SOLA, 2014, Vol. 10, 15−18, doi:10.2151/sola.2014-004

15Impact of Boreal Summer Intraseasonal Oscillation on Environmentof Tropical Cyclone Genesis over the Western North Pacific
Ryuji Yoshida1, Yoshiyuki Kajikawa1, and Hirohiko Ishikawa2
RIKEN, Advanced Institute for Computational Science, Kobe, Japan
2
Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan
1

Abstract
The impact of intraseasonal oscillation on tropical cyclone
genesis (TCG) over the western North Pacific during boreal
summer was examined in association with categorized synoptic-scale low-level flow patterns. Five synoptic-scale flow patterns
were considered as synoptic-scale environment connecting thelarge-scale intraseasonal oscillation and meso-scale TCG: a shear
line, a confluence region, an easterly wave, a monsoon gyre, anda pre-existing tropical cyclone. The phase and amplitude of theboreal summer intraseasonal oscillation (BSISO) were used. It isfound that the phase 7 was favorable for TCG associated with theshear line, and the phases 7 and 8 were also favorable for TCG
associated with the confluence region. During the favorable phases
for each flow pattern associated with TCG, the BSISO affected
the large-scale zonal wind distribution, and the large-scale zonal
wind forms synoptic-scale flow patterns over the suitable environment for TCG. The link between the intraseasonal oscillation andsynoptic-scale flow patterns is an important factor in TCG.
(Citation: Yoshida, R., Y. Kajikawa, and H. Ishikawa, 2014:
Impact of boreal summer intraseasonal oscillation on environmentof tropical cyclone genesis over the Western North Pacific. SOLA,
10, 15−18, doi:10.2151/sola.2014-004).

1. Introduction
To understand the physical mechanisms governing tropical
cyclone genesis (TCG) is one of the most challenging issues intropical meteorology. Meso-scale organization process of TCG
and its environmental conditions are still open questions. TCG israther complicated spanning over multiple scales in both space
and time. Meso-scale organization processes of TCG are identified
in some typical synoptic-scale conditions, and these conditions
are organized in a large-scale environment (Fudeyasu et al. 2010).
Equatorial waves also have significant role on TCG. They enhance
convective activity and low-level vorticity, and meso-scale disturbance in the waves sometimes develops into tropical cyclone (TC)
(Dickinson and Molinari 2002; Aiyyer and Molinari 2003). Frankand Roundy (2006) discussed the importance of the way thatanomalous waves affect the low-level vorticity. It is worthwhile
to discuss organization of low-level vorticity from the view of
low-level flow pattern. In TCG cases over the western North Pacific, synoptic-scale processes are categorized into five characteristic
patterns, based on the low-level flow (Ritchie and Holland 1999).
These are the shear line (SL), the confluence region (CR), theeasterly wave (EW), the monsoon gyre (GY), and the pre-existing
TC (PTC). Some flow patterns supply vorticity to meso-scale
processes of TCG, and others assist the TCG processes. Yoshida
and Ishikawa (2013) introduced an objective method to categorize
the five patterns, and applied it to all 908 TCG cases from 1979
to 2008. They found that the SL most frequently associates withTCG (42%), followed by CR and EW (16% and 18%, respectively). Mechanisms controlling this proportion are still an open
question, and also it is important to know the background largeCorresponding author: Ryuji Yoshida, RIKEN/Advanced Institute for
Computational Science, 7-1-26 Minatojima-minami-machi, Chuo-ku,
Kobe, Hyogo 650-0047, Japan. E-mail: ryoshida@riken.jp. ©2014, theMeteorological Society of Japan.

