Clim Dyn
DOI 10.1007/s00382-016-3247-8

Decadal change in the boreal summer intraseasonal oscillation
Tsuyoshi Yamaura1 · Yoshiyuki Kajikawa1

Received: 9 November 2015 / Accepted: 17 June 2016
© The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract A decadal change in activity of the boreal summer intraseasonal oscillation (BSISO) was identified ata broad scale. The change was more prominent during
August–October in the boreal summer. The BSISO activity during 1999–2008 (P2) was significantly greater thanthat during 1984–1998 (P1). Compared to P1, convection in the BSISO was enhanced and the phase speed ofnorthward-propagating convection was reduced in P2.
Under background conditions, warm sea surface temperature (SST) anomalies in P2 were apparent over the tropicalIndian Ocean and the western tropical Pacific. The former
supplied favorable conditions for the active convection of
the BSISO, whereas the latter led to a strengthened Walker
circulation through enhanced convection. This induced
descending anomalies over the tropical Indian Ocean.
Thermal convection tends to be suppressed by descending
anomalies, whereas once an active BSISO signal enters
the Indian Ocean, convection is enhanced through convective instability by positive SST anomalies. After P2, the
BSISO activity was weakened during 2009–2014 (P3).
Compared to P2, convective activity in the BSISO tended
to be inactive over the southern tropical Indian Ocean in
P3. The phase speed of the northward-propagating convection was accelerated. Under background conditions during
P3, warmer SST anomalies over the maritime continent
enhance convection, which strengthened the local Hadleycirculation between the western tropical Pacific and thesouthern tropical Indian Ocean. Hence, the convection in* Tsuyoshi Yamaura
tyamaura@riken.jp
1

RIKEN Advanced Institute for Computational Science,
Computational Climate Science Research Team,
Kobe 650‑0047, Hyogo, Japanthe BSISO over the southern tropical Indian Ocean was
suppressed. The decadal change in BSISO activity correlates with the variability in seasonal mean SST over thetropical Asian monsoon region, which suggests that it ispossible to predict the decadal change.

1 Introduction
Asian summer monsoon variability is directly linked towater resources and natural disasters due to the rainfall
amount and heavy rainfall events. Hence, many studies haveexamined its physical mechanism as well as the climatological dynamics (e.g., Charney and Shukla 1981; Murakami
and Matsumoto 1994; Wang and LinHo 2002). Interannual variability is one of the most important features of the
Asian summer monsoon. Because the El Niño/Southern
Oscillation (ENSO) has a large impact on the interannual
variability of the Asian summer monsoon, ENSO–monsoon
relationships have been considered in many studies (e.g.,
Kawamura 1998; Wang et al. 2001; Yamaura and Tomita2014). Variability at a timescale longer than interannual variability has become a major issue for studies of the global
climate due to the increase in the available observational
datasets and improvements in the global circulation model.
Thus, the climate change events around 1976 in the North
Pacific (e.g., Nitta and Yamada 1989; Trenberth and Hurrell 1994) and around 1990 (e.g., Watanabe and Nitta 1999)
were detected and investigated. The decadal variability of
the Asian summer monsoon has also been studied in recent
years, e.g., in the East Asian summer monsoon (Tomita
et al. 2007; Zhou et al. 2009) and the western north Pacificmonsoon (Kajikawa and Wang 2012; Tomita et al. 2013).
The Asian summer monsoon largely consists of theconvective activity associated with the intraseasonal

