Diversity on the Interannual Variations of Spring Monthly Precipitation in Southern China and the Associated Tropical Sea Surface Temperature Anomalies

There is a continuous and relatively stable rainy period every spring in southern China(SC).This spring precipitation process is a unique weather and climate phenomenon in East Asia.Previously,the variation characteristics and associated mechanisms of this precipitation process have been mostly discussed from the perspective of seasonal mean.Based on the observed and reanalysis datasets from 1982 to 2021,this study investigates the diversity of the interannual variations of monthly precipitation in spring in SC,and focuses on the potential influence of the tropical sea surface temperature(SST)anomalies.The results show that the interannual variations of monthly precipitation in spring in SC have significant differences,and the correlations between each two months are very weak.All the interannual variations of precipitation in three months are related to a similar western North Pacific anomalous anticyclone(WNPAC),and the southwesterlies at the western flank of WNPAC bring abundant water vapor for the precipitation in SC.However,the WNPAC is influenced by tropical SST anomalies in different regions each month.The interannual variation of precipitation in March in SC is mainly influenced by the signal of El Nino-Southern Oscillation,and the associated SST anomalies in the equatorial central-eastern Pacific regulate the WNPAC through the Pacific-East Asia(PEA)teleconnection.In contrast,the WNPAC associated with the interannual variation of precipitation in April can be affected by the SST anomalies in the northwestern equatorial Pacific through a thermally induced Rossby wave response.The interannual variation of precipitation in May is regulated by the SST anomalies around the western Maritime Continent,which stimulates the development of low-level anomalous anticyclones over the South China Sea and east of the Philippine Sea by driving anomalous meridional vertical circulation.

Interannual Variation and Statistical Prediction of Summer Dry and Hot Days in South China from 1970 to 2018

The frequent occurrence of dry and hot(DH)days in South China in summer has a negative impact on social development and human health.This study explored the variation characteristics of DH days and the possible reasons for this knotty problem.The findings revealed a notable increase in the number of DH days across most stations,indicating a significant upward trend.Additionally,DH events were observed to occur frequently.The number of DH days increased during 1970-1990,decreased from 1991 to 1997,and stayed stable after 1997.The key climate factors affecting the interannual variability of the number of DH days were the Indian Ocean Basin warming(IOBW)in spring and the East Asian Summer Monsoon(EASM).Compared with the negative phase of IOBW,in the positive phase of IOBW,500 hPa and 850 hPa geopotential height enhanced,the West Pacific subtropical high strengthened and extended abnormally to the west,more solar radiation reached the surface,surface outgoing longwave radiation increased,and there was an anomalous anticyclone in eastern South China.The atmospheric circulation characteristics of the positive and negative phases of ESAM were opposite to those of IOBW,and the abnormal circulation of the positive(negative)phases of ESAM was unfavorable(favorable)for the increase in the number of DH days.A long-term prediction model for the number of summer DH days was established using multiple linear regression,incorporating the key climate factors.The correlation coefficient between the observed and predicted number of DH days was 0.65,and the root-mean-square error was 2.8.In addition,independent forecasts for 2019 showed a deviation of just 1 day.The results of the independent recovery test confirmed the stability of the model,providing evidence that climatic factors did have an impact on DH days in South China.

Influence of Monsoon over the Warm Pool on Interannual Variation on Tropical Cyclone Activity over the Western North Pacific

The relationship between the interannual variation in tropical cyclone （TC） activity over the western North Pacific （WNP） and the thermal state over the warm pool （WP） is examined in this paper. The results show that the subsurface temperature in the WP is well correlated with TC geographical distribution and track type. Their relation is linked by the East Asian monsoon trough. During the warm years, the westward-retreating monsoon trough creates convergence and vorticity fields that are favorable for tropical cyclogenesis in the northwest of the WNP, whereas more TCs concentrating in the southeast result from eastward penetration of the monsoon trough during the cold years. The steering flows at 500 hPa lead to a westward displacement track in the warm years and recurving prevailing track in the cold years. The two types of distinct processes in the monsoon environment triggering tropical cyclogenesis are hypothesized by composites centered for TC genesis location corresponding to two kinds of thermal states of the WP. During the warm years, low-frequency intraseasonal oscillation is active in the west of the WNP such that eastward-propagating westerlies cluster TC genesis in that region. In contrast, during the cold years, the increased cyclogenesis in the southeast of the WNP is mainly associated with tropical depression type disturbances transiting from equatorially trapped mixed Rossby gravity waves. Both of the processes may be fundamental mechanisms for the inherent interannual variation in TC activity over the WNP.

