A NUMERICAL STUDY FOR THE EFFECT OF TROPICAL 40-50 DAY OSCILLATION ON ASIAN SUMMER MONSOON

By using a five-layer primitive equation model with P-sigma coordinates,the effect of convective heating source with the oscillation of a dipole pattern over the tropical Indian Ocean-Western Pacific on Asian summer monsoon is investigated.The results from simulations show that the oscillatary heating source may cause oscillations in east-west zonal circulation at the equator,in cross-equatorial flow,in meridional monsoon circulation and in the phase of high-low level circulation over Asia,with period same as that of the oscillating heat source.Furthermore,the influence mechanism of the tropical heating source associated with oscillations on Asian summer monsoon circulation is also studicd.It is clearly shown that the westward propagation of thermally-forced Rossy waves to the west of the oscillatary heating source and the northward propagation of disturbances can give rise to oscillations of the equatorial east-west zonal circulation and monsoonal meridional circulation.Finally,the oscillation of all the Asian summer monsoon circulation is formed.     

赤道西太平洋-印度洋海温异常对亚洲夏季风的影响

本文采用了p-σ五层原始方程模式模拟并研究了赤道西太平洋-印度洋海温距平场对亚洲夏季风的影响,计算了四种不同的海温距平试验方案。试验结果表明赤道西太平洋海温正距平使对流层下层的印度低压明显加强,副高北挺,季风槽加深,同时加强了对流层上层的反气旋环流。赤道西印度洋暖海温的模拟结果与赤道西太平洋暖海温对上述系统的影响相反,而赤道西印度洋冷海温对季风环流的影响与赤道西太平洋暧海温的影响一致。试验进一步表明赤道西太平洋-印度洋海温距平的纬向梯度方向对亚洲夏季风的影响是主要的,这一结论与实际观测结果一致。本文进一步讨论了赤道海温距平对越赤道气流、印度洋赤道东-西纬向环流和非绝热加热场的影响,结果都表明赤道西太平洋海温正距平和赤道西印度洋海温负距平的模拟特征与反El Nino年亚洲夏季环流特征类似,而赤道西印度洋海员正距平的模拟特征与El Nino年亚洲夏季坏流特征类似。     

Mechanisms of low frequency intraseasonal oscillations of the Indian summer monsoon

Summary This work deals with idealized modelling experiments designed to understand the dynamical evolution of low frequency intraseasonal monsoonal oscillations that result from interactions between the large scale monsoon Reverse Hadley Cell (RHC) and moist convective processes. The monsoon differential heating, which primarily determines the low-level convergence of the large-scale monsoon flow, is found to play a decisive role in affecting the northward progression of the monsoonal modes. A strong north-south differential heating leads to a robust generation and steady maintenance of northward propagating monsoonal oscillations. A weaker land-ocean thermal contrast leads to feeble low frequency monsoonal modes that have relatively longer periods in the 30–50 day band. This increase in the period of the monsoonal oscillations due to weak north-south thermal contrast is in good agreement with the observational findings of Yasunari (1980) and Kasture and Keshavamurty (1987). It is speculated that such an increase in the oscillatory period may be an outcome from an elongation in the meridional scale of the transient Hadley type cells which act as resonating cavities for the monsoonal modes.A Mobile Wave CISK (MWC) form of interaction between the large scale monsoon and the transient circulations associated with the Madden Julian Oscillation (MJO) is projected as a viable physical mechanism for the northward movement of low frequency modes. It is demonstrated that the effective low level convergence, following such an interaction, tends to shift northward relative to the site of interaction. This enables the heating perturbations to be displaced northward which in turn causes the secondary circulations and wind perturbations to follow. The essential criterion for the occurrence of a prolonged northward propagation of the low frequency modes is that the heating perturbations should phase lead the wind perturbations at all times.An examination of the - interactions on the 30–50 day time scale reveals that the conversion from the transient divergent motions to rotational motions is quite intense (feeble) in the strong (weak) monsoon differential heating experiments. Because of the closer proximity to the monsoon heat source and also due to the latitudinal variation of earth's rotational effects, the - interactions tend to be more pronounced to the north of 15°N while they are less robust in the near equatorial latitudes.The regularity of the monsoonal modes is found to depend on the strength of the monsoon differential heating and also on the periodic behaviour of the equatorial intraseasonal oscillations. The monsoonal modes are quite steady and exhibit extreme regularity in the presence of a weak north-south differential heating provided the equatorial forcing due to the MJO varies in a periodic manner. This result supports the findings of Mehta and Krishnamurti (1988) who found greater regularity of the 30–50 day modes during bad monsoon years.The low frequency monsoonal modes are found to be quite sensitive to the moisture availability factor (m) and the vertical profile of heating used in the MWC parameterization. A small increase in the value of (m) is found to significantly intensify the amplitude of the monsoonal oscillations while there is no considerable shift in the spectral frequency within the 30–50 day band as such. The 30–50 day motions show significant enhancement, with a relatively sharp spectral peak around 45 days, when the vertical profile of MWC heating has a maximum in the lower troposphere. However an upward displacement of the heating maximum tends to weaken the low frequency oscillations.With 19 Figures     

