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Revisiting the Second EOF Mode of Interannual Variation in Summer Rainfall over East China


doi: 10.1007/s00376-015-5010-1

  • The second EOF (EOF2) mode of interannual variation in summer rainfall over East China is characterized by inverse rainfall changes between South China (SC) and the Yellow River-Huaihe River valleys (YH). However, understanding of the EOF2 mode is still limited. In this study, the authors identify that the EOF2 mode physically depicts the latitudinal variation of the climatological summer-mean rainy belt along the Yangtze River valley (YRRB), based on a 160-station rainfall dataset in China for the period 1951-2011. The latitudinal variation of the YRRB is mostly attributed to two different rainfall patterns: one reflects the seesaw (SS) rainfall changes between the YH and SC (SS pattern), and the other features rainfall anomalies concentrated in SC only (SC pattern). Corresponding to a southward shift of the YRRB, the SS pattern, with above-normal rainfall in SC and below-normal rainfall in the YH, is related to a cyclonic anomaly centered over the SC-East China Sea region, with a northerly anomaly blowing from the YH to SC; while the SC pattern, with above-normal rainfall in SC, is related to an anticyclonic anomaly over the western North Pacific (WNP), corresponding to an enhanced southwest monsoon over SC. The cyclonic anomaly, related to the SS pattern, is induced by a near-barotropic eastward propagating wave train along the Asian upper-tropospheric westerly jet, originating from the mid-high latitudes of the North Atlantic. The anticyclonic anomaly, for the SC pattern, is related to suppressed rainfall in the WNP.
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  • Adler, R. F., Coauthors, 2003: The version-2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979-present). J. Hydrometeor., 4, 1147- 1167.10.1175/1525-7541(2003)004<1147:TVGPCP>2.0.CO;228942e6d-8b3e-4278-b81c-e1bdaf53e50fd567cc68010323a20c6188f2b04817b5http://www.researchgate.net/publication/23837598_The_Version_2_Global_Precipitation_Climatology_Project_(GPCP)_Monthly_Precipitation_Analysis_(1979-Present)http://www.researchgate.net/publication/23837598_The_Version_2_Global_Precipitation_Climatology_Project_(GPCP)_Monthly_Precipitation_Analysis_(1979-Present)The Global Precipitation Climatology Project (GPCP) Version-2 Monthly Precipitation Analysis is described. This globally complete, monthly analysis of surface precipitation at 2.517 latitude 17 2.517 longitude resolution is available from January 1979 to the present. It is a merged analysis that incorporates precipitation estimates from low-orbit satellite microwave data, geosynchronous-orbit satellite infrared data, and surface rain gauge observations. The merging approach utilizes the higher accuracy of the low-orbit microwave observations to calibrate, or adjust, the more frequent geosynchronous infrared observations. The dataset is extended back into the premicrowave era (before mid-1987) by using infrared-only observations calibrated to the microwave-based analysis of the later years. The combined satellite-based product is adjusted by the rain gauge analysis. The dataset archive also contains the individual input fields, a combined satellite estimate, and error estimates for each field. This monthly analysis is the foundation for the GPCP suite of products, including those at finer temporal resolution. The 23-yr GPCP climatology is characterized, along with time and space variations of precipitation.
    Chen J. L., R. H. Huang, 2007: The comparison of climatological characteristics among Asian and Australian monsoon subsystems. Part II: Water vapor transport by summer monsoon. Chinese J. Atmos. Sci., 31, 766- 778. (in Chinese)93133be7-c2ee-4a58-94b1-dbfe9773c1a0a7a1825c702fb570845390adbf854e06http%3A%2F%2Fwww.oalib.com%2Fpaper%2F1557104refpaperuri:(22b6aa0e307885342c139c516479c38d)http://www.oalib.com/paper/1557104利用1979~2002年的ERA-40和NCEP/NCAR逐日再分析资料以及CMAP降水资料探讨了亚澳季风各夏季风子系统(南亚夏季风、东亚夏季风、北澳夏季风)水汽输送的气候学特征及其与夏季降水的关系。分析表明:各夏季风子系统水汽输送通量主要取决于低层季风气流,南亚夏季风和北澳夏季风以纬向水汽输送为主,而东亚夏季风有很强的经向水汽输送。分析也证实,亚澳季风区的夏季风降水主;要源于水汽输送的辐合,而且ERA-40资料对夏季风水汽输送辐合的描述能力强于NCEP/NCAR资料。此外,受低层季风气流结构的影响,三夏季风子系统水汽输送辐合的动力机理存在明显差异,南亚夏季风和北澳夏季风的水汽输送辐合主要由低层西风气流的风场辐合所造成,而东亚夏季风的水汽输送辐合则由低层南风气流的风场辐合和季风湿平流共同作用造成。因此,东亚夏季风降水有别于南亚夏季风降水和北澳夏季风降水。
    Chen W., L. H. Kang, and D. Wang, 2006: The coupling relationship between summer rainfall in China and global sea surface temperature. Climatic and Environmental Research, 11, 259- 269. (in Chinese)10.1016/S1003-6326(06)60040-X3b3d4989-6406-4463-b27b-d6dc241e3137c14a186fcb96dcd06a52f59c70ca95aehttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QHYH200603001.htmrefpaperuri:(a3d0cdf72abf6ab3b0019d28cc909241)http://en.cnki.com.cn/Article_en/CJFDTOTAL-QHYH200603001.htmBy using the monthly rainfall in 160 stations of China and the sea surface temperature(SST) data from NCEP/NCAR,the spatial and temporal distributions of summer(JJA) rainfall and their correlations with SST are analyzed.The coupling relationship between the anomalous distribution in summer rainfall and the variations of SST has been studied with the Singular Value Decomposition(SVD) analysis.The results show that there are mainly three patterns in the anomalous distributions of summer rainfall.Their temporal variations present the time scales of not only interannual but also interdecadal.Especially,the rainfall in North China decreases from about 1965 and more obviously after 1976.The coupling relationship between the summer rainfall and global SST with SVD analysis presents a dominate time scales of interdecadal variation.The remarkable decrease of summer rainfall in North China and southern part of Northeast China after 1976 has been shown to be closely associated with the SST anomalies in Pacific Ocean,Indian Ocean and the tropical and south of Atlantic Ocean.Particularly,the SST becomes warmer obviously from 1976 in the tropical central and eastern Pacific Ocean,nearly the whole Indian Ocean and the tropical and south of Atlantic Ocean.The relationship between persistent drought in North China and the interdecadal variations of SST in Indian and Atlantic Oceans has not been revealed before.The coupling relationship between the summer rainfall and SST has also been presented in the significant correlations between the rainfall anomalies in the middle and lower reaches of the Yangtze River and the SST anomalies in the Pacific and Atlantic Oceans.When the SST anomalies are positive in South China Sea,Kuroshio region and the neighbouring area,and in the tropical and north of Atlantic Ocean,and negative in the tropical central and eastern Pacific Ocean,the rainfall in the middle and lower reaches of the Yangtze River tends to be above normal.In the reverse situation,the rainfall tends to be below(normal.)
    Ding Y. H., J. C. L. Chan, 2005: The East Asian summer monsoon: An overview. Meteor. Atmos. Phys., 89, 117- 142.10.1007/s00703-005-0125-za6a3f3f9-bd68-45e0-8ac3-3aaf0f4fc5db4fc03ef7f52d18a6b06360a88b350048http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00703-005-0125-zrefpaperuri:(2f0a00476f74c1e849176e5b36130297)http://link.springer.com/10.1007/s00703-005-0125-zThe 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 -渟witch- 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.
    Gill A. E., 1980: Some simple solutions for heat-induced tropical circulation. Quart. J. Roy. Meteor. Soc., 106, 447- 462.10.1002/qj.49710644905ba60c0ea-dc4a-4dbb-a64c-0e0fd9c79640fb9168cefb4f58c6237bda35a1f5e297http://onlinelibrary.wiley.com/doi/10.1002/qj.49710644905/pdfhttp://onlinelibrary.wiley.com/doi/10.1002/qj.49710644905/pdfABSTRACT A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line.
    Han J. P., R. H. Zhang, 2009: The dipole mode of the summer rainfall over East China during 1958-2001. Adv. Atmos. Sci.,26, 727-735, doi: 10.1007/s00376-009-9014-6.10.1007/s00376-009-9014-6c05e92f0-3ba4-429b-bc72-d1202e5690f2538eac94446995ea83c30121a76fdf3bhttp://link.springer.com/10.1007/s00376-009-9014-6http://d.wanfangdata.com.cn/Periodical_dqkxjz-e200904013.aspxBy examining the second leading mode (EOF2) of the summer rainfall in China during 1958-2001 and associated circulations, the authors found that this prominent mode was a dipole pattern with rainfall decreasing to the north of the Yangtze River and increasing to the south. This reverse relationship of the rainfalls to the north and to the south of the Yangtze River was related with the meridional circulations within East Asia and the neighboring region, excited by SST in the South China Sea-northwestern Pacific. When the SST was warmer, the geopotential heights at 500 hPa were positive in the low and high latitudes and negative in the middle latitudes. The anticyclone in the low latitudes favored the subtropical high over the northwestern Pacific (SHNP) shifting southwestward, leading to additional moisture transport over southern China. The anomalous atmospheric circulations along the East Asian coast tends to enhance upward movement over the region. Subsequently, rainfall in southern China is enhanced.
    He M., X. Li, 1992: The relationship between summer rainfall in China and tropical circulation anomaly. Quarterly Journal of Applied Meteorology, 3, 181- 189. (in Chinese)3deec118-aaab-48d9-9fed-51af2bb997be188ebbe616e38db7fbd549323bc75aa7http://en.cnki.com.cn/Article_en/CJFDTOTAL-YYQX199202007.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-YYQX199202007.htmTotal rainfall percentage anomaly of summer in China has been estimated by using EOF method.The first three characteristic vectors of EOF represent three typical rainfall distributionpatterns,respectively.In this paper,the features of tropical circulation of three rainfall distributionpatterns are analysed,and correlation analyses of the time coefficients corresponding to the threecharacteristic vectors are discussed.The results show that there exists positive(negative)correla-tion between southern rainfall pattern and 850hPa(200hPa)equatorial zonal wind in the middleand eastern Pacific.Northern rainfall pattern is closely associated with 850hPa and 200hPa zonalwind in the vicinity of Australia.Middle rainfall pattern is related to the circulation anomalies inIndian ocean.
    Huang R. H., F. Y. Sun, 1992: Impacts of the tropical western Pacific on the East Asian summer monsoon. J. Meteor. Soc.Japan, 70, 243- 256.10.1175/2008JCLI2183.1afcb244f-8f5a-468b-95eb-79769c343f19d9ce16864dec2a0f14bc9fc0033b88e3http%3A%2F%2Fwww.diva-portal.org%2Fsmash%2Fget%2Fdiva2%3A713362%2FFULLTEXT01.pdfrefpaperuri:(0677c5217d4f0b8c00c61d00d248b67b)http://www.diva-portal.org/smash/get/diva2:713362/FULLTEXT01.pdfDefining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979092006 period. The existing indices can be classified into five categories: the east09est thermal contrast, north09outh thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Ni01±o, while the first category reflects the second mode that corresponds to the developing El Ni01±o. The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring. The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.
    Huang R. H., Y. H. Xu, and L. T. Zhou, 1999: The inter-decadal variation of summer precipitations in China and the drought trend in North China. Plateau Meteorology, 18, 465- 476. (in Chinese)5209e6f5-a011-4efa-8b6e-b7398b3c5e1183b24ac36b43d90b61f06d2b4aaf47b4http://en.cnki.com.cn/Article_en/CJFDTOTAL-GYQX199904000.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-GYQX199904000.htmThe interdecadal variations of summer precipitation in China and the drought trend in North China are analysed by using the observed data of summer precipitations (June~August) at 336 stations in China and SST in the Pacific from 1951 to 1994. The analysed results show that a climate jump of summer precipitation in China occurred in 1965. Since 1965, the summer precipitations in North China have obviously decreased and drought trend is obvious there. This drought trend is analogous to that in the Sahel area of West Africa. The analysed results also show that the climate in China in the 1980's was obvious different from that in the 1970's. This difference is that the precipitation in the Yangtze River-the Huai River valley increased and flood disasters obviously increased from the later 1970's, while the precipitations in South and North China in the period from the 1980's to the early 1990's were obvious less than those in the 1970's and drought trend was more and more severe there. However, from the middle 1990's, there was an increasing trend in the precipitation in the northern part of North China. The above mentioned climatic change may be due to the obvious temperature warmings in the middle 1960's and the period from the 1980's to the early 1990's and the obvious cooling in the 1970's in the equatorial eastern and central Pacific, respectively, This phenomenon is like an interdecadal “ENSO Cycle”phenomenon, and it may has a larger impact on the global climatic change and the climatic change in China, especially on the drought trend in North China. However, there is a cooling trend in the SST of the tropical eastern and central Pacific from the middle 1990's, and this is helpful to the increase of precipitation in North China.