scale environment controlling the synoptic-scale flow processes.
Westerly winds in the Asian monsoon trough or waves in the
trade winds are considered as the typical large-scale environment
affecting TCG over the western North Pacific (Gray 1968, 1998).
The Madden–Julian Oscillation (MJO; Madden and Julian 1994)
is reported as one of a typical intraseasonal oscillation in largescale environments for TCG (Nakazawa 1988; Liebmann et al.
1994; Zhu et al. 2003). Camargo et al. (2009) evaluated thepotential for TCG in the lifecycle of the MJO. They found that themost favorable phase of the MJO for TCG cases over the western
Pacific was 6 or 7 in the Wheeler and Hendon diagram (Wheeler
and Hendon 2004). Camargo et al. (2009) suggested that variations in relative humidity and vorticity significantly contribute to
the creation of a TCG environment. In previous studies, however,
it is not explicitly shown how the MJO modifies the TCG environment.
Ritchie and Holland (1999) described the roles of the monsoon
trough and trade winds in the formation process of the synopticscale flow patterns. However the intraseasonal oscillation was notincluded in their scope. Although Camargo et al. (2009) found
the favorable phases of the MJO for TCG, they did not link theirresults with synoptic-scale flow pattern. The most favorable
phase would be different among the different synoptic-scale flow
patterns. One example can be seen if we compare TCG on 29
September, 2007 and 14 October, 1983 (Fig. 1). In this figure, theboreal summer intraseasonal oscillation (BSISO) index was used
to identify lifecycle of the intraseasonal oscillation. The TCG
location of these cases was quite similar (around 120°E, 17°N),
and both occurred near the boundary between westerly and easterly winds. However, the BSISO phase and synoptic-scale flow
pattern were different at the time of genesis; the former TCG was
associated with SL, while the latter was associated with CR. Thisdifference of favorable phase among synoptic-scale flow patterns
should be clarified for better understanding of TCG mechanism.
Therefore, in this study we investigate the impact of the BSISO
on the synoptic-scale flow patterns and discuss the possible mechanism in relations between large-scale and synoptic-scale flows.

2. Data and Analysis Methodology
The genesis of each TC was defined using best-track datafrom the Joint Typhoon Warning Center. In total, 738 TCG cases
are recorded during the months of May–November between 1979
and 2008. We also used the daily mean outgoing longwave radiation (OLR) data (Liebmann and Smith 1996) and the six-hourly
wind data from the Japanese 25-year reanalysis (JRA-25)/Japan
Meteorological Agency Climate Data Assimilation System (Onogi
et al. 2007) for the period 1979−2008. A Lanczos band-pass filter(Duchon 1979) with cut-off periods of 25 and 90 days and 139
weights was applied to the OLR and wind data to extract the intraseasonal variability. Because the majority of TCG occurs during
May–November in the western North Pacific, we calculated an
index of the BSISO following Kikuchi et al. (2012) to highlightthe dominant mode of the intraseasonal oscillation during theboreal summer. The lifecycle of the BSISO mode is constructedon the basis of a phase space consisting of the first two principal
components of extended empirical orthogonal function (EEOF)
analysis. The lifecycle of the BSISO is divided into eight phases.
The convective anomaly associated with the BSISO propagateseastwards over the equatorial Indian Ocean and then northwards

16

Yoshida et al., Impact of BSISO on Environment of Tropical Cyclone Genesis

Fig. 1. TCG cases for the a) shear line and b) confluence regions and the active MJO. Blue dots (yellowdots) show shear line (confluence) detected by objective analysis. The wind fields (vectors) and zonal windspeeds (shading) at 850 hPa three days before TCG
are shown. The green contours are OLR anomalies[W m−2] from a 30-year average. The red stars denotethe locations of TCG. The bottom-left diagram in eachpanel is the daily trace of the phase diagram of the
BSISO index, and the red dot is the time of TCG.

to the Philippine Sea. The positive convective anomaly is locatedover the western Pacific in phases 6 and 7, when the BSISO may
have maximum influence on TCG in that region. The definition of
the synoptic-scale flow pattern categories for TCG was the sameas in Yoshida and Ishikawa (2013).

