GRL Seminar Dec 11, Advanced Asian summer monsoon onset in recent decades Yoshiyuki Kajikawa RIKEN, Advanced Institute for Computational Science Kajikawa, Y., T. Yasunari, S. Yoshida, and H. Fujinami, 2012: Advanced Asian monsoon onset in recent decades. Geophys. Res. Lett. Kajikawa, Y., and B. Wang, 2012: Interdecadal change of the South China Sea summer monsoon onset. J. Climate., 25,

Issues and used data sets [Q] How, and to what extent has Asian monsoon changed since the late 20th century? [Q] How are its regionality and seasonality?  Rainfall (1) CMAP, (2) GPCP, (3) APHRODITE, (4) University of Delaware (  Water vapor flux NCEP DOE reanalysis II (Kanamitsu et al., 2002)  Hadley Center SST dataset (Rayner, 2003) (Kajikawa et al., 2012) We analyze the trends ( ) on a monthly mean basis

3 Trend of rainfall in May and June Jun-May Jun May Increasing trend in May along 10N Decreasing trend in June Rainfall pattern in May during recent decades is become closer to that in June during previous. (Kajikawa et al., 2012)

4 Inter-comparison of the rainfall trend in May CMAP Delaware APHRODITE GPCP (Kajikawa et al., 2012)

5 Trend of rainfall along 10-15N w/ climatology 10-15N, E (Kajikawa et al., 2012)

6 Trend of seasonal mean rainfall JJA Jun MJJAS The trend of boreal summer mean rainfall changes significantly depending upon whether it includes rainfall in May. (Kajikawa et al., 2012)

7 Stepwise Asian summer monsoon onset (climatology) (Wang and LinHo, 2002) The Julian pentad in which the relative CPM rainfall rate exceeds 5 mm day is defined as the onset pentad.

8 Asian summer monsoon onset difference Onset pentad [ ] – Onset Pentad [ ] EarlierLater Earlier Later (Kajikawa et al., 2012)

9 Trend of atmospheric thickness ( hPa) 30N-5N30NSST(5N) Warming trend over the continent in May (Right: 30N) has induced earlier seasonal overturning of land-sea heat contrast (Left: 30N-5N). (Kajikawa et al., 2012)

10 Advanced Indian summer monsoon onset was affected by earlier seasonal overturning of land-sea heat contrast. [Q] is it also primary factor for the advanced western Pacific monsoon onset? We’ll Focus on the South China Sea summer monsoon onset change in detail …

11 SCSSM index= U850 (5-15N, E) - (Wang et al.2004)  Decadal change of the SCSSM is clear only in May and June.  The SCSSM starts earlier in Significant Westerly Easterly Wind Focus on the South China Sea summer monsoon onset (Kajikawa and Wang, 2012)

/305/14 Previous study (Kajikawa and Wang 2012)  the SCSSM onset date is shifted 2 weeks around 1993/1994. (Kajikawa and Wang, 2012)

13 SCSSM onset evolution (OLR and U 850 ) Mean onset date is early June The SCSSM onset is primarily associated with northward seasonal march of the ITCZ (Kajikawa and Wang, 2012)

14 Mean onset date is mid-May The SCSSM onset is affected by enhanced northwestward moving TC from the Eq WP SCSSM onset evolution (OLR and U 850 ) (Kajikawa and Wang, 2012)

15 Factors responsible for the SCSSM onset variability around 1993/1994 Abrupt convection enhancement for the monsoon onset Shifted two weeks Thermal condition Convective instability no significant change Tropical disturbances as triggers for monsoon onset Possibly changed … Tropical Cyclone genesis Intraseasonal variability Possible mechanism of SCSSM onset change (Kajikawa and Wang, 2012)

# #7 Difference between and SST diff GPI diff Possible mechanism of SCSSM onset change (Kajikawa and Wang, 2012)

day ISV30-70-day ISV Difference of the intraseasonal variation activities during 4/15- 5/15 between and Previous study (Kajikawa and Wang 2012) Enhanced ISV activity over the western Pacific in the latter epoch significantly.

18 We have elucidated the significant seasonality in long-term trends in the Asian monsoon. 1.Increasing rainfall trend in May along 10N correspond to the advanced monsoon onset. 2.Rainfall trends in July and August showed less significant. 3.The advanced monsoon onset (continental monsoon region) was most likely due to the heat contrast between land and Ocean. The heating trend over the Asian landmass primarily contributed. One plausible factor for the warming trend would be dust aerosol loading along the Himalaya-Tibetan Plateau. 4.The enhancements of the ISV and tropical cyclone activity, which are attributed to significant SST warming over the western Pacific, could be also a stronger trigger of the advanced monsoon onset over the South China Sea and western Pacific. Summary

19 Decadal change of seasonal cycle (precipitation) (110E-120E) SCS China 1.Northward moving of Mei- Yu front is sharp and abrupt in Rainfall in June over China in is stronger than Double peak of convection over the SCS is clear in SCSSM may be key area for Mei-yu front variability…

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