Large-scale Circulation Anomalies Associated with Interannual Variation in Monthly Rainfall over South China from May to August

Interannual variation in summer rainfall over South China （SC） was investigated on the monthly timescale.It was found that monthly rainfall from May to August exhibits different features of variation,and the amounts are basically independent of each other.There is a significant negative correlation,however,between May and July SC rainfall,which is partially related to the developing phases of ENSO events.It was also found that stronger （weaker） lower-tropospheric winds over SC and the upstream parts are responsible for more （less） SC rainfall in every month from May to August.Despite this monthly consistent enhancement of horizontal winds,the wind anomalies exhibit distinct differences between May-June and July-August,due to the remarkable change in climatological winds between these two periods.More SC rainfall is associated with a lower-tropospheric anticyclonic anomaly over the SCS and the Philippine Sea in May and June,but with a cyclonic anomaly centered over SC in July and August.

Prediction of the Asian-Australian Monsoon Interannual Variations with the Grid-Point Atmospheric Model of IAP LASG(GAMIL)

Seasonal prediction of Asian-Australian monsoon （A-AM） precipitation is one of the most important and challenging tasks in climate prediction. In this paper, we evaluate the performance of Grid Atmospheric Model of IAP LASG （GAMIL） on retrospective prediction of the A-AM interannual variation （IAV）, and determine to what extent GAMIL can capture the two major observed modes of A-AM rainfall IAV for the period 1979-2003. The first mode is associated with the turnabout of warming （cooling） in the Nifio 3.4 region, whereas the second mode leads the warming/cooling by about one year, signaling precursory conditions for ENSO. We show that the GAMIL one-month lead prediction of the seasonal precipitation anomalies is primarily able to capture major features of the two observed leading modes of the IAV, with the first mode better predicted than the second. It also depicts the relationship between the first mode and ENSO rather well. On the other hand, the GAMIL has deficiencies in capturing the relationship between the second mode and ENSO. We conclude： （1） successful reproduction of the E1 Nifio-excited monsoon-ocean interaction and E1 Nifio forcing may be critical for the seasonal prediction of the A-AM rainfall IAV with the GAMIL; （2） more efforts are needed to improve the simulation not only in the Nifio 3.4 region but also in the joining area of Asia and the Indian-Pacific Ocean; （3） the selection of a one-tier system may improve the ultimate prediction of the A-AM rainfall IAV. These results offer some references for improvement of the GAMIL and associated seasonal prediction skill.

The Interannual Variation in Monthly Temperature over Northeast China during Summer

The interannual variations of summer surface air temperature over Northeast China （NEC） were investigated through a month-to-month analysis from May to August. The results suggested that the warmer temperature over NEC is related to a local positive 500-hPa geopotential height anomaly for all four months. However, the teleconnection patterns of atmospheric circulation anomalies associated with the monthly surface air temperature over NEC behave as a distinguished subseasonal variation, although the local positive height anomaly is common from month to month. In May and June, the teleconnection pattern is characterized by a wave train in the upper and middle troposphere from the Indian Peninsula to NEC. This wave train is stronger in June than in May, possibly due to the positive feedback between the wave train and the South Asian rainfall anomaly in June, when the South Asian summer monsoon has been established. In July and August, however, the teleconnection pattern associated with the NEC temperature anomalies is characterized by an East Asia/Pacific （EAP） or Pacific/Japan （PJ） pattern, with the existence of precipitation anomalies over the Philippine Sea and the South China Sea. This pattern is much clearer in July corresponding to the stronger convection over the Philippine Sea compared to that in August.

