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MONTHLY WEATHER REVIEWVOLUME 142

Characteristics of Low Pressure Systems Associated with Intraseasonal
Oscillation of Rainfall over Bangladesh during Boreal Summer
DAISUKE HATSUZUKA
Graduate School of Environmental Studies, Nagoya University, Nagoya, JapanTETSUZO YASUNARI
Research Institute for Humanity and Nature, Kyoto, JapanHATSUKI FUJINAMI
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan(Manuscript received 28 September 2013, in final form 29 August 2014)
ABSTRACT
Characteristics of low pressure systems (LPSs) responsible for submonthly-scale (7–25 days) intraseasonal
oscillation (ISO) in rainfall over Bangladesh and their impact on the amplitude of active peaks are investigated for 29 summer monsoon seasons. Extreme and moderate active peaks are obtained based on theamplitude of 7–25-day-filtered rainfall series. By detecting the LPSs that formed over the Indian monsoon
region, it was found that about 59% (62%) of extreme (moderate) active peaks of rainfall are related to LPSs.
These LPSs have horizontal scale of about 600 km and vertical scale of about 9 km. For the extreme active
peak, the locations of the LPS centers are clustered significantly over and around Bangladesh, accompaniedby the maximum convergence in the southeast sector of the LPSs. After their formation, they tend to remain
almost stationary over and around Bangladesh. In contrast, for the moderate active peak, the LPS centers arelocated over the Ganges Plain around 858E, and the maximum convergence of the LPSs occurs around theircenters. This difference in the convergence fields is closely associated with the geographical features to thenorth and east of Bangladesh and the horizontal scale of the LPSs. These features suggest that the amplitudeof the active peaks in the submonthly-scale ISO is modulated by small differences in the locations of the LPS
centers. These findings suggest that improved predictions of both genesis location and the tracks of the LPSs
are crucial to forecasting seasonal rainfall over Bangladesh.

1. Introduction
During the summer monsoon season (June–September),Bangladesh often receives heavy rainfall and the maximum
seasonal rainfall is in excess of 6000 mm (Matsumoto et al.
1996). Bangladesh is characterized by very flat lowland (lessthan 10 m above sea level), but the Shillong Plateau andChittagong Hill Tracts, situated near the northeast and eastsoutheast borders with India, respectively, rise up to;2000 m above sea level (Fig. 1). As the southwesterly/
southerly monsoonal flow from the Bay of Bengal dominates during this season, these higher-elevation regions

Corresponding author address: Daisuke Hatsuzuka, Graduate
School of Environmental Studies, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8601, Japan.
E-mail: hatsuzuka.daisuke@f.mbox.nagoya-u.ac.jp
DOI: 10.1175/MWR-D-13-00307.1
Ó 2014 American Meteorological Society

act as orographic barriers against the prevailing winds.
Thus, these geographical features are closely associatedwith both the spatial distribution of seasonal mean
rainfall and the development of individual precipitation
systems over Bangladesh (e.g., Kripalani et al. 1996;
Ohsawa et al. 2000, 2001; Islam et al. 2005; Kataoka andSatomura 2005; Terao et al. 2006; Murata et al. 2008;
Rafiuddin et al. 2010). Heavy rainfall during this season
often causes disastrous floods over the flat lowland of
Bangladesh (e.g., Matsumoto et al. 1996; Chowdhury
2003; Murata et al. 2008). To aid the prediction of heavy
rainfall that leads to flooding over Bangladesh, it isnecessary to have precise understanding of the processes
and precipitation systems causing the heavy rainfall.
In the South Asian monsoon region, intraseasonal
oscillations (ISOs) are a dominant mode of rainfall/convective variability. Many previous studies have shownDECEMBER 2014

HATSUZUKA ET AL.

FIG. 1. Topography in and around Bangladesh. The boxed region
denotes the area used in calculating the area-averaged rainfall timeseries.

