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Effects of the South Asian Monsoon Intraseasonal Modes on
Genesis of Low Pressure Systems over Bangladesh
DAISUKE HATSUZUKAa
Graduate School of Environmental Studies, Nagoya University, Nagoya, JapanHATSUKI FUJINAMI
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan(Manuscript received 6 May 2016, in final form 9 January 2017)
ABSTRACTThe quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh.
Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This
study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis of LPSs over Bangladesh during 29 summer monsoon seasons. Daily lag compositesof convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case)and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward
propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in bothcases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case,which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an
opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equatorto the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances
the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of
cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results
suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS
genesis over Bangladesh.

1. Introduction
Rainfall and tropospheric atmospheric circulation both
exhibit pronounced intraseasonal oscillations (ISOs) in
the South/Southeast Asian region during the boreal
summer. These ISOs can be divided into two types basedon time scale (e.g., Yasunari 1979; Annamalai and Slingo2001): one with a period of 25–60 days and the otherwith a period of 10–20 days, which are referred to here as
the boreal summer intraseasonal oscillation (BSISO) and

Denotes content that is immediately available upon publication as open access.
aCurrent affiliation: Faculty of Environmental Earth Science,
Hokkaido University, Sapporo, Japan.

Corresponding author e-mail: Daisuke Hatsuzuka, hatsuzuka@
ees.hokudai.ac.jp

quasi-biweekly oscillation (QBW), respectively. These
two ISO modes have differing spatiotemporal structures.
The BSISO mode exhibits northward propagation of convection and circulation anomalies from the equatorial Indian
Ocean (e.g., Yasunari 1981; Murakami et al. 1984; KemballCook and Wang 2001; Lawrence and Webster 2002; Jiang
et al. 2004; Wang et al. 2006), whereas the QBW mode ischaracterized by westward propagation of convection andcirculation anomalies from the western Pacific (e.g.,
Krishnamurti and Ardanuy 1980; Chen and Chen 1993;
Yokoi and Satomura 2005; Hoyos and Webster 2007;
Kikuchi and Wang 2009; Fujinami et al. 2014).
The area around Bangladesh exhibits a distinct QBW
during summer monsoon rainfall (e.g., Ohsawa et al.
2000; Murata et al. 2008; Fujinami et al. 2011, 2014;
Sato 2013; Hatsuzuka et al. 2014). Fujinami et al. (2011)
Publisher’s Note: This article was revised on 22 March 2017 toinclude the open access designation that was missing when originallypublished.

DOI: 10.1175/JCLI-D-16-0360.1
Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright
Policy (www.ametsoc.org/PUBSReuseLicenses).

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FIG. 1. Geographical features in and around Bangladesh. The
boxed region (only over land) denotes the area used to calculate
a representative rainfall time series for Bangladesh.

found that the rainfall spectrum in Bangladesh has apronounced peak at about 14 days, while no statistically significant peak appears within the 30–60-day
period. They also reported that the interannual variability of the total summer monsoon rainfall overBangladesh is correlated significantly with the 7–25-day
rainfall variance, suggesting that the QBW controls
the interannual variability of the total seasonal rainfall
in the country. Recently, Hatsuzuka et al. (2014) foundthat a vortex-type low pressure system (LPS) is often
observed in the active phase of the QBW overBangladesh. The typical horizontal scale of these LPSs is;600 km, which is considerably smaller than the socalled Indian monsoon depressions (e.g., Krishnamurti
et al. 1975; Godbole 1977), and most of these systems
occur over and around Bangladesh. During the 29
summer monsoon seasons considered by Hatsuzuka
et al. (2014), it was revealed that about 60% of theextreme and moderate active peaks were related tosuch vortex-type LPSs and that small differences intheir central locations caused modulation of the amplitude of the active peaks. Furthermore, they reported
that heavy rainfall over the lowland area of Bangladesh
was more likely when the LPS was present (LPS case)than during its absence (non-LPS case), under extreme
active peaks. Thus, it is evident that LPS activity profoundly affects ISO activity and total summer rainfall
over Bangladesh. Contrastingly, in non-LPS cases,
rainfall is significantly enhanced around the mountainous areas such as the Meghalaya Plateau and

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Chittagong Hill Tracts (Fig. 1) because of the orographic lifting of moist air over the windward areas ofthe mountains. However, the processes that determine
the occurrence of LPS and non-LPS cases in the active
phase of the QBW over Bangladesh remain unclear.
Previous studies have revealed that the distinct QBW in
rainfall over Bangladesh is caused by the interaction between the low-level zonal wind fluctuation around theGanges Plain and the topographical features around
Bangladesh (Fujinami et al. 2011, 2014). In the active(break) phase of the QBW over Bangladesh, a low-level
westerly/southwesterly (easterly/southeasterly) dominatesalong the plain. The strong westerly/southwesterly flowenhances the orographic lifting of low-level moisture over
the windward area of the Meghalaya Plateau and Chittagong Hill Tracts during the active phase, which results in asignificant increase in rainfall over the country. Recently,
Fujinami et al. (2014) demonstrated that the QBW-scale
low-level zonal wind fluctuation is controlled by awestward-moving n 5 1 equatorial Rossby wave with awavelength of ;6000 km. However, their analyses werebased on all active peaks without consideration of LPS andnon-LPS cases. Hatsuzuka et al. (2014) suggested that the
LPS over Bangladesh might be triggered by a westwardpropagating anticyclonic anomaly in the QBW mode.
However, this conjecture was based on the analysis of asingle LPS event. Thus, a more rigorous statistical analysis
is necessary to elucidate the relationship between the
QBW mode and LPS formation over Bangladesh.
Some studies have focused on the relationship between the BSISO mode and the formation of tropical
cyclones over the north Indian Ocean (e.g., Kikuchi andWang 2010; Yanase et al. 2012). Tropical cyclones areintense systems with maximum wind speeds exceeding
17 m s21, and they generally occur under conditions of
weak vertical shear during the pre- and postmonsoon
seasons. These studies reported that the formation oftropical cyclones is enhanced during the active convective phases of the BSISO mode. With regard to LPSs
during the mature monsoon season (e.g., monsoon lows
and depressions), previous studies have reported similarfindings to those above regarding tropical cyclones [i.e.,high genesis frequencies over the Bay of Bengal (BoB)
and adjoining land areas during the active phases of the
BSISO mode; e.g., Yasunari 1981; Goswami et al. 2003;
Krishnamurthy and Shukla 2007; Krishnamurthy andAjayamohan 2010]. In contrast, few studies have addressed the relationship with the QBW mode. For instance, Saha et al. (1981) reported that most of themonsoon depressions that form over the BoB are associated with the redevelopment of westward-propagating
residual lows from the western Pacific, suggesting that
their genesis is affected by the QBW mode that