    Huang R. H., L. T. Zhou, and W. Chen, 2003: The progresses of recent studies on the variabilities of the East Asian monsoon and their causes. Adv. Atmos. Sci.,20, 55-69, doi: 10.1007/BF03342050.10.1007/BF033420501c7ab91d-206d-4056-982f-69b290766fa01fc22ae417fae42bb8963fd04c5f11e8http%3A%2F%2Fwww.springerlink.com%2Fopenurl.asp%3Fid%3Ddoi%3A10.1007%2FBF03342050refpaperuri:(3bd2d1e7bc71e2f924f42875f8b451c9)http://en.cnki.com.cn/Article_en/CJFDTOTAL-DQJZ200301006.htmThe variabilities of the East Asian summer monsoon arc an important research issue in China, Japan, and Korea. in this paper, progresses of recent studies on the intrascasonal, interannual, and interdecadal variations of the East Asian monsoon, especially the East Asian summer monsoon, and their causes are reviewed. Particularly, studies on the effects of the ENSO cycle, the western Pacific warm pool, the Tibetan Plateau and land surface processes on the variations of the East Asian summer monsoon are systematically reviewed.
    Huang R. H., J. L. Chen, G. Huang, and Q. L. Zhang, 2006: The quasi-biennial oscillation of summer monsoon rainfall in China and its cause. Chinese J. Atmos. Sci., 30, 545- 560. (in Chinese)10.1016/S1001-8042(06)60011-00cd17b31-3720-4687-b3da-04a11de940a5e6bacce6b991f7ab182204cdf6916915http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK200604000.htmrefpaperuri:(b68a1e91129b806f5f129656cce8ac8f)http://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK200604000.htmThe observed data of precipitation at 160 observational stations of China,the ERA-40 reanalysis data and the Empirical Orthogonal Function(EOF) and the entropy spectral analysis methods are applied to analyze the interannual variations of summer(JuneAugust) rainfall in China and water vapor transport fluxes over East Asia.The results show that there is an obvious oscillation with a period of two or three years,i.e.,the quasi-biennial oscillation,in the interannual variations of summer monsoon rainfall in China,especially in the eastern and southern parts of China including South China,the Yangtze River valley and the Huaihe River valley and North China.And it is also shown that this oscillation is closely associated with the quasi-biennial oscillation in the interannual variations of the water vapor transport fluxes by summer monsoon flow over East Asia.Furthermore,the interannual variations of sea temperature in the surface and subsurface of the tropical western Pacific are analyzed by using the sea surface temperature(SST) data from the NCEP/NCAR reanalysis dataset and the sea temperature data in the subsurface of the western Pacific along 137 E from Japan Meteorological Agency,respectively.And it is revealed that there is also a significant quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific.In this paper,the correlative and composite analysis methods are applied to discuss the influence of the quasi-biennial oscillation of thermal state of the tropical western Pacific on summer rainfall in China and water vapor transport over East Asia,and it is shown that the quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific has a great impact on the East Asian summer monsoon and the water vapor transport driven by the monsoon flow.Besides,the influence of the quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific on the quasi-biennual oscillation in the interannual variations of the summer monsoon rainfall in China is simply discussed by using the teleconnection theory of the East Asia/Pacific(EAP) pattern. From the above-mentioned analyses,the physical mechanism of the quasi-biennial oscillation of summer rainfall in China may be summarized as follows: If the thermal state of the tropical western Pacific is in a warming state during a winter,then the convective activities will be intensified around the Philippines in the following spring and summer,which can cause weak summer monsoon rainfall in the Yangtze River and the Huaihe River valleys through the EAP pattern teleconnection.And due to the intensification of the convective activities around the Philippines,a strong convergence of atmospheric circulation will appear over the tropical western Pacific.This will trigger a strong upwelling in the tropical western Pacific.As a consequence,the thermal state of this region will turn into a cooling one in the following winter.On the other hand,since the tropical western Pacific will in a cooling state during the following winter,the convective activities will weaken around the Philippines in the spring and summer of the third year,which can cause strong summer monsoon rainfall in the Yangtze River and the Huaihe River valleys through the EAP pattern teleconnection.And due to the weakening of the convective activities around the Philippines,a divergence of atmospheric circulation will appear over the tropical western Pacific in the spring and summer of the third year.As a consequence,the thermal state of these ocean regions will again turn into a warming one in the winter of the third year.
    Huang R. H., J. L. Chen, and G. Huang, 2007: Characteristics and variations of the East Asian monsoon system and its impacts on climate disasters in China. Adv. Atmos. Sci.,24, 993-1023, doi: 10.1007/s00376-007-0993-x.10.1007/s00376-007-0993-x56e275b8-449d-417a-85cd-82592ac40a370a6ab4a9be9c5431e1637a464329e3f1http%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqkxjz-e200706005.aspxrefpaperuri:(9dd801f38fad75c0910775065c6eb68e)http://d.wanfangdata.com.cn/Periodical_dqkxjz-e200706005.aspxRecent advances in studies of the structural characteristics and temporal-spatial variations of the East Asian monsoon (EAM) system and the impact of this system on severe climate disasters in China are reviewed.Previous studies have improved our understanding of the basic characteristics of horizontal and vertical structures and the annual cycle of the EAM system and the water vapor transports in the EAM region.Many studies have shown that the EAM system is a relatively independent subsystem of the AsianAnstralian monsoon system,and that there exists an obvious quasi-biennial oscillation with a meridional tripole pattern distribution in the interannual variations of the EAM system.Further analyses of the basic phvsical processes,both internal and external,that influence the variability of the EAM system indicate that the EAM systern may be viewed as an atmosphere-ocean-land coupled system,referred to the EAM climate system in this paper.Further,the paper discusses how the interaction and relationships among various components of this system can be described through the East Asia Pacific (EAP) teleconnection pattern and the teleconnection pattern of meridional upper-tropospheric wind anomalies along the westerly jet over East Asia.Such reasoning suggests that the occurrence of severe floods in the Yangtze and Huaihe River valleys and prolonged droughts in North China are linked,respectively,to the background interannual and interdecadal variability of the EAM climate system.Besides,outstanding scientific issues related to the EAM system and its impact on climate disasters in China are also discussed.
    Huang R. H., J. L. Chen, L. Wang, and Z. D. Lin, 2012: Characteristics,processes, and causes of the spatio-temporal variabilities of the East Asian monsoon system. Adv. Atmos. Sci., 29, 910-942, doi: 10.1007/s00376-012-2015-x.10.1007/s00376-012-2015-x3633ae73-7798-4138-862d-e19d2a79b1efb828067a1cc41440b37e544ccfeda46ahttp://www.cqvip.com/QK/84334X/201205/42901340.htmlhttp://d.wanfangdata.com.cn/Periodical_dqkxjz-e201205002.aspxRecent advances in the study of the characteristics, processes, and causes of spatio-temporal variabilities of the East Asian monsoon (EAM) system are reviewed in this paper. The understanding of the EAM system has improved in many aspects: the basic characteristics of horizontal and vertical structures, the annual cycle of the East Asian summer monsoon (EASM) system and the East Asian winter monsoon (EAWM) system, the characteristics of the spatio-temporal variabilities of the EASM system and the EAWM system, and especially the multiple modes of the EAM system and their spatio-temporal variabilities. Some new results have also been achieved in understanding the atmosphere-ocean interaction and atmosphere-land interaction processes that affect the variability of the EAM system. Based on recent studies, the EAM system can be seen as more than a circulation system, it can be viewed as an atmosphere-ocean-land coupled system, namely, the EAM climate system. In addition, further progress has been made in diagnosing the internal physical mechanisms of EAM climate system variability, especially regarding the characteristics and properties of the East Asia-Pacific (EAP) teleconnection over East Asia and the North Pacific, the -ilk Road- teleconnection along the westerly jet stream in the upper troposphere over the Asian continent, and the dynamical effects of quasi-stationary planetary wave activity on EAM system variability. At the end of the paper, some scientific problems regarding understanding the EAM system variability are proposed for further study.
    Huffman G.J., Coauthors, 1997: The global precipitation climatology project (GPCP) combined precipitation dataset. Bull. Amer. Meteor. Soc., 78, 5- 20.10.1175/1520-0477(1997)0782.0.CO;26408b2a5-cc33-4289-8ab3-96c67ba0904b8d41c9f14c72ff096e849cfb2b6baab6http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F24321979_The_Global_Precipitation_Climatology_Project_%28GPCP%29_Combined_Precipitation_Dataset%2Ffile%2F00b4952983eeea9895000000.pdfrefpaperuri:(03ade519844677d80fdbf1eca94c48e6)http://www.researchgate.net/publication/24321979_The_Global_Precipitation_Climatology_Project_(GPCP)_Combined_Precipitation_Dataset/file/00b4952983eeea9895000000.pdfAbstract The Global Precipitation Climatology Project (GPCP) has released the GPCP Version 1 Combined Precipitation Data Set, a global, monthly precipitation dataset covering the period July 1987 through December 1995. The primary product in the dataset is a merged analysis incorporating precipitation estimates from low-orbit-satellite microwave data, geosynchronous-orbit-satellite infrared data, and rain gauge observations. The dataset also contains the individual input fields, a combination of the microwave and infrared satellite estimates, and error estimates for each field. The data are provided on 2.5° × 2.5° latitude–longitude global grids. Preliminary analyses show general agreement with prior studies of global precipitation and extends prior studies of El Ni09o–Southern Oscillation precipitation patterns. At the regional scale there are systematic differences with standard climatologies.