two dominant spectral peaks: one with a period of 30–
60 days (e.g., Yasunari 1979, 1980; Hartmann andMichelsen 1989; Singh et al. 1992; Annamalai and Slingo2001) and the other with a period of 7–25 days, which inthis study is referred to as a submonthly-scale ISO (e.g.,
Krishnamurti and Bhalme 1976; Yasunari 1979; Chen andChen 1993; Annamalai and Slingo 2001; Hoyos andWebster 2007). A few studies have addressed specificallythe ISO of rainfall/convection over Bangladesh. Ohsawa
et al. (2000) examined the ISO of rainfall over Bangladesh for the summer of 1995 and found that the 7–25-day
variation is dominant, whereas the 30–60-day variation is
weak. Murata et al. (2008) presented distinct submonthly
rainfall variability at Cherrapunjee (located on thesouthern slopes of the Shillong Plateau; 25.258N, 91.738E)
in the 2004 monsoon season. Recently, Fujinami et al.
(2011) reported similar findings over Bangladesh whenusing long-term data from a network of 25 rain gauges
obtained between 1981 and 2000. These results indicatethat the rainfall variation over and around Bangladesh is
dominated by the submonthly-scale ISO. These ISO activities are also strongly related to interannual variabilityof the total summer monsoon rainfall in India and Bangladesh (Goswami and Mohan 2001; Fujinami et al.
2011). Fujinami et al. (2011) found that the interannual
variability of the total summer monsoon rainfall in Bangladesh is correlated significantly with the submonthly
rainfall variance, suggesting that the ISO activity controls
the interannual variability of the total summer monsoon
rainfall. They also examined probable effects of thesubmonthly-scale ISO on the interannual variability ofsummer rainfall and showed that high-amplitude active

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peaks occur more frequently during wet monsoon years
than during dry monsoon years. However, it is still unclear
which processes enhance the amplitude of the active peaks.
Monsoon lows and depressions, classified by the India
Meteorological Department based on their maximum
surface wind speeds (Raghavan and Rajesh 2003), arethe main rain-producing systems over the Indian monsoon region. In this study, these systems are referred tocollectively as low pressure systems (LPSs). Most LPSs
form over the head of the Bay of Bengal and then movenorthwestward, accompanied by maximum rainfall in
their southwest sectors. Their typical horizontal scale isa few thousand kilometers. These characteristic features
have been well documented in many previous studies(e.g., Mooley 1973; Krishnamurti et al. 1975; Godbole
1977; Sikka 1977; Mooley and Shukla 1989). LPS activity
is related strongly to the active and break phases of theISOs (Yasunari 1981; Murakami et al. 1984; Goswami
et al. 2003). Goswami et al. (2003) showed that the frequency of occurrence of LPSs is ;3.5 times higher
during the active phase of an ISO than during its breakphase, and that the tracks of the LPSs exhibit strong
spatial clustering along the monsoon trough during theactive phase of the monsoon. Recently, Krishnamurthyand Ajayamohan (2010) revealed that the number of
LPS days during the active period is ;1.7 times greaterthan that during the break period. These results indicatethat the spatial and temporal clustering of LPSs plays anessential role in increasing (decreasing) rainfall overcentral India during active (break) phases. As Bangladesh adjoins the head of the Bay of Bengal, the area inwhich the LPSs form most frequently, such systems are
expected to augment the country’s rainfall. However, noprevious studies have addressed the relationship between
the activity of the LPSs and the ISO over Bangladesh.
The objectives of this study are to 1) reveal the relationship between LPS activity and the submonthlyscale ISO of rainfall over Bangladesh during the summer
monsoon season (June–September), 2) reveal the characteristics of the LPSs related to the ISO in Bangladesh,and 3) explain how the amplitude of active peaks in
the ISO is modulated by the LPSs. Section 2 describes
the datasets and analysis methods used in this study. The
spatiotemporal structure of rainfall and atmospheric circulation associated with the submonthly-scale ISO overBangladesh is presented in section 3. The detailed characteristics of LPSs related to the ISO and their impact on
the amplitude of active peaks are discussed in section 4.
In section 5, other processes that enlarge the amplitude ofactive peaks are discussed. In addition, the detailed
structure of the LPSs is also discussed here, in comparison
with monsoon depressions over India. Finally, the results
are summarized in section 6.

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FIG. 2. Time series of area-averaged rainfall in APHRODITE dataset (black bars; left axis)
and 7–25-day-filtered rainfall anomaly (solid line; right axis) from 1 Jun to 30 Sep 1989. Blackand white circles denote extreme and moderate active peaks of the ISO in this study, respectively. The ‘‘L’’ indicates the extreme and moderate active peaks associated with LPS
events. Black squares denote break peaks. Dashed lines indicate the criteria (61.0s, 11.5s,
and 12.0s of climatological 7–25-day rainfall variance) used to select the peaks.