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propagates westward in a similar manner. Goswami
et al. (2003) showed, based on their ISO index containing the two dominant periods of the BSISO and
QBW modes, that the occurrence frequency of LPSs is;3.5 times higher in the active phase than during the
break phase and that the remarkable difference in LPS
occurrence is due to modulation of the large-scalemonsoon circulation by the ISO. As mentioned above,
the area around Bangladesh exhibits a distinct QBW
during summer monsoon rainfall, whereas the BSISO
signal is very weak. However, in the low-level circulation fields, the BSISO mode also has some signals overthe country [e.g., Fig. 8 of Hoyos and Webster (2007)].
This implies that it is probable that the BSISO mode
could modulate the structure of atmospheric circulation
associated with the active phases of rainfall over Bangladesh or induce favorable conditions for LPS genesis
in this region. Therefore, to understand fully the processes that determine the LPS and non-LPS cases in theactive phase of the QBW over Bangladesh, the effects of
the QBW and BSISO modes should be investigated.
The objectives of this study were 1) to reveal the spatiotemporal structures of the atmospheric circulation in
the QBW and BSISO modes based on LPS and non-LPS
cases in the active peak of the QBW mode over Bangladesh and 2) to show how the two ISO modes affect theenvironmental conditions for LPS genesis. The findingsof this study further the understanding of the formation
processes of LPSs under the influence of these two ISO
modes. The remainder of this paper is organized as follows. Section 2 describes the datasets and analysis
methods used in this study. Section 3 presents the spatiotemporal structures of the atmospheric circulation andconvection related to the two ISO modes based on LPS
and non-LPS cases. The impacts of two ISO modes on theenvironmental conditions for LPS genesis are also discussed in this section. Section 4 discusses the interaction
between the BSISO and QBW modes and some environmental factors associated with LPS genesis. Finally,
the results are summarized in section 5.

2. Data and analysis method
a. Dataset
The APHRODITE rainfall dataset was used to analyze the rainfall variations associated with the ISO
over Bangladesh from 1979 to 2007 (29 years). The
APHRODITE dataset is a high-resolution (0.258 3
0.258) daily rainfall product over land based on rain
gauge observations (Yatagai et al. 2009, 2012). To examine the large-scale atmospheric circulation, the Japanese 25-year Reanalysis Project (JRA-25) and Japan
Meteorological Agency Climate Data Assimilation

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System (JCDAS) datasets on a 1.258 3 1.258 grid (Onogi
et al. 2007) were used. The JRA-25 dataset covers theperiod from 1979 to 2004, and JCDAS data are available
from January 2005 to January 2014. Because the JCDAS
is based on the same assimilation system as the JRA-25,
the two products provide a homogeneous dataset for theperiod 1979–2007. Daily interpolated outgoing longwave radiation (OLR) data on a 2.58 3 2.58 grid werealso used as a proxy for large-scale convective activity(Liebmann and Smith 1996).

b. Method
The data processing and analysis methods used in thisstudy were similar to those of Hatsuzuka et al. (2014).
To remove annual cycles, daily anomalies of rainfall,
OLR, and the reanalysis data were computed bysubtracting a 121-day (about 4 months) running meanfrom the original time series for each year. Then, aLanczos filter (Duchon 1979) was applied to the daily
anomalies to extract the two ISO signals (i.e., the BSISO
and QBW). A 25–60-day band was chosen as the BSISO
because the major spectral peak of the BSISO appears at
the higher frequency at around 30 days compared withthat of the eastward-propagating Madden–Julian oscillation (MJO) (Wang et al. 2006). A 7–25-day band wasapplied for the QBW because a statistically significant
peak often appears at around 10 days in each year, although the ensemble spectrum shows a peak at around
14 days (Fujinami et al. 2011). To check the dependenceof the results on the bandwidth of the filter, otherbandwidths (i.e., a 30–60-day band for the BSISO and a
10–20-day band for the QBW) were applied to the OLR
and reanalysis data; however, the results were little different. A representative rainfall time series for Bangladesh
was created by averaging over the region 218–258N, 888–
928E, which encompasses a large area of Bangladesh(Fig. 1). As an example, Fig. 2 presents the daily rainfall
time series for the 1979 monsoon season (black bars) andthe 7–25-day-filtered rainfall anomaly (solid line) with thepower spectrum of each time series. Both time series show aclear 7–25-day variation with a significant spectrum peak at
about 18 days. Based on the 7–25-day rainfall anomalies,
positive extremes that exceeded the 29-summer climatological one standard deviation (s 5 ;6.3 mm day21) were
defined as active peaks (filled circles in Fig. 2). In total, 148
active peaks were identified during the 29 summer monsoon seasons (June–September).
This study used LPS data obtained by Hatsuzuka
et al. (2014), who detected the locations of LPS centers that formed over the BoB and adjoining land
areas during the 29 summer monsoon seasons. Using
JRA-25 data, they have developed a method for
identifying not only large-scale LPSs such as Indian

