The tropical intraseasonal oscillation in SAMIL coupled and uncoupled general circulation models

Simulations of tropical intraseasonal oscillation(TISO) in SAMIL,the Spectral Atmospheric Model from the Institute of Atmospheric Physics(IAP) State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics(LASG) coupled and uncoupled general circulation models were comprehensively evaluated in this study.Compared to the uncoupled model,the atmosphere-ocean coupled model improved the TISO simulation in the following aspects:(1) the spectral intensity for the 30-80-day peak eastward periods was more realistic;(2) the eastward propagation signals over western Pacific were stronger;and(3) the variance distribution and stronger signals of Kelvin waves and mixed Rossby gravity waves were more realistic.Better performance in the coupled run was assumed to be associated with a better mean state and a more realistic relationship between precipitation and SST.In both the coupled and uncoupled runs,the unrealistic simulation of the eastward propagation over the equatorial Indian Ocean might have been associated with the biases of the precipitation mean state over the Indian Ocean,and the unrealistic split of maximum TISO precipitation variance over the Pacific might have corresponded to the exaggeration of the double Intertropical Convergence Zone(ITCZ) structure in precipitation mean state.However,whether a better mean state leads to better TISO activity remains questionable.Notably,the northward propagation over the Indian Ocean during summer was not improved in the mean lead-lag correlation analysis,but case studies have shown some strong cases to yield remarkably realistic northward propagation in coupled runs.     

Recent Advance in Understanding the Dynamics of the Madden-Julian Oscillation

The Madden-Julian oscillation （MJO） is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The effect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher- frequency eddies are delineated.     

The East Asia-western North Pacific boreal summer intraseasonal oscillation simulated in GAMIL 1.1.1

We evaluate the performance of GAMIL1.1.1 in a 27-year forced simulation of the summer intraseasonal oscillation (ISO) over East Asia (EA)-western North Pacific (WNP). The assessment is based on two measures: climatological ISO (CISO) and transient ISO (TISO). CISO is the ISO component that is phase-locked to the annual cycle and describes seasonal march. TISO is the ISO component that varies year by year. The model reasonably captures many observed features of the ISO, including the stepwise northward advance of the rain belt of CISO, the dominant periodicities of TISO in both the South China Sea-Philippine Sea (SCS-PS) and the Yangtze River Basin (YRB), the northward propagation of 30--50-day TISO and the westward propagation of the 12--25-day TISO mode over the SCS-PS, and the zonal propagating features of three major TISO modes over the YRB. However, the model has notable deficiencies. These include the early onset of the South China Sea monsoon associated with CISO, too fast northward propagation of CISO from 20^oN to 40^oN and the absence of the CISO signal south of 10^oN, the deficient eastward propagation of the 30--50-day TISO mode and the absence of a southward propagation in the YRB TISO modes. The authors found that the deficiencies in the ISO simulation are closely related to the model's biases in the mean states, suggesting that the improvement of the model mean state is crucial for realistic simulation of the intraseasonal variation.     

Synoptic climatology of transient tropical intraseasonal convection anomalies: 1975–1985

Summary Pentad mean anomaly maps were used to study the climatology of tropical intraseasonal convection anomaly (TICA) as a dynamic system. One hundred and twenty-two events were identified and classified into three categories: eastward (77), independent northward (27), and westward (18) propagation. The eastward propagation is more active in boreal winter than in summer, while the independent northward propagation, which is not associated with equatorial eastward propagation, occurs in boreal summer from May to October.The eastward moving TICA exhibits three major paths: 1) eastward along the equator from Africa to the mid-Pacific, 2) first eastward along the equator, then either turning north-east to the northwest Pacific or turning southeast to the southwest Pacific at the maritime continent, and 3) the main anomaly moves eastward along the equator with split center(s) moving northward over the Indian and/or western Pacific Oceans. The equatorial Indian Ocean and the western Pacific intertropical convergence zone are preferred geographic locations for their development, while the maritime continent and central Pacific are regions of dissipation.Independent northward propagation is confined to the Indian and western Pacific monsoon regions. Its existence suggests that the mechanism responsible for meridional propagation may differ from that for eastward propagation.The dynamic effect of the equator and the thermodynamic effect of the underlying warm ocean water are basic factors in trapping TICA in the deep tropics, while the annual march of maximum SST (thermal equator) and the monsoon circulation have profound influences on the annual variation and meridional movement of TICA.With 12 FiguresContribution No. 89-11, Department of Meteorology, University of Hawaii.     

