1. Zhang HY, Wen ZP. , Wu RG. 2016: Climate Dynamics. DOI 10
Zhang HY, Wen ZP*, Wu RG. 2016: Climate Dynamics. DOI /s

3. In recent years, inter-decadal variability of the EASM has drawn great attention.
A lot of studies have investigated the variability of the EASM, especially on interseasonal to interannual timescale.
(Bin Wang et al., 2009)

4. Summer rainfall anomaly
A significant climate shift occurred around the late 1970s, featuring an increase in summer rainfall over the Yangtze River region and a decrease in rainfall over South China and North China (Nitta and Hu 1996; Wu and Chen 1998; Zhai et al. 2005; Ding et al. 2008).
(Ding et al., 2008)
(a)
(b)
(c)
tripole structure
dipole structure
Summer rainfall anomaly

5. Marked variability—the 1970s shift
EASWJ moved southward
A significant southward displacement of the East Asian subtropical westerly jet (EASWJ)
Intensity of the EASM weakened abruptly at the same time (Wang 2001; Guo et al. 2003).
(Yu et al. 2004)
UV diff
(Li et al. 2010)

6. Decrease in land-sea contrast
SST change
Marked variability—the 1970s shift
Influences of the Pacific decadal oscillation (PDO), and Increase in TP snow were emphasized. (Nitta and Yamada 1989; Trenberth and Hurrell 1994; Ding et al. 2009)
Decrease of land–sea thermal contrast in summer leds to weakening of monsoon.
Increase in TP snow
(Ding et al. 2009)
Decrease in land-sea contrast
(Ding et al. 2009)
On the plausible reasons for the 1970s shift,

7. Marked variability—the 1990s change
Decadal change in the EASM
(Zhu et al, 2014)
Weakening in the EASWJ
(Kwon et al., 2007)

8. Marked variability—the 1990s change
SST warming
Increase in snow
dif_vpot850
dif_wind10 A
(Wu et al, 2010)

9. Most of the previous studies investigated the climate changes separately.
Are there any common features between the late 1970s and early 1990s shifts?
What may be responsible for the intercommunity? How to interpret the amplitude asymmetry?
Is there any SST forcing that might be responsible for both of these inter-decadal changes?
Most of the previous studies investigated the climate changes separately.

10. Datasets:
Japan Meteorological Agency (JMA) 55-year Reanalysis (JRA-55, Kobayashi et al. 2015) for the period 1958–2012.
The 40-year European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40, Uppala et al. 2005) for the period 1958–2002
Monthly SST from the Hadley Centre of the UK Met Office (HadISST, Rayner et al. 2003). The SST is available on a 1° × 1° grid starting from January 1847.
Model:
The National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 4.0 (CAM4, Neale et al. 2013) is used.
The model horizontal resolution is about 2.5° longitude by 1.9° latitude and 26 vertical-levels.

11. a remarkable meridional tripole pattern
associated weakened EASWJ.
capture the common features
enhanced northerly winds, enhanced southerly winds and weakened
displays an increasing trend
To illustrate the variability ……
Imply the interplay between natural decadal variability and anthropogenic climate change
Fig.1 The MV-EOF modes of the EASM and the corresponding time coefficients from 1958 to 2012.

12. A barotropic cyclonic circulation anomaly.
For the first change around the end of 1970s
A barotropic cyclonic circulation anomaly.
A prominent anomalous cyclone.
An increase in divergence at upper level is detected.
Enhanced upper-level convergence over the SCS-Philippines are seen.
How about the inter-decadal …?
The circurlation anomalies are more robust for the first change than for the second one.
For the second change around the early of 1990s
Fig. 2 Inter-decadal changes in winds at low level and upper level

13. For the first change around the end of 1970s
the second mode is most important in determining features of the first change.
MVEOF2-related
signals of natural decadal variability and global warming are overlying.
a positive contribution to the tropical circulation change occurring in the late 1970s.
MVEOF3-related
what are the relative contributions of second mode and third mode?
Spatial patterns of changes caused by natural decadal variability
Fig, 3 changes in monsoon circulation for MVEOF2-related components and MVEOF3-related components

14. For the second change around the end of 1970s
Natural decadal variability of the EASM also makes the most important contribution.
What differs from the former change is that parts of anomalies over the SCS-Philippines caused by MV-EOF2 are offset by anomalies caused by MV-EOF3.
The total anomalies are weaker in contrast to the 1970s change.
MVEOF2-related
MVEOF3-related
Fig. 4 As in Fig. 3, but for the difference of the 1994–2002 mean minus 1980–1993 mean

