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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L06810, doi:10.1029/2009GL037174, 2009

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ArticleDecadal change in intraseasonal variability over the South China Sea
Yoshiyuki Kajikawa,1 Tetsuzo Yasunari,2 and Bin Wang1,3
Received 5 January 2008; revised 20 February 2009; accepted 24 February 2009; published 27 March 2009.

[1] Evidence is presented to reveal a decadal change aroundmid-1990s in the behavior of intraseasonal variability (ISV)over the South China Sea (SCS). During 1979 –1993, the ISV
has a spectral peak around 64 days, which is longer thanin the recent epoch of 1994 – 2007 (around 42 days). The
ISV event in 1979 – 1993 involves a merging process ofthe northward and westward propagating convection
anomalies over the western North Pacific. The ISV in
1994 – 2007 has no such a merging process but exhibits atilted band structure extending from the northern Indian
Ocean to the SCS, which is strongly connected to theequatorial eastward propagating Madden-Julian Oscillation.
The merging process during 1979– 1993 modified the ISVover the SCS, resulting in the prolonged period, enhanced
convective activity and a weakened relationship with theeastward propagating MJO. The possible cause of this change
is discussed. Citation: Kajikawa, Y., T. Yasunari, and B. Wang(2009), Decadal change in intraseasonal variability over the South
China Sea, Geophys. Res. Lett., 36, L06810, doi:10.1029/
2009GL037174.

1. Introduction[2] The South China Sea summer monsoon (SCSSM) is
an important component of the Asian summer monsoon.
After the discovery of the eastward propagating MaddenJulian Oscillation (MJO) [Madden and Julian, 1972], theactive/break cycle of the Asian summer monsoon have beenrecognized as a results of northward propagation of the intraseasonal convection system [Yasunari, 1980, 1981], which
is sometimes but not always linked to the MJO [Wang andRui, 1990]. The SCSSM has strongest intraseasonal variability (ISV) over the entire Asian summer monsoon regions[Kemball-Cook and Wang, 2001]. Furthermore, the ISV over
the South China Sea (SCS) has relatively large interannual
variability compared with those over the Arabian Sea and the
Bay of Bengal [Kajikawa and Yasunari, 2005]. Better understand the behavior of the SCSSM system is important as it
can affect the weather and climate over the East Asia [Nitta,
1987], Indo-China, the western North Pacific and even the
Indian Ocean and maritime continent [Kajikawa et al., 2003].
[3] Previous studies have described the mean temporal
evolution of the ISVover the Indian Ocean and western North
Pacific. Some differences were found in the composite spatial
structure of the ISV. For instance, using OLR data for the
1

International Pacific Research Center, University of Hawai’i at Manoa,
Honolulu, Hawaii, USA.
2
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japan.
3
Department of Meteorology, University of Hawai’i at Manoa,
Honolulu, Hawaii, USA.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL037174$05.00

period 1974 – 1997, Goswami and Mohan [2001] showedthat the ISV based on convection anomalies over the Bay of
Bengal has a zonally elongated structure of convection
anomalies across the northern Indian Ocean and western
North Pacific. On the other hand, using OLR data for theperiod 1979 – 1993, Hsu and Weng [2001] showed a quadrapole structure of convection anomalies, which are zonally outof phase between the northern Indian Ocean and the western
North Pacific. This discrepancy could be due to their data
covering different periods but could also arise from the nonstationary behavior of the ISV.
[4] Kwon et al. [2005] found a significant decadal change
in the relationship between the East Asian monsoon and thewestern North Pacific monsoon before and after (inclusive)
1994. Kwon et al. [2007] also discussed the climate shift ofsummertime circulation over the East Asia and suggested
that the increasing the number of typhoon in the western
North Pacific may have impacted the precipitation increaseover the southern China since the mid-1990s. This decadal
shift also induced the decadal change of the SCSSM [Wanget al., 2009] and potentially the ISV over the SCS.
[5] Therefore, this study investigates the possible longterm change of the ISV properties over the SCS with a
special focus on periodicity and spatial-temporal evolution
during the 29 years from 1979 to 2007. The data used herein
include daily interpolated outgoing longwave radiation (OLR)
from the NOAA satellite [Liebmann and Smith, 1996] and thewind data from the NCEP/NCAR reanalysis project [Kalnay
et al., 1996].