Intraseasonal variability of latent-heat flux in the South China Sea

Intraseasonal variability (ISV) of latent-heat flux in the South China Sea (SCS) is examined using 9 years of weekly data from January 1998 to December 2006. Using harmonic and composite analysis, some fundamental features of the latent-heat flux ISVs are revealed. Intraseasonal latent-heat flux has two spectral peaks around 28–35 and 49–56 days, comparable with the timescales of the atmospheric ISV in the region. Active monsoon is clearly correlated with positive and negative phases of the ISV of latent-heat flux in the SCS. The characteristics of the intraseasonal latent-heat flux variations in summer are remarkably different from those in winter. The amplitudes of significant intraseasonal oscillations are about 35 and 80 W?m^?2 during summer and winter monsoons, respectively. In summer, the intraseasonal latent-heat flux perturbations are characterized by slow eastward (about 1° latitude/day) and slower northward (about 0.75° longitude/day) propagations, probably in a response to eastward and northward propagating Madden-Julian oscillations (MJOs) from the equatorial Indian Ocean. In contrast, the perturbations appear to remain in the northern SCS region like a quasi-stationary wave in winter. In summer, the intraseasonal latent-heat flux fluctuations are highly correlated with wind speed. In winter, however, they are primarily associated with winds and near-surface air humidity. In addition, the intraseasonal SST variation is estimated to significantly reduce the amplitude of the intraseasonal latent-heat flux by 20% during winter.     

POSSIBLE INFLUENCES OF THE TROPICAL INDIAN OCEAN DIPOLE ON THE EASTWARD PROPAGATION OF MJO

Based on multiple datasets, correlation and composite analyses, and case studies, this paper investigated possible influences of the Indian Ocean dipole （IOD） mode on the eastward propagation of intraseasonal oscillation in the tropical atmosphere. The results showed that （1） the 30-60 day outgoing longwave radiation anomalies in the southeastern Indian Ocean and the 30-60 day 850-hPa zonal wind anomalies over the equatorial central Indian Ocean were significantly correlated with the IOD index; （2） during positive IOD years, the anomalously cold water in the southeastern Indian Ocean and the 850-hPa anomalous easterlies over the equatorial central Indian Ocean might act as barriers to the continuously eastward propagation of the intraseasonal convection, which interrupts the Madden-Julian oscillation （MJO） propagation in the eastern equatorial Indian Ocean and western Pacific; and （3） during negative IOD years, the anomalously warm water in the southeastern Indian Ocean and the low-level westerly anomalies over the equatorial central Indian Ocean favor the eastward movement of MJO.     

THE ROLE OF THE MERIDIONAL CIRCULATION IN STATIONARY WAVE PROPAGATION

In a general baroclinic atmosphere,when the basic state includes meridional circulation,the sta-tionary waves might not only pass through the equatorial easterlies,but also strengthen significantly.The orographic forcing in the Northern Hemisphere mid-latitude might cause marked responses in thelow latitude atmosphere.This suggests that the meridional circulation plays an important role in theconnection of stationary responses in mid and low latitudes,and so does the heating forcing in theNorthern Hemisphere mid-latitude.Forced by the heating forcing in the Northern Hemisphere mid-latitude,the features similar to the Northern Hemisphere summer monsoon circulation can be ob-tained.It appears that the meridional circulation plays certain role in the formation of summer mon-soon circulation.The heating anomaly forcing located at the eastern equatorial Pacific makes the sta-tionary waves present PNA(Pacific-North America)pattern in the winter hemisphere,but it doesnot in the summer hemisphere.It suggests that the meridional circulation has a marked influence onthe route of stationary wave propagation both in the winter and summer hemispheres.     