The Indian summer monsoon drought of 2002 and its linkage with tropical convective activity over northwest Pacific

The Indian subcontinent witnessed a severe monsoon drought in 2002, which largely resulted from a major rainfall deficiency in the month of July. While moderate El Nino conditions prevailed during this period, the atmospheric convective activity was anomalously enhanced over northwest and north-central Pacific in the 10–20°N latitude belt; and heavy rainfall occurred over this region in association with a series of northward moving tropical cyclones. Similar out-of-phase rainfall variations over the Indian region and the northwest (NW) Pacific have been observed during other instances of El Nino/Southern Oscillation (ENSO). The dynamical linkage corresponding to this out-of-phase rainfall variability is explored in this study by conducting a set of numerical experiments using an atmospheric general circulation model. The results from the model simulations lend credence to the role of the tropical Pacific sea surface temperature anomalies in forcing the out-of-phase precipitation variability over the NW Pacific and the Indian monsoon region. It is seen that the ENSO induced circulation response reveals an anomalous pattern comprising of alternating highs and lows which extend meridionally from the equatorial region into the sub-tropic and mid-latitude regions of west-central Pacific. This meridional pattern is associated with an anomalous cyclonic circulation over NW Pacific, which is found to favor enhanced tropical cyclonic activity and intensified convection over the region. In turn, the intensified convection over NW Pacific induces subsidence and rainfall deficiency over the Indian landmass through anomalous east-west circulation in the 10–20°N latitude belt. Based on the present findings, it is suggested that the convective activity over NW Pacific is an important component in mediating the ENSO-monsoon teleconnection dynamics.     

印度洋—西太平洋对流涛动的季节内特征

本文利用30~60天带通滤波资料, 考察了不同季节印度洋—西太平洋区域对流活动季节内尺度变率的主要模态, 发现在不同季节赤道东印度洋(5°S~10°N, 70°E~100°E)和西北太平洋(5°N~20°N, 110°E~160°E)对流活动均存在反相变化的关系, 将之称为季节内尺度的印度洋—西太平洋对流涛动(Indo-West Pacific Convection Oscillation), 简称IPCO。对IPCO两极子区域对流活动进行超前滞后相关分析, 发现IPCO事件形成—发展—消亡的生命周期是由对流活动季节内振荡及其传播造成的。对流扰动首先在赤道中西印度洋形成, 随后逐渐向东发展变强, 在其继续变强的过程中将分两支传播:一支由赤道印度洋向北传播, 至印度半岛南部后逐渐减弱消失;另一支沿赤道继续东传, 在海洋大陆受到抑制, 快速越过海洋大陆到达赤道西太平洋后又开始发展变强, 随后北传至西北太平洋区域逐渐减弱, 最终至我国长江流域中下游到日本区域消失。将这一过程划分为8个位相, 详细分析了不同位相对应的环流场和降水场特征, 最后给出了IPCO事件演化示意图。     

Simulation of monsoon intraseasonal variability in NCEP CFSv2 and its role on systematic bias

We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass.     

Daily modes of South Asian summer monsoon variability in the NCEP climate forecast system

