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Interdecadal Change of the South China Sea Summer Monsoon Onset*
YOSHIYUKI KAJIKAWA
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan

BIN WANG
Department of Meteorology and International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii(Manuscript received 16 April 2011, in final form 22 August 2011)
ABSTRACT
A significant advance in the onset dates of the South China Sea summer monsoon (SCSSM) is detectedaround 1993/94: the epochal mean onset date is 30 May for 1979–93 and 14 May for 1994–2008. The relatively
late onset during the first epoch is primarily determined by the northward seasonal march of the intertropicalconvergence zone, whereas the advanced onset during the second epoch is affected by the enhanced activity ofnorthwestward-moving tropical disturbances from the equatorial western Pacific. During 1994–2008, theintraseasonal variability (ISV) over the western Pacific was enhanced during the period from mid-April to
mid-May; further, the number of tropical cyclones (TCs), which passed through the South China Sea (SCS)
and Philippine Sea during the same period, is about doubled compared with those occurring during 1979–93.
This enhanced ISV and TC activity over the SCS and Philippine Sea are attributed to a significant increase in
SST over the equatorial western Pacific from the 1980s to 2000s. Therefore, the advanced SCSSM onset isrooted in the decadal change of the SST over the equatorial western Pacific.

1. Introduction
The South China Sea summer monsoon (SCSSM) is
an important component of the Asian summer monsoon
because the South China Sea (SCS) is geographicallylocated at the center of the Asian–Australian monsoon
system and a joint region of the East Asian monsoon andthe western North Pacific monsoon (Murakami andMatsumoto 1994; Wang et al. 2009). One of the mostspectacular features of the SCSSM is its abrupt climatological onset across the entire SCS basin in the middleof May (Lau and Yang 1997; Wang and Xu 1997; Wang
and LinHo 2002). It is accompanied by a reversal of the
low-level premonsoon easterly to monsoon westerly over
the SCS between 58 and 158N and an enhanced convection activity with rainfall bursts in the northern SCS* International Pacific Research Center Publication Number 827
and School of Ocean and Earth Science and Technology Publication Number 8522.

Corresponding author address: Yoshiyuki Kajikawa, Hydrospheric Atmospheric Research Center, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8601, Japan.
E-mail: ykaji@hyarc.nagoya-u.ac.jp
DOI: 10.1175/JCLI-D-11-00207.1
Ó 2012 American Meteorological Society(108–208N). The SCSSM onset signifies the commencement of the rainy season in eastern Asia (Tao and Chen
1987; Ding 1992) and the western North Pacific (Wangand LinHo 2002). In addition, the interannual variationof the SCSSM has a broad impact on the East Asian
weather in boreal summer (Nitta 1987) and the Indian
Ocean climate variability (Kajikawa et al. 2003).
Recently, the interdecadal change of the western
North Pacific monsoon, including the SCSSM, has been
discussed (Kwon et al. 2005, 2007). Kwon et al. (2005)
found a significant decadal change in the East Asiansummer monsoon and its relationship with the western
North Pacific monsoon before and after (inclusive) 1994.
Yim et al. (2008) found a similar interdecadal change
in the long-term integration of a hybrid coupled model
simulation and suggested a potential reason for thischange. Wang et al. (2009) also found that the summer
and autumn rainfall has increased over southern China
and the northern SCS, but decreased over the central
SCS after 1993. They also noted an interdecadal change
in seasonality based on the season-reliant empirical orthogonal function (SEOF) analysis. However, the detailof this interdecadal change was not fully explored yet
and the possible mechanism remains elusive. In addition, the interdecadal change around 1993/94 was also

