The interannual variability of East Asian Winter Monsoon and its relation to the summer monsoon

1.IntroductionOvertheEastAsiaregion,themostprominentsurfacefeatureofthewintermonsoonisstrongnortheasterliesalongtheeastflankoftheSiberianhighandthecoastofEastAsia.At500hPathereisabroadtroughcenteredaboutatthelongitudesofJapan.Thedominantfea-tureat2O0hPaistheEastAsianjetwithitsmaximumlocatedatjustsoutheastofJapan.Thisktisassociatedwithintensebaroclinicity,largeverticalwindshearandstrongadvectionofcoldair(StaffmembersofAcademiaSinica,l957,LauandChang,1987;BoyleandChen,1987;Chenetal.,1991…     

南亚和东亚热带夏季风强弱不同配置及其对中国夏季降水的影响北大核心

利用NOAA向外长波辐射(OLR)、NCEP/NCAR再分析资料和CN05.1降水资料,研究了南亚和东亚热带夏季风强度年际变化关系,及其强弱不同配置对中国夏季降水的影响。结果表明:南亚和东亚热带夏季风强度变化之间存在同相和反相两种配置,定义的强度同相和反相变化指数可以很好地表征该关系。同相变化模态可能与海温异常时的强El Nino(La Nina)影响有关,其反相变化模态受El Nino(La Nina)以及印度洋海盆一致模的影响,同时西太平洋副热带高压和伊朗高压位置东西偏移和强度变化也影响着不同配置的出现。两者不同配置时,对中国夏季降水的影响不同。当变化呈同相偏强时,夏季中国东部地区降水为“中间少南北多”的雨型。当变化呈反相,东亚热带夏季风偏强南亚夏季风偏弱时,夏季中国东部地区降水为“一致偏少”雨型。     

AGCM simulations of intraseasonal variability associated with the Asian summer monsoon

The intraseasonal variability associated with the Asian summer monsoon as simulated by a number of atmospheric general circulation models (AGCMs) are analyzed and assessed against observations. The model data comes from the Monsoon GCM Intercomparison project initiated by the CLIVAR/Asian–Australian Monsoon Panel. Ten GCM groups, i.e., the Center for Ocean–Land–Atmosphere Studies (COLA), Institute of Numerical Mathematics (DNM), Goddard Space Flight Center (GSFC), Geophysical Fluid Dynamics Laboratory (GFDL), Institute of Atmospheric Physics (IAP), Indian Institute of Tropical Meteorology (IITM), Meteorological Research Institute (MRI), National Center for Atmospheric Research (NCAR), Seoul National University (SNU), and the State University of New York (SUNY), participated in the intraseasonal component of the project. Each performed a set of 10 ensemble simulations for 1 September 1996–31 August 1998 using the same observed weekly SST values but with different initial conditions. The focus is on the spatial and seasonal variations associated with intraseasonal variability (ISV) of rainfall, the structure of each model's principal mode of spatial-temporal variation of rainfall [i.e. their depiction of the Intraseasonal Oscillation (ISO)], the teleconnection patterns associated with each model's ISO, and the implications of the models' ISV on seasonal monsoon predictability. The results show that several of the models exhibit ISV levels at or above that found in observations with spatial patterns of ISV that resemble the observed pattern. This includes a number of rather detailed features, including the relative distribution of variability between ocean and land regions. In terms of the area-averaged variance, it is found that the fidelity of a model to represent NH summer versus winter ISV appears to be strongly linked. In addition, most models' ISO patterns do exhibit some form of northeastward propagation. However, the model ISO patterns are typically less coherent, lack sufficient eastward propagation, and have smaller zonal and meridional spatial scales than the observed patterns, and are often limited to one side or the other of the maritime continent. The most pervasive and problematic feature of the models' depiction of ISV and/or their ISO patterns is the overall lack of variability in the equatorial Indian Ocean. In some cases, this characteristic appears to result from some models forming double convergence zones about the equator rather than one region of strong convergence on the equator. This shortcoming results in a poor representation of the local rainfall pattern and also significantly influences the models' representations of the global-scale teleconnection patterns associated with the ISO. Finally, analysis of the model ensemble shows a positive relationship between the strength of a model's ISV of rainfall and its intra-ensemble variability of seasonal monsoon rainfall. The implications of this latter relation are discussed in the context of seasonal monsoon predictability.     