The leading modes of daily variability of the Indian summer monsoon in the climate forecast system (CFS), a coupled general circulation model, of the National Centers for Environmental Predictions (NCEP) are examined. The space?Ctime structures of the daily modes are obtained by applying multi-channel singular spectrum analysis (MSSA) on the daily anomalies of rainfall. Relations of the daily modes to intraseasonal and interannual variability of the monsoon are investigated. The CFS has three intraseasonal oscillations with periods around 106, 57 and 30?days with a combined variance of 7%. The 106-day mode has spatial structure and propagation features similar to the northeastward propagating 45-day mode in the observations except for its longer period. The 57-day mode, despite being in the same time scale as of the observations has poor eastward propagation. The 30-day mode is northwestward propagating and is similar to its observational counterpart. The 106-day mode is specific to the model and should not be mistaken for a new scale of variability in observations. The dominant interannual signal is related to El Ni?o-Southern Oscillation (ENSO), and, unlike in the observations, has maximum variance in the eastern equatorial Indian Ocean. Although the Indian Ocean Dipole (IOD) mode was not obtained as a separate mode in the rainfall, the ENSO signal has good correlations with the dipole variability, which, therefore, indicates the dominance of ENSO in the model. The interannual variability is largely determined by the ENSO signal over the regions where it has maximum variance. The interannual variability of the intraseasonal oscillations is smaller in comparison.     

Influence of the Eastern Pacific and Central Pacific Types of ENSO on the South Asian Summer Monsoon

Based on observational and reanalysis data,the relationships between the eastern Pacific(EP)and central Pacific(CP)types of El Ni?o?Southern Oscillation(ENSO)during the developing summer and the South Asian summer monsoon(SASM)are examined.The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s.While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically,the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s.The drought-producing El Ni?o warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line.Accordingly,compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent,leading to suppressed convection and decreased precipitation over these regions.Moreover,anomalously less moisture flux into South Asia associated with developing EP El Ni?o and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Ni?o,may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods,respectively.El Ni?o events with the same“flavor”may not necessarily produce consistent Indian monsoon rainfall anomalies,while similar Indian monsoon droughts may be induced by different types of El Ni?o,implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.     

INTERRELATION BETWEEN EAST-ASIAN WINTER MONSOON AND INDIAN/PACIFIC SST WITH THE INTERDECADAL VARIATION*

Investigated statistically is the interrelation between East Asian winter monsoon (EAWM) and SST over sensitive areas of the Indian and Pacific Oceans.with focus on the relation of EAWM to strong ENSO signal area.i.e.,the equatorial eastern Pacific (EEP) SST.Evidence suggests that the EAWM variation is intimately associated not only with the EEP SST but with the equatorial western Pacific "warm pool" and equatorial Indian/northwestern Pacific Kuroshio SST as well:the EAWM and ENSO interact strongly with each other on the interannual time scales,exhibiting pronounced interdecadal variation mainly under the joint effect of the monsoon QBO and the monsoon/SST background field features on an interdecadal basis-when both fields are in the same phase(anti-phase).strong EAWM contributes to EEP SST rise(drop)in the following winter,corresponding to a warm(cold)ENSO cycle;the EAWM QBO causes ENSO cycle to be strong phase-locked with seasonal variation,making the EEP SST rise lasting from April-May to May-June of the next year,which plays an important role in maintaining a warm ENSO phase.     

Eastward propagating MJO during boreal summer and Indian monsoon droughts

Improved understanding of underlying mechanism responsible for Indian summer monsoon (ISM) droughts is important due to their profound socio-economic impact over the region. While some droughts are associated with ‘external forcing’ such as the El-Niño and Southern Oscillation (ENSO), many ISM droughts are not related to any known ‘external forcing’. Here, we unravel a fundamental dynamic process responsible for droughts arising not only from external forcing but also those associated with internal dynamics. We show that most ISM droughts are associated with at least one very long break (VLB; breaks with duration of more than 10 days) and that the processes responsible for VLBs may also be the mechanism responsible for ISM droughts. Our analysis also reveals that all extended monsoon breaks (whether co-occurred with El-Niño or not) are associated with an eastward propagating Madden–Julian Oscillation (MJO) in the equatorial Indian Ocean and western Pacific extending to the dateline and westward propagating Rossby waves between 10° and 25°N. The divergent Rossby wave associated with the dry phase of equatorial convection propagates westward towards Indian land, couple with the northward propagating dry phase and leads to the sustenance of breaks. Thus, the propensity of eastward propagating MJO during boreal summer is largely the cause of monsoon droughts. While short breaks are not accompanied by westerly wind events (WWE) over equatorial western Pacific favorable for initiating air–sea interaction, all VLBs are accompanied by sustained WWE. The WWEs associated with all VLB during 1975–2005 initiate air–sea interaction on intraseasonal time scale, extend the warm pool eastward allowing the convectively coupled MJO to propagate further eastward and thereby sustaining the divergent circulation over India and the monsoon break. The ocean–atmosphere coupling on interannual time scale (such as El-Niño) can also produce VLB, but not necessary.     