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found in the periodicity and spatial structure of the intraseasonal variability (ISV) over the SCS during boreal
summer (Kajikawa et al. 2009). This interdecadal changeof the ISV would potentially impact on the SCSSM seasonal cycle.
These previous results of the interdecadal change insummer mean field and the ISV over the SCS around1993/94 raise a question—that is, whether this interdecadal change has seasonal dependency. The long-termclimate variability or change may affect the SCSSM differently from season to season. Therefore, in this study
we examine the interdecadal change in the seasonalevolution of the SCSSM with a special focus on its transition phase, in particular the summer monsoon onset.
Our objectives are to 1) detect the interdecadal change
in the SCSSM evolution, especially its onset and withdrawal; 2) examine the nature and processes of such aninterdecadal change; and 3) determine the potential factors responsible for the change in the timing of SCSSM
onset, if any.
Section 2 describes data used in this study. In section
3, we examine the interdecadal change of the SCSSM
evolution and show a significant change in the SCSSM
onset date around 1993/94. We examine the different
SCSSM onset processes before and after (inclusive) 1994
in section 4. Potential factors to determine the SCSSM
onset timing are examined in section 5. Concluding remarks with discussions are presented in section 6.

2. Datasets and method
We used the daily mean interpolated outgoing longwave radiation (OLR) data from the National Oceanicand Atmospheric Administration (NOAA) satellite forthe period 1979–2008 (Liebmann and Smith 1996) as anindicator of tropical convection activity, daily mean
wind data from the National Centers for Environmental
Prediction (NCEP)–National Center for AtmosphericResearch (NCAR) reanalysis project for the period
1949–2008 (Kalnay et al. 1996), and the monthly mean
Hadley Center sea surface temperature (SST) dataset
for the period 1979–2008 (Rayner et al. 2003). The
tropical cyclone track data over the western Pacificfrom the International Best Track Archive for Climate
Stewardship (IBTrACS; Knapp et al. 2010) were used
for the period 1979–2008. The Lanczos bandpass filter(Duchon 1979) was applied to the OLR dataset to extract the intraseasonal signals—10–25- and 30–80-day
variations, respectively. Since we have a 30-yr satellite
dataset available, we focus on interdecadal change between two epochs: 1979–93 and 1994–2008 in this study.
Both the period of 1979–93 and 1994–2008 have 15-yr
data samples.

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FIG. 1. Time series of climatological mean 850-hPa zonal wind
averaging over 58–158N, 1108–1208E as the SCSSM index during 1)
1979–93 (dotted line) and 2) 1994–2008 (solid line). The period
when the difference between the two indices is statistically significant is shaded.

3. Interdecadal change of the SCS summer
monsoon onset
First, we examined the climatological difference of the
SCSSM evolution between 1979–93 and 1994–2008. In
this study, we used the daily mean SCSSM circulationindex rather than rainfall index to capture large-scalevariations. This is also because of the unavailability ofdaily rainfall data over the ocean. Using a wind circulation index has the advantage of avoiding random noise
associated with small-scale rainfall variation. Figure 1
shows the time series of the SCSSM circulation index
that is defined as 850-hPa zonal wind averaging over 58–
158N, 1108–1208E (Wang et al. 2004) during the two
epochs. During 1979–93, the SCSSM index turns fromnegative (easterly) to positive (westerly) in late May,
which indicates the mean SCSSM onset time. The index
reaches first peak in late June and a second major peakin the middle of August. The index turns to negative inthe middle of October, signifying the mean withdrawal
date of the SCSSM. In contrast, during 1994–2008 theSCSSM starts from early May and reaches the first peakin the middle of May with the major peak in early August.
However, the monsoon withdrawal remains unchangedin the middle of October. Of great interest is that thesignificant difference of the SCSSM index between thetwo epochs is mainly seen during early summer, especially in May. The epochal difference of the SCSSM
circulation index in July and August is not statisticallysignificant, although seemingly large. The SCSSM during 1994–2008 is also significantly stronger in the middleof September. However no significant difference canbe found in the withdrawing of the SCSSM around the
middle of October.
To detect the interdecadal change of broad-scaleseasonal evolution in an extending monsoon area, thetime–longitude distribution of the epochal difference in1 MAY 2012

