Interdecadal change of the controlling mechanisms for East Asian early summer rainfall variation around the mid-1990s

East Asian (EA) summer monsoon shows considerable differences in the mean state and principal modes of interannual variation between early summer (May–June, MJ) and late summer (July–August, JA). The present study focuses on the early summer (MJ) precipitation variability. We find that the interannual variation of the MJ precipitation and the processes controlling the variation have been changed abruptly around the mid-1990s. The rainfall anomaly represented by the leading empirical orthogonal function has changed from a dipole-like pattern in pre-95 epoch (1979–1994) to a tripole-like pattern in post-95 epoch (1995–2010); the prevailing period of the corresponding principal component has also changed from 3–5 to 2–3 years. These changes are concurrent with the changes of the corresponding El Nino-Southern Oscillation (ENSO) evolutions. During the pre-95 epoch, the MJ EA rainfall anomaly is coupled to a slow decay of canonical ENSO events signified by an eastern Pacific warming, which induces a dipole rainfall feature over EA. On the other hand, during the post-95 epoch the anomalous MJ EA rainfall is significantly linked to a rapid decay of a central Pacific warming and a distinct tripolar sea surface temperature (SST) in North Atlantic. The central Pacific warming-induced Philippine Sea anticyclone induces an increased rainfall in southern China and decreased rainfall in central eastern China. The North Atlantic Oscillation-related tripolar North Atlantic SST anomaly induces a wave train that is responsible for the increase northern EA rainfall. Those two impacts form the tripole-like rainfall pattern over EA. Understanding such changes is important for improving seasonal to decadal predictions and long-term climate change in EA.     

东亚冬夏季风关系在1970s末的年代际转变

利用NCEP/NCAR和Hadley中心的大气与海洋再分析资料,选取具有代表性的东亚冬、夏季风指数,采用滑动相关和线性回归等方法,主要讨论了受ENSO影响的东亚冬季风分量和后期夏季风之间关系的年代际变化,并分析了二者关系发生变化的原因。结果表明:在1965—1979年,受ENSO影响的冬季风与后期夏季风强度的对应关系并不明显。在1980—2004年,受ENSO影响的冬季风强,对应后期的夏季风偏弱,弱冬季风对应的后期夏季风偏强。当受ENSO影响的冬季风较强时,冬季在对流层低层西北太平洋出现了异常气旋并可以维持到次年夏季,低纬地区位势高度偏低,削弱了西太平洋副热带高压,异常气旋西部的偏北气流阻碍了西南风的北进,导致夏季风偏弱。海表温度异常在1980年前后春、夏季不同的分布型可以解释环流在不同时段内的差异。     

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.     

有关东亚夏季风北边缘的定义及其特征

采用欧洲中心(ECMWF)44年冉分析(ERA40)日总可降水量(TPW)资料,用标准化可降水量指数(NPWI)定义了夏季风北边缘,并进一步研究了亚洲夏季风北边缘的气候特征及其年际、年代际变化特征.结果表明,用标准化町降水量指数定义的夏季风北边缘在哑洲可以确定出印度夏季风系统和东亚夏季风系统;就夏季风北边缘的平均位置而言,其在100°E以西沿青藏高原南侧呈东一西走向,年际变化极小;在100°E以东呈东北-西南走向,从青藏高原东侧北上经西北地区东部、华北地区北部、东北地区西部延伸到东北亚地区,并存在明显的年际、年代际变化.     

A preliminary attempt on decadal prediction of the East Asian summer monsoon

Theoretical and Applied Climatology - The East Asian summer monsoon (EASM) is one of the major synoptic systems that affect the summer climate in China. Decadal prediction of the EASM is of great...     

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。     

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。     

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.     

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:...     

Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases简

The variability of the East Asian winter monsoon （EAWM） can be divided into an ENSO-related part （EAWMEN） and an ENSO-unrelated part （EAWMres）.The influence of EAWMres on the ENSO-East Asian summer monsoon （EASM） relationship in the decaying stages of ENSO is investigated in the present study.To achieve this,ENSO is divided into four groups based on the EAWMres：（1） weak EAWMres-E1Ni（n）o （WEAWMres-EN）; （2） strong EAWMres-E1Ni（n）o （SEAWMresEN）; （3） weak EAWMres-La Ni（n）a （WEAWMres-LN）; （4） strong EAWMres-La Ni（n）a （SEAWMres-LN）.Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN.The corresponding low-level anticyclonic （cyclonic） anomalies over the western North Pacific （WNP） associated with El Ni（n）o （La Ni（n）a） tend to be strong.Importantly,this feature may persist into the following summer,causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases.In contrast,for the SEAWMres-EN and WEAWMres-LN groups,the EAWMres tends to weaken the atmospheric circulation anomalies associated with E1 Ni（n）o or La Ni（n）a.In these cases,the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes,which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN.Further study suggests that anomalous EAWMres may have an effect on the extra-tropical sea surface temperature anomaly,which persists into the ensuing summer and may interfere with the influences of ENSO.     

A tripolar pattern as an internal mode of the East Asian summer monsoon

A tripolar anomaly pattern with centers located around the Philippines, China/Japan, and East Siberia dominantly appears in climate variations of the East Asian summer monsoon. In this study, we extracted this pattern as the first mode of a singular value decomposition (SVD1) over East Asia. The squared covariance fraction of SVD1 was 59?%, indicating that this pattern can be considered a dominant pattern of climate variations. Moreover, the results of numerical experiments suggested that the structure is also a dominant pattern of linear responses, even if external forcing is distributed homogeneously over the Northern Hemisphere. Thus, the tripolar pattern can be considered an internal mode that is characterized by the internal atmospheric processes. In this pattern, the moist processes strengthen the circulation anomalies, the dynamical energy conversion supplies energy to the anomalies, and the Rossby waves propagate northward in the lower troposphere and southeastward in the upper troposphere. These processes are favorable for the pattern to have large amplitude and to influence a large area.     

东亚和南亚季风协同作用对西南地区夏季降水的影响

为探究东亚夏季风(EASM,East?Asian?summer?monsoon)和南亚夏季风(SASM,South?Asian?summer?monsoon)相互作用及其强弱变化对西南地区夏季降水的影响,利用1979—2019年西南地区161站逐日降水观测资料和ERA-5提供的1979—2019年全球再分析资料,通过对...     

The effects of land-surface characteristics on the East Asian summer monsoon

 The Community Climate Model version 2 (CCM2) of the National Center for Atmospheric Research (NCAR) was used to investigate the effects of the land-surface characteristics on the East Asian summer monsoon. Four numerical experiments were performed in this study. They include the control run, the biosphere–atmosphere transfer scheme (BATS) run, the heavy snow run, and the light snow run. The results show that CCM2 can reasonably simulate many characteristics of the East Asian summer monsoon, such as the 850-hPa southwesterlies, 200-hPa easterlies, high precipitation rate, two monsoon subsystems, the low-level subtropical high, and the upper level South Asian anticyclone. Nevertheless, the model still exhibits some systematic errors, including oversimulation of the temperature over the Eurasian continent, which in turn intensifies the monsoon circulations. In the BATS run, the model can significantly relieve the temperature bias over the continent in spring and early summer. However, the effect of BATS decreases in the summer due to excessive incoming solar radiation. The Eurasian continent is still occupied by an oversimulated thermal low in summer. In the heavy snow case, the high albedo of snow and larger soil moisture suppress the warming rate of the surface and atmosphere in the early summer and hence the cooler troposphere results in a weaker monsoon circulation. Moreover, anomalous cyclonic flows are found in the leeside of Tibetan Plateau (i.e. the southwest vortex in China) in the heavy snow case. This may shed a light on the precipitation anomalies (floods) over Yangtze River Valley (Central China) and eastern Asia due to intensified baroclinic disturbances. Received: 8 September 1999 / Accepted: 5 June 2000     