2. Data and analysis method
a. Dataset
The Asian Precipitation–Highly-Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE) dataset (Yatagai et al. 2009,2012) was used to analyze the rainfall variations overBangladesh. APHRODITE is a daily rainfall producton a 0.258 3 0.258 grid based on rain gauge observations. The period analyzed in this study is from June to
September covering 29 years (1979–2007). To examine
the temporal variation of rainfall over Bangladesh, therainfall data obtained from an area encompassed by
the coordinates 218–258N, 888–928E (rectangular box in
Fig. 1) were averaged. Rain gauge stations were distributed uniformly over Bangladesh during the study
period (Yatagai et al. 2009); thus, the area-averaged
rainfall series does not depend on a specific region. Thisdataset was also used to depict the spatiotemporal
structure of rainfall associated with the submonthlyscale ISO over Bangladesh. However, because this dataset is restricted to land areas, data from the TropicalRainfall Measuring Mission (TRMM) 3B42 rainfall
product, on a 0.258 3 0.258 grid, were also used for thecase study in section 4.
To examine the spatiotemporal structure of atmospheric circulation associated with the submonthly-scale
ISO over Bangladesh, we used Japanese 25-year Reanalysis Project (JRA-25) data on a 1.258 3 1.258 grid
(Onogi et al. 2007). In accordance with APHRODITE,
the period from June to September for 29 years (1979–2007) is used for the analysis. Moreover, we detected
and tracked LPSs that formed over the Indian monsoon
region during the study period by using the reanalysis
data. The detailed procedure is described in section 4.

b. Definition of active and break peaks
To remove annual cycles, daily rainfall anomalies werecomputed by subtracting a 121-day (about 4 months)
running mean from the original time series for each year.
Submonthly (7–25 days) fluctuations were then extracted
by applying a Lanczos filter (Duchon 1979) to theanomaly series throughout the entire year. This method issimilar to that described by Fujinami et al. (2011). As an
example, Fig. 2 presents the daily rainfall time series forthe 1989 monsoon season (black bar) and the 7–25-dayfiltered rainfall (solid line). The daily rainfall time series
exhibits clear active and break cycles on the submonthly
time scale, particularly during June, July, and September. Active and break peaks of the 7–25-day-filteredrainfall time series correspond well with those of theunfiltered time series. To extract the active peaks of theISO, the 29-summer climatological standard deviation based on the 7–25-day-filtered anomalies (1.0s:;6.8 mm day21) was used as a criterion. As the purposeof this study is to clarify the process that causes highamplitude active peaks of the ISO, positive extremes
that exceeded 12.0s were selected as extreme active
peaks (filled circles in Fig. 2). To demonstrate the distinct difference between the active peaks, we also extracted moderate active peaks defined as those lying
between 11.0s and 11.5s (open circles in Fig. 2). Notethat active peaks between 11.5s and 12.0s (e.g., theactive peak in early July 1989) were not included in either classification. In total, 49 extreme and 58 moderate
active peaks were identified over the 29 years. The mean
daily rainfall is 47.1 and 22.0 mm day21 for the extreme
and moderate active peaks, respectively. Additionally,
148 break peaks, defined as negative extremes of lessthan 21.0s, were also identified to show general

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FIG. 3. Composites of APHRODITE rainfall (shading), 850-hPa geopotential height (contour), and verticallyintegrated (from the surface to 100 hPa) moisture flux vectors for the (a) extreme active, (b) moderate active, and(c) break peaks. The thick solid lines indicate the axis of the monsoon trough from India to Bangladesh. (d) Composite
differences between active and break peaks. Only 95% statistically significant vectors are plotted.

features of the submonthly-scale ISO over Bangladesh(open squares in Fig. 2).

3. Synoptic features of submonthly-scale ISO overBangladesh
To investigate the spatiotemporal structure of rainfall andatmospheric circulations associated with the submonthlyscale ISO over Bangladesh, composite analyses wereconducted for extreme active, moderate active, andbreak peaks. Figure 3 shows composites of the total atmospheric circulation fields and rainfall for each peak.
The position of the monsoon trough was identified using
the 850-hPa streamfunction. In the extreme active peak,heavy rainfall ($30 mm day21) is observed over bothBangladesh and the high-elevation regions of the Shillong Plateau and Chittagong Hill Tracts (Fig. 3a). The
monsoon trough is located along the foot of the Himalayas in the low-level troposphere and has its eastern end

over Bangladesh. In this synoptic situation, southwesterly moisture flow dominates from the Bay of Bengal to
Bangladesh. In the moderate active peak, the area ofheavy rainfall is restricted to the Shillong Plateau andChittagong Hill Tracts (Fig. 3b). Note that rainfall of
more than 10 mm day21 still can be observed over theentire country. The atmospheric circulation features
show close similarity to those of the extreme active peak.
In contrast, in the break peak, the rainfall amount exhibits a significant decrease over and around Bangladesh(Fig. 3c). The monsoon trough shifts southward and its
eastern end is located over the head of the Bay of
Bengal. The wind direction over Bangladesh becomes
southerly/southeasterly, instead of southwesterly, owing
to the shift of the monsoon trough. The features of thesynoptic-scale circulation in both the active and breakpeaks are similar to those obtained by Ohsawa et al.
(2000) and Fujinami et al. (2011). Figure 3d shows thecomposite differences between the active (147 peaks