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FIG. 2. Time–longitude diagram of OLR and 850-hPa wind difference between 1979–93 and
1994–2008 along 108N (1994–2008 minus 1979–93). Only those that are significant at a 95%
level by Student’s t test are plotted. Counter denotes the climatology during 1979–93. The
vector unit is 10 m s21 and the shading and contour interval is 10 W m22.

mean OLR and 850-hPa zonal wind between 1979–93
and 1994–2008 along 108N are shown in Fig. 2. Significant westerly wind anomalies in early to middle May
are prominent not only over the SCS but also over thenorthern Indian Ocean. These westerly anomalies areassociated with the enhanced convection anomalies over
the SCS and Bay of Bengal in early May. The negative
OLR anomalies in early to middle May and positive
OLR anomalies in early June over the SCS are consistent with the results of the SCSSM circulation index(Fig. 1). Notable westward propagation of negative
OLR anomalies from 1508E to the SCS in early May isalso significant. The other westward-propagating negative OLR anomalies from 1008 to 608E are also noteworthy in late April to early May. Most significant
convection anomalies are seen only in May, which means

the monsoon over the SCS and the Bay of Bengal tends
to start earlier in the second epoch than in the first one.
It has been described that the climatological abrupt
SCSSM onset occurs in the middle of May with longrecord dataset starting from 1950 (Wang et al. 2004,2009). The results of the possible interdecadal change
between 1979–93 and 1994–2008 in the SCSSM circulation index, OLR, and 850-hPa wind field should beconnected with the long-term SCSSM onset date variability. Here, we defined the SCSSM onset date in eachyear by using the daily mean SCSSM circulation index
following previous studies (Wang et al. 2004; Kajikawa
et al. 2010). The onset day is defined as the first day after
25 April that satisfies the following criteria: 1) on theonset day and during the 5 days after the onset day theaveraged SCSSM index must be greater than 0 (meaning

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FIG. 3. Time series of the SCSSM onset date. The definition of the onset date is explained indetail in the main text.

that the westerly is steadily established); 2) in the subsequent 20 days, the SCSSM index must be positive inat least 15 days; and 3) the cumulative 20-day mean
SCSSM index must be greater than 1 m s21 (meaning apersistent seasonal transition). Because the large-scaleseasonal transition and the connection of southwesterly
over the SCS and southwesterly over the Indian Ocean
basically occur after late April, we defined the cutoff day
as 25 April to eliminate the false detection of the bogus
onset.
Figure 3 shows the time series of the SCSSM onset
date from 1948 to 2008 by using the NCEP–NCAR reanalysis data. Clear interdecadal shift of the monsoon
onset date between 1993 and 1994 is seen. The climatological mean onset date during 1979–93 is around 30 May;in contrast, that during 1994–2008 is around 14 May. The
difference is about two weeks and this is statistically significant. The interannual variability of the SCSSM onset
variation during 1994–2008 is relatively smaller than theprevious epoch. Although some other interdecadal variability seems to occur before 1979, we will mainly focus
on the interdecadal shift between 1993 and 1994. In addition, both the SCSSM onset in 1979 and the second
epoch occur around mid-May, but we define 1979–93 as
the first epoch in this study. The interannual and interdecadal change of the SCSSM onset day is consistent withthose calculated with pentad mean data as Wang et al.
(2004), whose interannual correlation is more than 0.80.

4. Differences in the onset processes during
the two epochs
To examine the differences in the evolution of themean SCSSM onset, we calculated the composite of
OLR and 850-hPa winds based on the SCSSM onset day
in each year for both epochs—1979–93 and 1994–2008.
Figure 4 shows the composite map of the SCSSM onset
processes in 1979–93 from 10 days before the onset day.