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FIG. 2. Time series of area-averaged daily rainfall (black bars; left y axis) and 7–25-day-filtered rainfall anomaly(solid line; right y axis) from 1 Jun to 30 Sep 1979. Closed circles denote selected active peaks for the rainfall
anomaly series. (bottom left) The power spectrum of the daily rainfall time series (with only the seasonal cycle ofrainfall removed). Dashed line indicates red-noise spectrum and solid line shows 90% significance level. (bottom
right) As in (bottom left), but for the 7–25-day-filtered rainfall anomaly.

monsoon depressions but also small-scale systems(;600 km) that bring heavy rainfall over Bangladesh.
The Indian monsoon region is characterized by strong
meridional cyclonic shear in the lower troposphere.
Therefore, to distinguish vortex-type LPSs from cyclonic shear (e.g., monsoon trough), a combination ofrelative vorticity and geopotential height at 850 hPa wasused in the identification of the LPSs. An LPS center was
identified when a grid of minimum 850-hPa geopotential
height satisfied the following two criteria: 1) the difference between the center and surrounding 8 (16) grids
was smaller than 22 (21) gpm, and 2) the 850-hPa relative vorticity of the center was more than 6.0 3 1025 s21.
A full description of the identification procedure isprovided in section 4 of Hatsuzuka et al. (2014). Here, an
LPS case was defined as a day on which at least one LPS
existed between day 21 and day 11 of an active peakday. In the summer of 1979 (Fig. 2), two of six active
peaks were classified as LPS cases. The remaining active
peaks occurred without LPSs (non-LPS case). In total,
82 (66) active peaks were classified as LPS (non LPS)
cases during the 29 summer monsoon seasons. In

this study, daily lag composites of convection and atmospheric circulation were constructed relative to these
active peaks.

3. Resultsa. Synoptic-scale features
To illustrate the spatiotemporal structure of the atmospheric circulation for the LPS and non-LPS cases
associated with the QBW over Bangladesh, Fig. 3 shows
the temporal sequence of composites for the total
850-hPa wind vectors and zonal wind speed from day 23
to day 0. Active rainfall peaks in the QBW mode overBangladesh are referred to as day 0. Because Indian
monsoon depressions form in regions with strong horizontal shear of the zonal wind, several studies havesuggested that barotropic dynamics play an importantrole in the formation of these depressions (e.g., Shukla
1977; Goswami et al. 1980; Lindzen et al. 1983). Hence,
this study also focuses attention on the horizontal shearof the zonal wind as an environmental factor responsible
for LPS genesis. In the LPS case (Fig. 3a), a cyclonic1 APRIL 2017

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FIG. 3. Composites of total 850-hPa wind (vectors) and zonal wind speed (shading) from day 23 to day 0 for(a) LPS and (b) non-LPS cases based on 7–25-day rainfall variation in Bangladesh. Day 0 corresponds to the active
rainfall peak in Bangladesh. The thick dashed line indicates the topographic 1500-m contour line.

circulation is observed over Bangladesh, reflecting thepresence of the LPS. During this period, a monsoonal
westerly flow prevails from the equator to around 208N.
The monsoon trough (approximately the zero line of thezonal wind speed) is located over the Ganges Plain andsouthwesterly/southerly winds dominate over Bangladesh.
In this synoptic situation, a strong meridional cyclonicshear of zonal winds is observed over the head of the BoB
and Bangladesh, which results in favorable environmental

conditions for LPS genesis. In the non-LPS case (Fig. 3b),the monsoonal westerlies over the BoB are much weaker
than in the LPS case. The monsoon trough is located along
the foot of the Himalayas; hence, stronger westerly winds
are observed around the Ganges Plain. This synoptic situation leads to a weaker meridional shear of zonal winds
over the head of the BoB and Bangladesh.
Figure 4 shows the composite differences of the
850-hPa wind vectors and relative vorticity between the2486

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LPS and non-LPS cases. On day 23, a significant positive(cyclonic) vorticity anomaly appears over the BoB between the westerly and easterly anomalies. On day 22,
the cyclonic vorticity anomaly is enhanced over thenorthern BoB, which then gradually moves northwardwith expansion to Bangladesh and northern India by
day 0. These composite results indicate that environmental
conditions more favorable for LPS genesis are created in
the LPS case than in the non-LPS case. Although thesignificant positive difference in the relative vorticity
around the BoB includes the effects of the LPS itself, thehorizontal scale of the LPS (;600 km) is much smaller
than that area. Significant westerly and easterly anomalies
associated with the positive vorticity anomaly also appear
broadly from India to the South China Sea. Thus, thesesignificant differences between LPS and non-LPS cases
cannot be explained fully only by the effects of the LPS. In
addition, because the LPSs occur most frequently on
day 21, they seem to have an insignificant effect on thecyclonic vorticity anomaly on days 23 and 22. Therefore,
this study considered the synoptic-scale situations ondays 23 and 22 as the environmental fields responsible for LPS genesis. The following subsections
show the impacts of the two ISO modes on the environmental fields related to LPS genesis.

b. QBW mode

FIG. 4. Composite difference of 850-hPa wind (vectors) andrelative vorticity (shading) between LPS and non-LPS cases.
Vectors denote only 95% statistically significant difference. The
green dashed line shows the 95% significance level for the relative
vorticity difference. The thick dashed line indicates the topographic 1500-m contour line.