    Kalnay E., Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437- 471.10.1175/1520-0477(1996)0772.0.CO;24f641748c1fe7c7d954de7018f8e59a5http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127325%2Fhttp://ci.nii.ac.jp/naid/10013127325/Abstract The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided by different countries and organizations. The system has been designed with advanced quality control and monitoring components, and can produce 1 mon of reanalysis per day on a Cray YMP/8 supercomputer. Different types of output archives are being created to satisfy different user needs, including a “quick look” CD-ROM (one per year) with six tropospheric and stratospheric fields available twice daily, as well as surface, top-of-the-atmosphere, and isentropic fields. Reanalysis information and selected output is also available on-line via the Internet (http//:nic.fb4.noaa.gov:8000). A special CD-ROM, containing 13 years of selected observed, daily, monthly, and climatological data from the NCEP/NCAR Re-analysis, is included with this issue. Output variables are classified into four classes, depending on the degree to which they are influenced by the observations and/or the model. For example, “C” variables (such as precipitation and surface fluxes) are completely determined by the model during the data assimilation and should be used with caution. Nevertheless, a comparison of these variables with observations and with several climatologies shows that they generally contain considerable useful information. Eight-day forecasts, produced every 5 days, should be useful for predictability studies and for monitoring the quality of the observing systems. The 40 years of reanalysis (1957–96) should be completed in early 1997. A continuation into the future through an identical Climate Data Assimilation System will allow researchers to reliably compare recent anomalies with those in earlier decades. Since changes in the observing systems will inevitably produce perceived changes in the climate, parallel reanalyses (at least 1 year long) will be generated for the periods immediately after the introduction of new observing systems, such as new types of satellite data. NCEP plans currently call for an updated reanalysis using a state-of-the-art system every five years or so. The successive reanalyses will be greatly facilitated by the generation of the comprehensive database in the present reanalysis.
    Lau K.-M., K.-M. Kim, and S. Yang, 2000: Dynamical and boundary forcing characteristics of regional components of the Asian summer monsoon. J.Climate, 13, 2461- 2482.10.1175/1520-0442(2000)0132.0.CO;2c0abba8a-2cae-492d-b68a-59f496d06056faed6341b2f5775abe51bec798978086http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127177%2Frefpaperuri:(4d288e64cabade3cf0f2ddde76d9ea00)http://ci.nii.ac.jp/naid/10013127177/In this paper, the authors present a description of the internal dynamics and boundary forcing characteristics of two major subcomponents of the Asian summer monsoon (ASM), that is, the South Asian monsoon (SAM) and the East-Southeast Asian monsoon (EAM). The description is based on a new monsoon-climate paradigm in which the variability of ASM is considered as the outcome of the interplay of a 'fast' and an 'intermediate' monsoon subsystem, under the influence of 'slow' external forcings. Two sets of regional monsoon indices derived from dynamically consistent rainfall and wind data are used in this study. Results show that the internal dynamics of SAM are representative of a 'classical' monsoon system in which the anomalous circulation is governed by Rossby wave dynamics, where anomalous vorticity induced by an off-equatorial heat source is balanced by the advection of planetary vorticity. On the other hand, the internal dynamics of EAM are characterized by a 'hybrid' monsoon system featuring multicellular meridional circulation over the East Asian sector, extending from the deep Tropics to the midlatitudes. These meridional cells link tropical heating to extratropical circulation systems via the East Asian jet stream and are responsible for the observed zonally oriented anomalous rainfall patterns over East and Southeast Asia and the subtropical western Pacific. In the extratropical regions, the major upper-level vorticity balance is between the advection and generation by anomalous divergent circulation and basic-state circulation. A consequence of the different dynamical underpinnings is that EAM is associated with stronger extratropical teleconnection patterns to regions outside ASM compared to SAM. The interannual variability of SAM is linked to basin-scale SST fluctuation with pronounced signals in the equatorial eastern Pacific. During the boreal spring, warming of the Arabian Sea and the subtropical western Pacific may lead to a strong SAM...
    Lu R.Y., 2001: Interannual variability of the summertime North Pacific subtropical high and its relation to atmospheric convection over the warm pool. J. Meteor. Soc.Japan, 79, 771- 783.10.2151/jmsj.79.771746202d7-b422-43d0-b5fe-266327779a3f6e5897706bf4e081cdaa91212205ca4ahttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001807686refpaperuri:(aa83f485df861460a45020dd407170a8)http://ci.nii.ac.jp/naid/110001807686Using to National Centers for Environmental Prediction/National Center of Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110 150E, 1030N) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one. Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40. Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere (925 hPa), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.
    Lu R. Y., 2004: Associations among the components of the East Asian summer monsoon system in the meridional direction. J. Meteor. Soc.Japan, 82, 155- 165.10.2151/jmsj.82.15554b6f291-b63e-4ddd-b8c3-a38a4e02c74d9203f3479aa6518d8879aae60e6496ebhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F250139918_Associations_among_the_Components_of_the_East_Asian_Summer_Monsoon_System_in_the_Meridional_Directionrefpaperuri:(35bb7fbc3274ae3eef2104566876810f)http://www.researchgate.net/publication/250139918_Associations_among_the_Components_of_the_East_Asian_Summer_Monsoon_System_in_the_Meridional_DirectionThe East Asian summer monsoon is characterized by strong interactions among its components in the meridional direction. The atmospheric convection over the Philippine Sea (PSAC), the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet stream (EAJ) are all closely related to the summer rainfall in East Asia. In this study, we examined the relationship on the interannual time-scale among these factors, by using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Re-analysis data and satellite-observed outgoing long-wave radiation (OLR) data for the 20-year period from 1979 to 1998. It is found that the PSAC-EAJ relation is weak in June, but strong in July and August. Similar difference exists in the relationship between the PSAC and convective activity along the East Asian summer rainy belt. Corresponding to enhanced PSAC, the EAJ is weakened in July and strengthened in August, and tends to exhibit a slight poleward displacement in both months. All these variations in the EAJ intensity and meridional displacement, on the other hand, correspond to suppressed convection along the East Asian summer rainy belt. Finally, the monthly difference in the PSAC-EAJ relation is interpreted by the role of vertical shear of zonal wind. The easterly shear in July and August over the Philippine Sea excites external modes, which are necessary for the tropical-extratropical teleconnection mechanism according to previous numerical studies, but the neutral shear in June is inefficient in exciting external modes.
    Lu R. Y., Z. D. Lin, 2009: Role of subtropical precipitation anomalies in maintaining the summertime meridional teleconnection over the western North Pacific and East Asia. J.Climate, 22, 2058- 2072.10.1175/2008JCLI2444.1c276cb14-3e86-458e-b1ec-eb83729da878cdfb4093683a4ee8541d264f1bda2573http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093162924.htmlrefpaperuri:(edc15401675a909a5dd9029f9ad325ca)http://www.cabdirect.org/abstracts/20093162924.htmlThe meridional teleconnection patterns over the western North Pacific and East Asia (WNP–EA) during summer have a predominant role in affecting East Asian climate on the interannual time scale. A well-known seesaw pattern of tropical–subtropical precipitation is associated with the meridional teleconnection, and the subtropical precipitation anomaly has been previously viewed as a result of anomalous circulations associated with the teleconnection. In this study, however, the authors suggest that subtropical precipitation anomalies, in turn, can significantly affect large-scale circulations and may be crucial for maintenance of the meridional teleconnection. Diagnosis by using observational and reanalysis data indicates that the meridional teleconnection patterns are clearer in summers when the subtropical rainfall anomalies are greater. The simulated results by a linear baroclinic model indicate that a subtropical heat source, which is equivalent to the diagnosed positive subtropical precipitation anomaly, induces zonally elongated zonal wind anomalies that resemble the diagnosed ones in both the upper and lower troposphere over the extratropical WNP–EA. The simulated results also indicate that the horizontal and vertical structures of circulation responses are insensitive to the locations and shapes of imposed subtropical heat anomalies, which implies the important role of basic flow in circulation responses. This study suggests that, for confidential dynamical seasonal forecasting in East Asia, general circulation models should be required to capture the features of interannual subtropical rainfall variability and basic-state flows in WNP–EA.
    Nitta T., 1987: Convective activities in the tropical western Pacific and their impact on the Northern Hemi sphere summer circulation. J. Meteor. Soc.Japan, 64, 373- 390.
    North G. R., T. Bell, R. F. Cahalan, and F. J. Moeng, 1982: Sampling errors in the estimation of empirical orthogonal functions. Mon. Wea. Rev., 110, 699- 706.10.1175/1520-0493(1982)110<0699:SEITEO>2.0.CO;221c9be4a-263e-4dfc-a764-540077536c8653336b77ebdea68afe1f59c9cd3400eehttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127186%2Frefpaperuri:(b6591628d000770558c7101c7cbb9e20)http://ci.nii.ac.jp/naid/10013127186/Abstract Empirical Orthogonal Functions (EOF's), eigenvectors of the spatial cross-covariance matrix of a meteorological field, are reviewed with special attention given to the necessary weighting factors for gridded data and the sampling errors incurred when too small a sample is available. The geographical shape of an EOF shows large intersample variability when its associated eigenvalue is “close” to a neighboring one. A rule of thumb indicating when an EOF is likely to be subject to large sampling fluctuations is presented. An explicit example, based on the statistics of the 500 mb geopotential height field, displays large intersample variability in the EOF's for sample sizes of a few hundred independent realizations, a size seldom exceeded by meteorological data sets.
    Shen B. Z., Z. D. Lin, R. Y. Lu, and Y. Lian, 2011: Circulation anomalies associated with interannual variation of early-and late-summer precipitation in Northeast China. Science China Earth Sciences, 54, 1095- 1104.10.1007/s11430-011-4173-68f094589-d445-4010-8f0e-e1c11aaa776d217353997658a1122e92231319006d0ehttp://www.cqvip.com/QK/60111X/201107/38327612.htmlhttp://www.cnki.com.cn/Article/CJFDTotal-JDXG201107014.htmSummer rainfall is vital for crops in Northeast China. In this study, we investigated large-scale circulation anomalies related to monthly summer rainfall in Northeast China using European Center for Medium-Range Weather Forecast ERA-40 reanalysis data and monthly rainfall data from 79 stations in Northeast China. The results show that the interannual variation in rainfall over Northeast China is mainly dominated by a cold vortex in early summer (May-june) and by the East Asian summer monsoon in late summer (July-ugust). In early summer, corresponding to increased rainfall in Northeast China, an anomalous cyclonic anomaly tilted westward with height appears to the northwest and cold vortices occur frequently. In late summer, the rainfall anomaly is mainly controlled by a northward shift of the local East Asian jet stream in the upper troposphere and the northwest extension of the western Pacific subtropical high (WPSH) in the lower troposphere. The enhanced southwesterly anomaly in the west of the WPSH transports more moisture into Northeast China and results in more rainfall. In addition, compared with that in July, the rainfall in Northeast China in August is also influenced by a mid- and high-latitude blocking high over Northeast Asia.
