Variation of East Asian Summer Monsoon and ItsRelationship with Precipitation of China in Recent111 Years

YANG Hao，ZHI Xie-fei * ，GAO Jie，LIU YingKey Laboratory of Meteorological Disaster of Ministry of Education，Nanjing University of Information Science ＆ Technology，Nanjing 210044

Abstract Based on the monthly average SLP data in the northern hemisphere from 1899 to 2009，East Asian summer monsoon intensity indexin recent 111 years was calculated，and the interdecadal and interannual variation characteristics of East Asian summer monsoon were analyzed．The results showed that East Asian summer monsoon in the 1920s was the strongest． The intensity of East Asian summer monsoon after themiddle period of the 1980s presented weakened trend． It was the weakest in the early 21st century． Morlet wavelet analysis found that the inter-decadal and interannual variations of East Asian summer monsoon had quasi-10-year and quasi-2-year significance periods． The interannual vari-ation of precipitation in the east of China closely related to intensity variation of East Asian summer monsoon． In strong ( weak) East Asian sum-mer monsoon year，the rainfall in the middle and low reaches of Yangtze River was less ( more) than that in common year，while the rainfall inNorth China was more ( less) than that in common year． The weakening of East Asian summer monsoon was an important reason for that it wasrainless ( drought) in North China and rainy ( flood) in the middle and low reaches of the Yangtze River after the middle period of the 1980s．Key words East Asian summer monsoon; Variation; Precipitation of China

Received: September 13，2011 Accepted: October 15，2011Supported by National Scientific and Technological Support Plan inChina ( 2009BAC51B03) ; " Six-Talent Peak" Item of Jiangsu Prov-ince ( 2005) ．* Corresponding author． E-mail: zhi@nuist． edu． cn

The monsoon is important component of global large-scale circulation system． Generally speaking，the monsoonformation closely relates to seasonal variation of thermody-namic circulation between ocean and land． The observationfact and research show that Asian monsoon zone is the mostsignificant one of interannual variability of wind field in globalmonsoon zone［1］． Asian monsoon zone can be divided intoSouth Asian and East Asian monsoon zones． They not onlyinterrelate，but also have their own characteristics and havecertain independence． The establishment，evolution and re-treatment of East Asian monsoon have important influenceson weather and climate in East Asian region［2］．

East Asian summer monsoon has multi-time scale varia-tion characteristics． It not only has seasonal and interannualvariations，but also has obvious interdecadal variation［3］．Many observation researches point out that East Asian sum-mer monsoon has close relationship with interannual variationof East Asian climate［4 －10］． The relationship between climateabnormality and monsoon abnormality in China is always animportant research subject． To deeply and quantitatively studythese relationships， it needs consistent understanding onstrong and weak monsoon years． Therefore，it is necessaryto define a monsoon intensity index which can reflect monsoonintensity． Shi Neng et al． considered thermodynamic differ-ence of east-west ocean and land，and used seal-level pres-sure difference to define East Asian summer monsoon intensi-ty index． The index reflected difference of pressure systemsof ocean and land in East Asian region． The difference deter-mined thermodynamic difference between ocean and land ofmonsoon formation to a certain degree［4 －6，11 －12］． Sun Xiu-

rong et al． used east-west and south-north thermodynamicdifferences between ocean and land to comprehensively de-fine an East Asian ocean-land thermodynamic difference in-dex［13］． The monsoon indices obtained by different definitionmethods had their own characteristics on reflecting summerprecipitation and precipitation abnormality in China，but hadcertain difference in describing capability of monsoon precipi-tation［13 －15］． By using the newest sea-level pressure and pre-cipitation data，we further revealed variation rule of East Asiansummer monsoon intensity index and its connection with pre-cipitation abnormality in the east of China．

Data and MethodsData

The monthly average SLP data ( 5° ×5° grid point) in thenorthern hemisphere during 1899 － 2009 provided by NCAR-UCAR and monthly precipitation data at 565 stations of Chinaduring 1961 －2009 provided by China National Climate Centerwere selected．MethodsDefinition of East Asian summer monsoon intensity indexAccording to the method of Shi Neng et al．［4］，the monsoon

intensity index of East Asia in winter and summer was definedas the sum of standardized sea-level pressure difference be-tween 110° and 160° E at 20°，25°，30°，35°，40°，45° and50° N． The obtained numerical sequence was carried outstandardization treatment again． The formula was:

MIt =Σ ( SLP*1jt － SLP

*2jt ) t = 1，2，3，…，111; j =1，

2，3，4，5，6，7The subscript of 1 and 2 respectively indicated 110° and

160° E． SLP* indicated seasonal standardized sea-level pres-sure． It was thus clear that the bigger summer monsoon indexwas，the weaker summer monsoon intensity was［4］．Wavelet analysis Fourier change method is that the timesequence is unfolded in frequency domain． The time se-quence is decomposed into different fluctuations． But the en-

Agricultural Science ＆ Technology，2011，12( 11) : 1711 －1716Copyright 2011，Information Institute of HAAS． All rights reserved． Resources and Environment

ergy variation of each frequency fluctuation as time can't begiven． The wavelet analysis is breakthrough of Fourier analy-sis method． Via wavelet base whose scale can change，thewavelet analysis method synchronously decomposes time se-quence f( t) in time and frequency domains． Then，the ampli-tudes of each " period" signal in time sequence and trans-formed information of these amplitudes as time are obtained．For wavelet transformation is local transformation of time andfrequency domains，the information can be effectively extrac-ted from signal． Via calculation functions of scaling and trans-lation， the signal is carried out multi-scale zoom analy-sis［16 －17］． The wavelet cross spectrum synthesizes main infor-mation of related structures of two time sequences in frequen-cy domain，reveals dependent relationship of relativity on fre-quency，and is important index of expressing related degreebetween two time sequences［11 －12］．Mann-Kendall test［18］ Mann-Kendall test method is non-pa-rameter method． For the time sequence X which has n sam-ples，an order sequence is constructed:

Sk =∑k

i =1ri ri =

1 xi ＞ xj{0 elsej =1，2，…，i

The order sequence Sk is accumulated number that thenumerical value at i time is bigger than that at j time． Sup-posed that the time sequence is random and independent，the

statistical quantity is defined: UFk =Sk －E( Sk )

Var( Sk槡 )，k =1，2，…，n．

UF1 =0． E ( Sk ) and Var ( Sk ) are mean and variance of Sk ．When x1，x2，…，xn are independent and have same continu-ous distribution，

E( Sk ) = k( k －1)4 ，Var ( Sk ) = k( k －1) ( 2k +5)

72 ． UFk is

standard normal distribution and is statistical quantity se-quence calculated by time sequence x ( x1，x2，…，xn ) ． Thesignificance level α is given，and the normal distribution tableis checked． If |UFk | ＞Ua，it indicates that the sequence hasobvious variation trend． According to the inverse sequence oftime sequence x ( xn，xn －1，…，x1 ) ，the above process is re-peated，and the statistical quantity UBk is obtained． UBk =－UFk，( k =n，n －1，…，1) ，UB1 =0．

The advantage of Mann-Kendall test is that it needn'tsample obeying certain distribution and isn't interrupted by fe-wer abnormal values． It is more suitable for type and se-quence variables． The calculation is also simple． It can clearstarting time of mutation and point out mutation zone．

Results and AnalysesVariation characteristics and analysis of East Asian sum-mer monsoon intensity

Based on calculation method of East Asian summer mon-soon intensity index defined by the literature［4］，East Asiansummer monsoon the intensity indices from 1899 to 2009 werecalculated ( Table 1 ) ． For summer monsoon ( from June toAugust) ，the bigger the index was，the weaker the monsoonintensity was［4］． From Fig． 1，the intensity of East Asian sum-mer monsoon in recent 100 years presented variation trendfrom strong to weak． The middle period of the 1980s was anobvious turning point． The intensity of East Asian summermonsoon in the 1920s was the strongest． At this time，Chinawas just in warm period，and summer precipitation was less．From the late 1960s to the early 1980s，East Asian summer

monsoon was weaker． Started from the middle period of the1980s，East Asian summer monsoon further obviously weak-ened． Since the 21st century，the intensity of East Asian sum-mer monsoon presented weakened trend． It was consistentwith the research conclusion of Shi Neng et al［4］．