13

T. Yamaura, Y. Kajikawa

oscillation (ISO). The boreal summer intraseasonal oscillation (BSISO), which is represented as the Madden Julian
Oscillation (MJO; Madden and Julian 1994) in summer, shows an active–break cycle of convective activity
with northward propagation for a 30–60 day period in thetropical Asian monsoon region (e.g., Ramamurthy 1969;
Murakami 1972; Hartmann and Michelsen 1989; Wangand Xie 1997; Jiang et al. 2004). Because this active–breakcycle directly corresponds to the rainfall over the Asianmonsoon region, many researchers have discussed thephysical mechanism of the active–break cycle (Wang and
Xie 1997; Stephens et al. 2004; Wang et al. 2005).
The relationship between BSISO activity and interannual variability in the Asian summer monsoon has been
examined in terms of its predictability. Sperber et al. (2000)
indicated that the dominant mode in the BSISO activity andinterannual variability of the Asian summer monsoon have
similar spatial features. The BSISO activity is correlatedwith the interannual variability of the South Asian monsoon
strength (Goswami and AjayaMohan 2001). These results
suggest that BSISO activity influences the interannual variability of the Asian summer monsoon. The contribution ofinternal variability primarily originating from the BSISO
activity, to external forcing, such as the component related
to the ENSO, is important for predictability of the interannual variability of the Asian summer monsoon. AjayaMohan and Goswami (2003) demonstrated that internal variability largely contributes to the interannual variability of
the Asian summer monsoon. On the other hand, limitationsin predictability of BSISO activity have been investigatedby many researchers (Goswami and Xavier 2003; Websterand Hoyos 2004). The predictability of the BSISO activity
may be linked to improvements in interannual variability in
the Asian summer monsoon.
In contrast to interannual variability, the long-term variability of broad-scale BSISO activity has rarely been considered due to data limitations. A better understanding of
the decadal variability controlled by external forcing could
improve the predictability of BSISO activity. Recently,
Kajikawa et al. (2009) reported that BSISO activity over
the South China Sea demonstrates decadal change before
and after 1994. Sabeerali et al. (2014) reported that tropical Indian Ocean warming in the 2000s can enhance South
Asian monsoon activity and reduce the phase speed of thenorthward propagation of convection. They used subseasonal variability of precipitation in the South Asian monsoon region. However, BSISO activity appeared at a broad
scale across the Asian summer monsoon region from thetropical Indian Ocean to the western Pacific. It remains to
be determined if the spatiotemporal structure of the BSISO
activity in the entire Asian monsoon region demonstrates
a decadal change. Kikuchi et al. (2012) considered thebimodal ISO index to be useful for displaying the phase13

and amplitude of the BSISO and MJO activity, respectively. This index could be used to investigate the long-termchange in BSISO activity on a broad scale. Because BSISO
activity has seasonal dependency (Kemball-Cook and Wang
2001), whether the decadal variability in the BSISO activity shows seasonal dependency is an interesting question.
Therefore, this study focused on the following three issues:(1) Does BSISO activity show decadal change on a broad
scale? (2) If decadal change exists, is there any seasonality?
(3) What are the differences in spatiotemporal structure of
the BSISO decadal change? In this study, we focused on
the BSISO with a periodicity of 25–90 days because thisISO has a large effect on the Asian summer monsoon thanthe biweekly mode has (Yasunari 1981; Hartmann andMichelsen 1989).
The rest of this paper is organized as follows. Section 2
describes the data and methodology used in this work.
We demonstrate the existence of long-term variability in
BSISO activity in Sect. 3. The characteristics and mechanism of the long-term variability in BSISO activity and its
possible source are discussed in Sects. 4 and 5. Finally, asummary of this work and further discussions are given in
Sect. 6.

2 Data and methodology
This study used the following three datasets: (1) dailymean outgoing longwave radiation (OLR) data compiledby the National Oceanic and Atmospheric Administration (NOAA) (Liebmann and Smith 1996); (2) 6-hourly
and monthly mean data for atmospheric parameters of theEuropean Centre of Medium-Range Weather Forecasts(ECMWF) Interim Re-Analysis (ERA-Interim; Dee et al.
2011); and (3) daily and monthly mean optimum interpolated sea surface temperature (SST) data compiled by
NOAA (Reynolds and Smith 1994). We used an interpolated OLR, which represents the period from 1979 to 2013,
and a non-interpolated OLR, which represents the period
from 2002 to 2015. The non-interpolated OLR includes
undefined values where the OLR was not observed. Weinterpolated the undefined value according to the methodof Liebmann and Smith (1996) for applying the bandpass
filter. The root mean square error between non-interpolated and interpolated OLR data during the overwrapped
period of 2002–2013 was 3.52 W m−2. The mean OLR
was 216.97 W m−2 in the original interpolated OLR data
and 216.90 W m−2 in the processed non-interpolated OLR
data. The error was about 1.6 % of the mean OLR. We concluded that the processed non-interpolated OLR data were
useful for the extension of the OLR data period. We used
the interpolated OLR data from 1979 to 2013 and the processed non-interpolated OLR data during 2014–2015.