    Shen S., K. M. Lau, 1995: Biennial oscillation associated with the East Asian summer monsoon and tropical sea surface temperatures. J. Meteor. Soc.Japan, 73, 105- 124.0c2c0630-5fcb-4688-8068-a55ae48f3d5148f4933be3d21ccf41f62485b5a8a926http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001807094refpaperuri:(7196d0a912e4cbfbc0c6899bdf022803)http://ci.nii.ac.jp/naid/110001807094In this paper, the interannual variability of the East Asian summer monsoon (EASM) rainfall and the tropical sea surface temperature (SST) have been studied. It is found that the EASM rainfall possesses a strong biennial signal, which is particularly pronounced over the southeast China. For the SST, the biennial oscillation is the second most significant quasi-periodic signal over the entire tropical Indian and Pacific Oceans. Results indicate that the biennial variations in the SST and EASM rainfall are closely linked. The SST pattern which is best correlated with EASM rainfall appears in the form of a double see-saw with quasi-stationary centers of action over the Indian Ocean, the Asian monsoon region and the eastern Pacific. The most pronounced SST signals are found in the equatorial eastern Pacific and Indian Ocean about two seasons preceding and following the EASM rainfall. Evidence is presented suggesting that the biennial variability of the EASM rainfall is phase-locked to a global scale biennial oscillation involving the interplay of the Asian monsoon, the Hadley and Walker circulations, and basin wide fluctuations in SST. In particular, the eastward propagation of zonal wind anomalies from the Indian Ocean to the western Pacific, which regulates the moisture fluxes from the western Pacific to the East Asian region, appears to be a key component of the biennial fluctuation associated with EASM rainfall. Results suggest that the relationship between the Asian monsoon and tropical SST is more robust in the biennial than the ENSO time scale, hence raising the possibility that the biennial oscillation may be more fundamentally related to monsoon-ocean-atmosphere interaction than ENSO itself.
    Smith T. M., R. W. Reynolds, T. C. Peterson, and J. Lawrimore, 2008: Improvements to NOAA's historical merged land-ocean surface temperature analysis (1880-2006). J.Climate, 21, 2283- 2296.
    Su Q., R. Y. Lu, and C. F. Li, 2014: Large-scale circulation anomalies associated with interannual variation in monthly rainfall over South China from May to August. Adv. Atmos. Sci.,31, 273-282, doi: 10.1007/s00376-013-3051-x.10.1007/s00376-013-3051-x911008c5-f08f-459c-abb4-697ef4ac06fc313c5b8b79b45d635ed5f360b5a14925http%3A%2F%2Fwww.cqvip.com%2FQK%2F84334X%2F201402%2F48829826.htmlrefpaperuri:(e72e0dbe50b400ae22575e8b2c8644a8)http://d.wanfangdata.com.cn/Periodical_dqkxjz-e201402004.aspx
    Takaya K., H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, 608- 627.10.1175/1520-0469(2001)058<0608:AFOAPI>2.0.CO;22eaffd83-f431-4eed-8090-96669e71b247cd8c40c8181e2ef17726a6d7ec840f85http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F249610023_A_Formulation_of_a_Phase-Independent_Wave-Activity_Flux_for_Stationary_and_Migratory_Quasigeostrophic_Eddies_on_a_Zonally_Varying_Basic_Flowrefpaperuri:(48fdd5619a2cc4d95799548d32c1f6e7)http://www.researchgate.net/publication/249610023_A_Formulation_of_a_Phase-Independent_Wave-Activity_Flux_for_Stationary_and_Migratory_Quasigeostrophic_Eddies_on_a_Zonally_Varying_Basic_FlowPresents a study which derived a formulation of an approximate conservation relation of wave-activity pseudomomentum applicable for stationary or migratory quasigeostrophic (QG) eddies on a zonally varying basic flow. Description of the formulation of a phase-independent wave-activity flux for QG eddies; Physical interpretations of phase-independent wave-activity fluxes; Conclusions.
    Wang B., R. G. Wu, and K. M. Lau, 2001: Interannual variability of the Asian summer monsoon: Contrasts between the Indian and the western North Pacific-East Asian monsoons. J.Climate, 14, 4073- 4090.10.1175/1520-0442(2001)014<4073:IVOTAS>2.0.CO;23603c0fe-6497-4279-8ffe-f37ba320128822442df9a8adfc36507e98e5d013c6d5http://ci.nii.ac.jp/naid/10013126171/http://ci.nii.ac.jp/naid/10013126171/Analyses of 50-yr NCEP070705NCAR reanalysis data reveal remarkably different interannual variability between the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) in their temporal070705spatial structures, relationships to El Ni070705o, and teleconnections with midlatitude circulations. Thus, two circulation indices are necessary, which measure the variability of the ISM and WNPSM, respectively. A weak WNPSM features suppressed convection along 1007070507070520070705N and enhanced rainfall along the mei-yu/baiu front. So the WNPSM index also provides a measure for the east Asian summer monsoon. An anomalous WNPSM exhibits a prominent meridional coupling among the Australian high, cross-equatorial flows, WNP monsoon trough, WNP subtropical high, east Asian subtropical front, and Okhotsk high. The WNP monsoon has leading spectral peaks at 50 and 16 months, whereas the Indian monsoon displays a primary peak around 30 months. The WNPSM is weak during the decay of an El Ni070705o, whereas the ISM tends to abate when an El Ni070705o develops. Since the late 1970s, the WNPSM has become more variable, but its relationship with El Ni070705o remained steady; in contrast, the ISM has become less variable and its linkage with El Ni070705o has dramatically declined. These contrasting features are in part attributed to the differing processes of monsoon070705ocean interaction. Also found is a teleconnection between a suppressed WNPSM and deficient summer rainfall over the Great Plains of the United States. This boreal summer teleconnection is forced by the heat source fluctuation associated with the WNPSM and appears to be established through excitation of Rossby wave trains and perturbation of the jet stream that further excites downstream optimum unstable modes.
    Weng H. Y., K. M. Lau, and Y. K. Xue, 1999: Multi-scale summer rainfall variability over China and its long-term link to global sea surface temperature variability. J. Meteor. Soc.Japan, 77, 845- 857.10.1175/1520-0469(1999)056<2728:OTFSIT>2.0.CO;21fb113df-357f-463a-b33f-dd6d549eccbadbace0ebcb5bb0802aaf81e6adde35cahttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F279669595_Multi-scale_summer_rainfall_variability_over_China_and_its_long-term_link_to_global_sea_surface_temperature_variabilityrefpaperuri:(ceb7d4e1ab24ce38803c7d84e778066d)http://www.researchgate.net/publication/279669595_Multi-scale_summer_rainfall_variability_over_China_and_its_long-term_link_to_global_sea_surface_temperature_variabilityABSTRACT
    Wu R. G., Z. P. Wen, S. Yang, and Y. Q. Li, 2010: An interdecadal change in southern China summer rainfall around 1992/93. J.Climate, 23, 2389- 2403.10.1175/2009JCLI3336.1c6f90cf9-b6ea-4e62-b63b-03a3d8c35173bb54b13159918336db2689939a8ccb97http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103193980.htmlrefpaperuri:(d7015f1144eb0bce23ff2bb232ae7322)http://www.cabdirect.org/abstracts/20103193980.htmlAbstract The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980–92 and above normal during 1993–2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)–western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS–subtropical WNP, and the other over north China–Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter–spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS–WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS–WNP.
    Ye H., R. Y. Lu, 2012: Dominant patterns of summer rainfall anomalies in East China during 1951-2006. Adv. Atmos. Sci.,29, 695-704, doi: 10.1007/s00376-012-1153-5.10.1007/s00376-012-1153-58970a788-eaaf-47c3-80ba-faf63ff6e36de66fc6e2fa688a62505ec196177a0fd6http%3A%2F%2Fwww.springerlink.com%2Fcontent%2F2701003588624410%2Frefpaperuri:(140236d2cce6bd5a91a597341c3e6d29)http://d.wanfangdata.com.cn/Periodical_dqkxjz-e201204005.aspxThe dominant patterns of summer rainfall anomalies in East China were studied using Empirical Orthogonal Function(EOF) analysis.The results indicate that after the late 1970s,the first and second dominant patterns switched.During the period before the late 1970s,the spatial pattern of the first(second) dominant mode was the "Yangtze River pattern"(the "South China pattern"),but this changed to the "South China pattern"(the "Yangtze River pattern") after the late 1970s.This decadal change in the dominant patterns resulted from a significant decadal change in summer rainfall over South China after the late 1970s,i.e.,a negative phase during 1978-1992 and a positive phase during 1993-2006.When the decadal variation of rainfall in East China is omitted from the analysis,the first and second dominant patterns represent the "Yangtze River pattern" and the "South China pattern",respectively.These results suggest that when decadal variation is included,the rainfall in China may be dominated by one mode during certain periods and by another in other periods.For the interannual variability when decadal variation is excluded,however,the first and second modes can be easily distinguished,and their order has been stable since at least 1951.
    You Y., Y. Zhou, X. Yang, and L. Fang, 2003: Using EOF method to analysis the spatial distribution and temporal variation of summer rainfall in China. Journal of Sichuan Meteorology, 23, 22- 23. (in Chinese)
    Zhou T.-J., R.-C. Yu, 2005: Atmospheric water vapor transport associated with typical anomalous summer rainfall patterns in China. J. Geophys. Res. , 110,D08104, doi:10.1029/ 2004JD005413.10.1029/2004JD005413783b212c-d86b-4926-bfe5-49bda5490bb2930ffed6d3f1de0f8f25dfae1109619ahttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2004JD005413%2Fabstractrefpaperuri:(d892552e32e34ca1c8b248fa552b83ac)http://onlinelibrary.wiley.com/doi/10.1029/2004JD005413/abstract[1] This paper attempts to reveal the atmospheric water vapor transports associated with typical anomalous summer rainfall patterns in China. The results show that origins of water vapor supply related to anomalous rainfall patterns are different from those related to the normal monsoon rainfall. Anomalous pattern 1, with a heavier rainbelt along the middle and lower reaches of the Yangtze River valley, follows from a convergence of the tropical southwest water vapor transport with the midlatitude northeast water vapor transport; the tropical water vapor transport comes directly from the Bay of Bengal and the South China Sea but originally from the Philippine Sea. The anomalous water vapor transport is associated with a southwestward extension of the western Pacific subtropical high and a southward shift of the upper East Asian jet stream. Anomalous pattern 2, with a main rainbelt along the Huaihe River valley, is supported by the convergence of the subtropical southwest water vapor with the midlatitude water vapor transport. The subtropical branch comes directly from the South China Sea but originally from the East China Sea and the adjacent subtropical Pacific to the further east along 20-25掳N. The background large-scale circulation change includes a northwestward extension of the western Pacific subtropical high and an eastward shift of the upper jet stream. Although the cross-equator flows including the Somali jet supply abundant water vapor for the normal condition of June, July, and August rainfall over China, the tropical water vapor transports related to typical anomalous rainfall anomalies originate from the tropical western Pacific Ocean. The northward transport of anomalous warm water vapor occurs mainly in the lower troposphere, while the transport of midlatitude cold water vapor occurs briefly in the upper troposphere.
    Zhu Q. G., X. G. Chen, 1992: Objective division of natural rainfall regions in China. Journal of Nanjing Institute of Meteorology, 15, 467- 475. (in Chinese)cce0a617-48a6-425f-9e00-d3d529395951da2b875d81a503bcf024143eaa980027http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-NJQX199204002.htmrefpaperuri:(8cd0aa44789ba6fb5163eebd6929db27)http://en.cnki.com.cn/Article_en/CJFDTOTAL-NJQX199204002.htmBy using rainfall data at 160 stations from 1951 through 1987 and the methodof EOF through time and space transformation, the temporal and spacial characteristicsof the rainfall in China are analysed. According to the geographical distribution of thefirst five eigenvectors. six natural rainfall regions in China are obtained. Results showthat the division is reasonable.
    Zou L., Y. Q. Ni, 1998: Impact of ENSO on the variability of the summer monsoon over Asia and the summer rainfall in China. Journal of Tropical Meteorology, 13, 306- 314. (in Chinese)5838958b-5c29-4c48-9b8c-62cac140ee75b394bb4f2fb80e58f270e7224a891f68http://www.cnki.com.cn/Article/CJFDTotal-RQXB199801004.htmhttp://www.cnki.com.cn/Article/CJFDTotal-RQXB199801004.htm1.INTRODUCTI0NBeingtW0oppositephases(ofwannandcoolwater)inthecycIeofENSO,theeventsofElNinoandLaNinaarereflectinghowtheequatorialSSTAevolves.Alotofobservationalfactshavesh0wnthatasasignalENSOisfoundnotonlyinthetropicalSSTA,sea-levelpressure,wind,cloudande
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Manuscript received: 10 January 2015
Manuscript revised: 05 June 2015
Manuscript accepted: 09 July 2015
通讯作者: 陈斌, bchen63@163.com
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Revisiting the Second EOF Mode of Interannual Variation in Summer Rainfall over East China