Interdecadal variability of the East Asian summer monsoon in an AGCM

It is well known that significant interdecadal variation of the East Asian summer monsoon （EASM） occurred around the end of the 1970s. Whether these variations can be attributed to the evolution of global sea surface temperature （SST） and sea ice concentration distribution is investigated with an atmospheric general circulation model （AGCM）. The model is forced with observed monthly global SST and sea ice evolution through 1958-1999. A total of four integrations starting from different initial conditions are carried out. It is found that only one of these reproduces the observed interdecadal changes of the EASM after the 1970s, including weakened low-level meridional wind, decreased surface air temperature and increased sea level pressure in central China, as well as the southwestward shift of the western Pacific subtropical high ridge and the strengthened 200-hPa westerlies. This discrepancy among these simulated results suggests that the interdecadal variation of the EASM cannot be accounted for by historical global SST and sea ice evolution. Thus, the possibility that the interdecadal timescale change of monsoon is a natural variability of the coupled climate system evolution cannot be excluded.     

Impacts of Coastal SST Variability on the East Asian Summer Monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions （EACRSST） on the East Asian summer monsoon （EASM） have been examined using a regional climate model （PδRCM9） in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM （IEASM） differences.
The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast （LSTC） which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.     

Seasonal evolution mechanism of the East Asian winter monsoon and its interannual variability

This study investigates the space–time evolution of the East Asian winter monsoon (EAWM) and its relationship with other climate subsystems. Cyclostationary Empirical Orthogonal Function (CSEOF) analysis and the multiple regression method are used to delineate the detailed evolution of various atmospheric and surface variables in connection with the EAWM. The 120 days of winter (November 17–March 16) per year over 62 years (1948–2010) are analyzed using the NCEP daily reanalysis dataset. The first CSEOF mode of 850-hPa temperatures depicts the seasonal evolution of the EAWM. The contrast in heat capacity between the continent and the northwestern Pacific results in a differential heating in the lower troposphere. Its temporal evolution drives the strengthening and weakening of the Siberian High and the Aleutian Low. The anomalous sea level pressure pattern dictates anomalous circulation, in compliance with the geostrophic relationship. Thermal advection, in addition to net surface radiation, partly contributes to temperature variations in winter. Latent and sensible heat fluxes (thermal forcing from the ocean to the atmosphere) increase with decreased thermal advection. Anomalous upper-level circulation is closely linked to the low-level temperature anomaly in terms of the thermal wind equation. The interannual variability of the seasonal cycle of the EAWM is strongly controlled by the relative strength of the Siberian High to the Aleutian Low. A stronger than normal gradient between the two pressure systems amplifies the seasonal cycle of the EAWM. The EAWM seasonal cycle in the mid-latitude region exhibits a weak negative correlation with the Arctic Oscillation and the East Atlantic/West Russia indices.     

Simulated interannual variation in summertime atmospheric circulation associated with the East Asian monsoon

The reproducibility of the interannual variability of the summertime East Asian circulation is examined using an atmospheric general circulation model (AGCM). An ensemble experiment is conducted using observed sea surface temperature (SST) of recent 20 years as a lower boundary condition. The spatial pattern associated with the first principal mode of observation of geopotential height at 500 hPa is characterized by a meridional wavy pattern extending over eastern Siberia, the vicinity of Japan and the subtropical western Pacific. The principal component (PC) time series of the leading mode is represented well by a high-resolution version of the AGCM with horizontal resolution T106 and with 56 vertical levels (T106L56), while with a lower resolution version, T42 and 20 vertical levels, the reproducibility is considerably degraded. The reproducibility by the AGCM suggests the importance of SST as a boundary condition. However, the simulated interannual variations show the alternating appearance of two distinct circulation regimes, a cold summer regime and a hot summer regime, exhibiting interesting bimodality in probability density distribution in PC phase space. This implies that the system’s response to the continuously varying boundary condition includes nonlinearity. The nature of this nonlinearity is suggested to be wave breaking in the westerly region of the high latitudes that requires high resolution for the reproduction. Using the T106L56 model, another ensemble experiment was carried out with doubled CO₂. The climate change appears as an increase in residence frequency of the cold summer regime of the principal patterns of the present-day climate. This paper is a contribution to the AMIP-CMIP Diagnostic Sub-project on General Circulation Model Simulation of the East Asian Climate, coordinated by W.-C. Wang.     