Decadal change in the boreal summer intraseasonal oscillation

The Lanczos bandpass filter (Duchon 1979) with cut-off
periods of 25 and 90 days and 141 weights was applied tothe daily data to extract the ISO signal. To obtain the amplitude and phase of BSISO, we used the daily BSISO index,
which is one of bimodal ISO indexes (Kikuchi et al. 2012).1
This index is based on the extended empirical orthogonal
function (EEOF) analysis results of bandpass-filtered OLR
data during the boreal summer with −10 and −5 day lags.
The amplitude and phase of the BSISO are described using
a combination of the first two principal components for
EEOF. Therefore, the amplitude of the BSISO index corresponds to the strength of BSISO activity. Normalized
amplitude of more than 1 was regarded as a significant
BSISO event. Using the OLR data, two types of daily
BSISO index were prepared: a historical daily BSISO
index, which was computed from the interpolated OLR
data during 1979–2013 and a real-time daily BSISO index,
which was evaluated from the non-interpolated OLR data
during 2002–2015. The average amplitudes of the two daily
indexes during 2002–2013 were both 1.01 W m−2, and theroot mean square error of the real-time BSISO index was
0.011 W m−2. Both the amplitude and phase errors wereless than 1.5 %. We merged the historical daily BSISO
index from 1979 to 2013 and the real-time daily BSISO
index from 2014 to 2015.
To investigate the meridional and vertical structure of
the BSISO activity, we used a composite analysis. A composite was constructed, with the first day of phase 1 of the
BSISO index displayed at day 0. Composite data were subtracted from the long-term mean and applied to the above
bandpass filter before the composite analysis.

3 Decadal change in BSISO activity
The climatological BSISO is active from May to October (Kikuchi et al. 2012). Figure 1 shows the year-to-year
variation in BSISO activity averaged from May to October (MJJASO). The red line denotes the composition of
the first three harmonics, with a Fourier decomposition of
the normalized BSISO index, which indicates the decadal
component. The decadal component of the BSISO index
was large and positive during 1999–2008, but was relatively small in 1984–1998. It is interesting to note that the
BSISO activity weakened in 2009–2014. Here, we define
the BSISO weak period (1984–1998) as P1, the enhanced
period (1999–2008) as P2, and the period after P2 (2009–2014) as P3. The differences in the averaged BSISO
activity between P1 and P2 and between P2 and P3 weresignificant at the 1 % level (Table 1). This implies that a1

http://iprc.soest.hawaii.edu/users/kazuyosh/Bimodal_ISO.html.

Fig. 1  Time series of the normalized BSISO index averaged from
May to October (black line), with the decadal component of the normalized BSISO index (red line with closed circle). The ordinate indicates the strength of BSISO activity (no unit), and the abscissa is theyear

climatological shift in BSISO activity occurred around1999 and 2009. Hence, the BSISO activity had significant
decadal variability. In addition, the decadal variability of
BSISO activity by using the Real-time Multivariate MJO
index (Wheeler and Hendon 2004) was not clear. This suggests that the decadal change of the BSISO has meridional
asymmetricity.
Because the climatological BSISO has a different spatial structure in early and late summer (Kemball-Cook andWang 2001), we examined the epochal difference in BSISO
activity on a monthly mean basis (Fig. 2). The decadalchange in the BSISO activity showed seasonal dependence.
The amplitude of the BSISO index in P2 (P1 and P3) was
larger (smaller) than the all-year average in all of the boreal
summer months. The amplitude of the BSISO index during P2 (red line) exceeded 1 from May to October. During
P1 (green line), it exceeded 1 from June to July, but wasless than 1 in other months. By contrast, the BSISO activity
in P3 (blue line) exceeded 1 only in August. Table 1 summarizes differences in the monthly and seasonally averaged BSISO activity between P1–P2 and between P2–P3.
The BSISO activity between P1 and P2 was significantly
different in May, August–October, and on average during
August–October (ASO) and MJJASO. The decadal changeof BSISO activity was insignificant on average during
May–July (MJJ). The decadal change in the BSISO activity
from P1 to P2 displayed seasonal dependency. The BSISO
activity between P2 and P3 was significantly different inall of the boreal summer months, except June and as a seasonal average (MJJ, ASO, and MJJASO). To investigate thecause of the decadal change in BSISO activity, we therefore focused on ASO for the changes from P1 to P2, and on
July–October (JASO) for the change from P2 to P3 in thefollowing sections.