3. Results and discussion
Figure 2 shows the frequency of TCG cases in different phasesof the BSISO index. The frequency is separately counted for theactive and inactive BSISO, where active (inactive) is defined as
the BSISO amplitude greater (less) than 1.0 at the genesis time. In
each figure, TCG frequency is shown by different bar for active
and inactive BSISO. TCG_TOTAL (Fig. 2a) denotes sum of all
TCG cases, and the total number of TCG is 738. TCG_TOTAL isclassified into five categories associated with synoptic-scale flow
patterns defined by Yoshida and Ishikawa (2013) (Fig. 2b−f). The
frequencies of TCG cases associated with SL (hereafter, TCG_
SL), TCG_CR, TCG_EW, TCG_GY, and TCG_PTC are 318, 142,
133, 51, and 94, respectively. The red line and blue line in eachpanel show the threshold to judge significance of the value. The
values of thresholds are defined as one standard deviation for each
amplitude group and each flow pattern of TCG. This straightforward judgment is adopted instead of more rigorous statistical tests,
since sample number is too small to conduct a reliable statistical
test.
In Fig. 2a, the frequency of TCG cases in the active BSISO

exceeds the threshold in phase 7, thus the most favorable phaseof the TCG_TOTAL is judged as phase 7. Phases 5 and 6 are alsofavorable in the active BSISO. On the other hand, the frequency
in the inactive BSISO does not have apparent favorable phases.
This phase preference is consistent with the findings in Camargo
et al. (2009). Of great interest is that the favorable phase for the
TCG cases associated with each flow patterns deviates from thatof TCG_TOTAL. The TCG_SL has frequency distribution similar
to TCG_TOTAL and it favors phase 7 when the BSISO is active.
The similarity of frequency distribution over different phases is
attributed to the fact that the TCG_SL accounts for more than
40% of the TCG_TOTAL cases. TCG_CR is the most favorable tooccur in phase 7 and 8 with the active BSISO. The frequency of
TCG_CR is considerably high even when the BSISO is inactive.
Among them some cases are identified as occurred at the leadingedge of westerly induced by pre-existing TC. TCG_EW cases
distribute more broadly over different phases than TCG_SL and
TCG_CR, and exhibits relatively high frequency in phases 3 to
5 in the inactive BSISO. High probability for TCG_GY is foundin phases 5 and 6 in the active BSISO. The TCG_PTC occurs inphases 7 and 8 in the inactive BSISO. A mature TC sometimes
organizes a wave train to the southeast of itself through a mechanism of Rossby wave energy dispersion (Li and Fu 2006; Li et al.
2006). TCG_PTC takes place in such a wave train organized bya mature TC. Because TCG_TOTAL (i.e., candidates of a matureTC) has high probabilities in phases 7 (Fig. 2a), TCG_PTC has ahigh probability in the same or the following phases.
Now, we reversely discuss as that which TCG pattern isFig. 2. Numbers of TCG cases during May to November from 1979 to 2008, which are divided into two groups (i.e. the active BSISO and the inactive
BSISO). The SL, CR, EW, GY and PTC are for specific synoptic-scale flow patterns and the TOTAL is the sum. Two lines denote thresholds to judge alarge number or not. The values of thresholds are defined as one standard deviation of each distribution for amplitude groups and flow patterns. Note thatthe range of vertical axis is different in each panel.

SOLA, 2014, Vol. 10, 15−18, doi:10.2151/sola.2014-004
favored by a specific BSISO phase. Discussion is given only for
the TCG pattern which frequency is greater than the threshold, redline in Fig. 2. Figure 3 shows climatological distribution of largescale zonal wind and the average location of TCG in each phases.
The superimposed contours show OLR anomaly from ‘climatological’ mean. To obtain ‘climatological’ field, the zonal wind and
OLR-anomaly are averaged over May to November of 1979 to2008 for only days with the active BSISO, and shown in therectangle area covering the major TC genesis area, 5°N–20°N and
100°E–180°E. The climatological TCG locations are also obtainedby averaging TCG locations in the active BSISO over the same
period. In each panel, the star indicates the average locationof TCG_TOTAL. The averaged TCG positions for individual
synoptic-scale flow pattern are computed in the same way and areshown by characters, S(SL), C(CR), E(EW), G(GY) and P(PTC).
The bracket denotes that the TCG pattern has not significantly
high frequency in the phase.
During phases 1 to 4 the center of the BSISO is in otherlongitudes, so that the convection is inactive over the western
North Pacific as seen in positive OLR anomaly in Fig. 3. In phase
5 the center of the BSISO is in Maritime Continents, and the OLR
anomaly turns to negative in the equatorial area. In phases 6 to 8,
it is seen that the center of convective activity shift to the north.
Thus, the climatological fields reflect the feature of the BSISO.
The large-scale westerly (easterly) wind is dominant around thestar in phase 6 to 8 (in phase 3 to 5). This feature is consistentFig. 3. 850 hPa zonal wind field averaged in the active BSISO during
May–November in 1979−2008 for each phase. Contours show averaged
anomaly of OLR anomaly [W m−2] in the active BSISO from climatological mean. Stars indicate average locations of TCG_TOTAL in the phasewith the active BSISO. Characters indicate average locations of TCG withthe flow pattern; S: shear line, C: confluence region, E: easterly wave,
G: monsoon gyre, and P: pre-existing TC. Blankets denote insignificant
occurrence frequency. The zonal wind is filtered with 25−90 days bandpass filter before averaging.