15. For the change from 1958–1975 to 1980–1993
Toral Precipitation anomaly displays a meridional quadrupole structure.
more precipitation to south China and less precipitation to northern China.
In South China and Northeast China, its contributions can be up to 80 %.
Monsoon precipitation.
Monsoon circulations are coupled with monsoon precipitation.
Fig. 5 Difference of the 1980– 1993 mean minus 1958–1975 mean for summer precipitation (mm day−1)

16. For the change from 1980–1993 to 191994–2002
Parts caused by decadal variability are offset by those related to global warming.
That may explain partly why climate change signals are indistinctive over North China and central China.
Contribution of natural decadal variability drops to about 40% in South China.
Fig. 6 As in Fig. 6, but for the difference of the 1994–2002 mean minus 1980–1993 mean

17. In order to find some command SST forcing
Inter-decadal variability of the EASM is closely correlated with the IPO,
Around the late 1970s, an El-Nino like decadal warming change, around the early 1990s, an La-Nina like decadal cooling change.
Remote SIO SST anomaly may be responsible for both inter-decadal changes of the EASM
Interdecadal
Total SST Ano
Above analyses indicate the interdecadal vriability of EASM mainly governed by natural decadal variability, and further modified by a weakening trend. In order to find some command SST forcing, we focus on natural decadal variability of ocean-atmosphere system.
Fig. 7 Correlations of summer SST with PC2. Time series of the anomalous summer SST (℃) averaged over SIO. Difference of inter-decadal component and total field.

18. A decadal cooling is accompanied by ……
Outflows from the eastern SIO and tropical IO converge over the Philippines, leading to …..
SST forcing in subtropical IO could exert an effect on monsoon circulation over tropical East Asia through anomalous vertical circulation.
Observation results 00 00
This figure shows ……
Fig.8 The regressed maps with respect to inter-decadal component of regional mean SST averaged over SIO

19. Observation results
An anomalous cyclone at 850 hPa, an increase in ascent at 500 hPa and a divergence at 200 hPa over the SCS-Philippines.
An increase in upper level convergence along with anomalous mid-tropospheric descent over the western Australia and equatorial IO are also seen.
The composited results indicate a linkage between the remote SIO SST and EASM discussed in above for both these inter-decadal changes.
And this fig shows the difference maps for level winds, mid-level p-vertical velocity, upper level winds and divergence based on inter-decadal components.
Fig. 9 Difference of the 1970s change and of the 1990s change, resperctively

20. Fig.10 The SST anomalies specified in the sensitive experiment
one with climatological seasonal cycle of SST and the other with climatological seasonal cycle of SST plus idealized JJA SST anomalies of −0.3 ℃ in SIO.
Althought the location of anomalies over the SIO displays some differences from observations. the model results support the impact of remote SIO SST forcing on the tropical East Asian circulation.
To further confirm possible influence of remote SIO SST forcing on the EASM, two simple numerical experiments with CAM4.0 are carried out:
Fig.10 The SST anomalies specified in the sensitive experiment

21. The present study compares characteristics of these two changes and analyzes plausible influences of the SIO SST change.
The two inter-decadal changes in the East Asian summer monsoon (EASM) , occurred around the late 1970s and early 1990s, share pronounced common features, characterized by an equivalent barotropic circulation anomaly over northern East Asia and a meridional vertical overturning circulation over the tropical region.
They display some distinct characteristics, especially over the tropics. The circulation anomalies are more robust for the first change than for the second one. Related amplitude asymmetry is partly attributed to a weakening trend in the EASM.
SST change in the SIO, featuring a decadal warming since the 1980s and a cooling after 1993, may contribute to both of these inter-decadal changes.

22. That’s my talk. Thanks for your attention.

23. We performed a parallel analysis using the ERA-40 Reanalysis dataset that is available for the period 1958–2002 which contains the main periods we focus on.
Both the JRA-55 and ERA-40 dada depict common characteristics between the late 1970s change and the early 1990s change, and their amplitude asymmetry over the tropics.
Fig. 12 The same as in Fig. 1 except based on ERA-40 data from 1958–2002

24. The spatial patterns of linear trends in summer 850 and 200-hPa winds are tested, as illustrated in Fig. 2.