  • 1. State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
  • 2. Department of Atmospheric Sciences, Yunnan University, Kunming 650091

Abstract: The second EOF (EOF2) mode of interannual variation in summer rainfall over East China is characterized by inverse rainfall changes between South China (SC) and the Yellow River-Huaihe River valleys (YH). However, understanding of the EOF2 mode is still limited. In this study, the authors identify that the EOF2 mode physically depicts the latitudinal variation of the climatological summer-mean rainy belt along the Yangtze River valley (YRRB), based on a 160-station rainfall dataset in China for the period 1951-2011. The latitudinal variation of the YRRB is mostly attributed to two different rainfall patterns: one reflects the seesaw (SS) rainfall changes between the YH and SC (SS pattern), and the other features rainfall anomalies concentrated in SC only (SC pattern). Corresponding to a southward shift of the YRRB, the SS pattern, with above-normal rainfall in SC and below-normal rainfall in the YH, is related to a cyclonic anomaly centered over the SC-East China Sea region, with a northerly anomaly blowing from the YH to SC; while the SC pattern, with above-normal rainfall in SC, is related to an anticyclonic anomaly over the western North Pacific (WNP), corresponding to an enhanced southwest monsoon over SC. The cyclonic anomaly, related to the SS pattern, is induced by a near-barotropic eastward propagating wave train along the Asian upper-tropospheric westerly jet, originating from the mid-high latitudes of the North Atlantic. The anticyclonic anomaly, for the SC pattern, is related to suppressed rainfall in the WNP.

1. Introduction
  • Summer rainfall in East China exhibits a strong year-to-year variability due to its typical monsoon climate, which leads to frequent flood and drought. Understanding the year-to-year variations of summer rainfall, therefore, is essential and many works have been devoted to this topic (Lau et al., 2000; Wang et al., 2001; Huang et al., 2003; Lu, 2004; Ding and Chan, 2005; Shen et al., 2011; Su et al., 2014).

    EOF analysis is the most common approach to investigate variations in summer rainfall in East China. Most such studies have been summarized in the recent works of (Ye and Lu, 2012) and (Huang et al., 2012). Their results showed that the first EOF (EOF1) mode of summer rainfall features a meridional tripole pattern in East China, with increased rainfall located in the middle-lower reaches of the Yangtze River valley and decreased rainfall to the both north and south (He and Li, 1992; Zhu and Chen, 1992; Shen and Lau, 1995; Zou and Ni, 1998; Weng et al., 1999; You et al., 2003; Zhou and Yu, 2005; Chen et al., 2006; Huang et al., 2006, 2007, 2012; Ye and Lu, 2012). This EOF1 mode is associated with the "Pacific-Japan (PJ)" teleconnection proposed by (Nitta, 1987) or the "East Asia-Pacific (EAP)" teleconnection, proposed by (Huang and Sun, 1992), and a Rossby wave train over the midlatitudes of continental Eurasia (Huang et al., 2007).