13

T. Yamaura, Y. Kajikawa
Table 1  The probability valueof the monthly (from May
to October), MJJ, ASO, and
MJJASO averaged BSISO
index following a Welch’s t-test
during P1 (1984–1998) and P2(1999–2008), and in P2 and P3(2009–2014)

May

Jun

Jul

Aug

Sep

Oct

MJJ

ASO

MJJASO

P2–P1

0.02

0.66

0.06

0.00

0.01

0.02

0.07

0.00

0.00

P3–P2

0.01

0.07

0.01

0.05

0.03

0.04

0.00

0.00

0.00

The bold figures indicate significance at the 5 % level

Fig. 2  The temporally averaged BSISO index in each month. The
black line displays the annual average (1979–2015). The green, red,and blue lines indicate the period in 1984–1998 (P1), the period in
1999–2008 (P2), and the period in 2009–2014 (P3), respectively. The
ordinate expresses the strength of the BSISO activity (no unit), andthe abscissa is the month

4 The intensified BSISO activity from 1984
to 1998 and from 1999 to 2008
First, we investigated the differences in the spatial patternof decadal change of BSISO activity. Figure 3 shows theratio of bandpass-filtered OLR amplitude to the total daily
variance for ASO during P1 and P2. This corresponds tothe strength of BSISO activity. A common feature in P1
and P2 was the large amplitude of OLR, which accounted
for about 40 % of the total daily variance in the tropicalIndian Ocean and around the Philippines. This spatial pattern is consistent with previous studies (e.g., Kemball-Cookand Wang 2001; Kajikawa and Yasunari 2005). Enhanced
BSISO activity in P2 was apparent over the equatorial
Indian Ocean and the South China Sea (Fig. 3c), which isconsistent with Sabeerali et al. (2014). In contrast, the ratio
was relatively small in the Philippine Sea.
The typical convective activity related to the BSISO
gradually moved northeastward from the tropical Indian
Ocean to the western Pacific (e.g., Jiang et al. 2004;
Drbohlav and Wang 2005; Kikuchi et al. 2012). The
northward propagation of convective activity tended tobe emphasized in late summer (Kemball-Cook and Wang
2001). To highlight the decadal change in the temporal
structure of convective activity, we produced a Hovmöller

13

Fig. 3  The ratio of bandpass-filtered OLR amplitude to the total variance (%) during ASO in a P1 and b P2. c The ratio of P2 to P1 (%).
Only the areas with a standard deviation of daily OLR more than
30 W m−2 are shown. The scales of shading are shown on the bottomof each panel

diagram of composite anomalies of OLR and SST during
P1 and P2, respectively (Fig. 4). A common feature in P1
and P2 was the enhanced convection that started over theDecadal change in the boreal summer intraseasonal oscillation
Table 2  The day displaying the minimum composite OLR at theequator and 10°N (70°E and 130°E) in P1 during ASO, P2 during
ASO, P2 during JASO, and P3 during JASO
P1 (ASO) P2 (ASO) P2 (JASO) P3 (JASO)
Northward
Minimum OLR
at 0N

5

7

7

7

Minimum OLR at
10N

14

18

17

13

Phase Speed
Eastward
Minimum OLR at
70E

1.11

0.91

1.00

1.67

6

11

10

9

Minimum OLR at
130E

22

29

24

23

Phase Speed

0.63

0.56

0.71

0.71

Day 0 is the start of a BSISO event. The phase speed was estimatedbetween the equator and 10°N (70°E and 130°E)

