grl seminar dec 11, 2013 1 advanced asian summer monsoon onset in recent decades yoshiyuki kajikawa...
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GRL Seminar Dec 11, 2013
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Advanced Asian summer monsoon onset in recent decades
Yoshiyuki Kajikawa ([email protected])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, 3207-3218
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 (http://www.esrl.noaa.gov/psd/) 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 (1979-2008) on a monthly mean basis
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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)
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Inter-comparison of the rainfall trend in May
CMAP
DelawareAPHRODITE
GPCP
(Kajikawa et al., 2012)
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Trend of rainfall along 10-15N w/ climatology
10-15N, 60-140E
(Kajikawa et al., 2012)
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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)
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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.
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Asian summer monsoon onset difference
Onset pentad [1994-2008] – Onset Pentad [1979-1993]
Earlier Later
Earlier
Earlier
Later
(Kajikawa et al., 2012)
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Trend of atmospheric thickness (200-500hPa)
30N-5N 30NSST(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)
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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 …
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SCSSM index= U850 (5-15N, 110-120E) - (Wang et al.2004)
Decadal change of the SCSSM is clear only in May and June.
The SCSSM starts earlier in 1994-2008.
1994-2008
1979-1993
Significant
Westerly
Easterly
Wind
Focus on the South China Sea summer monsoon onset
(Kajikawa and Wang, 2012)
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1994-20081979-1993
5/30 5/14
Previous study (Kajikawa and Wang 2012)
the SCSSM onset date is shifted 2 weeks around 1993/1994.
(Kajikawa and Wang, 2012)
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SCSSM onset evolution (OLR and U850)
Mean onset dateis early June
The SCSSM onset is primarily associated
with northward seasonal march of
the ITCZ
1979-1993
(Kajikawa and Wang, 2012)
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Mean onset dateis mid-May
The SCSSM onset is affected by enhanced
northwestward moving TC from the
Eq WP
1994-2008
SCSSM onset evolution (OLR and U850)
(Kajikawa and Wang, 2012)
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Factors responsible for the SCSSM onset variability around 1993/1994
Abrupt convection enhancement for the monsoon onset
Shifted two weeks
Thermal conditionConvective 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)
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1994-2008#13
1979-1993#7
Difference between
1979-1993 and 1994-2008
SST diffGPI diff
Possible mechanism of SCSSM onset change
(Kajikawa and Wang, 2012)
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10-25-day ISV 30-70-day ISV
Difference of the intraseasonal variation activities during 4/15-5/15 between 1994-2008 and 1979-1993
Previous study (Kajikawa and Wang 2012)
Enhanced ISV activity over the western Pacific in the latter epoch 1994-2008 significantly.
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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
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Decadal change of seasonal cycle (precipitation)
(110E-120E)
SCS
SCS
China
China
1. Northward moving of Mei-Yu front is sharp and abrupt in 1994-2007.
2. Rainfall in June over China in 1994-2007 is stronger than 1979-1993.
3. Double peak of convection over the SCS is clear in 1979-1993.
1979-1993
1994-2007
SCSSM may be key area for Mei-yu front variability…
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