2. Decadal Change in Temporal Characteristics[6] First, we applied the Morlet wavelet analysis to original daily OLR data averaged over the northern SCS (110°E–
120°E, 10°N–20°N) for highlighting the interannual andinterdecadal features of the convective activity. Figure 1
shows the wavelet power that is significant above 95%
confidence level for red noise. On the ISV (30-70-day) timescale, the wavelet power is remarkable during boreal summerof 1979, 1980, 1984, 1985, 1988, 1994, 1996, 1998, 2000,
2001, 2002, 2005, 2006 and 2007. It is of interest to note thatthe wavelet power for active ISV in 1980s shows a significant
peak that extends to beyond 60 days; in contrast, the significant wavelet peak after 1990 occurs on a time scale
shorter than 60 days. This calls the decadal change of the
ISV between 1988 and 1994. Considering the climate shift
over the East Asia and western North Pacific in 1993/1994 by
Kwon et al. [2005, 2007], it is suggested that this periodicity
difference in the ISVover the SCS is possibly associated withthe decadal change in mid-1990s. Therefore, we decide tolook into changes of the ISV behavior before and after(inclusive) 1994.
[7] In order to verify the decadal change of the ISV over
the SCS, we first applied the spectrum analysis to original

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KAJIKAWA ET AL.: DECADAL CHANGE IN ISV OVER THE SCS

L06810Figure 1. Wavelet power spectrum of daily OLR averaged over the SCS (110°E– 120°E, 10°N –20°N) (dashed contour)
and the ratio to the wavelet power of the 95% confidence level for red-noise processes (shading). Contour interval is 4.0 
103 (w2/m4).
daily OLR data over the SCS (110°E – 120°E, 10°N –20°N)
for May–September during 1979–2007. Then, we computed
ensemble mean spectrum power for the two epochs: 1979–

1993 and 1994– 2007. A significant spectral peak at 64 day
in 1979–1993 is found in Figure 2a (red line), while thesignificant peak at 42 day is seen in 1994–2007 (blue line).

Figure 2. (a) Ensemble power spectrum of daily OLR time series (May to September) averaged over the South China Sea(110°E – 120°E, 10°N –20°N) during 1979 – 1993 (red line) and 1994 –2007 (blue dashed line). (b) Same as Figure 2a but
for over the Arabian Sea (65°E–75°E, 10°N – 20°N). Red Noise spectrum and its 95% confidence level are also shown as
dotted line.
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Figure 3

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KAJIKAWA ET AL.: DECADAL CHANGE IN ISV OVER THE SCS

L06810Figure 4. Time-longitude diagram of lag-regression of band-pass filtered anomalies of OLR (Wm 2) along 20°N (shaded)and 850 hPa relative vorticity along 25°N (contour) in (a) 1979 – 1993 and (b) 1994 – 2007. All anomalies are plotted in thesame manner of Figure 3.
This period difference (about 20 days) is statistically significant between the two epochs. The spectrum difference
around 42days is also significant by the Welch’s two-sample
t-test. In contrast, the spectrum power of OLR over the
Arabian Sea (110°E – 120°E, 10°N – 20°N; Figure 2b) forMay – September does not show significant difference
between 1979 – 1993 and 1994 – 2007. The periodicity of
the ISV over the SCS in 1994 – 2007 is very close to thatover the Arabian Sea in 1994 – 2007, whereas different in
1979 – 1993. The ensemble mean spectrum over the Bayof Bengal also shows no significant difference between
1979 – 1993 and 1994 – 2007 (figure not shown). Thus, it
is suggested that the decadal change of the ISV in terms of
periodicity occurs only over the SCS and this also implies
a change in the spatial structure of the ISV between thetwo epochs, especially the phase relationship between the
SCS and the Arabian Sea/Bay of Bengal.

3. Epochal Differences in the Spatial
and Temporal Evolution[8] Based on the results of the wavelet and spectrum
analysis, the band-pass filtering of the data was performed
for the entire period by using the 30 –80 day Lanczos filter[Duchon, 1979] with 121 weights. In order to identify thedifference in the spatial-temporal evolution of the ISV between the two epochs, we calculated the lagged regression
coefficient between the filtered OLR time series over the
SCS (110°E– 120°E, 10°N–20°N) and the OLR and 850 hPa
relative vorticity anomalies over the entire domain of interest