At day 210, an active convection center is located overthe eastern Indian Ocean and the Sumatra and Borneo
Islands that is accompanied by twin cyclonic circulation
to the west of the convections in both Northern andSouthern Hemispheres (Fig. 4a). Westerly wind has
maximum speed along the equator at day 210. Easterly
wind is robust over the entire western North Pacific(WNP). These active convections gradually move northward with eastward expansion from day 28 to day 22(Figs. 4b–e). The developing convection center reaches
the Arabian Sea and the Bay of Bengal, and maximum
westerly wind axis reaches 108N at day 22. Eventually,
the active convection area is abruptly expanded into thesouthern part of the SCS and the westerly wind penetrates to the SCS at day 0 (Fig. 4f). The WNP subtropicalhigh is shifted eastward, concurring with the SCSSM
onset. The convection over the southern Indian Ocean is
weakened along with the SCSSM onset. It is suggested that
the SCSSM onset during the period of 1979–93 is primarily
associated with a northward migration of the seasonal
march of the intertropical convergence zone (ITCZ).
On the other hand, the SCSSM onset processes during
1994–2008 are different from the previous epoch (Fig. 5).
Before the onset, the WNP subtropical high extends
farther westward than that in 1979–93, with prominent
active convection over the equatorial Indian Ocean atday 210 (Fig. 5a). The convection is more restricted along
the equator than that in 1979–93’s climatology. Becauseof the confinement of the convection along the equator,
the northern part of the twin cyclonic circulation overthe Bay of Bengal is weak. Another convection center isseen over the equatorial western Pacific. The convection
over the Indian Ocean moves northward in a similarmanner as in the previous epoch from day 28 to day 22(Figs. 5b–e). However, this convection is confined only
over the Bay of Bengal and is gradually weakened.
Of great interest are the convections over the western
Pacific that move northwestward and intensify. Eventually,1 MAY 2012

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FIG. 4. Composite evolution of OLR and 850-hPa wind based on the SCSSM onset date in each year in the epoch 1979–93 from 10 days
before onset (day 210) to onset date (day 0). Shading interval is 20 W m22 and vector unit is 10 m s21.

the active convections move into the SCS and the winddirection over the SCS abruptly changes from easterly towesterly at day 0 (Fig. 5f). Thus, the SCSSM onset
during 1994–2008 is affected by the enhanced activity ofnorthwestward-moving tropical convections, which areassociated with tropical disturbances, from the equatorial western Pacific.
In short, the most prominent difference in the SCSSM
onset processes between the two epochs lies in the direction of active convection and the associated low-level
westerlies coming into the SCS. Figure 6 shows the time–
longitude diagram of the SCSSM onset composite along

108N. It is highlighted that the westward-moving convection clearly affected the SCSSM onset during 1994–
2008. The development of the convection over the
Arabian Sea and the west side of the Bay of Bengal also
differs between the two epochs.

5. Factors responsible for the SCSSM onset
variability
What factors determine the interdecadal change of the
SCSSM onset? It is well known that convective instability
with near-neutral to unstable temperature profile in the3212

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FIG. 5. As in Fig. 4, but for the epoch 1994–2008.

lower troposphere is necessary for the abrupt convection
enhancement for the monsoon onset. Meanwhile, theSCSSM has large ISV. The ISV over the SCS has two
dominant time scales: one is a 10–25-day variation called‘‘quasi-biweekly mode,’’ and the other is a 30–80-day variation, which is coupled with the eastward-propagating
Madden–Julian oscillation (MJO; Madden and Julian
1994) (Kajikawa and Yasunari 2005). Several studies
have discussed the importance of the tropical disturbances and ISV on the triggers of the SCSSM onset. Chanet al. (2002) found the SCSSM onset in 1998 was mainly
controlled by the 30–60-day variation. Zhou and Chan(2005) investigated the role of the two types of theISV—10–20- and 30–60-day variations—in the establishment of the SCSSM and concluded that these two
types of the ISV activities interact to cause a weakeningof the subtropical high over the western Pacific. Somestudies also claimed that the SCSSM onset was frequently
triggered by the MJO (Straub et al. 2006; Tong et al.
2009). Meanwhile, Mao and Wu (2008) found that thetropical cyclone ‘‘Chanchu’’ entering the SCS acted as
an immediate trigger for the onset and contributed tothe establishment of the SCSSM in 2006. Based on theseprevious studies, we explore the roles of the thermal
precondition, the ISV activity, and the tropical cyclone
activity in interdecadal change of the SCSSM onset.