Figure 5 shows the temporal sequence of compositesof the 850-hPa circulation and OLR anomalies related
to the QBW mode for the LPS case. The composite
anomalies exhibit a westward-propagating feature fromthe western Pacific. On day 25, Bangladesh is dominated by suppressed convection accompanied by a local
anticyclonic circulation anomaly. Active convection
and cyclonic anomalies are located over the BoB, andthey move gradually northward from day 25 to day 22.
Note that the cyclonic anomaly moves westward fromthe South China Sea and that it reaches the BoB by
day 25 (not shown). Alternatively, an anticyclonicanomaly over the western Pacific starts to move westward toward the Indochina Peninsula from day 25. On
day 22, when the anticyclonic anomaly is located over
the South China Sea, a westerly/southwesterly flow issignificantly enhanced over the BoB because of an increase in the pressure gradient between the cyclonic andanticyclonic anomalies. Simultaneously, a narrow cyclonic circulation deepens around Bangladesh, in conjunction with an active convection anomaly overBangladesh. On day 0, when the convection/rainfall
reaches a maximum over Bangladesh, the anticyclonicanomaly is centered on the western coast of the Indochina Peninsula, and the westerly/southwesterly flow isfurther enhanced around Bangladesh.

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FIG. 5. Composites of 7–25-day-filtered 850-hPa geopotential height (contour), wind (vectors), and OLR(shading) anomalies from day 25 to day 0 for LPS case based on 7–25-day rainfall variation in Bangladesh. Day 0
corresponds to the active rainfall peak in Bangladesh. The contour interval is 2 m. Only 95% statistically significant
vectors are plotted. The green circles denote statistically significant grids at the 95% confidence level for the OLR
anomalies. The thick dashed line indicates the topographic 1500-m contour line.

The time sequence of composites for the non-LPS
case is shown in Fig. 6. In the non-LPS case, as in the LPS
case, the QBW mode is characterized by the westward
propagation of atmospheric circulation and convection.
From day 25 to day 22, an anticyclonic circulation
anomaly moves westward along 158N from the western
Pacific to the western coast of the Indochina Peninsula.
On day 22, convection is enhanced over Bangladesh
simultaneously with the enhancement of a westerly/
southwesterly flow over the head of BoB and Bangladesh.
By day 0, the anticyclonic anomaly moves into the BoB,
and the westerly flow and convection are further enhanced over and around Bangladesh. In addition,

statistically significant circulation signals are also observed over the midlatitudes from day 21 to day 0. As
shown by Fujinami et al. (2014), the airflow from themidlatitudes intrudes over Bangladesh along the southwestern periphery of the Tibetan Plateau, which seems to
be related to the enhancement of the westerly flow overBangladesh.
As discussed in the introduction, previous studies
have revealed that the anticyclonic circulation anomalyof the QBW mode provides strong low-level westerly/
southwesterly winds over and around Bangladesh in theactive phase (Fujinami et al. 2011, 2014). The prevailing
low-level flow transports abundant moisture to the2488

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FIG. 6. As in Fig. 5, but for the non-LPS case.

windward areas of the Meghalaya Plateau and Chittagong Hill Tracts, which triggers forced lifting of the
moist air and results in a large increase in rainfall overthe country. The results of this study for both the LPS
and non-LPS cases also demonstrate that the westwardpropagating anticyclonic anomaly is closely associatedwith the active peaks of rainfall over Bangladesh.
However, the central position of the anticyclonicanomaly is slightly different between the LPS and nonLPS cases. For instance, on day 22, the anticyclonic
center is located over the South China Sea in the LPS case(Fig. 5d). This situation brings the westerly/southwesterly
anomaly over the BoB, which enhances the horizontal
shear of zonal wind over and around Bangladesh where
the LPSs occur most frequently. In contrast, in the nonLPS case (Fig. 6d), the anticyclonic center is located overthe Indochina Peninsula, and the westerly/southwesterly
anomaly dominates over northern India and Bangladesh.
This acts to weaken the low-level cyclonic shear aroundthe genesis region of the LPSs. To highlight the difference
in environmental conditions between the two cases, Fig. 7
shows the composites of the 7–25-day-filtered 850-hPa
wind and relative vorticity anomalies on day 22. In the
LPS case (Fig. 7a), a large positive vorticity anomaly
dominates over and around Bangladesh. This means that
the QBW mode provides favorable environmental conditions for LPS genesis in the LPS case. In contrast, in the
non-LPS case (Fig. 7b), the low-level cyclonic vorticity ismainly enhanced along the foot of the Himalayas, in association with the enhancement of westerly/southwesterly flow from northern India to Bangladesh. Hatsuzuka
et al. (2014) noted that the LPS that occurred on 26 July

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between the LPS and non-LPS cases are discussed
further in section 4.

c. BSISO mode

FIG. 7. Composite difference of 7–25-day-filtered 850-hPa wind(vectors) and relative vorticity (shading) anomalies between (a) LPS
and (b) non-LPS cases on day 22. Vectors denote only 95% statistically significant difference. The green dashed line shows the 95%
significance level for the relative vorticity difference. The thickdashed line indicates the topographic 1500-m contour line.

1989 was related to the westward propagation of the
anticyclonic anomaly in the QBW mode [Fig. 12 of
Hatsuzuka et al. (2014)]. In their case study, an anticyclonic anomaly was located over the South China Sea
one day before its occurrence (25 July). On 26 July, theanticyclonic anomaly intruded into the BoB and enhanced the westerly flow over the northern tip of the
BoB. These spatial features are similar to the composite structures on day 22 and day 21 in the LPS case.
Unlike those of the non-LPS case, a remarkable westerly anomaly was absent over Bangladesh on thesedays. Thus, the case study by Hatsuzuka et al. (2014)
seems to provide a typical example of the relationshipbetween the QBW mode and LPS genesis. These results suggest that modulation of the low-level circulation by the QBW mode affects the environmental
conditions for LPS genesis over Bangladesh. The differences in the spatial patterns of the QBW mode