东亚夏季风和ENSO关系的不稳定性

通过本项研究，发现了东亚夏季风和ENSO的相互关系在长期变化中是不稳定的。不稳定指的是在一段时期两者关系比较紧密而在另一段时期两者关系比较微弱。文章揭示：在东亚季风和ENSO关系紧密时期（HCP）和关系微弱时期（LCP）夏季大气环流的年际变率有显著差别。在关系紧密时期，南热带东太平洋区的信风、热带东太平洋区的低层大气温度、两个半球的副热带高压系统等的年际变率均显著高于关系微弱时期。并且，HCP和LCP时期中国夏季降水和ENSO的关系也有明显差异。     

一个新的东亚副热带夏季风指数的定义

利用1958—2014年夏季NCEP/NCAR大气环流资料和中国486站降水观测资料,通过多种统计诊断方法,从与夏季中国东部3类不同雨型分布相联系的东亚高低层风场变化特征出发,依据与雨带变化密切联系的高层200 hPa纬向风定义了一个新的东亚副热带夏季风指数。分析表明,该指数不仅能反映夏季东亚大气环流的变化特征,兼顾北方冷空气活动和南方东亚夏季风环流变化,同时还能反映夏季中国东部降水南北差异的年际特征。强东亚副热带夏季风指数年,高层中纬度西风急流位置偏北,低层西太平洋副热带高压偏强偏北,有利于冷空气活动位置偏北和东亚东部西南暖湿气流向北推进,中国东部多以Ⅰ类雨型为主;弱东亚副热带夏季风指数年的环流变化刚好相反,中国东部多以Ⅲ类雨型为主。与现有东亚夏季风指数的对比分析表明,该指数在反映中国东部南北区域降水变化的差异方面有很大改进。     

东亚夏季风环流对气溶胶分布的影响

用2001—2012年逐月的MODIS-TERRA卫星观测气溶胶光学厚度(AOD)资料和NCEP/NCAR风场资料,分析了5—8月东亚地区AOD的时-空分布特征,研究东亚夏季风环流对气溶胶时-空分布的影响。主要结论如下:5—8月的中国东部及邻近海洋上AOD有着显著的季节演变特征,尤其是32.5 °N附近的AOD高值区,其强度和范围在5—8月逐渐增强然后又减弱。东亚夏季风通过环流输送作用对各地的AOD产生了不同程度的影响,使中国南部AOD减少,而华北和东北地区AOD增加。在强、弱季风年背景下,7月观测的AOD差异与环流输送作用差异的分布特征有着一定的相似性,体现出东亚夏季风年际变化对气溶胶分布的影响。在东亚夏季风演变的不同阶段,季风环流对气溶胶输送大部分情况下,可解释局地气溶胶变化10%~20%的方差。     

The effect of regional warming on the East Asian summer monsoon

Climate Dynamics - East Asian summer monsoon (EASM) precipitation has changed significantly due to regional warming. In this study, effect of regional warming on the EASM summer precipitation is...     

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.     

Dynamic-analogue correction of the decadal change of East Asian summer precipitation in the late 1990s

This paper systematically evaluates the deviations that appear in the hindcasts of the East Asian summer precipitation (EASP) decadal change in the late 1990s in two global coupled models (BCC_CGCM and BCC_CSM). The possible causes for the deviations between the model hindcasts and observations are analyzed. The results show that the hindcasts of EASP by BCC_CGCM and BCC_CSM deviate from observations, with the anomaly correlation coefficient (ACC) being -0.01 and -0.09 for the two models, respectively. The SST anomalies in North and West Pacific and the SST index values predicted by the two models also deviate from the observations, indicating that inconsistent SST fields may be the key factor leading to the deviation in the prediction of the EASP decadal shift. Thus, a dynamic-analogue scheme is proposed to correct the precipitation hindcasts by using SSTs, where SST and EASP are highly correlated, to select historical analogue cases. Cross validations show that the average ACC of the temporal-latitude distribution of the EASP between the corrected hindcasts and observations is 0.18 for BCC_CGCM and 0.02 for BCC_CSM; both are much higher than the uncorrected hindcasts. Applying the dynamic-analogue correction scheme in both models successfully improves prediction of the EASP decadal change in the late 1990s.