A primary study of the correlation between the net air-sea heat flux and the interannual variation of western North Pacific tropical cyclone track and intensity

A singular value decomposition （SVD） analysis is carried out to reveal the relationship between the interannual variation of track and intensity of the western North Pacific tropical cyclones （WNPTCs） in the tropical cyclone （TC） active season （July–November） and the global net air-sea heat flux （Q net ） in the preceding season （April–June）. For this purpose, a tropical cyclone track and intensity function （TIF） is defined by a combination of accumulated cyclone energy （ACE） index and a cyclone track density function. The SVD analysis reveals that the first mode is responsible for the positive correlation between the upward heat flux in the tropical central Pacific and the increased activity of western North Pacific （WNP） TIF, the second mode for the positive correlation between the upward heat flux in the North Indian Ocean and the northeastward track shift of WNPTCs and the third mode for the negative correlation between the upward heat flux in mid-latitude central Pacific and the northwest displacement of the WNP TC-active center. This suggests that Q net anomalies in some key regions have a substantial remote impact on the WNP TC activity.

Numerical Simulation of the Impact of Vegetation Index on the Interannual Variation of Summer Precipitation in the Yellow River Basin

Two sets of numerical experiments using the coupled National Center for Environmental Prediction General Circulation Model （NCEP/GCM T42L18） and the Simplified Simple Biosphere land surface scheme （SSiB） were carried out to investigate the climate impacts of fractional vegetation cover （FVC） and leaf area index （LAI） on East Asia summer precipitation, especially in the Yellow River Basin （YRB）. One set employed prescribed FVC and LAI which have no interannual variations based on the climatology of vegetation distribution; the other with FVC and LAI derived from satellite observations of the International Satellite Land Surface Climate Project （ISLSCP） for 1987 and 1988. The simulations of the two experiments were compared to study the influence of FVC, LAI on summer precipitation interannual variation in the YRB. Compared with observations and the NCEP reanalysis data, the experiment that included both the effects of satellite-derived vegetation indexes and sea surface temperature （SST） produced better seasonal and interannual precipitation variations than the experiment with SST but no interannual variations in FVC and LAI, indicating that better representations of the vegetation index and its interannual variation may be important for climate prediction. The difference between 1987 and 1988 indicated that with the increase of FVC and LAI, especially around the YRB, surface albedo decreased, net surface radiation increased, and consequently local evaporation and precipitation intensified. Further more, surface sensible heat flux, surface temperature and its diurnal variation decreased around the YRB in response to more vegetation. The decrease of surface-emitting longwave radiation due to the cooler surface outweighed the decrease of surface solar radiation income with more cloud coverage, thus maintaining the positive anomaly of net surface radiation. Further study indicated that moisture flux variations associated with changes in the general circulation also contributed to the precipitation interannual variation.

Studies on the Northern Early Summer Teleconnection Patterns, Their Interannual Variations and Relation to Drought / Flood in China

By using the one-point correlation method, calculations have been made of the northern early summer 500 hPa teleconnection patterns. Seven teleconnection patterns are revealed, namely, the Western Atlantic (WA), the Eastern Atlantic (EA), the Eurasian (EU), the Bengal / Northern Pacific (BNP), the Western Pacific (WP), the East Asian / Pacific (EAP), and the Huanghe / East Asian (HEA) patterns. Their centers are determined and their yearly intensity indices (1951-1990) are calculated. On this basis the relationship between their interannual variations and the drought / flood in China is examined. It is noted that the EU, HEA and EAP wave trains are closely related to the drought / flood in China. The HEA and EAP patterns strongly influence the precipitation in eastern China. For example, the fierce floods experienced in 1991 early summer over China are related to the weak EAP and strong HEA patterns.

Relationship between land cover and monsoon interannual variations in east Asia

Asian monsoon have multiple forms of variations such as seasonal variation, intra-seasonal variation, interannual variation, etc. The interannual variations have not only yearly variations but also variations among several years. In general, the yearly variations are described with winter temperature and summer precipitation, and the variations among several years are reflected by circulation of ENSO events. In this study, at first, we analyze the relationship between land cover and interannual monsoon variations represented by precipitation changes using Singular Value Decomposition method based on the time series precipitation data and 8km NOAA AVHRR NDVI data covering 1982 to 1993 in east Asia. Furthermore, after confirmation and reclassification of ENSO events which are recognized as the strong signal of several year monsoon variation, using the same time series NDVI data during 1982 to 1993 in east Asia, we make a Principle Component Analysis and analyzed the correlation of the 7th component eigenvectors and Southern Oscillation Index (SOI) that indicates the characteristic of ENSO events, and summed up the temporal-spatial distribution features of east Asian land cover’s inter-annual variations that are being driven by changes of ENSO events.