To illustrate the relationship between the BSISO
mode and QBW in rainfall over Bangladesh, Fig. 8
shows the temporal sequence of composites for the
25–60-day-filtered 850-hPa circulation and OLR anomalies for the LPS and non-LPS cases. Note that day
0 corresponds to the active rainfall peak in the QBW
mode over Bangladesh. The BSISO mode is characterized by the northward propagation of the anomalies
from the equatorial Indian Ocean in both LPS and nonLPS cases. However, interestingly, the BSISO mode
exhibits an opposite phase between the two cases. In the
LPS case (Fig. 8a), an active convection anomaly appears over the equatorial Indian Ocean on day 220,
which then moves northward. On day 25, the active
convection anomaly covers a broad area from central
India to the BoB, accompanied by a cyclonic circulation
anomaly. Simultaneously, a following suppressed convection anomaly starts to develop over the equatorial
Indian Ocean. On day 0, the convection is further enhanced over the head of the BoB. Note that the enhanced convection significantly weakens by day 15
without intruding into Bangladesh (not shown). In
contrast, the BSISO mode in the non-LPS case (Fig. 8b)shows the northward propagation of the suppressed
convection. From day 220 to day 25, the suppressed
convection shifts from the equatorial Indian Ocean to
the Indian subcontinent. On day 0, the suppressed convection signals are observed broadly from northwestern
India to the western Pacific, accompanied by anticyclonic anomalies centered around 858 and 1208E. In bothcases, the convection signals are weak over Bangladesh
on day 0. This result indicates that the BSISO mode does
not directly produce the convection/rainfall related to
the QBW over Bangladesh. However, a significant
contrast in the circulation anomaly between the LPS andnon-LPS cases is observed over the BoB and Bangladesh, suggesting that the BSISO mode has considerable
effect on the modulation of the environmental conditions for LPS genesis.
To illustrate the spatial pattern of circulation associated with the BSISO mode for the LPS and non-LPS
cases more clearly, Fig. 9 presents the time–latitude crosssections of the 850-hPa zonal wind anomaly averaged
over 808–958E. Lag day 0 corresponds to the active rainfall peak in the QBW mode over Bangladesh. As shownin Fig. 8, a northward propagation signal is evident fromthe equator to Bangladesh but with opposite signs between the LPS and non-LPS cases. In the LPS case(Fig. 9a), a statistically significant westerly (easterly)

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FIG. 8. Composites of 25–60-day-filtered geopotential height (contour), wind (vectors), and OLR (shading) anomalies from day 220 to day 0 for (a) LPS and (b) non-LPS cases based on 7–25-day rainfall variation in Bangladesh. Day 0
corresponds to the active peak of the 7–25-day rainfall variation in Bangladesh. The contour interval is 1 m. Only 95%
statistically significant vectors are plotted. The green circles denote statistically significant grids at the 95% confidence
level for the OLR anomalies. The thick dashed line indicates the topographic 1500-m contour line.

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FIG. 9. Time–latitude cross section of 25–60-day-filtered 850-hPa zonal wind anomaly averaged between 808 and 958E for (a) LPS and(b) non-LPS cases. Lag 0 corresponds to the active peak of the 7–25-day rainfall variation in Bangladesh. The white contour shows the
95% significance level. Bangladesh is located roughly within the two dashed lines. A map is shown to the right of (b).

anomaly appears over the BoB (northern Bangladesh)from at least day 25 to day 15. This situation enhances
the meridional cyclonic shear of the zonal wind over thenorthern BoB and Bangladesh, which provides favorable
environmental conditions for LPS genesis. Conversely, inthe non-LPS case (Fig. 9b), a significant easterly (westerly) anomaly is observed over the BoB (Bangladesh)
during almost the same period. In this situation, the anticyclonic shear of the zonal wind is enhanced over thenorthern BoB and Bangladesh, which leads to environmental conditions unfavorable for LPS genesis.
Figure 10 shows the occurrence frequency of the active rainfall peaks in the QBW mode over Bangladesh
as a function of the phase of the BSISO. The BSISO
phases are represented by the amplitude of the 25–
60-day-filtered 850-hPa relative vorticity averaged over
158–258N, 808–958E, which covers the region from thenorthern BoB to Bangladesh. To focus on the activityof the BSISO before LPS genesis, the results presented
here are only for day 25 because the LPSs occur
most frequently on day 21. In Fig. 10a, about 65% of
the total active peaks in the LPS cases (54 out of 82)
occur in the positive (cyclonic) phase of the BSISO. In
contrast, about 60% of the total active peaks in the

non-LPS cases (40 out of 66) occur in the negative(anticyclonic) phase (Fig. 10b). The frequency distributions from day 24 to day 0 also show similar percentages as day 25 in both LPS and non-LPS cases (notshown). This phase preference of the BSISO mode in
the two cases is consistent with the results of the composites shown in Figs. 8 and 9. Thus, these results
strongly suggest that the phase of the BSISO has animportant role in determining the LPS or non-LPS
cases in the active peak of the QBW over Bangladesh.

d. Impact of the two ISO modes on LPS genesis
To understand how these two ISO modes affect thesynoptic-scale environment for LPS genesis, Fig. 11
shows the composite differences of the 850-hPa windvectors and zonal wind speed between the LPS and nonLPS cases for the total field, BSISO mode, and QBW
mode. The overall patterns of anomalies are similarfrom day 23 to day 0 in each field. In the total field(Fig. 11a), the spatial pattern is characterized by awesterly anomaly over the BoB and an easterly anomaly
over Bangladesh. The favorable environment for LPS
genesis is created between these westerly and easterly
anomalies, where the low-level cyclonic vorticity isFIG. 10. Occurrence frequency of active peaks for (a) LPS and (b) non-LPS cases as a function of amplitude of
25–60-day-filtered 850-hPa relative vorticity on day 25 averaged over 158–258N, 808–958E.