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FIG. 6. Hovmöller diagram of composite evolution of OLR and 850-hPa wind along 108N based on the SCSSM onset datefrom 210 days to 15 days during (a) 1979–93 and (b) 1994–2008. Shading interval is 20 W m22 and vector unit is 10 m s21.

a. Thermal condition over the SCS
Figure 7 shows the time evolution of the vertical profile of equivalent potential temperature over the SCS,

averaging over 108–208N, 1108–1208E during 1979–93 andthe difference between the two epochs. Time axis covers
from 1 month before the climatological SCSSM onset
during 1994–2008. The thermal condition in the lower

FIG. 7. Time–height section of epochal difference in the equivalent potential temperature (K)over the SCS (108–208N, 1108–1208E) (1994–2008 minus 1979–93) (shading). Contours denote theclimatology of equivalent potential temperature during 1979–93.

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FIG. 8. The epochal difference in the OLR anomalies (W m22) on (a) 10–25- and (b) 30–80-day
time scales during 15 Apr–15 May (1994–2008 minus 1979–93). The contours denote climatological intraseasonal variation during 1979–93. Black dots indicate areas where the difference issignificant above a 95% confidence level.

troposphere up to 500 hPa is unstable in April but
gradually weakening through May during 1979–93. A
large potential temperature difference between the two
epochs appears around 11 May, which indicates theabruptly enhanced convective anomalies due to the
SCSSM onset itself during 1994–2008. No significant
difference in the thermal condition over the SCS appears before the SCSSM onset. This implies that thepreconditioning for the onset during boreal spring issimilar in both epochs at least over the SCS and has
little impact on the interdecadal change of the SCSSM
onset.

b. Intraseasonal variability
To detect the interdecadal difference of the ISV activity, we calculated mean standard deviations of the 10–25- and 30–80-day filtered OLR during 15 April–15 May,
which covers 1 month before the climatological SCSSM
onset during 1994–2008. Figure 8 shows the differenceof this mean standard deviation between 1979–93 and
1994–2008. The enhancement of the quasi-biweekly modeover the SCS and Philippine Sea during 1994–2008 is significant. This is associated with northwestward-moving
disturbances during 1994–2008 and weakening of the1 MAY 2012

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FIG. 9. Tropical cyclone tracks that passed through the SCS and/or
the Philippine Sea (108–208N, 1058–1308E) in 15 Apr–15 May during(a) 1979–93 and (b) 1994–2008.

WNP subtropical high. Many interesting features appear in the difference on the MJO time scale. Enhancement of the ISV activity over the Sumatra Islands and
WNP in the second epoch is significant. The suppressed
ISV activity is seen over the southwestern Pacific andMaritime Continent. Overall, these results indicate thatboth types of the ISV are more active in the epoch of the
1994–2008 before the mean onset date. This implies thatthe enhancement of the ISV may be a trigger for theearlier SCSSM onset during 1994–2008.

c. Tropical cyclones
Figure 9 shows the tropical cyclone tracks in 1979–93
and 1994–2008, respectively, that passed over the SCS or
Philippine Sea (58–208N, 1108–1308E) during 15 April
and 15 May. The number of the tropical cyclones in
1994–2008 (13) is about twice that of the TCs in 1979–93(7). It is interesting that the region of the most tropical
cyclone generations in 1994–2008 is confined to thewestern Pacific—to the west of 1408E. Several more
tropical cyclones were also formed in the SCS during3215

