GRL Seminar Dec 11, 2013

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Advanced Asian summer monsoon onset in recent decades

Yoshiyuki Kajikawa (ykaji@riken.jp)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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