Table 1 East Asian summer monsoon intensity index from 1899 to2009

Year Intensity index Year Intensity index Year Intensity index

1899 0． 029 3 1936 0． 202 8 1973 －0． 110 71900 0． 512 2 1937 －0． 980 8 1974 0． 770 61901 －1． 056 7 1938 －1． 068 5 1975 －0． 761 41902 2． 266 0 1939 －1． 571 7 1976 0． 563 01903 0． 599 8 1940 －0． 695 5 1977 －0． 305 51904 0． 437 3 1941 0． 023 4 1978 －0． 161 31905 －1． 132 3 1942 1． 082 8 1979 0． 700 81906 －0． 570 1 1943 2． 070 4 1980 0． 884 11907 －0． 581 9 1944 0． 196 5 1981 －1． 147 01908 －0． 787 4 1945 1． 365 3 1982 －0． 049 81909 0． 300 5 1946 －0． 760 3 1983 0． 698 31910 －1． 359 2 1947 －0． 576 8 1984 －0． 176 21911 －0． 068 4 1948 －1． 039 9 1985 －0． 227 51912 －0． 121 1 1949 －0． 441 5 1986 1． 018 21913 0． 546 1 1950 －0． 959 3 1987 0． 591 51914 0． 222 6 1951 －0． 053 3 1988 －0． 737 91915 1． 037 7 1952 0． 164 9 1989 －0． 035 01916 －0． 283 4 1953 －0． 272 8 1990 0． 089 51917 －1． 834 2 1954 0． 020 7 1991 1． 614 31918 0． 021 6 1955 0． 824 9 1992 0． 651 41919 －1． 948 8 1956 1． 176 6 1993 0． 514 11920 －2． 019 0 1957 0． 153 1 1994 0． 650 21921 －0． 823 6 1958 0． 175 3 1995 0． 938 71922 －2． 741 8 1959 －0． 049 0 1996 1． 033 31923 －1． 100 0 1960 －1． 495 4 1997 1． 229 31924 －1． 308 3 1961 －1． 622 0 1998 －0． 443 21925 －1． 743 1 1962 0． 007 0 1999 0． 644 81926 －0． 157 6 1963 －0． 083 0 2000 0． 779 41927 －2． 291 9 1964 0． 363 9 2001 2． 385 41928 －1． 369 5 1965 0． 765 8 2002 1． 416 91929 －0． 608 7 1966 －0． 276 9 2003 0． 957 01930 0． 164 7 1967 0． 489 7 2004 2． 506 51931 －0． 456 7 1968 0． 577 9 2005 1． 151 41932 －1． 475 9 1969 0． 089 1 2006 0． 487 01933 －0． 258 7 1970 0． 097 4 2007 2． 163 81934 0． 561 0 1971 0． 560 5 2008 0． 840 51935 －0． 576 9 1972 －0． 276 5 2009 0． 666 8

Fig． 1 Interannual variation of East Asian summer monsoonintensity index ( straight square bar ) and its 11-yearmoving average ( curve)

If the year when the absolute value of index was≥ 1 wasstrong ( weak) monsoon intensity year，the strong ( weak )

2171 Agricultural Science ＆ Technology Vol． 12，No．11，2011

monsoon intensity years from 1899 to 2009 could be conclu-ded as below: the strong monsoon years were in 1901，1905，1910，1917，1919，1920，1922，1923，1924，1925，1927，1928，1932，1938，1939，1948，1960，1961 and 1981; theweak monsoon years were in 1902，1915，1942，1943，1945，1956，1986，1991，1996，1997，2001，2002，2004，2005 and2007．

By using Morlet wavelet analysis method，East Asiansummer monsoon intensity index from 1899 to 2009 was ana-lyzed ( Fig． 2) ． It was found that the significant quasi-2-yearoscillation period existed from the early 20th century to the1920s; the quasi-2-year and quasi-10-year significance peri-ods simultaneously existed from the 1940s to the 1950s; the3-4-year significant period existed in the 1980s and the early21st century． It was clear that the significant period of EastAsian summer monsoon intensity index was different in differ-ent times． On average，East Asian summer monsoon intensi-ty index from 1899 to 2009 had significant 2-3-year period．Many researches pointed out that the precipitation in the eastof China also had significant quasi-2-year oscillation［19 －21］．

The cross shaded area denoted boundary extension influencezone of wavelet analysis，and the region enclosed by black sol-id line denoted that the power spectrum passed 90% confi-dence level．Fig． 2 Morlet wavelet energy spectrum

To study the mutation phenomenon of monsoon intensityindex，East Asian summer monsoon intensity index in 111years was tested by M-K test method． Seen from Fig． 3，EastAsian summer monsoon intensity index from the early 20th

century to the 1920s presented a decline trend，and sur-passed significance level in the late 1920s． Since the 1950s，East Asian summer monsoon intensity index had an obviousincrease trend． The increase trend from the 1990s to the early21st century was more significant． East Asian summer mon-soon intensity index mutated in the middle and later periods ofthe 1980s．