Medium-range oscillations in the summer tropical easterlies at 200 hPa

By the use of space-time spectral analysis and band-pass filter, some of the features of the medium-range Oscillations in the summer tropical easterlies (10oS-20o) at 200 hPa are investigated based on a two-year (1980 and 1982) wind (u, v) data set for the period from May to September. Space-time power spectral analysis shows that the total energy of the westward moving waves was the largest and that of the standing waves and eastward moving waves was relatively small in the 200 hPa easterlies; the total energy of the eastward moving waves was at minimum at 10oN. Three kind of the medium-range oscillations with about 50 day, 25 day and quasi-biweekly periods were found in the easterlies, which all show a remarkable interannual variation and latitudinal differences in these two years. The wave energy of zonal wind is mainly associated with the planetary waves (1-3), which all may make important contributions to the 50 day and 25 day oscillations in different years or different latitudes. The quasi-biweekly oscillation is mainly related to the synoptic waves (4-6). In equatorial region, the 50 day oscillation was dominant with a eastward phase propagation in 1982 while the dominant oscillation in 1980 was of 25day period with a westward phase propagations in 1980. Both of them are of the mode of zonal wavenumber 1. Strong westward 50 day oscillation was found in 10oN-20oN in these two years. Regular propagations of the meridional wind 50 day oscillation were also found in the easterlies.The 50 day and 25 day oscillation of zonal wind all demonstrate southward phase propagation over the region of the South Asia monsoon and northward phase propagation near interational date line, where are the climatic mean position of the tropical upper-tropospheric easterly jet and the tropical upper tropospheric trough (TUTT), respectively.     

夏季MJO持续异常的主要特征分析

在MJO传播过程中,其活动中心并不总是规律地沿赤道东传。本文通过资料分析发现,夏季MJO的活动中心会出现东传停滞的情况,表现为MJO在赤道太平洋持续异常活跃或者在印度洋持续异常活跃两种形式。为更好描述MJO这种东传不明显的异常特征,本文定义了一个描述MJO持续异常的指数,并据此对夏季MJO持续异常的主要特征进行分析。通过小波分析的方法,发现夏季MJO持续异常时其振荡周期会出现缩短或变弱。通过对MJO持续异常状况下的大气环流进行合成对比分析后发现,夏季MJO的持续异常会对热带大气环流造成显著的影响。具体表现为:MJO夏季在赤道太平洋（印度洋）持续活跃的时候,赤道沃克环流减弱（增强）,西太平洋哈得来环流增强（减弱）,西太平洋副高位置偏北（偏南）,赤道太平洋（印度洋）高层辐散且对流活跃。     

Effects of air�Csea coupling on the boreal summer intraseasonal oscillations over the tropical Indian Ocean

The effects of air?Csea coupling over the tropical Indian Ocean (TIO) on the eastward- and northward-propagating boreal summer intraseasonal oscillation (BSISO) are investigated by comparing a fully coupled (CTL) and a partially decoupled Indian Ocean (pdIO) experiment using SINTEX-F coupled GCM. Air?Csea coupling over the TIO significantly enhances the intensity of both the eastward and northward propagations of the BSISO. The maximum spectrum differences of the northward- (eastward-) propagating BSISO between the CTL and pdIO reach 30% (25%) of their respective climatological values. The enhanced eastward (northward) propagation is related to the zonal (meridional) asymmetry of sea surface temperature anomaly (SSTA). A positive SSTA appears to the east (north) of the BSISO convection, which may positively feed back to the BSISO convection. In addition, air?Csea coupling may enhance the northward propagation through the changes of the mean vertical wind shear and low-level specific humidity. The interannual variations of the TIO regulate the air?Csea interaction effect. Air?Csea coupling enhances (reduces) the eastward-propagating spectrum during the negative Indian Ocean dipole (IOD) mode, positive Indian Ocean basin (IOB) mode and normal years (during positive IOD and negative IOB years). Such phase dependence is attributed to the role of the background mean westerly in affecting the wind-evaporation-SST feedback. A climatological weak westerly in the equatorial Indian Ocean can be readily reversed by anomalous zonal SST gradients during the positive IOD and negative IOB events. Although the SSTA is always positive to the northeast of the BSISO convection for all interannual modes, air?Csea coupling reduces the zonal asymmetry of the low-level specific humidity and thus the eastward propagation spectrum during the positive IOD and negative IOB modes, while strengthening them during the other modes. Air?Csea coupling enhances the northward propagation under all interannual modes due to the persistent westerly monsoon flow over the northern Indian Ocean.     