during boreal summer (June –September). Since the periodicity of the ISV is different in the two epochs, we identify
eight phases as one cycle of the ISV with the day of maximum
convection over the SCS as 0 day in the two epochs respectively. The interval between each phase is 8 days for theepoch 1979–1993 and 5 days for the epoch 1994– 2007.
[9] Figure 3 shows the temporal evolution of the ISV for
the two epochs from 1/2 cycle ( 32 days for the 1979–
1993 and 20 days for the 1994– 2007) to phase lag 0. The
eastward propagating convection along the equator to 10°S
and the northward propagating convection over the Arabian
Sea and the western North Pacific are seen in both epochs.
Also, a positive vorticity anomaly is always located to thenorthwest of the negative OLR anomaly. In spite of thesecommon features, distinctive ISV evolutions are seen during
the two epochs.
[10] During the epoch 1979 – 1993, a prominent process
is the merging the two convection anomalies over the SCS
and Philippine Sea. One convection anomaly moves northward from the maritime continent from day 16 to day 8(Figures 3c – 3d), which comes originally from the eastward
moving convection anomaly over the equatorial Indian
Ocean (Figures 3a – 3b). The other is a westward moving
convection anomaly over the western North Pacific. Thisis seen in the northwestern Pacific (150°E, 25°N) in
Figures 3a – 3b and moves westward accompanied bysignificant strong cyclonic circulation in transition phase(Figures 3c – 3d). It is suggested that the origin of theseconvection and vorticity anomalies come from the vicinityof the dateline by considering the opposite sign of day 16

Figure 3. Lag-regression of band-pass filtered anomalies of OLR (Wm 2; shaded) and relative vorticity at 850 hPa level(contour) upon the filtered OLR index over the SCS during boreal summer (JJAS) in 1979– 1993 from (a) a lag of 1/2 cycleto (e) a day with maximum convection over the SCS with 1/8 cycle interval. Figures 3f – 3j are the same as Figures 3a – 3ebut for 1994 – 2007. The period of 1/8 cycle is 8 (5) days for the epoch 1979– 1993 (1994 –2007). The contour interval is
10 7 [s 1], and the dashed contour is negative. All anomalies are opposite in sign to original for setting maximum
convection peak over the SCS in Figure 3e day 0. The regression coefficients have been multiplied by one standard
deviation of band-pass filtered OLR anomalies over the SCS, and only those that are significant at the 95% level byStudent’s t-test are plotted.
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and day 8 (170°E, 25°N) in Figures 3d – 3e. These twoconvection anomalies merge over the SCS and Philippine
Sea, and the convection is abruptly strengthened after themerge at day 0 (Figure 3e). Another interesting feature is
the out-of-phase convection anomalies between the northern Indian Ocean and SCS. This is also associated with the‘quadrapole’ pattern of OLR over the Indian Ocean and thewestern Pacific in the transition phase day 16, (1/4 phaselag). In addition, the convection anomaly over the Bay of
Bengal is not associated with the convection over the SCS
during the epoch 1979 – 1993.
[11] On the other hand, the temporal evolution during theepoch 1994 – 2007 has no merging process but exhibits aclear west-northwest to east-southeast tilted ‘‘band’’ structure, which extends from the northern Indian Ocean to thewestern North Pacific. This tilted band structure includes
significant anomalies over the Bay of Bengal, where convective anomalies were absent in the epoch 1979 –1993.
This spatial structure also indicates that the convection
anomalies over the northern Indian Ocean and the western
North Pacific tend to have the same sign. The elongatedstructure is originated in the enhanced convection anomalies
over the southeastern Indian Ocean at day 20 (Figure 3f).
These anomalies are clearer and become stronger than thosein 1979– 1993. After eastward propagation along the equator, the elongated structure moves northward from day 10
to day 0 (Figures 3h– 3j).
[12] Figure 4 shows time-longitude diagram of lagregression of OLR along 20°N and relative vorticity at
850 hPa along 25°N to further reveal the horizontal
structure difference between the two epochs. The westward propagating convection and vorticity from 170°E isprominent only in the epoch 1979 – 1993. The merging of
this westward propagating convection with the northward
propagating convection from equatorial area (Figure 3)
over the SCS enhance convection and prolong the active
convection period in 1979 – 1993. The OLR anomalies
over the northern Bay of Bengal and the Arabian Sea are
not clear in 1979 – 1993. In contrast, coherent signal of
OLR anomalies over the Bay of Bengal and the ArabianSea, which is associated with northward propagation of
convection anomalies extending from west-northwest to
east-southeast, is clear during the epoch 1994 – 2007. Thiselongated structure also extends to 140°E after the convection maximum over the SCS.