Fig. 4  Hovmöller diagram of composite OLR (shading; W m−2) andSST (contour; K) averaged between 70° and 90°E in a P1, b P2, andc the differences (P2–P1) during ASO. The contour interval is 0.04 K.
Negative values are indicated by dashed contours. The scales of shading are shown at the right of each panel. The ordinate is latitude, andthe abscissa is lead-lag days

equator from day 0 and propagated the convection anomalies northward and southward. At the same time, negative
SST anomalies appeared near the equator. Before day 0,
suppressed convection anomalies and positive SST anomalies were apparent in the tropical Indian Ocean. Positive
SST anomalies reinforced by suppressed convection enableproduction of the next active convection (Wang et al. 2005).
Air–sea interactions can enhance the convection related to
BSISO activity. Compared with P1 (Fig. 4a), the amplitudeof the OLR and SST anomalies was large in P2 (Fig. 4b).
The phase speed of the northward propagation of convection decreased from P1 to P2 (Table 2). The temporal structure of eastward propagation of the BSISO was also investigated, but its phase speed difference between P1 and P2
was not statistically significant (Table 2).

Figure 5 shows the average composite vertical structureof moisture and atmospheric circulation when a convection
center appeared between the equator and 10°N (P1: days
5–14, P2: days 7–18; Table 2). The composite anomaliesof ascending motion, moisture convergence in the lower
troposphere, and vertically consistent positive vorticity
appeared near the region with active convection between
the equator and 10°N as common features in P1 and P2(Fig. 5a–d). These characteristics were identified as active
convection related to BSISO activity, which was consistent with previous studies (Jiang et al. 2004; Sabeerali et al.
2014). Compared with P1, these composite anomalies were
further emphasized in P2 (Fig. 5e, f). This implies the convection anomalies were strengthened in P2, which is linked
to the enhanced BSISO activity.
We then investigated the possible mechanism of the decadal change in BSISO activity from P1 to P2. The longterm variability can be connected to the changes in ocean
conditions because of its large heat capacity. To diagnose
the oceanic and atmospheric conditions associated with thedecadal change of BSISO activity, we demonstrated theepochal differences in OLR, SST, vertical wind shear composed of 200 hPa zonal winds subtracted from 850 hPa,moisture at 1000 hPa, and the vertically (1000–300 hPa)
integrated moisture flux during ASO between P1 and P2(Fig. 6). Negative OLR anomalies were elongated along
20°N in the region with the Asian summer monsoon andnorthwest–southeast direction from the western equatorial
Pacific to the South Pacific convergence zone (Fig. 6a).
Positive OLR anomalies were apparent at both the east andwest sides of the negative OLR anomalies. With the convective anomalies, easterly moisture flux anomalies in thelower troposphere were apparent to the east of the negative13

T. Yamaura, Y. Kajikawa

Fig. 5  Latitude-height cross section averaged between 70° and 90°E
in a–b P1, c–d P2, and e–f the differences (P2–P1) during ASO. The
left column displays the vertical p-velocity (shading; 10−2 Pa s−1) andspecific humidity (contour; g kg−1), while the right column shows
the relative vorticity (shading; 10−5 s−1), horizontal divergence (con-tour; 10−5 s−1), and vectors from meridional wind (m s−1) and vertical p-velocity (10−2 Pa s−1). The contour intervals are 0.1 g kg−1 inthe left column and 5 × 10−7 s−1 in the right column. The verticalp-velocity was multiplied by −1. The scales of shading and vectors
are displayed at the bottom of each panel

OLR anomalies along the equator (Fig. 6c). These differences imply that the enhanced Walker circulation induced
descending anomalies over and around the maritime continent and eastern tropical Indian Ocean. The vertical wind
shear was strengthened over the western tropical Indian
Ocean and central tropical Pacific (Fig. 6b), which corresponded to the enhanced convection anomalies over thewestern Pacific. Positive SST anomalies and relatively
weak negative OLR anomalies were located over the equatorial Indian Ocean (Fig. 6a, b). Because the convective
activity with the BSISO was initiated from the equatorial

Indian Ocean (Kemball-Cook and Wang 2001; Wang et al.
2005), the positive SST anomalies can strengthen the convective activity associated with the BSISO through anenhanced moisture convergence in the lower troposphere(Sabeerali et al. 2014). Positive (negative) SST anomalies
appeared in the western (central) tropical Pacific (Fig. 6b),which corresponded to the convection anomalies over thewestern and central Pacific, and resembled the negative pattern of the Interdecadal Pacific Oscillation (IPO). The IPO
index often has a negative sign after 2000 (England et al.
2014). The differences in moisture at 1000 hPa significantly