17

with the typical wind distribution associated with the BSISO
discussed in Kemball-Cook and Wang (2001) and Kikuchi et al.
(2012).
In phase 1 to 4 when the convection is relatively inactive over
the TCG region, the averaged TCG locations are scattered in theeastern portion of the western North Pacific. In phase 5 when the
BSISO migrates in between the eastern Indian Ocean and maritime continent from the west, convection becomes rather active.
Westerly wind appears over the equatorial western Pacific, andcyclonic horizontal wind shear is enhanced between westerly wind
and trade wind. Thus, the favorable environment appears over
the western portion of the displayed area where the convectionis active and the low-level vorticity is enhanced. Reflecting this,the average location moves to the west comparing with previous
phases. The average location shifts to the north of the locationin phase 5 during phase 6 to 8 following the shift of active convection and westerly wind associated with the BSISO. The highfrequency of TCG in phase 5 to 8 shown in Fig. 2 seems to beconsistent with active convection and distribution of large-scale
flow in phase 5 to 8.
We discuss the distribution of large-scale flow from a viewof TCG_SL, TCG_CR, and TCG_EW, which have larger sampleof cases. In phase 7, large-scale westerly wind is climatologicallystrong over the west of the star, and likely to form synoptic-scale
horizontal shear and confluence flow of zonal wind around thestar. It seems that this distribution of zonal wind promotes theoccurrence of TCG_SL and TCG_CR, and in fact, the S and C areplotted closely to the star in phase 7. As well as phase 7, largescale westerly wind in phase 8 is strong over the area to the westof the star, and synoptic-scale confluence flow seems to be formed
there. Thus, the C is plotted near the star. On the other hand,large-scale easterly winds are dominant climatologically aroundthe star in phase 3 to 5. The large-scale environment which dominant easterly wind is not suitable for TCG_SL and TCG_CR, thus
these patterns do not have high frequency in phase 3 to 5 as shownin Fig. 2b, c. In phase 5, which is the favorable phase for TCG_
EW, easterly wind is remarkable in the northeast of the star, andthe E is plotted over the area where easterly wind is remarkable.
In phase 3 or 4, easterly wind is climatologically dominant overthe equatorial western Pacific. However, convection is not strong
enough to TCG in these phases. In phase 5, easterly wind is stillprevailing and convection becomes active, and thus TCG_EW
takes place. In the following phases after 5, convection is active
enough but easterly wind shifts northwards.
It is revealed that large-scale flow tends to form the favorable
background environment for the each synoptic-scale flow patterns
corresponding with the favorable phases seen in Fig. 2. In order
to see the impact of the BSISO clearer, we investigated whether
large-scale flow associating with the active BSISO enhances synoptic-scale flow pattern or not. We discuss only for the TCG_SL
and TCG_CR, which have larger frequencies than the other patterns. Figure 4 shows composited fields of zonal wind centering
each locations of TCG_SL and TCG_CR dividing with activenessof the BSISO. Comparing TCG_SL in the active BSISO (Fig. 4a)and in the inactive BSISO (Fig. 4b), westerly wind to the southwest of the TCG location (a center of the figure) is stronger in theactive BSISO than that in the inactive BSISO. It suggests that astronger shear line is organized near the TCG location in the active
BSISO. This is consistent with significant westerly winds in phase
7 with the active BSISO shown in Fig. 3. The westerly winds inthe inactive BSISO would be controlled by the Asian-monsoon
activity. Similar characteristic is found for TCG_CR by comparing Figs. 4c and 4d. Distributions of sea level pressure in TCG_
CR show that TCG_CR occurs at the eastern edge of the tropicaltrough. Westerly wind to the west of the TCG location is stronger
in the active BSISO than in the inactive BSISO. Stronger westerly
wind in the western side of the TCG location can intensify a zonal
gradient of zonal wind, and it contributes to the TCG_CR.
Synoptic-scale low-level flow patterns in this study would
correspond with the equatorial waves. Easterly wave correspondswith Tropical Depression (TD)-type disturbance. Dickinson andMolinari (2002) pointed out that many TD-type disturbance were18