    The second EOF (EOF2) mode is characterized by rainfall anomalies with opposite sign in South China (SC) and the Yellow River-Huaihe River valleys (YH) (Zhu and Chen, 1992; Shen and Lau, 1995; Weng et al., 1999; You et al., 2003; Zhou and Yu, 2005; Chen et al., 2006; Huang et al., 2007, 2012; Ye and Lu, 2012). The corresponding principal component (PC2) shows a decadal shift around the early 1990s, consistent with significantly enhanced rainfall in SC after 1992 (Wu et al., 2010).

    Some previous studies investigated circulation anomalies associated with the EOF2 mode (e.g., Zhou and Yu, 2005; Han and Zhang, 2009). The negative phase of the EOF2 mode, with increased rainfall in the YH and decreased rainfall in SC, is related to a northwestward extension of the western North Pacific subtropical high (WNPSH) and an eastward shift of the upper jet stream (Zhou and Yu, 2005). On the contrary, the positive phase of the EOF2 mode is associated with a southwestward extension of the WNPSH (Han and Zhang, 2009). Consequently, the subtropical southwest moisture transport in the west of the WNPSH converges with midlatitude moisture transport over the YH in the negative phase (Zhou and Yu, 2005) and over SC in the positive phase (Han and Zhang, 2009). Indeed, these studies concentrated on the decadal change of the EOF2 mode around the early 1990s; similar circulation anomalies associated with the decadal change of summer rainfall in SC after 1992 have also been identified (Wu et al., 2010).

    In addition to the significant decadal change around the early 1990s, the EOF mode with opposite rainfall change between the YH and SC has also been identified from interannual variations of summer rainfall in East China (Ye and Lu, 2012). (Ye and Lu, 2012) revealed that this mode accounts for 10.9% of the total variance of interannual variations of summer rainfall in East China during 1955-2002 and, as the EOF2 mode, is distinguishable from the EOF1 mode. Unfortunately, their results did not show any associated circulation change, so the physical processes involved are still not clear.

    The objective of the present study is to investigate the EOF2 mode of summer rainfall in East China on interannual timescales and to discuss the possible underlying mechanisms. The text is organized as follows: Section 2 introduces the data used in the study. In section 3, the first two dominant modes of interannual summer rainfall in East China are obtained based on EOF analysis. In section 4, based on the variation of summer rainfall in the YH and SC, the authors identify two main patterns in the EOF2 mode: one features a seesaw (SS) rainfall change between the YH and SC, and the other with rainfall anomalies concentrated in SC only. The possible mechanisms responsible for the SS and SC pattern are investigated in section 5. Finally, conclusions and discussion are provided in section 6.

2. Data
  • In this study, the 160-station observed monthly rainfall data in mainland China during 1951-2011, provided by the National Climate Center of the China Meteorological Administration, are used to obtain the first two EOF leading modes in East China. Also used are global precipitation data derived by the Global Precipitation Climatology Project (GPCP) (Huffman et al., 1997; Adler et al., 2003) during 1979-2010, and precipitable water for the entire atmosphere from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data (Kalnay et al., 1996) during 1951-2011. The resolution is 2.5°× 2.5° for both the GPCP data and the NCEP-NCAR reanalysis data. In addition, monthly National Oceanic and Atmospheric Administration ERSST (extended reconstructed sea surface temperature) data (Smith et al., 2008) are also used.

    The monthly atmospheric data are from the NCEP-NCAR reanalysis datasets (Kalnay et al., 1996) during 1951-2011, with a horizontal resolution of 2.5°× 2.5° at 17 pressure levels. Also used are the 6-h specific humidity, zonal and meridional wind at the eight pressure levels from 1000 hPa to 300 hPa, and surface pressure data from the NCEP-NCAR reanalysis datasets (Kalnay et al., 1996) for the period 1951-2011. The 6-h data are applied to calculate moisture transport and its divergence. Similar to (Chen and Huang, 2007), the column moisture flux Q is integrated vertically from 300 hPa to the surface, $$ {Q}=\dfrac{1}{g}\int_{300}^{P_s}{q{V}dp} , $$ and its divergence D is calculated as $$ D=\nabla \cdot {Q}=\dfrac{1}{g}\nabla\cdot \int_{300}^{P_s}{q{V}dp} , $$ where q is specific humidity, p is pressure, and V is the horizontal wind vector including zonal (u) and meridional (v) wind components. The constant g is gravitational acceleration and the variable Ps is atmospheric pressure at the surface. The variables Q and D are first calculated at 6-h temporal resolution, and then their seasonal means are derived. When D>0, moisture flux diverges, acting as a sink of moisture; when D<0, it converges, acting as a source of moisture and favors rainfall.

    In this study, summer is defined as June-July-August (JJA). To obtain interannual anomalies, a 9-yr Guassian filter is employed to remove long-term trends and decadal variations of summer rainfall and circulation anomalies.

3. The EOF2 mode of interannual summer rainfall in East China
  • Figure 1a shows the climatology of summer-mean rainfall in East China east of 105°E and south of 45°N, which includes 108 stations as depicted by black dots. There are two main rainy belts with summer-mean rainfall exceeding 5 mm d-1: the rainy belt along the Yangtze River (YRRB), and along the southeast coast of China. Along these two belts, summer rainfall also exhibits strong interannual variation, with the maximum of standard deviation being larger than 2 mm d-1 during 1951-2011 (Fig. 1b).

    Figure 1.  (a) Climatology and (b) interannual standard deviation of summer rainfall for 1951-2011 in East China. The filled circles depict 108 stations east of 105°E and south of 45°N. The (c) first and (d) second EOF modes of interannual summer rainfall in East China. (e) The normalized PCs corresponding to the first (line) and second (bars) modes. Spatial patterns in (c, d) are shown as regressed anomalies against the corresponding normalized PCs. Shading indicates rainfall exceeding 5 and 2 mm d-1 in (a) and (b), respectively, and depicts significant rainfall anomalies at the 95% confidence level in (c, d). The contour interval is 1, 0.5, 0.3 and 0.3 mm d-1 in (a-d), respectively. The two regions depicted by the rectangles in (d) are the YH (north region), which includes 25 stations (open circles), and SC (south region), which includes 30 stations (filled circles).

    Figure 2.  Fractions (%) of (a) interannual and (b) total variance of summer rainfall explained by the EOF2 mode.

    We perform the EOF analysis on interannual summer rainfall in East China and present the first two EOF modes in Figs. 1c and d. The spatial pattern of rainfall anomalies of the EOF1 mode features increased rainfall in the Yangtze River valley and decreased rainfall over the southeast coast of China (Fig. 1c). It physically depicts a rainfall oscillation between the YRRB and the rainy belt along the southeast coast of China. This mode explains 17% of the total variance. A similar spatial pattern was also obtained in the EOF1 mode of the interannual component of summer rainfall in East China during 1955-2002 by (Ye and Lu, 2012). Note that the rainfall anomalies in North China are weak and positive, which are different from those revealed by (Huang et al., 2012) using total rainfall data. This difference is probably due to the effect of opposite decadal variations of summer rainfall between North China and the Yangtze River valley around the late 1970s (Huang et al., 1999; Huang et al., 2012).

    The EOF2 mode features inverse rainfall changes between South China (SC) and the Yellow River-Huaihe River valleys (YH) (Fig. 1d). This mode accounts for 12% of the total interannual variance of summer rainfall in East China, which is distinguishable from the EOF1 mode (17%) and from the third EOF mode (8%) in terms of the criterion of (North et al., 1982). Moreover, the maximum of interannual variance of summer rainfall explained by the EOF2 mode is more than 50% in SC (Fig. 2a), and that of total variance is more than 40% (Fig. 2b). As for the YH summer rainfall, this mode explains the maximum of more than 30% of its interannual variance (Fig. 2a) and 20% of total variance (Fig. 2b). The area-averaged explicable fractions of interannual variance are larger than 20% and 10% in SC and the YH, respectively, and those of total variance larger than 17% and 8%.

    Figure 3.  Composite results of (a, d) interannual summer rainfall anomalies and (b, e) total rainfall in East China, and (c, f) case results of total summer rainfall (shading) and interannual anomalies (contours), in the (a-c) positive and (d-f) negative phases of the EOF2 mode. Shading indicates statistical significance at the 95% confidence level in (a, d) and depicts rainfall exceeding 5 mm d-1 in (b, c, e, f) (scale bar at the bottom). The contour interval is 0.5 mm d-1 in (a, d) and 1 mm d-1 in (b, c, e, f).

    To reveal the physical meaning of the EOF2 mode, 19 positive-phase cases, corresponding to the associated principal component (PC2) being larger than 0.5, and 20 negative-phase cases, with the corresponding PC2 being smaller than -0.5, are chosen (Table 1). Based on the 39 cases, interannual rainfall anomalies (Figs. 3a and d) and total rainfall (Figs. 3b and e) in summer are then composited. Rainfall increases in SC and decreases in the YH in the positive phase of the EOF2 mode (Fig. 3a). Accordingly, the YRRB is located to the south of the Yangtze River (Fig. 3b). In the negative phase, rainfall decreases in SC and increases in the YH (Fig. 3d), and the YRRB moves northward to north of the Yangtze River (Fig. 3e). Meanwhile, the rainy belt along the southeast of China remains in both the positive (Fig. 3b) and negative (Fig. 3e) phases. The result indicates that the EOF2 mode depicts a meridional shift of the summer YRRB. Figures 3c and f show total summer rainfall (shading) and their interannual anomalies (contours) in two cases of the positive and negative phases of the EOF2 mode, respectively. The YRRB shifted southward in 1999 in the positive phase (Fig. 3c) and northward in 2003 in the negative phase (Fig. 3f), consistent with the composite results. It is also noted that the interannual rainfall anomalies in the YH are weaker than those in SC in the composite results (Figs. 3a and d), which is due to the effect of more than one third of cases with rainfall anomalies being concentrated mainly in SC only, in addition to the cases with inverse rainfall variation between the YH and SC, related to the EOF2 mode, identified in the next section.