13Decadal change in the boreal summer intraseasonal oscillation

Fig. 7  Longitude-height cross-section averaged between 10°S
and 10°N for the differences (P2–P1) of vectors from zonal wind(m s−1) and vertical p-velocity (10−2 Pa s−1). The contour interval is
0.5 m s−1. Shading indicates downward motion. The vertical p-velocity was multiplied by −1. The scales of the vectors are displayed at
the bottom of panel

Fig. 8  Same as Fig. 6b, but averaged from May to JulyFig. 6  a Differences (P2–P1) in the seasonal average in ASO for
OLR (shading and contour; W m−2), b SST (shading; K) and verticalwind shear composed of zonal wind at 200 hPa subtracted from thatat 850 hPa (contour; m s−1), and c water vapor at 1000 hPa (shading;g kg−1) and vertically integrated (1000–300 hPa) water vapor flux
(vector; kg m−1 s−1). The dotted grid indicates that shading is significant at the 5 % level. The scale of shading (vector) is on the right(bottom) of panel

decreased around the eastern tropical Indian Ocean andwestern tropical Pacific. This decreased moisture anomaly
weakened the north–south gradient near the equator, reducing the phase speed of northward propagation associatedwith the BSISO activity (Jiang et al. 2004).
Figure 7 displays the vertical structure of the differences in zonal wind and vertical p-velocity near the equator between P1 and P2. In the equatorial Indian Ocean, thewesterly (easterly) anomalies in the lower (upper) troposphere enhanced the vertical wind shear. The slowdown of
the northward propagation of convection seems to be limited (Table 2), because the enhanced vertical wind shear
can affect the phase speed far from the equator (Jiang et al.

2004). Ascending anomalies appeared at 150°E and compensatory descending anomalies were present on both sidesof the ascending anomalies. This implies that the strengthened Walker circulation can suppress convective activity
in the tropical Indian Ocean. In contrast, the positive SST
anomalies in the tropical Indian Ocean increased the convective instability (Fig. 6b). Thermal convection tends to be
suppressed by descending anomalies, whereas once a disturbance occurs with large-scale convergences, such as anarrival of the active BSISO signal, convection is enhanced
using the convective instability. Therefore, the descending anomalies and positive SST anomalies can amplify theactive and break cycle of BSISO in P2.
To determine why the enhanced BSISO activity was limited during ASO in P2 (Table 1), we displayed the epochal
differences in SST and vertical wind shear during MJJ
between P1 and P2 (Fig. 8). Positive (negative) SST anomalies were apparent in the western (central) tropical Pacific.
A strengthened Walker circulation was apparent during MJJ
and ASO (figure not shown), with SST anomalies over the
Pacific. On the other hand, the positive SST anomalies were13

T. Yamaura, Y. Kajikawa

relatively small in the tropical Indian Ocean compared to the
ASO condition. The relatively weak warming in the tropical Indian Ocean implies that active convection associatedwith the BSISO was small during MJJ. Thus, the seasonal
dependency of the decadal variability may be caused by thelimited tropical Indian Ocean warming in late summer.

5 Abrupt weakening of the BSISO in 2009–2014
BSISO activity rapidly weakened from P2 to P3 (Fig. 1;
Table 1). Figure 9 shows the ratio of bandpass-filtered OLR
to the total daily variance during JASO in P2 and P3. The