Links between tropical SST anomalies and precursory signalsassociated with the interannual variability of Asian summer monsoon

Summary ?The interannual variability of broad-scale Asian summer monsoon was studied using a general circulation model (GCM) and NCEP (National Center for Environmental Prediction) data set during 1979–95. In the GCM experiment, the main emphasis was given to isolate the individual role of surface boundary conditions on the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon. In order to understand the role of sea-surface temperatures (SSTs) alone on the existence of precursory signals, we have conducted 17 years numerical integration with a GCM forced with the real-time monthly averaged SSTs of 1979 to 1995. In this experiment, among the many surface boundary conditions only SSTs are varying interannually. The composite circulation anomalies simulated by the GCM have good resemblance with the NCEP circulation anomalies over subtropical Asia. This suggests that the root cause of the existence of winter-spring time circulation anomalies associated with the interannual variability of Asian summer monsoon is the interannual variability of SST. Empirical Orthogonal Functions (EOFs) of 200-mb winds and OLR were constructed to study the dynamic coupling between SST anomalies and winter-spring time circulation anomalies. It is found that the convective heating anomalies associated with SST anomalies and stationary eddies undergo systematic and coherent interannual variations prior to summer season. We have identified Matsuno-Gill type mode in the velocity potential and stream function fields. This suggests the existence of dynamic links between the SST anomalies and the precursory signals of Asian summer monsoon. Received June 9, 1999/Revised April 7, 2000     

Evaluation of the interannual variability in the East Asian summer monsoon in AMIP and historical experiments of CAS FGOALS-f3-L

The evaluation of East Asian summer monsoon (EASM) simulations could improve our understanding of Asian monsoon dynamics and climate simulations. In this study, by using Phase 6 of the Coupled Model Intercomparison Project (CMIP6) experiments of the Atmospheric Model Intercomparison Project (AMIP) and historical runs of the Chinese Academy of Sciences (CAS) Flexible Global Ocean–Atmosphere–Land System (FGOALS-f3-L) model, the model simulation skill for the interannual variability in the EASM was determined. According to multivariate empirical orthogonal function (MV-EOF) analysis, the major mode of the EASM mainly emerged as a Pacific-Japan pattern in the western Pacific accompanied by a local anticyclonic anomaly with a total variance of 24.6%. The historical experiment could suitably reproduce this spatial pattern and attained a closer total variance than that attained by the AMIP experiment. The historical experiment could also better simulate the time frequency of the EASM variability than the AMIP experiment. However, the phase of principal component 1 (PC1) was not suitably reproduced in the historical experiment since no initialization procedure was applied at the beginning of the integration in the historical simulation process, whereas the sea surface temperature (SST) was preset in the AMIP experiment. Further analysis revealed that air–sea interactions in the Indian Ocean and tropical western Pacific were important for the model to provide satisfactory EASM simulations, while El Niño–Southern Oscillation (ENSO) simulation was possibly related to the climate variability in the EASM simulations, which should be further analyzed.摘要对东亚夏季季风(EASM)模拟的评估可以提高我们对亚洲季风动力和气候模拟的理解. 在这项研究中, 通过使用中国科学院(CAS)全球海洋-大气-陆地系统(FGOALS-f3-L)模式参加的第六次耦合模式相互比较计划(CMIP6)中的大气模式相互比较计划(AMIP)和历史(historical)试验, 明确了EASM的年际变率的模拟能力. 通过多变量经验正交函数(MV-EOF)分析发现, 观测的EASM的主导模态为西太平洋上的太平洋-日本模态, 并伴有局部反气旋异常. 主导模态的方差贡献率为24.6%. 历史(historical)试验可以基本再现这种空间模态, 其方差贡献率较AMIP试验更接近于观测. 与AMIP试验相比, 历史(historical)试验还能更好地模拟EASM变率的时间频率. 然而, 由于历史(historical)模拟没有在积分开始时应用初始化过程, 而AMIP试验受到海表面温度(SST)的约束, 因此主成分(PC1)的位相在历史(historical)试验中没有得到较好地再现. 进一步分析发现, 印度洋和西太平洋热带地区的海气相互作用对EASM的模拟非常重要, 而EASM气候变率的模拟可能与厄尔尼诺-南方涛动(ENSO)的模拟能力有关, 这值得进一步分析.     

