GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L04710, doi:10.1029/2004GL021836, 2005

Interannual variability of the 10–25- and 30–60-day variation over theSouth China Sea during boreal summer
Yoshiyuki Kajikawa and Tetsuzo Yasunari
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan
Received 26 October 2004; revised 17 January 2005; accepted 31 January 2005; published 24 February 2005.

[1] The interannual variability (IAV) of the intraseasonalvariation (ISV) over the South China Sea (SCS) wasinvestigated, focusing on the relationship between the 10–25- and 30– 60-day variation. Both ISV activities over theSCS had larger IAV than those of the Bay of Bengal andArabian Sea. The 10– 25- (30 – 60-) day variation wasrelatively active when the 30– 60- (10 – 25-) day variation
was suppressed in June, July and September. The IAV of theISV during early summer (June – July) was related to theIAV of the SCS monsoon onset. A late (early) onset wasaccompanied by active 10– 25- (30 –60-) day variation. TheIAV of the ISV could be divided into early and late summer(September), because there was little persistence between
these two periods. Moreover, large-scale circulation
anomalies associated with the IAV had different spatial
structures in these two periods. Citation: Kajikawa, Y., andalso not clear whether there is a relationship between ISV
activity and seasonal or monthly mean anomalies of the IAV[Goswami and Mohan, 2001; Krishnamurthy and Shukla,2000]. The persistency and transitivity of the IAV throughout the boreal summer is also an important issue.
[4] This study investigated the IAV of the ISV over theSCS with a special focus on the relationship between the10– 25- and 30 –60-day variation, to answer these questions. Here, we used the daily interpolated outgoing longwave radiation (OLR) from the NOAA satellite [Liebmannand Smith, 1996] and wind data from the NCEP/NCAR
reanalysis project [Kalnay et al., 1996]. Anomalies weregenerated by removing the climatology computed for
1979– 2002. The band-pass filter used in this study was aLanczos filter [Duchon, 1979].

T. Yasunari (2005), Interannual variability of the 10 – 25- and 30 –60-day variation over the South China Sea during boreal summer,Geophys. Res. Lett., 32, L04710, doi:10.1029/2004GL021836.

2. Relationship Between the 10–25- and
30–60-Day Variation

1. Introduction[2] The South China Sea monsoon (SCSM) is an important component of the western North Pacific monsoon(WNPM) [Murakami and Matsumoto, 1994] and strongly
influences the interannual variability (IAV) in the WNPM
and surrounding regions. For instance, convection anomalies over the WNPM can affect the eastern Asian summer
weather condition via the Pacific-Japan teleconnection pattern [Nitta, 1987], while enhanced convection anomalies
over the South China Sea (SCS) can play an important rolein the onset processes of the Indian Ocean dipole mode
[Kajikawa et al., 2003]. Moreover, the SCSM also has large
intraseasonal variability (ISV) with two dominant timescales, one with a period of 30– 60 days and the other witha period of 10– 25 days [Lau et al., 1998; Annamalai andSlingo, 2001]. The 30– 60-day variation shows northward
propagation [Lau and Chan, 1988; Hsu and Weng, 2001],
while the 10– 25-day variation is characterized by westward
phase propagation [Chen and Chen, 1993; Chen and Weng,1999].
[3] The prominent periodicity of the ISV differs eachyear, for instance, the 30– 60-day variation was dominant in1998 [Chan et al., 2002; Zhu et al., 2003]. Some problems
with the IAV of the ISV remain. First, is the IAV of the ISVover the SCS actually significant compared with other
Asian monsoon regions? Is there a relationship between
the 10– 25- and 30– 60-day variation activity in IAV? It isCopyright 2005 by the American Geophysical Union.
0094-8276/05/2004GL021836$05.00[5] To clarify the IAV of the 10– 25- and 30 –60 day
variation, the distribution of the mean ISV activity and theinterannual standard deviation of the ISV activity during
boreal summer are presented for each period in Figure 1. In
advance, we defined the standard deviation of the eachband-pass filtered daily OLR data averaged over the SCS(110°– 120°E, 10°– 20°N) as the ISV activity index. Climatologically, the 10– 25-day variation is active over the Bayof Bengal (BOB), SCS and Philippine Sea (PS), while the30– 60-day variation is active over the Arabian Sea (AS),SCS and PS (Figures 1a and 1b). The IAV of the ISV activity
is remarkably strong over the SCS, PS and part of theFigure 1. The mean standard deviation of the (a) 10–25-day and (b) 30 –60-day filtered OLR (W/m2). The
standard deviation of the interannual variation of the (c) 10–25-day and (d) 30 –60-day variation activity.