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defined as positive extremes that exceed 11.0s) andbreak peaks. The vectors where either the zonal or
meridional moisture flux component surpasses the 95%
confidence level, according to a Student’s t test, are plotted. The circulation anomalies show a remarkable westerly
moisture flux anomaly around 208–258N, accompanied bya cyclonic circulation anomaly over northern Bangladesh
and an anticyclonic circulation anomaly over the Bay of
Bengal. Thus, these results suggest that the westerlycomponent toward Bangladesh plays an important role inincreasing rainfall over Bangladesh.
Although these composites demonstrate obvious differences between the active and break peaks, our focus
in this study is the differences between the active peaks.
Figure 4 shows the composite differences between theextreme and moderate active peaks. As shown in Fig. 3,
a remarkable positive rainfall anomaly is observed overnortheastern India, all of Bangladesh, and western
Myanmar. The circulation anomalies show a wavelike
pattern with a ridge axis at about 838E and a trough axis
at about 908E. A striking feature is a local cyclonic circulation anomaly centered on northern Bangladesh, enhancing westerly moisture flux anomalies over the headof the Bay of Bengal and Bangladesh. Associated withthe local cyclonic circulation anomaly, moisture flux
convergence is also enhanced remarkably over Bangladesh (not shown). The composite differences between theextreme and moderate active peaks reconfirm that theintensity of the westerly component is an important factor for increasing rainfall over Bangladesh. To understand the mechanism that causes the enhancement oflocal cyclonic circulation over Bangladesh, the following
section focuses on the activity of the LPSs.

4. Relationship between LPS activity andsubmonthly-scale ISO over Bangladesha. Procedure for identifying LPSs
In this section, the relationship between LPS activity
and active peaks of submonthly rainfall variability overBangladesh is examined. In the present study, an LPS
means a cyclonic circulation with a closed contour of
geopotential height in the low-level troposphere. We
attempt here to detect and track LPSs that formed overthe Bay of Bengal and adjoining land areas, during the
29-yr study period (1979–2007), using reanalysis data.
Some previous studies have used long-term data of thegenesis and tracks of lows and depressions that formed
over the Indian monsoon region (e.g., Goswami et al.
2003; Krishnamurthy and Ajayamohan 2010). These data
were derived from a sea level pressure analysis of daily
weather reports published by the India Meteorological
Department. Although we also tried to detect LPSs based

VOLUME 142

FIG. 4. Composite differences of APHRODITE rainfall (shading), 850-hPa geopotential height, and vertically integrated (fromthe surface to 100 hPa) moisture flux vectors between extreme andmoderate active peaks. Only 95% statistically significant vectors
are plotted.

on the distribution of sea level pressure using an objective
criterion, small-scale LPSs, which significantly affectrainfall variation over Bangladesh (shown in section 4b),
were not detected well. Takahashi and Yasunari (2008)
used the 700-hPa relative vorticity to identify tropicalstorms and weaker tropical cyclones around the Indochina domain. However, the Indian monsoon region ischaracterized by strong meridional cyclonic shear in thelower troposphere, which is due to the westerlies (easterlies) to the south (north) of the monsoon trough (seeFig. 3). As the region of strong cyclonic shear is usuallyaccompanied by large positive vorticity, it is difficult todistinguish closed LPSs from the cyclonic shear. Therefore, we detected LPSs using a combination of relative
vorticity and geopotential height at 850 hPa. The procedures are as follows. 1) A candidate LPS center is
identified by satisfying a criterion of a minimum 850-hPa
geopotential height. That criterion is defined by the difference from surrounding grids, the thresholds of whichare 2 and 1 gpm for the surrounding 8 and 16 grids, respectively. 2) If the 850-hPa relative vorticity of thecandidate center is more than 6.0 3 1025 s21, it is identified as a legitimate LPS. 3) To track the LPS, a searcharea is defined as a range of six grids (about 800 km) to
the west side and three grids (about 400 km) to the east,
south, and north sides from the LPS center. If, on thefollowing day, an LPS appears within the search area, it isconsidered the same one as detected previously. 4) Only
LPSs formed within the genesis region are detected andare tracked within the tracking region (each region isshown in Fig. 5). These regions cover a dense area of thegenesis and track of LPSs described by previous studies

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FIG. 5. Distributions of genesis points (red circles) and tracks(solid lines) of all LPSs identified during June–September (JJAS)
for the period 1979–2007. The size of red circles denotes thenumber of geneses in each grid point. The inner and outer domains
bounded by dashed lines indicate the regions used to detect andtrack the LPSs, respectively (see the text for details).