A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO

This study investigates how accurately the interannual variability over the Indian Ocean basin and the relationship between the Indian summer monsoon and the El Niño Southern Oscillation (ENSO) can be simulated by different modelling strategies. With a hierarchy of models, from an atmospherical general circulation model (AGCM) forced by observed SST, to a coupled model with the ocean component limited to the tropical Pacific and Indian Oceans, the role of heat fluxes and of interactive coupling is analyzed. Whenever sea surface temperature anomalies in the Indian basin are created by the coupled model, the inverse relationship between the ENSO index and the Indian summer monsoon rainfall is recovered, and it is preserved if the atmospherical model is forced by the SSTs created by the coupled model. If the ocean model domain is limited to the Indian Ocean, changes in the Walker circulation over the Pacific during El-Niño years induce a decrease of rainfall over the Indian subcontinent. However, the observed correlation between ENSO and the Indian Ocean zonal mode (IOZM) is not properly modelled and the two indices are not significantly correlated, independently on season. Whenever the ocean domain extends to the Pacific, and ENSO can impact both the atmospheric circulation and the ocean subsurface in the equatorial Eastern Indian Ocean, modelled precipitation patterns associated both to ENSO and to the IOZM closely resemble the observations.     

Climatology and Interannual Variability of the Southeast Asian Summer Monsoon

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Predictability of the East Asian winter monsoon interannual variability as indicated by the DEMETER CGCMS

The interannual variability of East Asian winter monsoon(EAWM) circulation from the Development of a European Multi-Model Ensemble(MME) System for Seasonal to Inter-Annual Prediction(DEMETER) hindcasts was evaluated against observation reanalysis data.We evaluated the DEMETER coupled general circulation models(CGCMs)’ retrospective prediction of the typical EAWM and its associated atmospheric circulation.Results show that the EAWM can be reasonably predicted with statistically significant accuracy,yet the major bias of the hindcast models is the underestimation of the related anomalies.The temporal correlation coefficient(TCC) of the MME-produced EAWM index,defined as the first EOF mode of 850hPa air temperature within the EAWM domain(20-60 N,90-150 E),was 0.595.This coefficient was higher than those of the corresponding individual models(range:0.39-0.51) for the period 1969-2001;this result indicates the advantage of the super-ensemble approach.This study also showed that the ensemble models can reasonably reproduce the major modes and their interannual variabilities for sea level pressure,geopotential height,surface air temperature,and wind fields in Eurasia.Therefore,the prediction of EAWM interannual variability is feasible using multimodel ensemble systems and that they may also reveal the associated mechanisms of the EAWM interannual variability.     

Potential impacts of East Asian winter monsoon on climate variability and predictability in the Australian summer monsoon region

This study explores potential impacts of the East Asian winter monsoon (EAWM) on summer climate variability and predictability in the Australia–Asian region through Australia–Asia (A-A) monsoon interactions. Observational analysis is conducted for the period of 1959 to 2001 using ERA-40 wind reanalysis and Climate Research Unit rainfall and surface temperature monthly datasets. Statistically significant correlations are established between the Australian summer monsoon and its rainfall variations with cross-equatorial flows penetrating from South China Sea region and northerly flow in the EAWM. The underlying mechanism for such connections is the response of the position and intensity of Hardley circulation to strong/weak EAWM. A strong EAWM is associated with an enhanced cross-equatorial flow crossing the maritime continent and a strengthened Australia summer monsoon westerlies which affect rainfall and temperature variations in northern and eastern part of the Australian continent. Furthermore, partial correlation analysis, which largely excludes El Niño-Southern Oscillation (ENSO) effects, suggests that these connections are the inherent features in the monsoon system. This is further supported by analyzing a global model experiment using persistent sea surface temperatures (SSTs) which, without any SST interannual variations, shows similar patterns as in the observational analysis. Furthermore, such interaction could potentially affect climate predictability in the region, as shown by some statistically significant lag correlations at monthly time scale. Such results are attributed to the impacts of EAWM on regional SST variations and its linkage to surface conditions in the Eurasian continent. Finally, such impacts under global warmed climate are discussed by analyzing ten IPCC AR4 models and results suggest they still exist in the warmed climate even though the EAWM tends to be weaker.     

The Variability of the Interannual Oscillations of the Indian Summer Monsoon Rainfall

A new method of analysis namely, Singular Spectrum Analysis (SSA) is applied to the Indian Summer Monsoon (June-September) Rainfall (ISMR) series. The method is efficient in extracting the statistically significant oscillations with periods 2.8 and 2.3 year from the white noise of the ISMR series. The study shows that 2.8 / 2.3 year cycle captures the variability of the ISMR related to Southern Oscillation / Quasi Biennial Oscillation. The temporal structure of these oscillations show that these are in phase in extreme (excess and drought) monsoon conditions as well as in El Nino Southern Oscillation (ENSO) years. Both these oscillations show minimum variability during the period 1920-1940 and there is an increasing trend in the variability of these oscillations in the recent decades. The study enables to obtain pure signal consisting of reconstructed time series using these two Oscillations, from the original white noise series.     