1994–2008. It is suggested that more tropical cyclone
during 15 April and 15 May can trigger more frequent
early SCSSM onset during 1994–2008.
To explore the possible factors that produce favorable
conditions for generating tropical cyclones in May during 1994–2008, we calculated the genesis potential index(GPI; Emanuel and Nolan 2004) in May for both epochs.
The GPI is a metric that combines 1) large-scale vorticity at 850 hPa, 2) relative humidity at 600 hPa, 3)
maximum potential intensity, and 4) wind shear between 850 and 200 hPa. Large value of the GPI indicatesthe enhancement of tropical cyclone development. The
potential intensity is primarily governed by SST andsurface relative humidity. Figure 10 shows the GPI ineach epoch and the difference between the two epochs.
The large GPI value is seen over the western Pacific andthe southern Indian Ocean in both epochs. However, asignificant difference between the two epochs is clearlyfound over the western Pacific, especially over thePhilippine Sea. The GPI over the western Pacific in 1994–
2008 is larger. This is consistent with the fact that more
TCs originate over the Philippine Sea (Fig. 9). We also
examined the contribution of each GPI term to the interdecadal change between the two epochs, and foundthat the primary contributor is 3) the maximum potential intensity, and the secondary contributor is 2) the
relative humidity at 600 hPa (figure not shown). This isassociated with interdecadal change of the SST, whichmeans that the SST over the western Pacific is warmer in
1994–2008 than 1979–93 (Fig. 11, lower panels). Interestingly, the warm pool area—the epochal mean SST
exceeding 288C in 1979–93—with the SST warming
during the two epochs is consistent with the area with
the GPI increasing. In addition, the enhancement of the
ISV during 1994–2008 (Fig. 9) can also accelerate thetropical cyclone genesis (Mao and Wu 2008; Kikuchi
et al. 2009).
In short, no significant interdecadal difference in theconvective instability prior to the onset can be foundover the SCS, but the following two factors show significant interdecadal difference: 1) the enhanced intraseasonal variability and 2) increased frequency of thetropical cyclones passing through the SCS and Philippine Sea during the second epoch. These provide
major triggers for the earlier monsoon onset in 1994–
2008.

6. Summary and discussions
In this study, we found a significant and abrupt interdecadal change in the onset of the South China Sea summer monsoon (SCSSM) onset around 1993/94 (Figs. 1–3).
The climatological mean onset date during 1979–93 appears

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FIG. 10. The tropical cyclone GPI in May during (a) 1979–93 and (b) 1994–2008. (c) The difference between (a) and(b) (1994–2008 minus 1979–93).

