Interdecadal Variability of East Asian Summer Monsoon and Precipitation

By Huijun WangInstitute of Atmospheric

Physics

2011-07-25

EAAM–seasonal migration of the rainbelt

Averaged monthly rainfall 1979 through 2006 (http://svs.gsfc.nasa.gov)

EAAM–seasonal migration of the rainbelt

Changes in observed precipitation, 1951-2000

Courtesy of T. Zhou

The East Asian monsoon experienced an interdecadal change at the end of 1970s

Wang H. J., 2001: The Weakening of the Asian Monsoon Circulation after the End of 1970’s, Adv. Atmos. Sci.,18, 376-386.

The East Asian summer monsoon index

（Wang, 2001, AAS; Wang, 2002, AAS）

Area： 110-125E, 20-40N

2 2EAMI U V Index of wind speed：

Data：NCEP reanalysis

Variability of the East Asian winter monsoon index

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Recent interdecadal Variation of the summer precipitation in

eastern China

Background

• Interdecadal variability: a main contributor to the variation in the summer rainfall in East China;▫ Late 1970’s change – global scale; southern flood and

northern drought (Wang 2001; Wu and Wang 2002; Han and Wang 2007; Ding et al. 2009; ……).

Possible contributors: global SST (Fu et al., 2009); PDO; Indian and Atlantic Ocean SST (Li et al. 2001; Yang and Lau 2004; Dong et al. 2006; Li et al. 2008; Wang et al. 2009; ……); snow cover over the Tibetan Plateau (Ding et al.,

2008); Arctic sea ice and Eurasian snow; ……

No consistent conclusion

•Recent changes in regional and global climate: (Kwon et al. 2007; Cummins et al. 2005; Bratcher and Giese 2002; Peterson and Schwing 2003; Swanson and Tsonis 2009; Easterling and Wehner 2009; ……).

▫Early 1990’s (south China rainfall increase);

▫Late 1990’s and beginning of 21st century (other regions China: a new interdecadal period?)

Data and the model employed

• (1) the 160-station precipitation dataset from the China Meteorological Administration;

• (2) the soil moisture, horizontal and vertical wind, specific humidity, geopotential height and air temperature from the NCEP/NCAR reanalysis;

• (3) the Hadley Centre sea surface temperature (SST);

• (4) the PDO index from the website http://jisao.washington.edu/pdo/

• Model: IAP 9L_AGCM

4° lat × 5° lon (Bi 1993; Wang and Bi 1996)

Results of the study

• (1) Features of the summer precipitation

pattern changes

• (2) Regional circulation

• (3) Background circulation

• (4) Sea Surface Temperature

• (5) AGCM experiments

1950 1960 1970 1980 1990 2000 2010-4

-2

0

2

4(a)

1950 1960 1970 1980 1990 2000 2010-2

-1

0

1

2(b) SC

YRHR

1960 1970 1980 1990 200020

30

40

50

-30

-20

-10

0

10

20

30

1960 1970 1980 1990 200020

30

40

50

-30

-20

-10

0

10

20

30

HR: 32-36°N, 110-121°EYR: 28-31°N, 110-121°ESC: 22-26°N, 110-121°E

percentage of the rainfall departure from the 1951-2008 summer mean (7-year smoothed)

(1) Features of the summer precipitation pattern changes

difference between 2000-2008 and 1979-1999 (a) station precipitation; (b) soil moisture

(reanalysis)

-+

+

-+

+Consistent change patterns.

(a) 850 hPa omega;

(b) vertically integrated water vapor content ;

(c) vertically integrated (1000-300 hPa) moisture flux ;

(d) climatology of (c).

(2) Regional circulation

Difference in the moisture flux divergence: HR-(- 0.78); YR-(0.29) (Sun et al., 2010)

(3) Background circulation

(a) 500 hPa gph;

(b) WPSH;

(c) 200 hPa jet;

(d) Temperature gradient.

(4) Sea Surface TemperatureSST difference and PDO index

0.51 1

1

0o 60oE 120oE 180oW 120oW 60oW 0o

60oS

30oS

0o

30oN

60oN

-1

-0.5

0

0.5

1

Correlation map between the summer-mean PDO index and 500 hPa geopotential height

during 1979-2008

Consistent with the Warming center over Lake Baikal

(5) AGCM experiments

0.51 1

1

0o 60oE 120oE 180oW 120oW 60oW 0o

60oS

30oS

0o

30oN

60oN

-1

-0.5

0

0.5

1

控制试验： 1980-2008年平均的夏季海温敏感性试验：方框区域内的海温差值叠加控制试验海温

500 hPa hgt

Observation

Model

surface air temperature

Observation

Model

Precipitation

Transport of water vapor（ 850 hPa

wind）

Observation

Model

Summary

• Two interdecadal variation: one in the end of 1970s, another around 1999

• After 1999, the summer precipitation in eastern China has changed significantly, with reduced rainfall over the middle-and-lower reaches of Yangtze River Basin

• The interdecadal change after 1999 is possibly related to variation of PDO, which is supported by the AGCM experiments with prescribed SST anomalies