Interannual Variation of Multiple Tropical Cyclone Events in the Western North Pacific

The interannual variability of occurrence of multiple tropical cyclone （MTC） events during June-October in the western North Pacific （WNP） was examined for the period 1979-2006. The number of the MTC events ranged from 2 to 9 per year, exhibiting a remarkable year-to-year variation. Seven active and seven inactive MTC years were identified. Compared to the inactive years, tropical cyclone genesis locations extended farther to the east and in the meridional direction during the active MTC years. A composite analysis shows that inactive MTC years were often associated with the E1 Nifio decaying phase, as warm SST anomalies in the equatorial eastern-central Pacific in the preceding winter transitioned into cold sea surface temperature （SST） anomalies in the concurrent summer. Associated with the SST evolution were suppressed low-level cyclonic vorticity and weakened convection in the WNP monsoon region. In addition to the mean flow difference~ significant differences between active and inactive MTC years were also found in the strength of the atmospheric intraseasonal oscillation （ISO）. Compared with inactive MTC years, ISO activity was much stronger along the equator and in the WNP region during active MTC years. Both westwardand northward-propagating ISO spectrums strengthened during active MTC years compared to inactive years. The combined mean state and ISO activity changes may set up a favorable environment for the generation of MTC events.

Interannual variation of nutrients along a transect across the Kuroshio and shelf area in the East China Sea over 40 years

Long-term datasets of water temperature,salinity,dissolved oxygen,phosphate and nitrate from 1973 to 2013 were used to study interannual variations of these parameters along a transect(section PN)across the Kuroshio and shelf area in the East China Sea.Water temperature and salinity at depths of 0–200 m showed a low-high-low-high pattern over the 40-year period.Water temperature and salinity at 500 m were relatively high from 1985 to 1990 and decreased continuously thereafter.Salinity at 800 m was lowest around 1993 and increased thereafter.Nutrients were highest between 1980 and 1985.After 1985,nutrients at 500 m were increasing.The range of variation in nutrients was large before 1985 and the magnitude increased with greater depths.Apparent oxygen utilization at 500 m depth was decreasing before 1990 and started to increase after 1992.Significant changes after 1990 at 500 m depth may be due to the upward trend of the upper boundary of Kuroshio intermediate water.Interannual variation of water temperature,salinity,dissolved oxygen,phosphate and nitrate along section PN has a significant correlation with the Pacific Decadal Oscillation index.

Latitudinal and interannual variations of the spring phytoplankton bloom peak in the East Asian marginal seas

Combined studies of latitudinal and interannual variations of annual phytoplankton bloom peak in East Asian marginal seas(17°–58°N, including the northern South China Sea(SCS), Kuroshio waters, the Sea of Japan and the Okhotsk Sea) are rarely. Based on satellite-retrieved ten-year(2003–2012) median timing of the annual Chlorophyll a concentration(Chl a) climax, here we report that this annual spring bloom peak generally delays from the SCS in January to the Okhotsk Sea in June at a rate of(21.20±2.86) km/d(decadal median±SD). Spring bloom is dominant feature of the phytoplankton annual cycle over these regions, except for the SCS which features winter bloom. The fluctuation of the annual peak timing is mainly within ±48 d departured from the decadal median peak date, therefore this period(the decadal median peak date ±48 d) is defined as annual spring bloom period. As sea surface temperature rises, earlier spring bloom peak timing but decreasing averaged Chl a biomass in the spring bloom period due to insufficient light is evident in the Okhotsk Sea from 2003 to 2012. For the rest of three study domains, there are no significant interannual variance trend of the peak timing and the averaged Chl a biomass. Furthermore this change of spring phytoplankton bloom timing and magnitude in the Okhotsk Sea challenges previous prediction that ocean warming would enhance algal productivity at high latitudes.