(e.g., Fig. 1 of Krishnamurthy and Ajayamohan 2010).
Some LPSs have also been seen over the Arabian Sea inthese previous studies; however, they are not expected toaffect directly the rainfall over Bangladesh. Figure 5 shows
the distribution of the genesis point and track of the all the
LPSs identified during the 29 summer monsoon seasonsaccording to this procedure. The size of the red circles
indicates the number of LPS geneses at each grid point.
During this period, 274 LPSs were identified with a seasonal average of 9.4 and an average lifetime of 4.9 days.
Most of these LPSs were generated over the head of the
Bay of Bengal and moved northwestward. This distribution captures well the characteristics of the classical LPSs,
such as monsoon lows and depressions. In addition, thesmall-scale systems that are the focus of this study are also
identified successfully by our method. To examine theactivity of the LPSs related to submonthly rainfall variability over Bangladesh, we defined an LPS case as a dayon which at least one LPS existed between day 21 and day
11 of a peak day. As a result, 29 out of 49 extreme (36 outof 58 moderate) active peaks were identified as LPS cases.
This result indicates that about 60% of the selected peaks
are related to the LPSs in both active peaks.

b. Case study
Figure 6 shows an example of an LPS related to an
extreme active peak in the summer of 2003 using
TRMM rainfall, 850-hPa geopotential height, and windvectors. The date of the extreme active peak was
21 June. This LPS formed over southwestern Bangladesh on 17 June with a weak cyclonic circulation, andremained almost stationary until its termination on
22 June. Interestingly, in general, tropical disturbances4763(such as tropical depressions, tropical storms, and typhoons) develop over the sea and weaken after landfall,
whereas this LPS formed over the coastal region andattained its maximum strength over land. In the initialstage of the LPS (17–19 June), heavy rainfall occurred
mainly in the low-level monsoon westerly region overthe Bay of Bengal. In the mature stage (20–22 June),heavy rainfall was observed in the southeast sector of the
LPS, corresponding to the region of strong southwesterly winds from the LPS. The low-level southwesterly
winds toward the coastal mountains of Myanmar cause
intense convection on the windward side via orographic
lifting (e.g., Xie et al. 2006; Houze et al. 2007). Therefore, this result suggests that the heavy rainfall in thesoutheast sector of the LPS was induced by the physicalinteraction between the southwesterly winds and themountainous terrain to the southeast of Bangladesh.
Note that high rainfall of more than 30 mm day21 can also
be observed near the LPS center (i.e., over Bangladesh)on 21–22 June. Moreover, as an important characteristic,
this LPS had a horizontal scale of about 500 km
throughout its lifetime.
As mentioned in section 1, LPSs such as monsoon
lows and depressions typically have horizontal scale of
1000–3000 km and move northwestward exhibiting
maximum rainfall in their southwest sectors. When these
LPSs move toward central India, associated southerly/
southeasterly winds dominate over Bangladesh (see Fig. 2of Krishnamurthy and Ajayamohan 2010). This situation
is similar to that during the break peak in our results(Fig. 4c); thus, Bangladesh receives little rainfall during
their northwestward passage. On the other hand, we
found that one LPS is closely related to heavy rainfall
over Bangladesh. The LPS examined here has obviously
different characteristics from the monsoon lows and depressions described in previous studies. However, thisresult is derived from the analysis of a single heavy
rainfall event over Bangladesh. Therefore, the statisticalcharacteristics of LPSs associated with active peaks overBangladesh are analyzed in section 4c.

c. Statistical characteristics
1) LPS LOCATION
Figure 7 shows a distribution of the appearance frequency of LPS centers associated with extreme andmoderate active peaks for 29 years, together with a histogram with respect to longitude. The frequency is obtained by counting the number of LPS centers detected
between day 21 and day 11 of peak days for each grid
point. In total, 85 and 92 LPSs were counted in the extreme and moderate active peaks, respectively. In bothactive peaks, the LPS centers are located mainly along

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FIG. 6. Time evolution of an LPS associated with an extreme active peak based on TRMM 3B42
rainfall (shading), 850-hPa geopotential height (contour), and wind vectors during 16–23 Jun 2003. The
date of 21 Jun corresponds to the extreme active peak. The dates of 17 and 22 Jun correspond to thegenesis and termination of the LPS, respectively, identified using our method (see the text).

