experments with positive anomalies over the equatorial Western Pacific,and positive and negative anomalies over the Western Indian Ocean,and zonal mean SST (the control case) are performed.The experimental results show that the South Asian low in the lower troposphere and the anticyclone over the South Asia in the uppet troposphere intensified when positive SST anomalies over the equatorial Western Pacific is included.A statistical test method for simulations is proposed.Finally,the influence mechanism of the SST anomalies over the equatorial oceans is discussed.It is worth stressing that the effects of the SST over the equatorial oceans on the Asian summer monsoon can arise as a result of interaction of SST anomalies,atmospheric flow field and heat sources and sinks in the atmosphere.     

Longitudinal Heating Gradient: Another Possible Factor Influencing the Intensity of the Asian Summer Monsoon Circulation

The largest longitudinal heating gradients in the tropics exist between the African desert and Asian convective regions during summer once the South Asian monsoon is established. The heating gradients are anchored by the latent heat release and net radiative flux convergence over the monsoon region, and by the dominant net radiative flux divergence over the desert.An apparent relationship is found between the intensity of the Asian summer monsoon circulation and the longitudinal heating gradients mentioned, in addition to the latitudinal heating gradients cross the monsoon region. The monsoon circulation measured in terms of the zonal wind component is stronger when the longitudinal heating gradients are large, and vice versa. Thus, we claim that the longitudinal heating gradient may be another important factor which influences the intensity of the Asian summer monsoon circulation. There is little evidence that the interannual variability of the longitudinal heating gradients between Africa and Asia and     

Recent Progress in the Impact of the Tibetan Plateau on Climate in China

Studies of the impacts of the Tibetan Plateau （TP） on climate in China in the last four years are reviewed. It is reported that temperature and precipitation over the TP have increased during recent decades. From satellite data analysis, it is demonstrated that most of the precipitation over the TP is from deep convection clouds. Moreover, the huge TP mechanical forcing and extraordinary elevated thermal forcing impose remarkable impacts upon local circulation and global climate. In winter and spring, stream flow is deflected by a large obstacle and appears as an asymmetric dipole, making East Asia much colder than mid Asia in winter and forming persistent rainfall in late winter and early spring over South China. In late spring, TP heating contributes to the establishment and intensification of the South Asian high and the abrupt seasonal transition of the surrounding circulations. In summer, TP heating in conjunction with the TP air pump cause the deviating stream field to resemble a cyclonic spiral, converging towards and rising over the TP. Therefore, the prominent Asian monsoon climate over East Asia and the dry climate over mid Asia in summer are forced by both TP local forcing and Eurasian continental forcing.
Due to the longer memory of snow and soil moisture, the TP thermal status both in summer and in late winter and spring can influence the variation of Eastern Asian summer rainfall. A combined index using both snow cover over the TP and the ENSO index in winter shows a better seasonal forecast.
On the other hand, strong sensible heating over the Tibetan Plateau in spring contributes significantly to anchor the earliest Asian monsoon being over the eastern Bay of Bengal （BOB） and the western Indochina peninsula. Qualitative prediction of the BOB monsoon onset was attempted by using the sign of meridional temperature gradient in March in the upper troposphere, or at 400 hPa over the TP. It is also demonstrated by a numerical experiment and theoretical study that the heating over the TP lea     

Characteristics of the Onset of the Asian Summer Monsoon and the Importance of Asian-Australian “Land Bridge”

Based on summarizing previous achievements and using data as long and new as possible, the onset characteristics of Asian summer monsoon and the role of Asian-Australian “land bridge” in the onset of summer monsoon are further discussed. In particular, the earliest onset area of Asian summer monsoon is comparatively analyzed, and the sudden and progressive characteristics of the onset of summer monsoon in different regions are discussed. Furthermore, the relationships among such critical events during the onset of Asian summer monsoon as the splitting of subtropical high belt over the Bay of Bengal (BOB), the initiation of convection over Indo-China Peninsula, the westward advance, reestablishment of South Asian High, and the rapid northward progression of convection originated from Sumatra in early summer are studied. The important impact of the proper collocation of the latent heating over Indo-China Peninsula and the sensible heating over Indian Peninsula on the splitting of the subtropical high belt, the deepening of BOB trough, the activating of Sri Lanka vortex (twin vortexes in the Northern and Southern Hemispheres), and the subsequent onset of South China Sea summer monsoon are emphasized.     

FURTHER RESEARCH ON MECHANISM OF TBO IN SOUTH ASIAN MONSOON REGION

The results of this study prove that there is significant troposphere biennial oscillation （TBO） in the South Asian climate, especially with the Indian summer monsoon rainfall. In order to explore the mechanism of TBO in the South Asian region, we defined a unified South Asian monsoon index to depict South Asian summer monsoon （SASM） and South Asian winter monsoon （SAWM） and the transition features between SASM and SAWM. Through further analysis, the connection between the abnormity of SASM and SAWM was discovered. Normally, a strong SAWM is beneficial for a weak SASM later, while a weak SAWM favors a strong SASM. Meanwhile, a strong SASM is favorable for a weak SAWM and a weak SAWM always happens after a weak SASM. Such results suggest the evolution of the South Asian monsoon, which may be an important mechanism to excite TBO in South Asia.     

INTERANNUAL VARIATION OF INDEX OF EAST ASIAN LAND-SEA THERMAL DIFFERENCE AND ITS RELATION TO MONSOON CIRCULATION AND RAINFALL OVER CHINA

This paper proposes an index of land-sea thermal difference(ILSTD)that describes its zonal andmeridional strength responsible for East Asian monsoon circulation to study its relation to the EastAsian monsoon circulation and the summer rainfall over China on an interannual basis.Results are asfollows:(1)ILSTD can be used to measure the strength of East Asian summer monsoon in such away that the strong(weak)ILSTD years are associated with strong(weak)summer monsooncirculation.(2)The index also reflects well summer rainfall anomaly over the eastern part of China.In the strong index years,rain belt is mainly located over the northern China,and serious droughtemerges in the Jianghuai valleys and mid-lower reaches of the Changjiang River,along with increaseof rainfall in North and South China,but in the weak years it is contrary.(3)Besides,the index hasobvious QBO and quasi 4-year oscillations,but the periods and amplitudes have significant changes onan interdecadal basis.