Fig. 9  Same as Fig. 3, but for P2 and P3 averaged from July to October

13

OLR pattern in JASO was almost the same as that in ASO(Figs. 3a, 9a). In P3, BSISO activity broadly decreased in
the tropical Indian Ocean and the western tropical Pacific(Fig. 9b). The ratio of P3 to P2 also clearly indicated theweakened BSISO activity in P3 (Fig. 9c). In particular,
BSISO activity weakened more over the southern tropicalIndian Ocean than over the northern part.
To examine the decadal change in the temporal patternof convective activity between P2 and P3, we produceda Hovmöller diagram of composite OLR and SST during
JASO (Fig. 10). The northward propagation of the anomalies of convection and SST was clear during P2, whereas
the amplitude of these anomalies was suppressed in P3.
In particular, the center of the SST anomalies shiftedslightly northward, which implied that the north–south
asymmetric convection anomalies were excited. Concurrently, the southward propagation of convective activity from the equator was unclear. Furthermore in P3,
this asymmetric structure was emphasized more in theFig. 10  Same as Fig. 4, but for P2 and P3 averaged from July to
OctoberDecadal change in the boreal summer intraseasonal oscillation

active condition of BSISO events than in the break condition. The anomalies of convection and SST appeared in
the South Indian Ocean around day −14, whereas theseanomalies were insignificant after day 0. This suggeststhat convective activity related to the BSISO activity was
suppressed over the southern tropical Indian Ocean in theactive state of the BSISO. The phase speed of northward
propagation of convective activity was accelerated in P3(Table 2). In contrast, the eastward propagating speed of
the BSISO was not significantly different between P2 andP3 (Table 2).
Figure 11 shows the average composite vertical structureof moisture and atmospheric circulation when the convection center appeared between the equator and 10°N (P2:days 7–17, P3: days 7–13; Table 2). Compared with P2,
the composite anomalies of ascending motion and moisture in the lower troposphere were slightly larger between
the equator and 10°N during P3 (Fig. 11a, c). On the other
hand, the ascending anomalies disappeared over the region
south of the equator, indicating the northward concentrated
vertical structure. Composite anomalies of atmospheric circulation over the southern tropical Indian Ocean were moresignificant than over the northern part. These characteristics
were consistent with the spatial pattern of the weakened
BSISO (Fig. 9c). The essential difference between P2 andP3 was the structural change of BSISO activity represented
as the weakened activity over the southern tropical Indian
Ocean.

Fig. 11  Same as Fig. 5, but for P2 and P3 averaged from July to October13

T. Yamaura, Y. Kajikawa

To determine what causes the northward shifting shownin the composite anomalies, we investigated the epochal
differences in OLR, SST, vertical wind shear composedof 200 hPa zonal winds subtracted from 850 hPa, moisture
at 1000 hPa, and the vertically (1000–300 hPa) integrated
moisture flux during JASO between P2 and P3 (Fig. 12).
Negative OLR anomalies appeared over the Arabian Sea
and to the north and south of the equator around the maritime continent (Fig. 12a), while positive OLR anomalies
were apparent over the South Indian Ocean and the central
tropical Pacific. Compared with the differences between
P1 and P2, OLR anomalies extended more northward in
the western tropical Pacific. With regard to the convective anomalies, ascending (descending) anomalies wereassumed to be east of the Indian subcontinent and aroundthe maritime continent (from the equator to the South
Indian Ocean), which implied that the local Hadley circulation was strengthened by the convection anomalies.

Fig. 13  Latitude-height cross-section averaged between 70° and
90°E for the differences (P3–P2) of vectors from meridional wind(m s−1) and vertical p-velocity (10−2 Pa s−1). The contour interval is
0.5 m s−1. Shading indicates downward motion. The vertical p-velocity was multiplied by −1. The scales of vectors are displayed at thebottom of panel

Suppressed BSISO activity during P3 may not be linked to
the SST anomalies over the tropical Indian Ocean, because
there was little change in SST over the region from P2 toP3 (Fig. 12b). Positive SST anomalies were apparent tothe south of 20°S in the South Indian Ocean and aroundthe maritime continent. The differences in moisture in thelower troposphere significantly increased in the tropicalIndian Ocean and western tropical Pacific (Fig. 12c). The
north–south gradient of these moisture anomalies strengthened near the equator, inducing acceleration of the phasespeed of northward propagation related to the BSISO activity (Jiang et al. 2004).
Figure 13 displays the vertical structure of the differences in meridional wind and vertical p-velocity in
the Indian Ocean from P2 to P3. Ascending anomalies
appeared to the north of the equator and compensatory
descending anomalies were apparent from the equator to
the South Indian Ocean, which suggests an enhanced local
Hadley circulation. The ascending anomalies were causedby the strengthened convective activity from the Arabian
Sea to the maritime continent (Fig. 12a), which is rooted
for the positive SST anomalies in the Indian Ocean andaround the maritime continent (Fig. 12b). The enhanced
local Hadley circulation can suppress convective activity
over the southern tropical Indian Ocean. Thus, it is thoughtthat the decadal change in BSISO activity between P2 andP3 was linked to the enhanced local Hadley circulation.