陕西夏季旱涝的主要环流特征

利用1961—2008年NCEP／NCAR再分析资料以及陕西地面月降水资料,采用EOF分解、合成分析等方法,分析了陕西夏季旱涝的时空分布特征以及前期气候系统的异常信号特征。结果表明：陕西夏季多雨年乌拉尔山高压脊和鄂霍次克海高压偏强,贝加尔湖低槽偏深,西太平洋副热带高压偏强,西伸脊点偏西。并且前期冬季中高纬度中亚长波脊偏强偏西,西太平洋副热带高压偏强,印缅槽偏弱,700hPa西北地区东部至华北偏北风异常偏强,赤道东太平洋出现暖水位相,西风漂流区海温偏低,印度洋海温偏高,陕西夏季易多雨;而陕西夏季少雨年西太平洋副热带高压偏弱,西伸脊点偏东,陕西主要受中亚高脊前西北气流控制。前期冬季中高纬度欧洲西北部低槽偏强,中亚长波脊偏弱,西太平洋副热带高压偏弱,印缅槽偏强,700hPa西北地区东部至华北偏南风异常偏强,赤道东太平洋出现冷水位相,西风漂流区海温偏高,印度洋海温偏低,陕西夏季易少雨。     

Application of UTCI in China from tourism perspective

This paper investigates the processes and mechanisms by which the East Asian winter monsoon (EAWM) affects the Madden-Julian oscillation (MJO) over the equatorial western Pacific in boreal winter (November–April). The results show that both the EAWM and MJO over the equatorial western Pacific have prominent interannual and interdecadal variabilities, and they are closely related, especially on the interannual timescales. The EAWM influences MJO via the feedback effect of convective heating, because the strong northerlies of EAWM can enhance the ascending motion and lead the convection to be strengthened over the equatorial western Pacific by reinforcing the convergence in the lower troposphere. Daily composite analysis in the phase 4 of MJO (i.e., strong MJO convection over the Maritime Continent and equatorial western Pacific) shows that the kinetic energy, outgoing longwave radiation (OLR), moisture flux, vertical velocity, zonal wind, moist static energy, and atmospheric stability differ greatly between strong and weak EAWM processes over the western Pacific. The strong EAWM causes the intensity of MJO to increase, and the eastward propagation of MJO to become more persistent. MJO activities over the equatorial western Pacific have different modes. Furthermore, these modes have differing relationships with the EAWM, and other factors can also affect the activities of MJO; consequently, the relationship between the MJO and EAWM shows both interannual and interdecadal variabilities.     

A FURTHER INQUIRY ON 30-60 DAY OSCILLATION IN THE TROPICAL ATMOSPHERE

Based on the ECMWF data(1980-1983) and others, a further inquiry on the activities and the structure feature of 30-60 day oscillation in the tropical atmosphere has been completed. The following results are obtained:There is stronger perturbation kinetic energy of 30-60 day atmospheric oscillation(AO) in the equatorial eastern Pacific. This means the equatorial eastern Pacific is a stronger activity region of 30-60 day AO in the tropics. Analyses also show that the AO system with the time scale of 30-60 days might consist of various spatial scale disturbances. The zonal propagation of 30-60 day oscillation in the tropical atmosphere is not all eastward. Some differences are found for different spatial scales, and for propagations in upper and lower tropospheres. The meridional propagation of the oscillation is even more different in the various regions and might be related to the low-frequency wave train in the atmosphere. The stronger activities of 30-60 day AO in the equatorial middle-western Pacific are related to the El Nino events and the weaker ones are correspondent to the inverse El Nino phenomena.     