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FIG. 7. (bottom) Distribution of the frequency of LPS centers in each grid point and (top) its histogram with respect
to longitude for the (a) extreme and (b) moderate active peaks. The thick solid line indicates the axis of the monsoon
trough from India to Bangladesh.

the monsoon trough, which extends from northwestern
India to Bangladesh. However, the longitudinal distributions show a considerable difference between theextreme and moderate active peaks. In the extreme active peak, the LPS centers are clustered over and around
Bangladesh (Fig. 7a). The maximum frequency appears
over Bangladesh at grid points of 23.758N, 908E and23.758N, 88.758E. In terms of location, this result supports that the LPS shown in Fig. 6 is a typical example ofthe extreme active peak. In the moderate active peak,the centers are concentrated over the Ganges Plainaround 258N, 858E (Fig. 7b). The high-frequency area
shifts slightly to the northwest side of that of the extreme
active peak. The locational difference between the extreme and moderate active peaks contributes to form thelocal cyclonic circulation anomaly over Bangladesh shownin Fig. 4. We also examined the location of LPS centerswhen active peaks in the submonthly-scale ISO lay between 11.5s and 12.0s (not shown). Despite a lower
percentage of LPS cases (;40%), the frequency with respect to longitude showed a maximum at 86.258 and 87.58E,
which is between the extreme and moderate active peaks.
Therefore, these results suggest strongly that the small
difference in the location of LPS centers has significant
impact on the amplitude of active peaks over Bangladesh.

2) LPS TRACK
Figure 8 shows the distributions of LPS tracks andgenesis points associated with extreme and moderate
active peaks. To show their spatial distributions morequantitatively, Fig. 8 also shows the frequencies thatwere computed on 2.58 3 2.58 grid boxes by counting thenumber of LPS within a given grid box. In both active
peaks, the LPS tracks and genesis points are concentrated around the monsoon trough. In the extreme active peak, most of the LPSs formed over the head of the
Bay of Bengal and around Bangladesh (Fig. 8a). The
distribution of LPS frequency shows the highest frequency around Bangladesh, indicating that it is rare for
an LPS to move northwestward like a monsoon depression(Fig. 8b). This result is consistent with that of the casestudy shown in Fig. 6. In the moderate active peak, the
LPSs occur most commonly over the northern tip of the
Bay of Bengal (Fig. 8c), and they tend to move northwestward to the Ganges Plain. A relatively large numberof LPSs are also formed over the Ganges Plain around
258N, 858E, which tend to remain almost stationary. The
characteristics of these LPS tracks are reflected in the highfrequency area in Fig. 8d. The difference in the distribution
between the extreme and moderate active peaks suggeststhat the LPSs related to the extreme active peak are distinct from those related to the moderate active peak.

3) SPATIAL STRUCTURE OF LPSS
To understand the processes responsible for increasing the amplitude of active peaks, we examined thespatial structures of LPSs associated with the extreme
and moderate active peaks. Figure 9 shows the composite structures of the LPSs for each active peak, which
are obtained by superposing the individual LPS centersat the origin of the coordinate system. In the extreme
and moderate active peaks, 85 and 92 samples were used
to construct the composites, respectively. Figures 9a and9c show the composites of 850-hPa geopotential height

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FIG. 8. (a) As in Fig. 5, but for the extreme active peak. (b) Distribution of the frequency of the LPS tracks for theextreme active peak, which is computed on 2.58 3 2.58 grid boxes by counting the number of LPS within a given gridbox. (c),(d) As in (a),(b), but for the moderate active peak. The gray shading in the top panels shows an elevation
higher than 500 m. The thick solid lines in the bottom panels indicate the axis of the monsoon trough from India to
Bangladesh.

and wind vectors. In both active peaks, closed cyclonic
circulations are clearly identified to the north of the
predominant westerly/southwesterly monsoon. A typical horizontal scale of these LPSs is estimated from thesize of the outermost closed contour. In the extreme
active peak, the horizontal scale of the LPS is estimatedto be 600–700 km (Fig. 9a). The low-level winds associated with the LPS have a maximum speed of 13–14 m s21
in the southeast sector. In the moderate active peak, thehorizontal scale is estimated to be about 600 km, which isalmost the same size as in the extreme active peak(Fig. 9c). The maximum wind speed is also seen in thesoutheast sector, but it is weak compared with that in theextreme active peak (;10 m s21). In both active peaks, it
is highly probable that the maximum winds in the southeast sector are caused by the superposition of the largescale monsoon westerlies and the LPS’s own flows. As wellas the maximum wind speed, the LPSs exhibit stronger
central vorticity in the extreme active peak than in
the moderate active peak (not shown). Although thehorizontal scale in the extreme active peak seems to beslightly larger than that of the moderate active peak, bothof these LPSs are much smaller than the monsoon depressions described by previous studies (e.g., Krishnamurti
et al. 1975; Godbole 1977). The similar horizontal scale of
the LPS between the extreme and moderate active peaks
emphasizes the importance of their locations for themodulation of the amplitude of active peaks.
Figures 9b and 9d show the composites of verticallyintegrated (from the surface to 100 hPa) moisture fluxand its divergence for each active peak. A striking feature in the extreme active peak is the remarkable convergence field in the southeast sector of the LPS(Fig. 9b). In the composite map, Bangladesh corresponds to the east side of the LPS. Thus, the spatiallylocalized feature of the strong convergence is influenced
undoubtedly by the surrounding mountainous terrain,such as the Shillong Plateau and Chittagong Hill Tracts.
This result is consistent with the maximum rainfall in thesoutheast sector of the LPSs during the mature phaseshown in Fig. 6. A relatively large convergence is also
located in the northeast sector away from the center ofthe LPS, which is likely caused by the orographic effectto the northeast of Bangladesh (i.e., the Shillong