around 30 May, whereas that during 1994–2008 occurs
around 14 May. This 2-week shift of the SCSSM onset
date is consistent with the decreased monthly mean
OLR in May, although negative (but less significant)
OLR anomalies are also seen in April (Fig. 11, upper
panels). The mean SCSSM onset processes between thetwo epochs, 1979–93 and 1994–2008, are also different.
The relatively late onset during 1979–93 is primarily
associated with the northward seasonal march of the
ITCZ, whereas the advanced onset during 1994–2008 isaffected by enhanced northwestward-moving tropicaldisturbances from the equatorial western Pacific. In otherwords, the most prominent difference in the SCSSM
onset process is the direction of active convection coming
into the SCS.
The possible factors for the interdecadal change of the
SCSSM onset were also identified. During 1994–2008, theISV activity over the western Pacific is enhanced during
15 April and 15 May. This enhancement of the ISV activity connected with the fact that the arrival of the MJO
over the SCS becomes earlier during 1994–2008. The
number of tropical cyclones, which passed through the
SCS and Philippine Sea during 15 April and 15 May in
1994–2008, is about doubled compared with those that
occurred during 1979–93. Therefore, the enhancementsof the ISV activity and tropical cyclone genesis after
1994 are major triggers for the advanced SCSSM onset.
The enhanced ISV activity and tropical cyclone genesis
are possibly affected by the sea surface warming over thewestern Pacific. The SST changes over the western Pacificand along the southeast coast of China are quite significant
both in April and May, although significant interdecadalchange of the convection anomalies is found robustly over
the SCS and Philippine Sea only in May (Fig. 11).
The interdecadal change of the GPI is also consistent
with the sea surface warming over the warm pool region,
especially along the 288C line. Because the convective
activity with strong rainfall is normally enhanced over
the area with SST exceeding 288C, the sea surface warming over the warm pool region can increase the atmospheric instability and the potential for tropical cyclone
genesis. Meanwhile, the spatial pattern of the SST changes
from the 1980s to the 2000s is similar to the horseshoeshaped SST warming during La Niña events. In theory, thewarm anomaly may induce a low-pressure anomaly to thewest of the warming itself due to the excitation of ascending Rossby waves. This low-pressure anomaly connects to the low-level convergence, which possibly
enhances the two types of the ISV activities as well as
synoptic disturbances. In fact, Goh and Chan (2010)

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FIG. 11. (top) Epochal mean OLR difference in (left) April and (right) May (1994–2008 minus 1979–93; shading)and the epochal mean during 1979–93 (contour). Black dots are plotted over the area where the significance is above
a 95% confidence level. (bottom) As in (top), but for SST.

pointed out that the SST anomalies in a La Niña event
can make a favorable condition for the tropical cyclones
moving into the SCS. The low-pressure anomaly also
produces the effects of the weakening of the WNP
subtropical high. Thus, the interdecadal change of the
SST warming can advance the SCSSM onset.
The interdecadal change of the SCSSM onset corresponds to the seasonal evolution change in the earlysummer season—May and June (Fig. 1 and Fig. 2). The
convective activity over the SCS is suppressed in June
during 1994–2008, while that over the southern part ofChina is enhanced in June (figure not shown). Several
previous studies noted the increase of the boreal summer (June–August) mean rainfall over southern China
from the 1980s to the 2000s (Ding et al. 2008; Wang et al.
2009; Li et al. 2010). It is suggested that the interdecadalchange of the SCSSM in June, which is induced by its
onset change, can affect boreal summer mean rainfall
variation. Wang et al. (2009) also showed westerly wind
anomalies during boreal spring (March–May) as an interdecadal change after 1993 (their Fig. 10). This may
also be affected by the large anomalies in May due to theearlier SCSSM onset. Thus, the interdecadal changeof the SCSSM onset can impact the boreal spring andsummer mean field over the western North Pacific monsoon and the East Asian monsoon.
We found an interdecadal change of the SCSSM onset
timing around 1993/94 and suggested the possible factors in this study. However, several interesting questions
still remain, such as the detailed processes of the impactof tropical cyclone activity on the monsoon onset. The
performance of the state-of-the-art model to simulate
this interdecadal change is also of great interest. A deeper
understanding of the mechanism of this interdecadalchange calls for further studies.
Acknowledgments. The authors are grateful to Dr.
Renguang Wu, the editor of the Journal of Climate, andanonymous reviewers for their useful comments. Special
thanks are due to Dr. Kazuyoshi Kikuchi in IPRC fortechnical help with the GPI dataset and lots of usefulcomments. The authors also would like to thank Prof.
Tetsuzo Yasunari in HyARC, Nagoya University for
helpful comments. The authors acknowledge support
from the International Pacific Research Center. This
work is supported by the Global Environment Research
Fund of the Ministry of the Environment, Japan (A0902).
The second author acknowledges support from National

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Science Foundation Award AGS-1005599 and APEC
Climate Center through an international project (Climate
Prediction and its Application to Society).
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