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FIG. 11. Composite difference of (a) total 850-hPa wind (vectors) and zonal wind speed (shading) between LPS and non-LPS cases fromday 23 to day 0. Day 0 corresponds to the active peak of the 7–25-day rainfall variation in Bangladesh. Vectors and shaded areas denote
only 95% statistically significant difference. (b) As in (a), but for BSISO mode. (c) As in (a), but for QBW mode.

enhanced (Fig. 4). Such circulation features are also
observed over the South China Sea and the western
Pacific. The spatial pattern of the composite differencein the total field is similar to that of the BSISO mode
(Fig. 11b). By comparing Figs. 11a and 11b, it is foundthat the westerly anomaly over the BoB, associated with
the BSISO mode, could explain approximately half thetotal difference. The easterly anomaly over Bangladesh
is also explained by the BSISO mode, but with a lower
percentage for the total difference. Although, in the
QBW mode, the spatial pattern of the atmospheric circulation is almost in phase between the LPS and nonLPS cases, it has been noted that the central position of
the anticyclonic anomaly is slightly different between
the two cases (Figs. 5 and 6). In Fig. 11c, the composite
difference of the QBW mode shows a statisticallysignificant westerly (easterly) anomaly over the BoB(Bangladesh), enhancing the low-level cyclonic vorticity
over the northern BoB. In addition, Fig. 11 indicates thatthe contribution of the QBW mode to the total difference is comparable with that of the BSISO modeover the BoB and Bangladesh. This result suggests againthat the locational difference in the anticyclonic anomaly in the QBW mode has an impact on the environmental conditions for LPS genesis.
This study demonstrated that the LPS genesis overBangladesh is fostered by the cyclonic circulation anomaly of the BSISO mode. This result is consistent with theresults of previous studies showing the relationship between the genesis of Indian monsoon depressions and the
BSISO mode (e.g., Yasunari 1981; Goswami et al. 2003;
Krishnamurthy and Ajayamohan 2010). With regard to1 APRIL 2017

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the QBW mode, Krishnamurti et al. (1977) and Sahaet al. (1981) have suggested that Indian monsoon depressions are initiated by the redevelopment of
westward-propagating residual lows from the western
Pacific. Furthermore, Goswami et al. (2003) reported
that the frequency of occurrence of LPSs increases significantly under the enhanced cyclonic circulation during the active phase over the BoB in both the 30–60- andthe 10–20-day modes. However, unlike the Indian
monsoon depressions, this study suggests that LPS
genesis over Bangladesh is induced by the anticyclonic
circulation of the QBW mode in combination with thecyclonic circulation of the BSISO mode. The cyclonic
circulation of the BSISO mode can be found from theIndian subcontinent through the BoB from approximately day 25 (Figs. 8a and 9a). On that day, the anticyclonic anomaly of the QBW mode is still located over
the western Pacific (Fig. 5a). Then, the anticyclonicanomaly of the QBW mode moves westward toward the
BoB, and in turn, the low-level cyclonic vorticity becomes enhanced around Bangladesh on day 22 andday 21 (Figs. 5d,e). Thus, the QBW mode likely acts as a
trigger for LPS genesis under the favorable cyclonicenvironment arranged by the BSISO mode. In addition, theregional-scale mountain topography around Bangladesh(i.e., the Meghalaya Plateau and Chittagong Hill Tracts)might help enhance the low-level cyclonic vorticity
within this region (Fig. 1). Therefore, these results suggest that the combination of the BSISO and QBW
modes is important for LPS genesis over Bangladesh.
Using composite analysis, this study revealed a robustfeature of atmospheric circulation associated with thetwo ISO modes. However, the fact remains that some
LPSs occur when the BSISO-related low-level vorticity
is anticyclonic over the BoB (Fig. 10a). This is likely due
to a greater contribution of the QBW mode or environmental factors other than the low-level zonal wind
shear to the environmental conditions for LPS genesis,
which should be studied in future work.

4. Discussiona. Interaction between the QBW and BSISO modes
In the previous section, it was reasoned that locational
differences in the anticyclonic anomaly of the QBW
mode have a significant impact on the environmental
conditions for LPS genesis. For instance, on day 21, theanticyclonic circulation center is located over the Indochina Peninsula in the LPS case (Fig. 5e), whereas it islocated over the BoB in the non-LPS case (Fig. 6e).
The northern pathway for the circulation anomalies of
the QBW mode (;158N) generally corresponds to thedominant area of zonal wind fluctuation in the BSISO

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mode (e.g., Goswami and Mohan 2001; Lawrence andWebster 2002; Hoyos and Webster 2007). The horizontal scale of the BSISO mode is also much larger than thatof the QBW mode, which extends from the Arabian Sea
to the western Pacific. This means that the BSISO mode
could act as a large-scale background condition for the
QBW mode and thus affect its spatial structure andevolution. To address this possibility, this study examined the preferred spatial structure of the QBW mode
under the two different phases of the BSISO mode. The
25–60-day-filtered 850-hPa zonal wind averaged over
108–208N, 608–1108E (the rectangle shown in Figs. 12a,b)
was used as a reference of the BSISO mode, where thezonal wind variance of the BSISO mode dominates [seeFig. 10d of Fujinami et al. (2011)]. Based on the reference time series, positive and negative extremes thatexceeded the 29-summer climatological one standard
deviation and their previous day and following day wereselected as westerly and easterly phases of the BSISO
mode, respectively. In total, 189 westerly days and 174
easterly days were used to compile the composites.
Figures 12a and 12b show the composites of the 850-hPa
wind anomaly for the westerly and easterly phases of the
BSISO mode. The westerly and easterly phases correspond to cyclonic and anticyclonic circulation anomalies
in the BSISO from northern India to Bangladesh, respectively. Figures 12c and 12d present the compositesof the 850-hPa wind and geopotential height anomalies
of QBW mode in the westerly and easterly phases of the
BSISO mode, respectively. Interestingly, in the westerly
phase of the BSISO, the QBW mode exhibits an anticyclonic circulation centered on the Indochina Peninsula (Fig. 12c), consistent with the features of the LPS
case (Fig. 5e). As shown in Figs. 5 and 6, this anticyclonicanomaly also shows a westward-propagating featurefrom the western Pacific to the BoB (not shown).
Moreover, a cyclonic circulation anomaly can be foundover Bangladesh in the BSISO westerly phase, reflecting
the presence of an LPS there. A remarkable anticyclonicanomaly also appears centered on the Arabian Sea inthe BSISO westerly phase (Fig. 12c). However, thisanticyclonic anomaly seems to be derived from a sudden
development over the Arabian Sea rather than the
westward propagation (not shown), suggesting that it
might not be part of the QBW mode. In contrast, during
the easterly phase of the BSISO, the anticyclonic circulation in the QBW mode is dominant over the BoB(Fig. 12d), consistent with the features of the non-LPS
case (Fig. 6e). However, no statistically significant
anomalies are shown in the midlatitudes, unlike the nonLPS case. These results are also consistent with the results found for the summer of 1979 by Chen and Chen
(1993), indicating that the cyclonic anomaly of the2494