    To further reveal the relationship between the meridional shift of the YRRB and rainfall variations in the YH and SC, two regions (32°-38°N, 107°-120°E) and (22°-30°N, 107°-120°E), as depicted by the two boxes in Fig. 1d, are chosen to represent the YH and SC regions, respectively. The YH box includes 25 stations and the SC box includes 30 stations.

    Figure 4.  Normalized time series of the rainfall indices of SC (SCRI) and the YH (YHRI). The SCRI is defined as interannual summer rainfall anomalies of the 30-station mean in SC and the YHRI of the 25-station mean in the YH, as shown in Fig. 1d.

    Accordingly, the YH rainfall index (YHRI) is defined as the 25-station mean rainfall and the SC rainfall index (SCRI) as the 30-station mean rainfall. Figure 4 shows time series of the two rainfall indices. The correlation coefficients of the PC2 with the SCRI and YHRI are 0.93 and -0.57, respectively, both significant at the 99% confidence level. In addition, the YHRI is also significantly correlated with the SCRI, with a correlation coefficient of -0.41, suggesting the interannual variation of the SC summer rainfall is, at least partially, related to that of the YH summer rainfall. A southward shift of the YRRB is related to a significant rainfall increase in SC and decrease in the YH.

4. Identification of the SS and SC patterns related to the EOF2 mode
  • The EOF2 mode, as identified in the last section, which depicts a meridional shift of the YRRB, is associated with the rainfall variations in both the YH and SC. In this section, the EOF2-mode cases are further classified into nine different groups based on the combined distribution of the normalized SCRI and YHRI (Fig. 5). A red dot depicts a strong, positive EOF2-mode year, with the PC2 value being larger than 0.5, and a blue dot represents a strong, negative EOF2-mode year, with the PC2 value being less than -0.5. The 39 strong EOF2-mode years are located mainly in four phases: Phase 1, with strong, positive rainfall anomalies in SC and strong, negative rainfall anomalies in the YH; Phase 4, which is the inverse of Phase 1; Phase 2, with strong, positive rainfall anomalies in SC and normal rainfall in the YH; and Phase 3, which is opposite to Phase 2.

    The phase distribution of the strong EOF2-mode years is summarized in detail in Table 2. There are six and seven years in Phases 1 and 4, and eight years in both Phases 2 and 3. The summed number of years in these four phases is 29, which accounts for approximately three quarters of the total 39 strong EOF2-mode years. Note that the differences between Phases 1 and 2 and between Phases 3 and 4 are whether or not rainfall anomalies in the YH are strong. The Student's t-test is used to obtain the statistical significance for the following composite results.

    Figure 5.  Phase distribution of the strong EOF2-mode years based on the normalized YHRI and SCRI. A positive (negative) EOF2-mode year with the PC2 value being larger (smaller) than 0.5 (-0.5), is depicted by red (blue) dots.

    Figure 6.  Composite interannual summer rainfall anomalies in East China based on (a, b, d, e) four categories of the EOF2-mode cases in Table 2 and (c, f) their difference. The spatial pattern of rainfall anomalies is referred to as the (a-c) SS pattern and (d-f) SC pattern. Phases 1 and 4 (2 and 3) represent the positive and negative phases of the SS (SC) pattern, respectively. Shading represents statistically significant anomalies at the 95% confidence level in (c, f), and the contour interval is 0.5 mm d-1 in (a, b, d, e) and 1 mm d-1 in (c, f).

    Figure 6 shows the composite interannual rainfall anomalies in the four phases. Phase 1 features an SS-like pattern, with increased rainfall in SC and decreased rainfall in the YH (Fig. 6a). The amounts of rainfall anomalies in these two regions are nearly equivalent. An opposite pattern is identified in Phase 4, with reduced rainfall in SC and enhanced rainfall in the YH (Fig. 6b). Composite differences of rainfall anomalies between these two phases show significant, positive anomalies in SC and significant, negative anomalies in the YH (Fig. 6c), resembling the EOF2 mode (Fig. 1d). To distinguish from the EOF2 mode, this pattern is, hereafter, referred to as the SS pattern. Phases 1 and 4 then depict the positive and negative phases of the SS pattern, respectively.

    In Phase 2 rainfall is also increased in SC (Fig. 6d), similar to that in Phase 1 (Fig. 6a). However, there are no strong rainfall anomalies in the YH in Phase 2, in contrast with those in Phase 1. In Phase 3 rainfall is reduced in SC (Fig. 6e), opposite to that in Phase 2 (Fig. 6d). Their difference shows significant, positive rainfall anomalies concentrated in SC and no significant anomalies in the YH (Fig. 6f). This pattern is named the SC pattern, in which Phases 2 and 3 represent its positive and negative phases, respectively.

    In the above analysis the EOF2-mode cases are then mainly classified into two patterns: the SS pattern and the SC pattern. The SS pattern reflects the fact that interannual variations of rainfall in SC are closely related to those in the YH, while the SC pattern represents rainfall varying locally over SC. The two cases in 1999 and 2003, as shown in Figs. 3c and f, distributed in Phases 1 and 4 (Table 2) respectively, are the SS-pattern cases with relatively equivalent amounts of interannual rainfall anomalies in both the YH and SC. On the other hand, the combination of the SS- and SC-pattern cases, leads to stronger interannual rainfall anomalies in SC than the YH in the composite results, as shown in Figs. 3a and d.

    But do both the SS and SC patterns lead to a meridional shift of the YRRB, the same as that in the composite results in Fig. 3? Figure 7 shows the latitudinal variation of the YRRB averaged between 107°E and 120°E for the SS and SC patterns, separately. The mean location of the YRRB moves from 29°N in the positive phase of the SS pattern to 33°N in the negative phase (Fig. 7a), and in the SC pattern from 29°N to 31°N (Fig. 7b). The SS and SC patterns both induce a meridional shift of the YRRB, though the YRRB moves more northward in the negative phase of the SS pattern, due to the contribution of both the decreased rainfall in SC and increased rainfall in the YH, compared to the SC pattern related to decreased rainfall in SC only.

5. Underlying mechanisms
  • To reveal the circulation anomalies responsible, Fig. 8 shows the composite results of horizontal wind anomalies at 850 hPa in summer associated with the SS and SC patterns, separately. Clearly, the SS pattern, with increased rainfall in SC and decreased rainfall in the YH in the positive phase (Phase 1), is significantly associated with a cyclonic anomaly over SC and the neighboring East China Sea (Fig. 8a). In contrast, an anticyclonic anomaly is identified in the negative phase (Phase 4, Fig. 8b). Their difference shows a significant cyclonic anomaly over the SC-East China Sea region (Fig. 8c). The cyclone-induced northerly anomaly blows from the YH to SC, which weakens the climatological southwest summer monsoon in East Asia. Subsequently, the weakened monsoon airflow suppresses northward moisture transport. The latter diverges over the YH and converges over SC (Fig. 9a). Rainfall decreases in the YH and increases in SC, concurrent with ascending motion anomalies over SC and descending motion anomalies over the YH (Fig. 9b).

    Figure 7.  Latitudinal variation of total summer rainfall (units: mm d-1) averaged zonally between 107°E and 120°E for each case (dotted line) and their mean (solid line) of the (a) SS and (b) SC pattern. Red lines represent the positive phase and blue lines the negative phase. The vertical solid lines indicate the latitude of the maximum of composite mean total rainfall, the location of the summer rainy belt in East China, in the positive (red) and negative (blue) phases.

    Figure 8.  As in Fig. 6 but for the composite results of the WNPSH (contours) and interannual horizontal wind anomalies at 850 hPa (vectors). The WNPSH is depicted by geopotential height (units: gpm) at 850 hPa, with the contours of 1470, 1490 and 1510. Statistically significant wind anomalies at the 95% confidence level are plotted with bold vectors in (c, f). The units for wind are given in the top-right corner of (c). The effect of the topography is masked by grey shading.

    Figure 9.  As in Figs. 6c and f but for the composite results of interannual anomalies of column (a, c) moisture transport (vectors), which is vertically integrated from 300 hPa to the surface, and its divergence (contours) and (b, d) vertical pressure velocity at 500 hPa. Statistically significant moisture transport anomalies at the 95% confidence level are plotted with bold vectors in (a, c), and the units are given at the bottom of (c). Dark and light shading indicates statistically significant anomalies at the 95% and 90% confidence levels, respectively. The contour interval is 0.5 mm d-1 in (a, c) and 0.01 Pa s-1 in (b, d).

    Figure 10.  As in Fig. 6c but for the composite results of (a) interannual anomalies of geopotential height and (b) associated Takaya and Nakamura flux (vectors) at 200 hPa, and (c) geopotential height at 850 hPa. Dark and light shading indicates statistically significant anomalies at the 95% and 90% confidence levels in (a, c). Shading in (b) depicts the location of the climatological westerly jet at 200 hPa. The contour interval is 6 gpm in (a, b) and 2 gpm in (c). The effect of the topography is masked by grey shading in (c).

    Figure 11.  As in Fig. 6f but for the composite results of interannual summer anomalies of (a) horizontal winds at 850 hPa (vectors) and GPCP precipitation (contours), (b) NCEP-NCAR reanalysis precipitable water for the entire atmosphere, (c) vertical pressure velocity at 500 hPa, and (d) SST. Dark and light shading indicates statistically significant anomalies at the 95% and 90% confidence levels, respectively. The contour interval is 1 mm d-1 in (a), 0.5 kg m-2 in (b), 0.01 Pa s-1 in (c), and 0.1°C in (d). The effect of the topography is masked by grey shading in (a).

    For the SC pattern, the increased rainfall in SC is related to an anticyclonic anomaly over the northern South China Sea (SCS) and the Philippine Sea in the positive phase (Phase 2, Fig. 8d), and the decreased rainfall in SC is associated with a cyclonic anomaly in the negative phase (Phase 3, Fig. 8e). Their difference is characterized by a significant anticyclonic anomaly (Fig. 8f), corresponding to the westward extension of the western North Pacific subtropical high (WNPSH). Due to the southerly anomaly in the west of the anticyclonic anomaly, more moisture is transported to SC (Fig. 9c), together with ascending motion anomalies (Fig. 9d), enhancing rainfall over SC.