Two lines denoted critical value U ( U = ±2． 56) at α =0． 01 sig-nificance level．Fig． 3 M-K test of East Asian summer monsoon intensity index

Relationship between interannual variation of East Asiansummer monsoon and precipitation of ChinaCorrelation relationship between precipitation of Chinaand East Asian summer monsoon intensity index Byusing precipitation data of 565 stations of China ( Fig． 4) andEast Asian summer monsoon index，the correlation coeffi-cients between precipitation anomaly percentages in summer( from June to August) and autumn ( from September to No-vember) of 1961 － 2009 in China and East Asian summermonsoon index were respectively calculated． The correlatedrelationships between East Asian summer monsoon and sum-mer，autumn precipitation of China were obtained ( Fig． 5 ) ．Seen from Fig． 5a，summer rainfall in the north of 35° N in theeast of China and East Asian summer monsoon intensity indexhad negative correlated relationship． That is to say，the varia-tion trend of summer rainfall in the zone from 1961 to 2009wasn't consistent with or presented opposite relationship withthat of East Asian summer monsoon intensity index． Summerrainfall in the south of 35° N had positive correlation with EastAsian summer monsoon intensity index． That is to say，thevariation trend of summer rainfall in the zone in 49 years wasconsistent with that of East Asian summer monsoon intensityindex． The distribution of significant relevant zone in autumn( Fig． 5b) and summer was different． Autumn rainfall in thewestern area in the north of 35° N positively correlated withEast Asian summer monsoon intensity index，while it wasnegative correlation in Northeast China and South China．

Fig． 4 The distribution of 565 precipitation stations in China

The zones where summer correlation was good were re-spectively the Yangtze River ( 24° － 35° N，104° － 123° E) ，Northeast China ( 40° －53° N，120° －130° E) and North Chi-na ( 35° －42° N，105° －117° E) ( Fig． 5a) ． Summer rainfallin the three zones from 1961 to 2009 was carried out stand-ardization treatment． By combination with corresponding EastAsian summer monsoon intensity index，their variation trendswere obtained ( Fig． 6) ． From Fig． 6，the standardized anom-aly of summer precipitation in the Yangtze River from 1961 to2009 was basically consistent with variation trend of East Asi-an summer monsoon intensity index，showing very good posi-tive correlation． Their correlated coefficient reached 0． 58，which passed α =0． 01 significance test． It illustrated that theinterannual variation of summer rainfall in the Yangtze Riverbasin had very good positive correlation with that of East Asi-an summer monsoon intensity index． In recent 49 years，thestandardized anomaly of summer rainfall in Northeast Chinaand North China was opposite to the variation trend of East A-sian summer monsoon intensity index． They presented anti-correlation relationship． Via the calculation，the correlation co-

3171YANG Hao et al． Variation of East Asian Summer Monsoon and Its Relationship with Precipitation of China in Recent 111 Years

efficients between standardized anomalies of summer rainfallsin Northeast China and North China and East Asian summer

monsoon intensity index were respectively － 0． 30 and－0．36，passing α = 0． 05 significance test． It illustrated that

4171 Agricultural Science ＆ Technology Vol． 12，No．11，2011

Fig． 8 The distribution of precipitation anomaly percentages of China during strong ( a) and weak ( b) summer monsoon years

the interannual variation trends of summer rainfalls in NorthChina and Northeast China had very good negative correla-tions with that of East Asian summer monsoon intensity index．It was thus clear that summer rainfall had very good correla-tion with intensity variation of East Asian summer monsoon inthe Yangtze River，Northeast China and North China． Itshowed that the interannual variation of East Asian summermonsoon intensity was an important reason of affecting sum-mer precipitation in China．