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FIG. 9. Horizontal structures of the composite LPS. (a) 850-hPa geopotential height (contour) and wind vectors forthe extreme active peak. (b) As in (a), but for vertically integrated (from the surface to 100 hPa) moisture flux and its
divergence (contour). The contours for moisture flux divergence are shown at 24.0, 23.0, 22.0, and 21.0. The unit is1 3 1024 kg m22 s21. (c),(d) As in (a),(b), but for the moderate active peak. In all panels, the vectors and shaded areas
indicate that the anomaly is statistically significant at the 99% confidence level.

Plateau). In the moderate active peak, a remarkable
convergence field is formed around the LPS center, but
the intensity is weak compared with that of the extreme
active peak (Fig. 9d). As Bangladesh corresponds to theeast side of the LPS in the composite map, the convergence around the center seems to be due to the effect of
surface friction rather than surrounding terrain. Notealso that weak convergence fields are observed over thesoutheast and northeast side of the LPS, suggesting
orographic rainfall on the windward slopes of the Shillong Plateau and Chittagong Hill Tracts. Thus, the difference in the moisture flux divergence field between theextreme and moderate active peaks suggests that thesegeographical features and the horizontal scale of the
LPSs are essential factors that enhance the convergenceof the LPSs. The submonthly-scale ISO of rainfall overBangladesh is caused by the low-level zonal windfluctuations associated with the north–south shift of themonsoon trough (e.g., Ohsawa et al. 2000; Fujinami et al.
2011). That is, when the southeasternmost portion of themonsoon trough is situated over Bangladesh during theactive phase of the ISO, the strong meridional cyclonicshear and abundant moisture provide favorable environmental conditions for inducing rainfall in this region.
Under this synoptic situation, the LPSs have a role infurther enhancing the moisture convergence locally and inenlarging the amplitude of active peaks over Bangladesh.
Figure 10 shows the vertical cross sections of eachvariable in the extreme active peak, which are based on
zonal vertical planes passing through the LPS center
described in Fig. 9. In the vertical structure of meridional
wind (Fig. 10a), a cyclonic circulation associated with the
LPS is identified from the surface up to the 300-hPa level,
which prevails particularly in the lower troposphere

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MONTHLY WEATHER REVIEWVOLUME 142

FIG. 10. Vertical cross sections of the composite LPS for the extreme active peak along the longitude of the LPS center shown in Fig. 9.
(a) Meridional wind. The contour interval is 2 m s21. (b) Temperature anomaly. The contour interval is 0.3 K. (c) Specific humidity
anomaly. The contour interval is 0.3 kg kg21. (d) Horizontal wind divergence. The contour interval is 0.5 3 1025 s21. In all panels, theshaded areas indicate that the anomaly is statistically significant at the 99% confidence level.

below the 600-hPa level. The vertical structure of thezonal wind also shows an associated cyclonic circulation
extending to about 300 hPa (not shown). These results
indicate that the vertical scale of the LPS is about 9 km.
Figure 10b shows the vertical structure of temperature
anomaly defined as a departure from the 29-yr summer
mean at each pressure level. The thermal structure shows
clearly a cold core in the lower troposphere and a warm
core in the upper levels. The reversal of the thermal
structure occurs between the 800- and 700-hPa levels.
Figure 10c shows the vertical structure of the specifichumidity anomaly. A deep moist layer is observed fromthe surface up to the 300-hPa level, in accordance with thedepth of the cyclonic circulation in Fig. 10a. The maximum anomaly appears in the middle troposphere near
the 700-hPa level. This moisture distribution seems to becaused by the vertical advection of water vapor due to theorganized convection. Figure 10d shows the verticalstructure of horizontal wind divergence. A striking feature is the strong convergence in the lower tropospherefrom the surface up to about 850 hPa. The low-level
convergence is stronger on the east side of the LPS center
than on the west side. This is consistent with the verticallyintegrated moisture flux convergence in Fig. 9, indicating
again the effects of the geographical features around
Bangladesh. In the moderate active peak, the LPS
shows vertical structures similar to the extreme active
peak, except for the divergence field with maximum convergence around the LPS center (not shown). These

structures of the LPS are compared with monsoon depressions over India and discussed in the following section.