印度洋海温年际异常与热带夏季季节内振荡的关系及其数值模拟研究

利用观测分析资料和SINTEX-F海气耦合长时间(70年)数值模拟结果,分析了印度洋海温年际异常与热带夏季季节内振荡(BSISO)各种传播模态之间关系及其物理过程。结果表明,印度洋海温年际异常与热带BSISO关系密切,当印度洋为正(负)偶极子情况,中东印度洋北传BSISO减弱(加强);当印度洋为正(负)海盆异常(BWA)情况,印度洋西太平洋赤道地区(40°E -180°)东传BSISO加强(减弱)。印度洋海温年际变化通过大气环流背景场和BSISO结构影响热带BSISO不同传播模态强度的年际变化。在负(正)偶极子年夏季,由于对流层大气垂直东风切变加强(减弱),对流扰动北侧的正压涡度、边界层水汽辐合加强更明显(不明显),导致形成BSISO较强(弱)的经向不对称结构,因此北传BSISO偏强(减弱)。印度洋BWA模态通过影响赤道西风背景以及海气界面热力交换,导致赤道东传BSISO强度产生变化。在正BWA年夏季,赤道地区西风较明显,当季节内振荡叠加在这种西风背景下,扰动中心的东侧(西侧)风速减弱(加强)更明显,海面蒸发及蒸发潜热减弱(加强)更明显,导致扰动中心的东侧(西侧)海温升高(降低)幅度更大,从而使边界层产生辐合(辐散)更强、水汽更多(少),因此赤道东传BSISO偏强;而在负BWA年,赤道地区西风背景减弱,以上物理过程受削弱使赤道东传BSISO偏弱。     

Forecast skill of the tropical intraseasonal oscillation in the NCEP GFS dynamical extended range forecasts

This study examines the forecast performance of tropical intraseasonal oscillation (ISO) in recent dynamical extended range forecast (DERF) experiments conducted with the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model. The present study extends earlier work by comparing prediction skill of the northern winter ISO (Madden-Julian Oscillation) between the current and earlier experiments. Prediction skill for the northern summer ISO is also investigated. Since the boreal summer ISO exhibits northward propagation as well as eastward propagation along the equator, forecast skill for both components is computed. For the 5-year period from 1 January, 1998 through 31 December, 2002, 30-day forecasts were made once a day. Compared to the previous DERF experiment, the current model has shown some improvements in forecasting the ISO during winter season so that the skillful forecasts (anomaly correlation>0.6) for upper-level zonal wind anomaly extend from the previous shorter-than 5 days out to 7 days lead-time. A similar level of skill is seen for both northward and eastward propagation components during the summer season as in the winter case. Results also show that forecasts from extreme initial states are more skillful than those from null phases for both seasons, extending the skillful range by 3–6 days. For strong ISO convection phases, the GFS model performs better during the summer season than during the winter season. In summer forecasts, large-scale circulation and convection anomalies exhibit northward propagation during the peak phase. In contrast, the GFS model still has difficulties in sustaining ISO variability during the northern winter as in the previous DERF run. That is, the forecast does not maintain the observed eastward propagating signals associated with large-scale circulation; rather the forecast anomalies appear to be stationary at their initial location and decay with time. The NCEP Coupled Forecast System produces daily operational forecasts and its predication skill of the MJO will be reported in the future.     

冬夏东亚季风环流对太平洋热状况的响应

冬夏隔季韵律关系一直是我国长期天气预报和短期气候预测的一个重要依据,然而迄今为止对它们之间的物理过程及成因机理并不十分清楚。利用NCEP/NCAR全球2.5°×2.5°网格月平均再分析资料,研究1951~2000年冬夏东亚季风环流异常变化与太平洋海面温度(SST)的关系及对关键海温区响应机理。研究指出:冬夏东亚季风环流隔季韵律关系及其年际变化与赤道东太平洋海面温度异常(SSTA)变化密切相关,冬季赤道东太平洋出现La Ni~na(El Ni~no)型的SST分布,有利冬、夏东亚季风环流加强(减弱),其影响过程通过赤道Walker环流强(弱)以及东亚地区Hadley环流强(弱)过程完成。冬季赤道东太平洋海温变化是冬、夏东亚环流季节以及年际变化的一个重要外强迫因子。     