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KAJIKAWA AND YASUNARI: INTERANNUAL VARIATION OF THE ISV OVER THE SCS

L04710Figure 2. (a) Time series of 5- (thin line) and 31- (thick line) day running mean OLR over the SCS and the sliding
correlation between 10– 25-day and 30– 60-day variation calculated using the 51-day time window (circle). The time seriesof each ISV activity calculated for (b) 61 days around June 27th, the maximum negative correlation day during earlysummer and for 41 days around September 14th, maximum negative correlation day during late summer.
southeastern Indian Ocean. In particular, prominent IAV ofboth ISV activities occurs only over the SCS (Figures 1cand 1d). Therefore, it is obvious that the SCS is an
important area for IAV of both the 10– 25– day and 30–60-day variation activity.
[6] Figure 2a shows the mean time series of the OLR andsliding correlations between the 10– 25- and 30– 60-day
variation activity calculated using a 51-day window, toinvestigate the temporal change in the correlation between
these two ISVs. Climatologically, the convection over theSCS abruptly becomes active in mid-May and the active
convection continues until late September with ISV. Nevertheless, there are significant negative correlations during
early (June – July) and late (September) summer. This
indicates that when one ISV is relatively active, the otheris suppressed during those two periods. In other words,
these two forms of ISV are rarely active or suppressedsimultaneously. The sliding correlations calculated withother windows (41-, 61-, and 71-day) gave the same result.
Figures 2b and 2c show the time series of each ISV activity
during early and late summer, centered on the day withmaximum negative correlation. The 10– 25-day variation
was relatively active in 1983, 1987, 1989, 1992, and 1993,
while the 30– 60-day variation was active in 1979, 1984,
1996, 1998, and 2001 during early summer. This is consistent with case studies conducted in 1979 [Chen andChen, 1995] and 1998 [Chan et al., 2002; Zhu et al., 2003].
[7] Figure 3 shows the time series of the auto-correlationof the 10– 25- and 30– 60-day variation activities from themaximum negative correlation day during early and latesummer in Figure 2. The seasonal persistence of the IAV ofthe ISV from early to mid summer was weak, showing
little correlation of the ISV activity between early and midsummer (August). The persistence from mid to late summer was also weak from little auto-correlations. Thissuggests that the IAV of the ISV over the SCS could be
divided into two periods across August. In following
section, the IAV during early summer is mainly focusedand investigated its relation to the SCSM onset and mean
anomaly field.

3. Relationship Between ISV Activity andSCSM Onset, Mean Anomaly Field[8] The SCSM onset implies the beginning of the monsoon over East Asia and the western Pacific [Wang et al.,2004]. To examine the relationship between ISV activity
and the onset date, the time series of ISV activity during
early summer and the SCSM onset day for the maximum
correlation day and time window are plotted in Figures 4aand 4b. The SCSM onset day was defined as the first day
when the OLR over the SCS fell below 220 (W/m2) for
more than five consecutive days. The 10– 25-day variation
activity shows a good positive correlation with the onset
date, except for 1985 and 1999 (r = 0.51). This indicatesthat a late (early) onset was accompanied by active (suppressed) 10– 25-day variation. In contrast, the 30– 60-day
variation activity was negatively correlated with the onsetday (r = 0.44). Thus, the IAV of the ISV activity during
early summer (June – July) was strongly correlated with theFigure 3. The time series of the auto-correlation of 10– 25and 30– 60-day variation activities from the center day ofearly summer peak (Jun. 27) and late summer peak (Sep. 14).