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FIG. 12. (a) Composite of 25–60-day-filtered 850-hPa wind anomaly (vectors) for the westerly phase of the
BSISO. The boxed region indicates the area in which the zonal wind is averaged to generate the BSISO index.
(b) As in (a), but for the easterly phase of the BSISO. (c) Composite of 7–25-day-filtered 850-hPa geopotential
height (shading) and wind (vectors) anomalies for the westerly phase of the BSISO. (d) As in (c), but for the easterly
phase of the BSISO. Only 95% statistically significant vectors are plotted. The thick dashed line indicates the
topographic 1500-m contour line.

10–20-day mode is reinitiated around Bangladesh by the
30–60-day monsoon trough over the northern BoB (i.e.,westerly phase over the BoB). Conversely, no cyclonicanomaly of the 10–20-day mode appears around Bangladesh
when the 30–60-day monsoon ridge is located over thenorthern BoB (i.e., easterly phase over the BoB). In
addition, during the transition phase from the westerly(easterly) to easterly (westerly) anomalies, the QBW
mode exhibits a remarkable cyclonic circulation anomaly centered on the BoB (figure not shown). Thus, thisfinding suggests that the BSISO mode might control thephase preference of the QBW mode around the BoB
and Bangladesh region. Further studies are needed toelucidate such a climatological relationship between the
QBW and BSISO modes over South Asia.
Fujinami et al. (2014) reported that the midlatitude
circulation around the Tibetan Plateau, in conjunction
with equatorial waves, helps induce the distinct QBW in
rainfall over Bangladesh. In the results of this study,
midlatitude circulation signals are not evident in the LPS
case (Fig. 5), whereas the non-LPS case shows that airflow from the midlatitudes does intrude over Bangladesh
along the southwestern periphery of the Tibetan Plateau

from day 21 to day 0 (Figs. 6e,f). The enhanced westerly
flow along the Ganges Plain probably weakens the meridional shear of zonal winds around Bangladesh, which
results in conditions unfavorable for LPS genesis in the
non-LPS case. Therefore, this result suggests that theenvironmental conditions for LPS genesis are influenced
by tropical–midlatitude interaction along with BSISO–
QBW interaction, although further analysis is needed
to elucidate the physical mechanism behind suchinteractions.

b. Other environmental factors for LPS genesis overBangladesh
The present study has demonstrated that low-level
shear is an important environmental factor for LPS
genesis over Bangladesh. However, previous studies
have also considered factors such as high sea surface
temperature, convective instability, high relative humidity in the midtroposphere, and weak vertical wind
shear with regard to the provision of favorable environment conditions for tropical cyclone genesis (e.g.,Gray 1979). These environmental factors are usuallyconsidered when diagnosing the formation of strong1 APRIL 2017

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2495

FIG. 13. Composites of total (a) 600-hPa relative humidity and (b) vertical zonal wind shear between 850 and
200 hPa on day 22 for (top) LPS cases, (middle) non-LPS cases, and (bottom) the difference between the LPS andnon-LPS cases. The green dashed line shows the 95% significance level.

systems such as hurricanes and typhoons that occur anddevelop over the oceans. Because the LPSs over Bangladesh
have vortex structures similar to such tropical cyclones(i.e., a cyclonic circulation extending through the entire
depth of the troposphere with a warm core at its upper
level), it is speculated that LPS genesis might be associated with not only low-level cyclonic vorticity but also
the other environmental factors mentioned above.
Here, the effects of three of the above environmental
factors were examined, excluding sea surface temperature because most of the LPSs form over land or over thesea near land. Figure 13 shows composites of total fields
associated with the environmental factors on day 22 for
the LPS and non-LPS cases and the difference between

the two cases. In Fig. 13a, the 600-hPa relative humidity
shows a remarkably high value along the southern slopeof the Himalayas and over the southeastern Tibetan
Plateau in both LPS and non-LPS cases, which is related
to the orographically induced convection over themountainous regions. The composite difference shows asignificant positive anomaly over the BoB and southern
Bangladesh, where the LPSs occur most frequently. As
greater low-level cyclonic vorticity in that area leads togreater convective activity, this result is consistent withthe result presented in Fig. 4. Moreover, it was demonstrated that the two ISO modes provide larger lowlevel cyclonic vorticity over the BoB in the LPS case
(Figs. 11b,c), suggesting that these ISO modes also