Classification of typical summer rainfall patterns in the East China monsoon region and their association with the East Asian summer monsoon

Theoretical and Applied Climatology - In this study, the summer rainfall patterns in the East China monsoon region during 1951–2015 were objectively classified into four typical categories:...     

The East Asian summer monsoon: an overview

Summary The present paper provides an overview of major problems of the East Asian summer monsoon. The summer monsoon system over East Asia (including the South China Sea (SCS)) cannot be just thought of as the eastward and northward extension of the Indian monsoon. Numerous studies have well documented that the huge Asian summer monsoon system can be divided into two subsystems: the Indian and the East Asian monsoon system which are to a greater extent independent of each other and, at the same time, interact with each other. In this context, the major findings made in recent two decades are summarized below: (1) The earliest onset of the Asian summer monsoon occurs in most of cases in the central and southern Indochina Peninsula. The onset is preceded by development of a BOB (Bay of Bengal) cyclone, the rapid acceleration of low-level westerlies and significant increase of convective activity in both areal extent and intensity in the tropical East Indian Ocean and the Bay of Bengal. (2) The seasonal march of the East Asian summer monsoon displays a distinct stepwise northward and northeastward advance, with two abrupt northward jumps and three stationary periods. The monsoon rain commences over the region from the Indochina Peninsula-the SCS-Philippines during the period from early May to mid-May, then it extends abruptly to the Yangtze River Basin, and western and southern Japan, and the southwestern Philippine Sea in early to mid-June and finally penetrates to North China, Korea and part of Japan, and the topical western West Pacific. (3) After the onset of the Asian summer monsoon, the moisture transport coming from Indochina Peninsula and the South China Sea plays a crucial “switch” role in moisture supply for precipitation in East Asia, thus leading to a dramatic change in climate regime in East Asia and even more remote areas through teleconnection. (4) The East Asian summer monsoon and related seasonal rain belts assumes significant variability at intraseasonal, interannual and interdecadal time scales. Their interaction, i.e., phase locking and in-phase or out-phase superimposing, can to a greater extent control the behaviors of the East Asian summer monsoon and produce unique rythem and singularities. (5) Two external forcing i.e., Pacific and Indian Ocean SSTs and the snow cover in the Eurasia and the Tibetan Plateau, are believed to be primary contributing factors to the activity of the East Asian summer monsoon. However, the internal variability of the atmospheric circulation is also very important. In particular, the blocking highs in mid-and high latitudes of Eurasian continents and the subtropical high over the western North Pacific play a more important role which is quite different from the condition for the South Asian monsoon. The later is of tropical monsoon nature while the former is of hybrid nature of tropical and subtropical monsoon with intense impact from mid-and high latitudes.     

Northern East Asian low and its impact on the interannual variation of East Asian summer rainfall

The strength of the East Asian summer monsoon and associated rainfall has been linked to the western North Pacific subtropical high (WNPSH) and the lower-tropospheric low pressure system over continental East Asia (EA). In contrast to the large number of studies devoted to the WNPSH, little is known about the variability of the East Asian continental low. The present study delineates the East Asian continental low using 850-hPa geopotential height. Since the low is centered over northern EA (NEA), we refer to it as the NEA low (NEAL). We show that the intensity of the NEAL has large interannual variation, with a dominant period of 2–4 years. An enhanced NEAL exhibits a barotropic structure throughout the whole troposphere, which accelerates the summer-mean upper-tropospheric westerly jet and lower-tropospheric monsoon westerly to its south. We carefully identify the anomalous NEAL-induced rainfall anomalies by removal of the tropical heating effects. An enhanced NEAL not only increases rainfall locally in northern Northeast China, but also shifts the East Asian subtropical front northward, causing above-normal rainfall extending eastward from the Huai River valley across central-northern Japan and below-normal rainfall in South China. The northward shift of the East Asian subtropical front is attributed to the following processes without change in the WNPSH: an enhanced NEAL increases meridional pressure gradients and the monsoon westerly along the East Asian subtropical front, which in turn induces a cyclonic shear vorticity anomaly to its northern side. The associated Ekman pumping induces moisture flux convergence that shifts the East Asian subtropical front northward. In addition, the frequent occurrence of synoptic cut-off lows is found to be associated with an enhanced NEAL. Wave activity analysis indicates that the interannual intensity change of the NEAL is significantly associated with the extratropical Polar Eurasian teleconnection, in addition to the forcing of the tropical WNP heating.     