Quantifying the Contribution of Track Changes to Interannual Variations of North Atlantic Intense Hurricanes

Previous studies have linked interannual variability of tropical cyclone(TC)intensity in the North Atlantic basin(NA)to Sahelian rainfall,vertical shear of the environmental flow,and relative sea surface temperature(SST).In this study,the contribution of TC track changes to the interannual variations of intense hurricane activity in the North Atlantic basin is evaluated through numerical experiments.It is found that that observed interannual variations of the frequency of intense hurricanes during the period 1958–2017 are dynamically consistent with changes in the large-scale ocean/atmosphere environment.Track changes can account for~50%of the interannual variability of intense hurricanes,while no significant difference is found for individual environmental parameters between active and inactive years.The only significant difference between active and inactive years is in the duration of TC intensification in the region east of 60°W.The duration increase is not due to the slow-down of TC translation.In active years,a southeastward shift of the formation location in the region east of 60°W causes TCs to take a westward prevailing track,which allows TCs to have a longer opportunity for intensification.On the other hand,most TCs in inactive years take a recurving track,leading to a shorter duration of intensification.This study suggests that the influence of track changes should be considered to understand the basin-wide intensity changes in the North Atlantic basin on the interannual time scale.

Interdecadal Change in the Interannual Variation of the Western Edge of the Western North Pacific Subtropical High During Early Summer and the Influence of Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific(WNP)subtropical high(WNPSH)in early summer experienced an interdecadal decrease around 1990.Correspondingly,the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP(WNPHA)in abnormal years possess smaller ranges after 1990.The different influences of the tropical SSTAs are important for this interdecadal change,which exhibit slow El Nino decaying pattern before 1990 while rapid transformation from El Nino to La Nina after 1990.The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences.Before 1990,the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation,respectively.Meanwhile,the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response,which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific.After 1990,the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation,respectively.Overall,the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990.Consequently,the WNPHA features narrower zonal range and less eastward extension after 1990,corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH.On the other hand,the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990,contributing to the change of the El Nino SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

Contrasts between the Interannual Variations of Extreme Rainfall over Western and Eastern Sichuan in Mid-summer

Rainfall amount in mid-summer(July and August)is much greater over eastern than western Sichuan,which are characterized by basin and plateau,respectively.It is shown that the interannual variations of extreme rainfall over these two regions are roughly independent,and they correspond to distinct anomalies of both large-scale circulation and sea surface temperature(SST).The enhanced extreme rainfall over western Sichuan is associated with a southward shift of the Asian westerly jet,while the enhanced extreme rainfall over eastern Sichuan is associated with an anticyclonic anomaly in the upper troposphere over China.At low levels,on the other hand,the enhanced extreme rainfall over western Sichuan is related to two components of wind anomalies,namely southwesterly over southwestern Sichuan and northeasterly over northeastern Sichuan,which favor more rainfall under the effects of the topography.Relatively speaking,the enhanced extreme rainfall over eastern Sichuan corresponds to the low-level southerly anomalies to the east of Sichuan,which curve into northeasterly anomalies over the basin when they encounter the mountains to the north of the basin.Therefore,it can be concluded that the topography in and around Sichuan plays a crucial role in inducing extreme rainfall both over western and eastern Sichuan.Finally,the enhanced extreme rainfall in western and eastern Sichuan is related to warmer SSTs in the Maritime Continent and cooler SSTs in the equatorial central Pacific,respectively.

Relationship between the Interannual Variations of Total Ozone in the Northern Hemisphere and the QBO of Basic Flow in the Tropical Stratosphere

The harmonic analyses of monthly mean total ozone in the atmosphere over the Northern Hemisphere for 26 years (1960-1985) are made by using the Fourier expansion. The analysed results show that there is obviously a quasi-biennial oscillation (QBO) in the interannual variations of the amplitudes of total ozone. Generally, the amplitudes of wavenumber 1 and 2 during the westerly of the equatorial QBO are larger than those during the easterly. In the early winter, the amplitude of wavenumber 1 during the easterly phase is larger, and in the late winter, it is larger during the westerly phase. These are in good agreement with the observational distributions.