ENSO循环的两个不同位相期印度海洋表温度异常的特征分析

通过对ENSO循环的两个不同位相中印度洋地区海表温度变化特征的分析，指出印度洋地区的海温变化与赤道东太平洋地区的海温变化有较好 的相关关系，是ENSO循环的重要组成部分，对应于赤道东太平洋暖位相期，印度洋地区的海温分布为东冷西暖，与此相反，在赤道东太平洋冷位相，印度洋地区的温分布为东暖西冷，进一步的分析还发现，印度洋东，西部地区海温变化纬向差异最明显的区域位于印度洋赤道以南0－25℃附近，且这种差异具有明显的年季变化特征，在整个夏季风期间差异较大，而冬季风期间较小，其中冷位相期间的纬向差异比暖位相期间的纬向差异大，代表印度洋纬向差异的IDM（偶极指数）变化与赤道东太平洋地区的海温变化有很好的正相关关系。     

Seasonal persistence and propagation of intraseasonal patterns over the Indian monsoon region

The space-time evolution of convection over the monsoon region containing the Indian subcontinent, the Indian Ocean and the West Pacific has been studied. A multi-channel singular spectrum analysis of the daily outgoing longwave radiation has yielded two intraseasonal oscillatory patterns and two large-scale standing patterns as the most dominant modes of intraseasonal variability. The oscillatory modes vary on time scales of about 45 and 28 days and their average cycles of variability are shown to correspond to the life cycles of active and break periods of monsoon rainfall over India. During an active (break) cycle, a convection (dry) anomaly zone first appears in the equatorial Indian Ocean, subsequently expands to cover the Indian subcontinent and finally contracts to disappear in the northern part of India. Some eastward and northward movements are found to be associated with both oscillatory modes, while westward movement may also be associated with the 28-day mode. The oscillatory modes are shown to have a large spatial scale extending to the West Pacific. One of the standing modes has anomalies of uniform sign covering the entire region and is related to El Niño and southern oscillation (ENSO) pattern. The other standing mode has a dipole structure in the equatorial Indian Ocean associated with large-scale anomalies over India with the same sign as those over the western part of the dipole. These two standing modes persist throughout the monsoon season, each maintaining its respective pattern. The seasonal mean monsoon is mainly determined by the two standing patterns, without much contribution from the oscillatory modes. The relative role of the standing patterns (ENSO mode and dipole mode) seems to be important in determining the seasonal mean during certain years.     

NCEP/CFSR再分析耦合资料中MJO对ENSO的影响研究

基于1979—2008年NCEP/CFSR再分析耦合数据集,研究了冬季MJO对ENSO事件的影响。结果表明,在年际时间尺度以及长期的年代际时间尺度上,热带印度洋MJO活动的强弱性都可以影响热带中东太平洋ENSO事件的发生和发展。在年际时间尺度上,ENSO发生前期征兆的赤道中东太平洋的西风爆发事件(Westerly Wind Burst,WWB),作为MJO影响ENSO的主要途径,存在着显著的次季节时间尺度的变化。相对于气候平均的赤道太平洋西部暖池区上升而东部下沉的Walker环流,MJO正位相东传后的西风异常,减弱了低层东风和赤道东太平洋海水上翻。这一上升海流的减弱导致了中东赤道太平洋的海温升高,从而有利于ENSO暖海温事件的发生。而在年代际时间尺度上,MJO范围和强度在1998年前后出现了明显的转变,1998年之前MJO的东移范围更东,强度更强,从而导致了西太平洋西风爆发区的次季节西风异常事件更加显著,在Bjeknes正反馈机制下对应了年代际时间尺度下的强尼诺事件出现,1998年之后则与之相反。冬季MJO对ENSO影响的这一年代际特征主要体现在晚冬季节,而在早冬伴随着印度洋的增暖,MJO强度一直在逐年增加。     

Possible role of warm SST bias in the simulation of boreal summer monsoon in SINTEX-F2 coupled model