By using wavelet cross spectrum，the relationships be-tween standardized anomalies of summer rainfalls in the threezones and East Asian summer monsoon intensity index wereanalyzed． Seen from Fig． 7，the standardized anomaly ofsummer precipitation in the Yangtze River basin and East Asi-an summer monsoon intensity index had quasi-3-year correla-tion oscillation in the 1980s and the early 21st century． Thestandardized anomaly of summer precipitation in NortheastChina and East Asian summer monsoon intensity index hadquasi-4-year correlation oscillation in the 1960s and the1990s． The standardized anomaly of summer precipitation inNorth China and East Asian summer monsoon intensity indexhad quasi-4-year correlation oscillation． The correlated oscilla-tion in the 1980s was significant． It was thus clear that the in-fluence of East Asian summer monsoon intensity on summerprecipitation of China embodied on interannual time scale． Itillustrated that the interannual variation of East Asian summermonsoon intensity had influence on precipitation of China，andthey significantly correlated at 2-4-year period．Analysis on rainfall situation of main rain belt in China instrong and weak monsoon years China is the zone wherethe monsoon climate is the most significant in the world． Therainfall has obvious seasonal variation． The precipitation inmost areas concentrates in summer half year，and it is coldand dry in many places in winter half year． The researchshowed that the relationship between East Asian summermonsoon and summer precipitation in China mainly showed insouth-north position of summer rain belt． When summer mon-soon was weak，the position of summer rain belt was bysouth，and it was flood year in the middle and low reaches ofthe Yangtze River． When summer monsoon was strong，itwas drought year in the middle and low reaches of the Yan-gtze River［22］． By using several strong ( weak) summer mon-soon years screened by summer monsoon intensity index inrecent 111 years，the abnormality of summer precipitation inChina since 1961 was analyzed． Seen from Fig． 8，in strongEast Asian summer monsoon year，the rainfall was 30% lessthan that in common year in the middle and low reaches of theYangtze River，while the rainfall was 20% more than that in

common year in North China． In weak East Asian summermonsoon year，the situation was completely opposite． Therainfall in the middle and low reaches of the Yangtze Riverwas more than that in common year，while the rainfall in NorthChina was less than that in common year．

In sum，it could be thought that the weakening of East A-sian summer monsoon was an important reason of the rain-less ( drought) in North China and rainy ( flood) in the middleand low reaches of the Yangtze River after the middle periodof the 1980s．

Conclusions( 1) From 1899 to 2009，the interdecadal variation of East

Asian summer monsoon was significant． The intensity of EastAsian summer monsoon was strong in the 1920s and presen-ted weakened trend after the middle period of the 1980s． Tothe early 21st century，it was the weakest．

( 2) By using Morlet wavelet analysis，it was found thatEast Asian summer monsoon intensity index also had signifi-cant interannual variation． It had significant quasi-2-year oscil-lation in the early 20th century and the 1920s，quasi-10-yearsignificant period from the 1940s to the 1950s，significant 3-4-year period in the 1980s and the early 21st century．

( 3) The intensity variation of East Asian summer mon-soon closely related to precipitation abnormality in the east ofChina． In strong East Asian summer monsoon year，the rain-fall was less than that in common year in the middle and lowreaches of the Yangtze River，while the rainfall was more thanthat in common year in North China． In weak East Asian sum-mer monsoon year，the rainfall in the middle and low reachesof the Yangtze River was more than that in common year，while the rainfall in North China was less than that in commonyear． The weakening of East Asian summer monsoon was animportant reason of that it was rainless ( drought ) in NorthChina and rainy ( flood) in the middle and low reaches of theYangtze River after the middle period of the 1980s．

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Responsible editor: SONG Ping Responsible proofreader:

檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪檪WU Xiao-yan

近 111 年来东亚夏季风的变异及其与中国降水的关系

杨 浩，智协飞* ，高 洁，刘 樱 ( 南京信息工程大学气象灾害省部共建教育部重点实验室，江苏南京 210044)

摘要 根据 1899 ～2009年北半球月平均海平面气压场资料，计算近 111年来东亚夏季风强度指数，分析了东亚夏季风的年代际及年际变化特征。结果表明，20世纪 20年代东亚夏季风达最强，80年代中期以后东亚夏季风强度一直呈减弱趋势，其中 21世纪初达最弱。Morlet小波分析发现，东亚夏季风年代际及年际变化中存在准 10年和准 2年显著性周期。我国东部降水的年际变化与东亚夏季风的强度变化密切相关，强( 弱) 东亚夏季风年份，长江中下游地区降水量比常年偏少( 偏多) ;而华北地区降水量比常年偏多( 偏少) 。东亚夏季风减弱是造成 20世纪 80年代中期以后华北地区干旱少雨，长江中下游地区洪涝多雨的一个重要原因。关键词 东亚夏季风;变异;中国降水

基金项目 国家科技支撑计划项目( 2009BAC51B03) ;江苏省“六大人才高峰”项目( 2005) 。作者简介 杨浩( 1986 － ) ，男，山东临沂人，硕士研究生，研究方向:短期气候预测。* 通讯作者，教授，博士，博士生导师，从事数值天气预报、短期气

候预测研究，E-mail: zhi@ nuist． edu． cn。收稿日期 2011-09-13 修回日期 2011-10-15

6171 Agricultural Science ＆ Technology Vol． 12，No．11，2011