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 experiments0.51 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