Reasonably realistic climatology of atmospheric and oceanic parameters over the Asian monsoon region is a pre-requisite for models used for monsoon studies. The biases in representing these features lead to problems in representing the strength and variability of Indian summer monsoon (ISM). This study attempts to unravel the ability of a state-of-the-art coupled model, SINTEX-F2, in simulating these characteristics of ISM. The coupled model reproduces the precipitation and circulation climatology reasonably well. However, the mean ISM is weaker than observed, as evident from various monsoon indices. A wavenumber–frequency spectrum analysis reveals that the model intraseasonal oscillations are also weaker-than-observed. One possible reason for the weaker-than-observed ISM arises from the warm bias, over the tropical oceans, especially over the equatorial western Indian Ocean, inherent in the model. This warm bias is not only confined to the surface layers, but also extends through most of the troposphere. As a result of this warm bias, the coupled model has too weak meridional tropospheric temperature gradient to drive a realistic monsoon circulation. This in turn leads to a weakening of the moisture gradient as well as the vertical shear of easterlies required for sustained northward propagation of rain band, resulting in weak monsoon circulation. It is also noted that the recently documented interaction between the interannual and intraseasonal variabilities of ISM through very long breaks (VLBs) is poor in the model. This seems to be related to the inability of the model in simulating the eastward propagating Madden–Julian oscillation during VLBs.     

Impact of Indian monsoon on the coupled atmosphere/ocean system in the tropical pacific

Summary This study addresses the relationship between the Indian summer monsoon (ISM) and the coupled atmosphere/ocean system in the tropical Pacific on the interannual time scales. High positive correlations are found between ISM rainfall and both mixed layer sea water temperature (SWT) and sea surface temperature (SST) anomalies of the tropical western Pacific in the following winter. Negative correlations between ISM rainfall and SST in the central/eastern Pacific also appear to be most significant in the following winter. These parameters are correlated with each other mainly on a biennial time scale. Lag-correlations between the zonal wind and SST along the the equatorial Pacific show that the westerly (easterly) surface wind stress anomalies over the central/western Pacific are greatly responsible for the formation of negative (positive) SST/SWT anomalies in the western Pacific and positive (negative) SST/SWT anomalies in the central/eastern Pacific. Furthermore, it is evidenced that these lagcorrelations are physically based on the anomalies in the large-scale convection over the Asian monsoon region and the associated east-west circulation over the tropical Pacific, which first appear during the Indian summer monsoon season and evolve during the following autumn and winter. These results strongly suggest that the Asian summer monsoon may have an active, rather than a passive, role on the interannual variability, including the ENSO events, of the coupled atmosphere/ocean system over the tropical Pacific.With 9 Figures     

Interannual spring Wyrtki jet variability and its regional impacts

The role of spring Wyrtki jets in modulating the equatorial Indian Ocean and the regional climate is an unexplored problem. The source of interannual variability in the spring Wyrtki jets is explored in this study. The relationship between intraseasonal and interannual variability from 1958 to 2008 and its relation with Indian Summer Monsoon is further addressed. Analysis reveals that the interannual variability in spring Wyrtki jets is controlled significantly by their intraseasonal variations. These are mostly defined by a single intraseasonal event of duration 20 days or more which either strengthens or weakens the seasonal mean jet depending on its phase. The strong spring jets are driven by such intraseasonal westerly wind bursts lasting for 20-days or more, whereas the weak jets are driven by weaker intraseasonal westerlies. During the years of strong jets, the conventional westward phase propagation of Wyrtki jets is absent and instead there is an eastward phase propagation indicating the possible role of Madden Julian Oscillation (MJO) in strengthening the spring Wyrtki jets. These strong intraseasonal westerly wind bursts with eastward phase propagation during strong years are observed mainly in late spring and have implications on June precipitation over the Indian and adjoining land mass. Anomalously strong eastward jets accumulate warm water in the eastern equatorial Indian Ocean (EIO), leading to anomalous positive upper ocean heat content and supporting more local convection in the east. This induces subsidence over the Indian landmass and alters monsoon rainfall by modulating monsoon Hadley circulation. In case of weak current years such warm anomalies are absent over the eastern EIO. Variations in the jet strength are found to have strong impact on sea level anomalies, heat content, salinity and sea surface temperature over the equatorial and north Indian Ocean making it a potentially important player in the north Indian Ocean climate variability.     