Yoshida et al., Impact of BSISO on Environment of Tropical Cyclone Genesis

Acknowledgements
The authors are grateful to the editor of SOLA and anonymous
reviewers for their useful and critical comments, and also thankDr. Yoshiaki Miyamoto, Dr. Tsuyoshi Yamaura, and Dr. Hirofumi
Tomita for valuable discussions.

References

Fig. 4. Composited zonal wind field centering each TCG location; a)
TCG_SL in phase 7 of the active BSISO, b) TCG_SL in phase 7 of theinactive one, c) TCG_CR in phase 8 of the active one, and d) TCG_CR inphase 8 of the inactive one. Color shade shows the zonal wind at 850-hPa
height, and contours shows sea level pressure [hPa]. Meteorological fields
are 69 hours before each genesis time (averaged data in 66 and 72 hours
before, but not filtered with 25−90 days bandpass filter). The objective
TCG cases are from May to November, 1979 to 2008.

found in a zonal wind confluence, thus CR is also associated with
TD-type disturbance. Mixed Rossby-gravity (MRG) wave transits
to TD-type disturbance in an environment of active convection
and zonal wind confluence, and then the TD-type disturbancedevelops into TC (Dickinson and Molinari 2002; Aiyyer and Molinari 2003). If we assume accordingly that TCG_CR and TCG_
EW are associated with TD-type disturbance, they explain 275
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during the active BSISO. Though equatorial waves closely relatewith TCG, it is not clear how much portion of TCG is triggered byequatorial waves. Frank and Roundy (2006) described that equatorial waves tend to organize large-scale cyclonic flow over thetropics, which eventually develops to TC. Since, the large-scale
cyclonic flow is associated with organization of various types of
low-level flow patterns, further study is necessary for addressing
the relationship between low-level flow and various equatorial
waves.

4. Concluding Remarks
We have described the impact of the BSISO, which is thedominant mode in the intraseasonal oscillation during boreal
summer, on the synoptic-scale flow pattern of the TCG environment. It is found that the most favorable phases of the BSISO
are different in each synoptic-scale flow pattern. In the BSISO,
northward propagation of convection and associated wind field isdominant, in particular westerly winds shift northward after phase
5. Since the shear line and confluence region are organized at thenorthern and eastern edges of the westerly region respectively,
the BSISO affects the position of the westerly winds and hence
affects the TCG locations for TCG_SL and TCG_CR. The BSISO
modulates the arrangement of westerlies such that the shear
line coincides with active convections in phase 7. On the other
hand, the confluence region does so in phase 7 and 8. For TCG_
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Although the variety of synoptic-scale features in TCG is nottreated in the investigation into the impact of the intraseasonal
oscillation on TCG in prior studies, it is revealed by the current
study that the favorable phases of the BSISO for TCG differ
among synoptic-scale flow patterns. The manifestation of a linkage between the intraseasonal oscillation and the synoptic-scale
flow pattern of TCG provides a better understanding and arrangement of the complicated processes of TCG.

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Manuscript received 22 November 2013, accepted 16 January 2014
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