The East Asian subtropical summer monsoon: Recent progress

The East Asian subtropical summer monsoon (EASSM) is one component of the East Asian summer monsoon system, and its evolution determines the weather and climate over East China. In the present paper, we firstly demonstrate the formation and advancement of the EASSM rainbelt and its associated circulation and precipitation patterns through reviewing recent studies and our own analysis based on JRA-55 (Japanese 55-yr Reanalysis) data and CMAP (CPC Merged Analysis of Precipitation), GPCP (Global Precipitation Climatology Project), and TRMM (Tropical Rainfall Measuring Mission) precipitation data. The results show that the rainy season of the EASSM starts over the region to the south of the Yangtze River in early April, with the establishment of strong southerly wind in situ. The EASSM rainfall, which is composed of dominant convective and minor stratiform precipitation, is always accompanied by a frontal system and separated from the tropical summer monsoon system. It moves northward following the onset of the South China Sea summer monsoon. Moreover, the role of the land–sea thermal contrast in the formation and maintenance of the EASSM is illustrated, including in particular the effect of the seasonal transition of the zonal land–sea thermal contrast and the influences from the Tibetan Plateau and midlatitudes. In addition, we reveal a possible reason for the subtropical climate difference between East Asia and East America. Finally, the multi-scale variability of the EASSM and its influential factors are summarized to uncover possible reasons for the intraseasonal, interannual, and interdecadal variability of the EASSM and their importance in climate prediction.     

东亚夏季风异常活动的空间多模态特征

利用ERA40再分析资料,采用相关、合成、自然正交函数展开（EOF分析）等方法,探讨了东亚地区夏季风活动的多空间模态特征及其与大气环流异常的可能联系。结果表明：1）东亚夏季风活动存在3种差异明显的典型空间模态。第一模态反映了夏季风活动在我国东部沿海及以东洋面与其以西地区的反相变化,主要体现了夏季风活动主体位置的东西变动;第二模态反映了自我国华南,经长江中下游、山东半岛、渤海湾至我国东北及朝鲜半岛一带夏季风活动的一致性变化,体现了东亚夏季风活动的整体强弱;第三模态主要反映了夏季风活动在中国以东洋面、朝鲜半岛、东北亚一带与我国华南地区的反相变化,主要体现了夏季风活动主体位置的南北移动。2）东亚夏季风活动的多空间模态对应的大气环流异常存在显著差异。东亚夏季风第一空间模态与亚洲南部区域以及鄂霍次克海上空的SLP呈负相关,而与北极极区、贝加尔湖地区及日本以东洋面的SLP呈正相关;而与同期500hPa高度场的相关分布主要表现为自极地经鄂霍次克海至日本以东洋面的“＋-＋”的波列分布特征。第二模态与SLP和500hPa高度场的相关分布具有非常相似的空间分布形势,均表现为东北、朝鲜半岛、日本海一带与菲律宾洋面、鄂霍次克海地区的反位相分布,自低纬向高纬呈现“＋-＋”的波列分布特征。第三模态与SLP和500hPa高度场的相关分布,主要表现为菲律宾附近洋面、日本及以东洋面、贝加尔湖到亚洲北部的负正相间的分布形势。     

西太平洋副高的东西进退与东亚夏季风系统的相互影响与关联

利用1980-2010年NCEP/NCAR再分析资料和美国NOAA向外长波辐射（outgoing longwave radiation,OLR）资料,根据关键区500 hPa位势高度的变化定义了西太平洋副高东西位置指标,利用该指标围绕东亚夏季风系统开展分析,详细对比了夏季6月、7月副高东西向活动异常时,季风区相应的环流及对流活动差异.结果表明：副高东西位置的年际变化反映了亚洲夏季风的强弱变化,副高偏西（东）年,南海夏季风偏弱（强）,副热带夏季风偏强（弱）;副高的东西进退与东亚夏季风系统成员之间相互影响、相互制约;副高偏西年,南亚高压偏东、偏强,季风槽不发展、强度偏弱,西风带长波槽发展加深,南半球马斯克林高压和澳大利亚高压减弱,越赤道气流减弱,而副高偏东年情况则反之.