    In summary, in the positive phase, the SS-pattern rainfall anomalies are induced by the cyclonic anomaly in the lower troposphere over the SC-East China Sea region, and the SC-pattern rainfall anomalies are caused by the anticyclonic anomaly over the northern SCS and the Philippine Sea. The physical processes responsible for the formation of the two cyclonic and anticyclonic anomalies in the lower troposphere are discussed in the following subsections 5.2 and 5.3, respectively.

  • The lower-tropospheric cyclonic anomaly over the SC-East China Sea region, in the positive phase of the SS pattern, is connected with an extratropical wave train originating from the mid-high latitudes of the North Atlantic in the upper troposphere (Fig. 10a). To reveal the characteristics of the associated Rossby wave propagation, the zonal and meridional components of a wave-activity flux for stationary Rossby waves (W) are employed, following (Takaya and Nakamura, 2001), which is defined as $$ {W}=\dfrac{1}{2|\overline{{V}}}\left( \begin{array}{c} \overline{u}(\psi'^{2}_x-\psi'\psi'_{xx})+\overline{v}(\psi'_x\psi'_y-\psi'\psi'_{xy})\\[1mm] \overline{u}(\psi'_x\psi'_y-\psi'\psi'_{xy})+\overline{v}(\psi'^{2}_y-\psi'\psi'_{yy}) \end{array} \right), $$ where |V| is the magnitude of the horizontal vector wind (u,v) and ψ is the stream function. Variables with an overbar represent their climatological summer mean averaged during 1951-2011, and variables with subscript and prime notations signify their partial derivatives and anomalies associated with the SS pattern.

    The wave-activity flux at 200 hPa, related to the wave train, originates in the mid-high latitudes of the North Atlantic, extends eastward into northern Europe, and then diverts over central Asia at approximately 60°E southeastward into the Asian westerly jet (Fig. 10b). Subsequently, it continues to propagate eastward into East Asia along the Asian westerly jet, causing a negative geopotential height at 200 hPa over East Asia. A similar spatial distribution of geopotential height at 850 hPa (H850) is also revealed (Fig. 10c), suggesting a barotropic nature of the extratropical wave train. The cyclonic anomaly over the SC-East China Sea region in the lower troposphere is formed as the barotropic response to the negative upper-tropospheric geopotential height over East Asia.

    The external forcing related to the SS pattern is also examined. The composite sea surface temperature (SST) anomalies show no significant signal in the tropical oceans and the North Atlantic in the concurrent summer (figure not shown).

  • The anticyclonic anomaly, responsible for enhanced rainfall over SC in the SC pattern, is associated with a rainfall decrease over the northern SCS and the Philippine Sea, based on the composite GPCP precipitation difference of the three cases in the positive phase, and three cases in the negative phase of the SC pattern since 1979 (Table 2). The decrease in rainfall over the SCS and the Philippine Sea is also supported by the composite results using the NCEP-NCAR reanalysis precipitable water data during 1951-2011 (Fig. 11b). The rainfall reduction-related descent (Fig. 11c) causes an anticyclonic anomaly over the northern SCS and the Philippine Sea, through divergence near the surface with friction. Meanwhile, the reduced rainfall-related WNP heating sink may excite a Rossby wave response in the west (Gill, 1980), further enhancing the anticyclonic anomaly (Lu, 2001; Lu and Lin, 2009).

    The SST related to the SC pattern warms in the Bay of Bengal, the SCS, and the Philippine Sea (Fig. 11d). The reduced rainfall over the warm SST anomalies suggests that the SST anomalies are a response against the anticyclonic anomaly related to the SC pattern. The anticyclonic anomaly may increase incoming shortwave radiation into the underlying oceans because of cloud cover reduction related to the suppressed rainfall. It may also suppress the Asian monsoon westerly in the south, reducing surface evaporation such that the SST warms.

    It is also noted that the WNP sinking-induced Rossby wave further propagates northeastward (Fig. 11a), resembling the PJ pattern (Nitta, 1987) or the EAP pattern (Huang and Sun, 1992). Some previous studies have revealed that the EOF1 mode is also affected by the EAP pattern triggered by the WNP heating (e.g. Huang et al., 2007). To distinguish their difference, Fig. 12 shows the regions with significant positive H850 anomalies related to the SC pattern and the EOF1 mode, separately. The significant H850 anomalies related to the SC pattern cover the SCS and Philippine Sea, while the anomalies related to the EOF1 mode expand northward, further covering the SC region. The different H850 responses to the WNP heating are possibly due to the change in the basic state, since monsoon rainfall peaks in June over SC and in June-July over the Yangtze River valley. This requires further investigation in future work. The northward expansion of the H850 anomalies causes the northward shift of the associated southwesterly anomaly and moisture transport in the west from SC to the Yangtze River valley. Consequently, rainfall increases in SC associated with the SC pattern, and in the Yangtze River valley associated with the EOF1 mode.

6. Conclusion and discussion
  • In this study, the authors reveal the EOF2 mode of interannual summer rainfall anomalies depicts a meridional shift of the YRRB. Moreover, the meridional shift of the YRRB is mostly due to two different types of rainfall anomaly patterns: one that reflects an SS pattern of rainfall anomalies between the YH and SC, and the other with the main rainfall anomalies concentrated in SC. The first pattern is referred to as the SS pattern and the latter is called the SC pattern. There are 13 SS-pattern years and 16 SC-pattern years, which account for three quarters of the total 39 strong EOF2-mode years during 1951-2011. Corresponding to a southward shift of the YRRB, rainfall increases in SC and decreases in the YH in the SS pattern, and it increases in SC only in the SC pattern.

    Figure 12.  The regions with statistically significant geopotential height at 850 hPa at the 95% confidence level related to the EOF1 mode (surrounded by the red contour) and the SC pattern (blue contour). The effect of the topography is masked by grey shading.

    The SS and SC patterns are related to different circulation anomalies. The SS pattern is associated with a lower-tropospheric cyclonic anomaly over SC and the East China Sea, with the northerly anomaly blowing from the YH to SC, weakening the climatological monsoon southwesterly. The weakened monsoon suppresses northward moisture transport and subsequent rainfall in the north, i.e., over the YH, and enhances rainfall in the south over SC. The SC pattern is, however, mainly related to an anticyclonic anomaly over the northern SCS and the Philippine Sea, suggesting a westward extension of the WNPSH. The enhanced southwesterly in the west transports more moisture to SC and favors rainfall.

    Moreover, two different mechanisms responsible for the formation of the cyclonic anomaly related to the SS pattern and the anticyclonic anomaly related to the SC patterns are proposed. The SS pattern results from an extratropical wave train originated from the mid-high latitudes of the North Atlantic. The wave train extends eastward into northern Europe, diverts over central Asia, at approximately 60°E, southeastward into the Asian westerly jet, and then propagates eastward to finally reach East Asia and cause a near-barotropic cyclonic anomaly over SC and the East China Sea. For the SC pattern, the anticyclonic anomaly over the northern SCS and the Philippine Sea is related to the suppressed rainfall in the WNP.

    The present study carefully examines the EOF2 mode of interannual summer rainfall in East China and reveals two different rainfall patterns in the EOF2 mode, with nearly equivalent numbers of cases (13 SS-pattern cases and 16 SC-pattern cases). The SS pattern reflects a dipole-like pattern with opposite rainfall variation in the YH and SC, and the SC pattern represents a monopole-like pattern with rainfall anomalies concentrated in SC. The existence of the monopole-like SC pattern associated with the EOF2 mode may challenge the traditional view of the "dipole" mode of summer rainfall in East China (Zhu and Chen, 1992; Shen and Lau, 1995; You et al., 2003; Zhou and Yu, 2005; Chen et al., 2006). Instead, in this study, we propose that the EOF2 mode may better depict the latitudinal variation of the summer-mean YRRB.

    We identify two main rainfall patterns associated with the EOF2 mode: the SS and SC patterns. It is interesting to note that there is no YH pattern, in which rainfall anomalies are concentrated in the YH only. In the 39 strong EOF2-mode years, there are only five years with strong rainfall anomalies in the YH and normal rainfall in SC (Table 2), far fewer than the 13 SS-pattern years and 16 SC-pattern years. The absence of the YH pattern is likely due to the much smaller interannual variance of summer rainfall in the YH than SC explained by the EOF2 mode (Fig. 2).

    The structure of the composite rainfall anomalies in the positive (Fig. 6d) and negative (Fig. 6e) phases of the SC pattern slightly deviate from their composite difference in Fig. 6f, though they are basically opposite. The deficient rainfall in SC in the negative phase deviates southeastward and the sufficient rainfall in the positive phase deviates northwestward. The nonlinearity indicated by the different deviation from the range is probably due to the different zonal extension of the WNPSH (Fig. 8f). Under the control of the westward-extended WNPSH, anomalous moisture transport by the southwesterly anomaly in the west shifts northwestward in the positive phase (figure not shown), while that transported by the northeasterly anomaly shifts southeastward under the eastward-retreated WNPSH in the negative phase (figure not shown). Accordingly, the rainfall anomalies move northwestward in the positive phase and southeastward in the negative phase.

    (Han and Zhang, 2009) investigated anomalies related to the EOF2 mode of summer rainfall in East China. They proposed that the mode, with increasing rainfall to the south of the Yangtze River over SC and decreasing rainfall to the north over the YH, is induced by a southwestward extension of the WNPSH. In this study, we show that the westward extension of the WNPSH only contributes to the increased rainfall in SC. The opposite change in summer rainfall between the YH and SC (the SS pattern), with above-normal rainfall in SC and below-normal rainfall in the YH, is related to a cyclonic anomaly over SC and the adjacent East China Sea, which is probably affected by the extratropical wave train originated from the mid-high latitudes of the North Atlantic.

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