多因子协同作用对1992年和1998年黄淮地区夏季降水异常的影响

1991年5月和1997年4月赤道中东太平洋均发生了El Ni?o事件,但是1992年夏季黄淮地区降水异常偏少,而1998年夏季却异常偏多。分析结果显示,1992年夏季西北太平洋副热带高压（以下简称副高）偏东,中高纬阻塞高压偏弱,黄淮地区降水异常偏少;而1998年夏季,副高偏西,中高纬阻塞高压活动频繁,黄淮地区降水异常偏多。对海温外强迫信号的诊断和数值模式试验显示:当西太平洋对流活动偏弱时,有利于副高西伸;鄂霍茨克海及以东海温偏高时,其上空的阻塞高压增强;北大西洋中纬度地区海温偏高时,有利于后期乌拉尔山高压脊明显增强。即在赤道中东太平洋发生El Ni?o事件的背景下,西太平洋对流、鄂霍茨克海附近亲潮区域和北大西洋中纬度区域海温异常可能是导致黄淮区域1992年夏季和1998年夏季降水差异大的主要原因。该工作显示仅根据El Ni?o事件的发生时间和强度无法完全预测黄淮地区夏季降水变化,需要综合考虑西太平洋对流、鄂霍茨克海附近海域和北大西洋中纬度区域海温异常对季风环流的影响,从多因子协同作用的角度诊断和预测黄淮地区夏季降水异常趋势,提高预测能力。     

Characteristics of Zonal Propagation of Atmospheric Kinetic Energy at Equatorial Region in Asia

Based on the daily NCEP/NCAR reanalysis dataset from 1980 to 1997, the zonal propagations of 850 hPa kinetic energy (KE) and meridional wind (v) at equatorial region are examined respectively. Results show that the strongest center of KE in the tropical Asian monsoon region is located at 75°-90°E, with the secondary over the Somalia low-level jet channel, i.e., about 50°E. East to 90°E, disturbances of both KE and v observed are mainly coming from the western Pacific Ocean and propagating westward to the Bay of Bengal (BOB) passing through the South China Sea. But the propagation directions of both KE and v are rather disorderly between the BOB and the Somalia jet channel. Therefore, the East Asian summer monsoon and the Indian summer monsoon are different in the propagating features of the disturbances of KE and v. Above facts indicate that East Asian monsoon system exists undoubtedly even at the equatorial region, and quite distinct from the Indian monsoon system, it is mainly affected by the disturbances coming from the tropical western Pacific rather than from the Indian monsoon region. The boundary of the two monsoon systems is around 95°-100°E, which is more westward than the counterpart as proposed in earlier studies by 5-10 degrees in longitude.     

NUMERICAL SIMULATION OF LAG INFLUENCE OF ENSO ON EAST-ASIAN MONSOON

By prescribing sea surface temperature anomalies(SSTAs)over eastern equatorial Pacific inJanuary—March,the lag influence of ENSO(El Nino and La Nina)on monsoon over East Asiahas been studied.The results suggest that,due to the excitation of atmospheric low-frequencyoscillation by the SSTA,ENSO has significant lag influence on the monsoon over East Asia.During the summer after E1 Nino,the subtropical high over western Pacific is intensified andshows the northward and westward displacement,meanwhile,the rainfall over East China isbelow normal,especially in North China:during the winter after E1 Nino,both the Asian troughand the winter monsoon over East Asia are strengthened.During the summer after La Nina,theanomalous subtropical high prevails over the lower reaches of Yangtze(Changjiang)River,therainfall between Yangtze and Huaihe Rivers is below normal:during the winter after La Nina,both the Asian trough and the winter monsoon over East Asia are weaker.Compared with LaNina,the effect of El Nino is stronger,but it is not always opposite to the one of La Nina.     

ABRUPT CHANGE IN ELEMENTS AROUND 1998 SCS SUMMER MONSOON ESTABLISHMENT WITH ANALYSIS OF ITS CHARACTERISTIC PROCESS

The latest dataset from the SCS(South China Sea)Monsoon Experiment is used to investigatethe features of abrupt change in some meteorological elements before,during and after the summermonsoon's establishment in 1998 and explore its onset characteristic process.We have arrived at apreliminary conclusion that the 1998 Asian summer monsoon is established first in the SCS as earlyas May 23,which is representative of the earliest indicator of the conversion from a winter into asummer monsoon situation in Asia;the continued retreat eastward of the western Pacificsubtropical high from the SCS region has direct effect on the SCS summer monsoon establishmentbecause the withdrawal favors the release of unstable energy,responsible for the sudden onset ofthe monsoon.Our tentative investigation indicates that the eastward extension of the westerly andrainfall band from the equatorial Indian Ocean into the Indo-China Peninsula and the southwardspreading of an active South-China stationary front,acting as the interaction between mid and lowlatitude systems,are likely to be the characteristic events contributing to the subtropical high'seastward retreating and the summer monsoon's onset over the SCS.