4. Summary and Discussions[13] This study detected a decadal change of the ISV over
the SCS in mid-1990s, which are distinctive in the periodicity
and the temporal-spatial evolution. The significant differenceof the period between the epoch 1979 – 1993 (peak at 64 day)and the epoch 1994 – 2007 (peak at 42 day) is found inspectrum analysis. In contrast, the ISVover the Bay of Bengal
and Arabian Sea does not show such a decadal change inperiodicity. In terms of the mean temporal-spatial evolutionof the ISV with respect to the convection over the SCS, four
different features between the two epochs are found; (1) the
merging process over the Philippine Sea and SCS (2) thewest-northwest to east-southeast tilted ‘‘band’’ structure,(3) the phase relationship of convection anomalies between
the northern Indian Ocean and the western North Pacific, andL06810(4) the convection anomalies over the Bay of Bengal. The
merging process of the northward propagating convection
from the maritime continent and the westward propagating
convection along 20°N –25°N from the central North Pacificis evident only in 1979 –1993. The existence of this westward
propagation of OLR and vorticity anomalies is consistent
with the results by Hsu and Weng [2001]. They used OLR
data from 1979 to 1993 and pointed out that this merging
process may reflect an interaction between the tropics andsubtropics. The tilted band structure of OLR and relative
vorticity anomalies in the west-northwest to east-southeast
direction can be seen only during the epoch 1994 – 2007
(Figures 3f – 3j). This structure resembles closely to thecomposite structure based on eastern equatorial Indian
Ocean convective anomalies derived by Wang et al.
[2006], who used the TRMM data for the period of
1997 – 2005. Waliser et al. [2004] also showed similar
results by using the rainfall data from 1979 to 1999 andextended empirical orthogonal function analysis. The convection anomalies between the northern Indian Ocean andthe western North Pacific are in (out of) phase during theepoch 1979 – 1993 (1994 – 2007). The quadrapole pattern of
OLR anomaly in the transition phase during the epoch
1979 – 1993 (Figure 3c) is similar to that described byAnnamalai and Slingo [2001], who used OLR data from1979 to 1995. The convection anomalies over the Bay of
Bengal are strongly connected with that over the SCS as
part of an elongated spatial structure only in the epoch of
1994 – 2007.
[14] The ISV over the over the Arabian Sea and Bay of
Bengal has 40-day period during both two epochs (Figure 2),
and the periodicity of the ISV over the SCS is close to it
during 1994 – 2007. Furthermore, this period is close totypical eastward propagating global MJO signal. Therefore, the ISV over the SCS during the epoch of 1994 – 2007
has stronger connection with the MJO signal (e.g. velocity
potential at 200 hPa) than during 1979 – 1993 (figure notshown). Thus, it is suggested that the northward propagating convection system of the Asian monsoon must be
associated with the eastward propagating MJO along theequatorial Indian Ocean. However, the westward propagation over the western North Pacific was strongly connectedwith the northward propagation from the maritime continent during the epoch of 1979 – 1993. This merging processenhanced the cyclonic circulation over the SCS and enhanced southwesterly embedded in the cyclonic circulationtransported more moisture in to the SCS and Philippine Sea[Hsu and Weng, 2001]. This could create favorable condition of deep convection longer and inhibited northward
propagation from the equator. The ISVover the SCS during
1979 – 1993 was modified with a prolonged periodicity. In
other words, the merge of tropical convection with subtropical convection becomes weak or disappears in recent
decades. Since the periodicity of the ISV over the Arabian
Sea and Bay of Bengal does not change in the two epochs,
this merging process also weakens the relationship between
SCS and northern Indian Ocean in the ISV time scale.
[15] Although this study indicates possible decadalchange of the ISV over the SCS, it leaves several interestingquestions unanswered. More examinations are needed toaddress the following interesting issues: What is the detailed
process of westward propagating convection along 20°N–

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25°N? How is the quadrapole structure in 1979 – 1993
formed? And why is the merging process prominent in
1979 – 1993? The convection anomalies over the southeastern Indian Ocean and the Bay of Bengal might be an
important area for the decadal change of the ISV structure.
The SST anomalies over the mid-western Pacific could be
associated with the existence of the westward propagating
convection and worthwhile to research for addressing above
questions.
[16] Acknowledgments. The authors acknowledge support from theInternational Pacific Research Center (IPRC). Bin Wang acknowledges the
support by the Climate Dynamics Program of the National Science
Foundation and the NOAA OGP through CLIVAR Program. This research
was supported by the JAMSTEC, which sponsors research at the IPRC.
IPRC /SOEST publication 598/7659.

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T. Yasunari, Hydrospheric Atmospheric Research Center, Nagoya
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