6 SummaryFig. 12  Same as Fig. 6, but for P2 and P3 averaged from July to
October13

We investigated the decadal change in the spatiotemporal
structure of the BSISO during 1979–2015. A two-decadalchange was found around 1998 and 2009, and was more

Decadal change in the boreal summer intraseasonal oscillation

Fig. 14  Schematic illustration of the environmental changes affecting the decadal change in BSISO activity a from P1 to P2 and b from
P2 to P3. The red (blue) shading indicates positive (negative) SST

anomalies. The green (blue) vector depicts zonal wind anomalies in
the lower (upper) troposphere. The purple vector indicates descending anomalies. The cloud icon indicates enhanced convection

prominent in late summer than in early summer. The decadal change was correlated with the difference in seasonalmean SST over the tropical Asian monsoon region during
1984–1998 (P1), 1999–2008 (P2), and 2009–2014 (P3).
The variability of seasonal mean SST may be linked to theimprovement of the decadal change in the BSISO activity.
Compared to P1, BSISO activity was significantly
large from August to October in P2. Convection in the
BSISO was strengthened in the tropical Indian Ocean and
the South China Sea. The phase speed of the northward
propagation of the convection was reduced. These decadalchanges in BSISO activity were consistent with the results
shown in Sabeerali et al. (2014). In contrast, the eastward
propagating speed of the convection was not significantly
different between P1 and P2. Under the background conditions in P2, positive SST anomalies over the western
tropical Pacific can encourage active convection (Fig. 14a).
The active convection reinforced the Walker circulationas ascending anomalies over the western tropical Pacificand compensated for the descending anomalies to the east
and west sides of the enhanced convection. The strengthened Walker circulation suppressed convective activity inthe tropical Indian Ocean. On the other hand, positive SST
anomalies in the tropical Indian Ocean favored to the active
convection in the BSISO. The background conditions of the
SST and Walker circulation contributed to a clearer contrast
between the active and break cycle of the BSISO, inducing
the decadal change between P1 and P2.
In contrast to P2, BSISO activity was suppressed during
P3. Convection in the BSISO tended to be inactive over thesouthern tropical Indian Ocean in P3, indicating northward
shifted convection anomalies. The structural changes canbe linked to the weakened BSISO activity. Additionally,
the phase speed of the northward propagation of convection
was accelerated, whereas the eastward propagating speed of
the convection displayed less significant differences from
P2 to P3. Under the background conditions in P3, positive SST anomalies around the maritime continent could
strengthen convective activity (Fig. 14b). This enhanced
convection intensified the local Hadley circulation between

the western tropical Pacific and the southern tropical Indian
Ocean. Hence, the convection in the BSISO over the southern tropical Indian Ocean was suppressed.
This study confirmed the existence of the decadal variability in BSISO activity and discussed the background conditions required to control decadal variability. However,some questions still remain. For example, why is the significant difference in SST over the tropical Indian Ocean
that affect the decadal change in BSISO activity limited inlate summer between P1 and P2? To answer this question,a detailed investigation of the variability, including internal variability of the Indian Ocean, may be needed. It isalso unclear how the background conditions quantitatively
contribute to the decadal variability of BSISO activity.
Because this study was limited in its analytical approach,sensitivity experiments are needed to verify the contribution of the background conditions to the decadal changes
in the BSISO activity in more detail, using an atmosphericmodel that is able to realistically reproduce the BSISO
activity.
Acknowledgments We thank Dr. Yoshida for fruitful discussions
to the physical mechanism of BSISO and its decadal change. Twoanonymous reviewers gave various constructive comments to further
improve the original manuscript. This work was supported by JSPS
KAKENHI Grant Number JP15K01177.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide alink to the Creative Commons license, and indicate if changes were
made.

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