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Figure 4. Time series of the ISV activity and SCSM onset
date. (a) The 10– 25-day variation activity was calculatedfor 31 days around July 2nd. (b) The 30– 60-day variation
activity was calculated for 51 days around June 15th.
IAV of the SCSM onset with the opposite sign for 10– 25and 30– 60-day variation.
[9] Figures 5a and 5b show composite fields for earlysummer during 5 active yeas (refer to Section 2) for velocity
potential and divergent wind anomalies at 200-hPa, and
OLR and 850-hPa wind anomalies for the 10 – 25-day
variation, respectively. The active 10– 25-day variation isaccompanied by enhanced convection over the equatorialL04710eastern and middle Pacific and suppressed convection along
the band 10°– 20°N over the Asian monsoon region andmaritime continent. Easterly and northerly wind anomalies
at 850-hPa also prevail over the Asian monsoon region.
This indicates that the active 10– 25-day variation during
early summer is related to weakening of the large-scalemonsoon circulation and east-west circulation over thePacific. Meanwhile, the composite fields for the active
30 – 60-day variation years show different features fromthose for the 10– 25-day variation (Figures 5c and 5d). The
active 30– 60-day variation is accompanied by suppressed
convection over the equatorial western-middle Pacific andenhanced convection over the equatorial eastern-middle
Indian Ocean. Northeasterly divergent wind anomalies at200-hPa are also remarkable only over the maritime continent. Consequently, weakening of the local Hadley circulation between the maritime continent and western Pacificis caused by east-west asymmetric convection anomalies
between the Indian and Pacific oceans. This suggests thatboth the 10– 25- and 30– 60-day variation activities during
early summer are associated with the large-scale anomalies
within the Indian and Pacific oceans.

4. Summary and Discussion[10] Both of the 10– 25- and 30– 60-day variation overthe SCS had large interannual variation compared withthose over the BOB and AS. The IAV of the 10 –25- (30 –60-) day variation was relatively active while 30– 60- (10 –25-) day variation was suppressed during early (June – July)and late (September) summer. The IAV of the ISV during
early summer was associated with the onset date of theSCSM. The 10– 25- (30 – 60-) day variation prevailed with alater (earlier) onset than usual. These ISV activities wereFigure 5. Composite diagram of (a) the 200-hPa velocity potential and divergent wind (the counter interval is 5  105 s 1and the units are 2 m/s) and (b) the OLR and 850-hPa wind (the counter interval is 5 W/m2 and the units are 3 m/s) for theyears with active 10– 25-day variation (1983, 1987, 1989, 1992, 1993) during northern early summer (June and July).
(c) and (d) are the same as (a) and (b), but for the years with the 30– 60-day variation (1979, 1984, 1996, 1998, 2001).
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L04710associated with a large-scale anomaly field during earlysummer. The active 10 – 25-day variation was related toenhanced convection over the equatorial middle Pacific.
The 10– 25-day variation with westward propagation originated over the equatorial west-mid Pacific [Annamalai andSlingo, 2001]. This implies that the active 10– 25-day variation is initiated by relatively enhanced convection of theearly summer mean field over the middle Pacific. In contrast,
the active 30– 60-day variation is related to enhanced convection over the equatorial eastern Indian Ocean. This agrees
with the observation that the warmer SST and enhanced
convection over the eastern Indian Ocean produce thepronounced activity of the 20– 60-day mode (H. Annamalai
and B. Wang, Interannual variation of the boreal summer
intra-seasonal variability: Role of the low-frequency circulations, submitted to Journal of Climate, 2004).
[11] The IAV of the ISV over the SCS could be divided
into the early and late summer across August as suggested
in section 2. The anomalous filed for the IAV of the ISV
during early and late summer showed different temporal andspatial pattern, respectively. The composite anomaly fields
for active ISV years during late summer showed conspicuous east-west asymmetric convection anomalies over theIndian Ocean and few significant signals over the Pacific(figure not shown). This implies that the IAV during latesummer is decoupled from anomalies over the Pacific. The
mechanism that separates the IAV in August remains
unclear. However, LinHo and Wang [2002] showed that
the Asian monsoon has two fast cycles, which are divided
by an abrupt change event: the first from mid-May to early
July, and the second from late July to early September.
Ueda et al. [1995] also described an abrupt northward shiftof convection over the WNP in late July. Therefore, we
speculate that a seasonal cycle within a drastic change could
play an important role in decreasing the anomalies of ISV
activity during early summer.
[12] This paper investigated the IAV of the ISV over theSCS, and found the relationships between 10– 25- and 30–
60-day variation, and the SCSM onset date and mean largescale anomalies. The processes causing the alternation in thetwo ISVs and their relation to the SCSM onset are aninteresting issue that will be examined in the future. Theseasonality of these relationships is also an important
subject. To solve this, it will be necessary to investigate
not only the activity but also phase of each ISV.

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[13] Acknowledgment. This study was supported in part by a Grantin-Aid for Scientific Research on Priority Area (A)(1) from the Japanese
Ministry of Education, Culture, Sports, Science, and Technology(No. 14204044).

Y. Kajikawa and T. Yasunari, Hydrospheric Atmospheric ResearchCenter, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8601, Japan.
(kajio@hyarc.nagoya-u.ac.jp)

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