Spatial distributions and interannual variations of snow cover over China in the last 40 years

By using the observational snow data of more than 700 weather stations,the interannual temporal and spatial characteristics of seasonal snow cover in China were analyzed.The results show that northern Xinjiang,northeastern China–Inner Mongolia,and the southwestern and southern portions of Tibetan Plateau are three regions in China with high seasonal snow cover and also an interannual anomaly of snow cover.According to the trend of both the snow depth and snow cover days,there are three changing patterns for the seasonal snow cover:The first type is that both snow depth and snow cover days simultaneously increase or decrease;this includes northern Xinjiang,middle and eastern Inner Mongolia,and so on.The second is that snow depth increases but snow cover days decrease;this type mainly locates in the eastern parts of the northeastern plain of China and the upper reaches of the Yangtze River.The last type is that snow depth decreases but snow cover days increase at the same time such as that in middle parts of Tibetan Plateau.Snow cover in China appears to have been having a slow increasing trend during the last 40 years.On the decadal scale,snow depth and snow cover days slightly increased in the 1960s and then decreased in the 1970s;they again turn to increasing in the 1980s and persist into 1990s.

Interannual variations in salt flux at 80°E section of the equatorial Indian Ocean

The interannual variations in salt flux on the 80°E section in the equatorial Indian Ocean were explored based on the ORAS5 data,which was quite consistent with the observational data among the four available reanalysis datasets.The results indicated that the area with significant interannual variations in salt flux coincided with that of significant climatological mean salt flux in general and was mainly located in the upper 150 m layer.Specifically,three important areas were identified in the north-south direction,i.e.,(1)the Equatorial Indian Ocean Area(EIOA,3°S–3°N),where the mean salt flux was eastward with the largest value on the section and associated with the most significant interannual variations mainly modulated by the Wyrtki Jets and the Equatorial Undercurrent;(2)the South Equatorial Indian Ocean Area(SEIOA,3°S–6°S),where the mean salt flux changed in the vertical direction from strong eastward flux in the upper layer to weak westward flux in the subsurface layer and associated with significant interannual variations in the upper 100 m layer,which was affected by the South Equatorial Countercurrent;and(3)the North Equatorial Indian Ocean Area(NEIOA,3°N–6°N),where the mean salt flux changed in the north-south direction from strong westward flux to the north of 5°N to weak eastward flux in the south and associated with relatively significant interannual variations,which was mainly influenced by the South Sri Lanka Coastal Current.Two leading interannual variation modes were revealed by the empirical orthogonal function decomposition.The first mode accounted for 39%of the total variance and had four significant spatial antinodes;two of those in-phase antinodes were located at SEIOA and upper 75 m of EIOA,and the other two opposite in-phase antinodes were located at NEIOA and below 75 m of EIOA.The second mode accounted for 17%of the total variance having four antinodes with two dominant out-of-phase antinodes located at the subsurface of EIOA and SEIOA.The temporal components of the two leading modes showed a 1–4 year variation with a main period of 2 years,in which the first mode showed a greater correlation with the Indian Ocean Dipole,while the second mode showed a weak correlation with the Indian Ocean Dipole and the El Niño-Southern Oscillation.Variance analysis showed that the interannual variations in salt flux were mainly determined by the variations in the current,and the spatial distribution was modulated by temporal mean salinity.Composite strong interannual events showed interannual variations in current,and so the salt flux was driven by the interannual anomaly of the wind field and sea surface temperature associated with the Indian Ocean Dipole.

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 and meridional strength responsible for East Asian monsoon circulation to study its relation to the East Asian monsoon circulation and the summer rainfall over China on an interannual basis.Results are as follows:(1)ILSTD can be used to measure the strength of East Asian summer monsoon in such a way that the strong(weak)ILSTD years are associated with strong(weak)summer monsoon circulation.(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 drought emerges in the Jianghuai valleys and mid-lower reaches of the Changjiang River,along with increase of rainfall in North and South China,but in the weak years it is contrary.(3)Besides,the index has obvious QBO and quasi 4-year oscillations,but the periods and amplitudes have significant changes on an interdecadal basis.