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contribute to the high level of midtropospheric humidity. In Fig. 13b, vertical wind shear is defined as themagnitude of the difference between the zonal winds at
200 and 850 hPa. During the monsoon season, easterly
vertical shear is climatologically significant south of
208N, whereas vertical shear is weak from the head of
the BoB to Bangladesh, which is where the LPSs occur
most frequently. The composite difference shows asignificant negative anomaly over the BoB, southern
Bangladesh, and central India, suggesting that vertical
shear also contributes to the favorable conditions for
LPS genesis. However, the low-level atmospheric circulation associated with the two ISO modes, together
with the total field, shows almost no statistically significant difference in the regions of the negative vertical shear anomaly (Fig. 11). Thus, the difference in thevertical wind shear might be due to the influence of theupper-level atmospheric circulation. In contrast, atmospheric instability in the lower troposphere (definedas the difference of equivalent potential temperature
between 850 and 500 hPa) exhibits no statistically significant difference between the LPS and non-LPS cases
over the northern BoB and Bangladesh; however, inboth cases, it shows convective instability that favors
LPS genesis (not shown). This is probably due to thewarm and humid air in the low-level troposphere fromthe BoB to Bangladesh. Thus, these results provide
further confirmation that environmental conditions
that are more favorable are created in the LPS casethan the non-LPS case. Furthermore, it has also beensuggested that the two ISO modes contribute to thehigh midlevel humidity as well as the strong low-level
cyclonic vorticity.
Only a handful of studies have investigated the physicalmechanisms responsible for the genesis and growth of
monsoon depressions. Sikka (1977) argued that barotropic dynamics play a significant role in the early stagesof the development of monsoon depressions, but thelatter stages of development, as well as the maintenanceof the depression, are related intimately with the baroclinic and conditional instability of the second kind
(CISK) dynamics. The results of this study also suggestthat LPS genesis over Bangladesh is related to the barotropic process rather than baroclinic dynamics because
the vertical wind shear is weaker in the LPS case compared with the non-LPS case (Fig. 13b). Recently,
Ditchek et al. (2016) examined the statistical associationof monsoon disturbance genesis and environmental
conditions using a genesis index that they developed.
They found that genesis frequency decreases in the region of strong vertical shear, suggesting again that baroclinic instability might not be a primary mechanismof genesis. As this study focused on LPS genesis, it is

VOLUME 30

considered that such dynamics might have an importantrole in the growth stages of the systems. In addition, they
revealed that genesis over the BoB is fostered by humid,
vorticity-rich, and convectively unstable environments.
The results also denoted the same tendency as Ditchek
et al. (2016), suggesting that similar mechanisms likely
govern the genesis of Indian monsoon depressions and
LPSs over Bangladesh. However, the details of the dynamics of the genesis and growth of LPSs are not the
subject of this study, and they should be investigated infuture work.

5. Summary
This study investigated the effects of two ISO modes(i.e., the BSISO and QBW modes) on LPS genesis overBangladesh for 29 summer monsoon seasons. The main
results were obtained from composite analyses of atmospheric circulation based on LPS and non-LPS casesin the active rainfall peak of the QBW mode overBangladesh. To summarize the relationship between thetwo ISO modes in LPS and non-LPS cases, schematic
drawings are provided in Fig. 14.
In the QBW mode, the westward propagation of alow-level anticyclonic anomaly from the western Pacificis common in both cases. It should be noted that the
anticyclonic center in the LPS case is located slightly tothe east of that in the non-LPS case. This locational
difference provides stronger low-level cyclonic shear
over and around Bangladesh in the LPS case. In contrast, the BSISO mode shows an opposite phase between
the two cases. In the LPS (non LPS) case, a cyclonic(anticyclonic) anomaly moves northward from theequator to the BoB. In the LPS case, the cyclonicanomaly in the BSISO mode enhances a westerly (easterly) flow over the BoB (northern Bangladesh), resulting
in enhancement of the meridional cyclonic shear of zonal
winds over the northern BoB and Bangladesh. It was alsosuggested that the two ISO modes contribute to the highlevel of midtropospheric humidity around the LPS genesis region, as well as the strong low-level cyclonic vorticity. Thus, in the LPS case, the two ISO modes provide
favorable environmental conditions for LPS genesis.
Conversely, in the non-LPS case, the westerly flow isenhanced from northern India to Bangladesh in association with the anticyclonic anomalies of the two ISO
modes. The enhanced westerly flow leads to the enhancement of orographic lifting of low-level moisture on
the windward slopes of mountains such as the Meghalaya Plateau and Chittagong Hill Tracts, which results
in a large increase in rainfall in the area. Thus, theseresults suggest that the phase relationship between thetwo ISO modes controls the selection of the LPS or1 APRIL 2017

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2497

FIG. 14. Schematic illustration of the relationship between BSISO mode (solid arrows) and QBW mode (dashedarrows) on day 21 when LPSs occur most frequently: (a) LPS case and (b) non-LPS case. Symbols L and H
represent the center of anticyclonic (orange arrows) and cyclonic (blue arrows) anomaly associated with the ISO
modes, respectively.

non-LPS case in the active peak over Bangladesh. Such a
relationship also suggests that the monitoring and prediction of the behaviors of these ISOs would aid theprediction of LPS genesis around the Bangladesh region. Because of their small scale and the complex features of the terrain around Bangladesh, numericalsimulation by regional and cloud-resolving models

would be necessary for furthering the understanding of
LPS formation processes.
Additionally, the results of this study suggest that
the QBW mode might be phase locked by the BSISO
mode over the BoB. The model results of Wang and Xie(1997) demonstrated that a westward-propagating Rossby
wave emanates from the equatorial eastward-propagating

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Kelvin–Rossby wave packet when it approaches thedate line from the western Indian Ocean. Thus,
the BSISO could be considered a system composed ofthe equatorial eastward, off-equatorial westward, andnorthward-propagating modes. Further studies areneeded to understand fully the complex behavior anddynamics of the BSISO.
Acknowledgments. The authors thank Drs. T. Yasunari,
T. Hiyama, T. Kumagai, H. Masunaga, and H. Kanamori
for their valuable comments and suggestions. We obtained
the APHRODITE dataset from the project website(http://www.chikyu.ac.jp/precip/). This study was conducted with the support of a Grant-in-Aid for Scientific
Research (C-26400465) and a Grant-in-Aid for Scientific
Research (S-26220202) from the Japan Society for the
Promotion of Sciences (JSPS). Portions of this study wereconducted as part of the joint research program of theInstitute for Space-Earth Environmental Research(ISEE), Nagoya University, Japan.
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