Role of TBO and ENSO scale ocean–atmosphere interaction in the Indo-Pacific region on Asian summer monsoon variability

Interannual variability of the Indian summer monsoon rainfall has two dominant periodicities, one on the quasi-biennial (2–3 year) time scale corresponding to tropospheric biennial oscillation (TBO) and the other on low frequency (3–7 year) corresponding to El Niño Southern Oscillation (ENSO). In the present study, the spatial and temporal patterns of various atmospheric and oceanic parameters associated with the Indian summer monsoon on the above two periodicities were investigated using NCEP/NCAR reanalysis data sets for the period 1950–2005. Influences of Indian and Pacific Ocean SSTs on the monsoon season rainfall are different for both of the time scales. Seasonal evolution and movement of SST and Walker circulation are also different. SST and velocity potential anomalies are southeast propagating on the TBO scale, while they are stationary on the ENSO scale. Latent heat flux and relative humidity anomalies over the Indian Ocean and local Hadley circulation between the Indian monsoon region and adjacent oceans have interannual variability only on the TBO time scale. Local processes over the Indian Ocean determine the Indian Ocean SST in biennial periodicity, while the effect of equatorial east Pacific SST is significant in the ENSO periodicity. TBO scale variability is dependent on the local factors of the Indian Ocean and the Indian summer monsoon, while the ENSO scale processes are remotely controlled by the Pacific Ocean.     

季节内印度洋-西太平洋对流涛动对次季节-季节尺度大气可预报性的影响

利用非线性局部Lyapunov指数和条件非线性局部Lyapunov指数定量估计了季节内印度洋-西太平洋对流涛动（IPCO）和实时多变量Madden-Julian指数（RMM指数）可预报期限,量化了季节内IPCO对S2S尺度大气可预报性的贡献,深入研究了季节内IPCO演变下S2S尺度可预报期限空间分布的变化规律。结果表明：（1）与RMM指数相比,季节内IPCO指数可预报性更强,可预报期限达到31天左右,比RMM指数高出2周以上;（2）印度洋-西太平洋区域S2S尺度大气可预报性最强,可预报期限达到30天以上,其中季节内IPCO是该地区的主要可预报性来源之一,其贡献达到6天,占总可预报期限的25%以上;（3）随着季节内IPCO的演变,印度洋-西太平洋地区S2S尺度大气可预报性有空间结构变化,表现为可预报期限异常的传播和振荡。S2S尺度大气可预报期限正负异常沿季节内IPCO传播路径,一支以赤道中西印度洋为起点北传至印度半岛,一支向东传播,经过海洋性大陆到赤道西太平洋后向北传播,到达日本南部。同时,可预报性异常的传播在在东印度洋和西太平洋表现出反向变化的特征,形成东西两极振荡,当季节内IPCO向正位相发展时,东印度洋具有更强的可预报性,西太平洋具有更弱的可预报性,反之亦然。季节内IPCO的发展（衰退）可使东印度洋（西太平洋）S2S尺度大气可预报性更强,表明模式预报技巧对此具有更大的提升空间。     

The summer monsoon of 1988

Summary Time averaged, monthly mean fields of a number of climate parameters such as sea surface temperature anomalies, outgoing longwave radiation anomalies, 200 mb velocity potential field, streamfunction anomaly at the lower and the upper troposphere and percentage rainfall amounts are presented, for the summer of 1988 over the regions of Asian summer monsoon. Above normal rainfall occurred over most of the Indian subcontinent, southeast Asia and eastern China during 1988. In comparison, 1987 was a drought year. This paper presents a comparison of some of the salient aforementioned parameters. The evolution of the planetary scale divergent motions and the streamfunction anomaly exhibit prominent differences during the life cycle of the monsoon in these two years. The velocity potential field exhibits a pronounced planetary scale geometry with the divergent outflows emanating from the monsoon region during 1988. The descending branches of these time averaged circulations are found over the Atlantic ocean to the west and over the eastern Pacific and North America to the east. The immense size of this circulation is indicative of an above normal monsoon activity. During 1987, the outflow center was located much further to the southeast over the western Pacific ocean. The longitudinal extent of the monsoonal divergent circulations were much smaller in 1987.The positive sea surface temperature anomaly of the El Nino year 1987 is seen to move westwards to the western Pacific in 1988, a warm anomaly also appears over the eastern equatorial Indian ocean and the Bay of Bengal at this time. The latter contributes to the supply of moisutre during the active monsoon season of 1988. The outgoing long wave radiation anomalies evolve with a westward propagation of strong positive anomalies from the central Pacific ocean consistent with the evolution of divergent circulation. The streamfunction anomalies basically show westerly zonal wind anomalies being replaced by easterly anomalies during 1988 over the upper troposphere of the monsoon region.The lower tropospheric streamfunction anomaly during the drought year 1987 showed a pronounced counter monsoon circulation. That feature was absent in 1988.With 11 Figures