5. Discussion
a. Regional-scale rainfall distribution and associatedmesoscale processes in LPS and non-LPS cases
In the previous section, we found that about 60% of
the selected active peaks are related to LPSs, and thatlocation is an important factor in determining the amplitude of the active peaks. However, about 40% ofthem occur without LPSs (hereafter referred to as a nonLPS case). This means that the non-LPS case also contributes to increasing rainfall over Bangladesh. Figure 11
shows the distributions of mean rainfall and 925-hPa windin the extreme and moderate active peaks. In the extreme
LPS case, heavy rainfall ($50 mm day21) is observed overmost of Bangladesh and high-elevation regions such as the
Shillong Plateau and Chittagong Hill Tracts (Fig. 11a).
Low-level winds are mainly southwesterly/southerly overBangladesh, in association with cyclonic circulation over
the west of the country. In contrast, in the extreme nonLPS case, the heavy rainfall is limited to the regions
around the Shillong Plateau and Chittagong Hill Tracts(Fig. 11b). Low-level winds are predominantly westerly/
southwesterly over Bangladesh, suggesting orographicallyinduced rainfall on the windward slopes of thesehigh-elevation regions. Corresponding to large-scale

DECEMBER 2014

HATSUZUKA ET AL.

geographical features, high rainfall ($20 mm day21) isalso observed along the foot of the Himalayas. Note thatthe rainfall over Bangladesh decreases compared withthe extreme LPS case, although more than 30 mm day21
is still observed over the entire country. This indicatesthe importance of LPSs for increasing rainfall over thelowland area of Bangladesh. In the moderate LPS case,high rainfall is located mainly over southern Bangladesh, accompanied by a cyclonic circulation around
858E (Fig. 11c), whereas in the non-LPS case it appears
over the northeast and southeast of Bangladesh(Fig. 11d). Comparing the non-LPS cases (Figs. 11b,d),the westerly/southwesterly winds are stronger in theextreme active peak than in the moderate active peak.
Using a regional atmospheric model, Sato (2013) indicatedthat during the active period in June–July 2004, thesouthwesterly wind speed in the lower troposphere isa crucial factor with regard to heavy rainfall over the
Shillong Plateau. Our results also suggest that the intensityof the low-level winds is one of the important factors forenhancing the amplitude of active peaks over Bangladesh.
The diurnal cycle during the active phase of submonthlyscale ISO also plays an important role in heavy rainfall overBangladesh. Ohsawa et al. (2000) showed that during theactive phases in 1995, convective activity over Bangladesh
has a clear diurnal cycle with its peak during the latenight–early morning hours. Using a nonhydrostatic
cloud-resolving model, Kataoka and Satomura (2005)
found that late night–early morning precipitation maxima in northeastern Bangladesh on 15–18 June 1995
were associated with squall-line precipitation systems,
triggered near the southern foot of the Shillong Plateau,
which moved southward. Recently, Sato (2013) reported
that the simulated diurnal cycle of rainfall around theShillong Plateau becomes more stimulated during theactive phase with a late night–early morning peak. He
also showed that the low-level jet develops around the
900-hPa level over windward areas such as Bangladesh
during the midnight–early morning hours, suggestingthat the diurnal cycle of the low-level jet contributes tothe ISO as well as the diurnal cycle of rainfall. Our results in this study also show similar synoptic features in the
non-LPS case (Figs. 11b,d). Thus, the same processes are
expected to be responsible for the non-LPS active peaks
over Bangladesh. However, the rainfall and circulation
fields in the extreme active peak show remarkable differences between the LPS and non-LPS cases (Figs. 11a,b),
suggesting that different mesoscale processes are responsible for increasing the rainfall over Bangladesh.
We did not examine the mesoscale processes associatedwith the LPS case in this study; however, this issue isimportant in understanding the processes that cause
heavy rainfall over Bangladesh.

4769

b. Vertical structure of LPSs over Bangladesh andIndian monsoon depression
In the previous section, we demonstrated that the
LPSs associated with active peaks over Bangladesh havedifferent horizontal scales and rainfall (and moisture flux
convergence) distributions from the monsoon depressions
over India. In contrast, the vertical scale of the LPS, which
is about 9 km (Fig. 10a), agrees roughly with that of themonsoon depressions obtained by Krishnamurti et al.
(1975) and Godbole (1977). The cold/warm core structure(Fig. 10b) also resembles well that of the monsoon depressions described by the above two studies, but theirresults show the cold core center near the 800-hPa leveland not at the surface level. Krishnamurti et al. (1975)
noted that this thermal structure is peculiar to themonsoon depressions over India, by comparing themwith other tropical disturbances. Murakami (1977) examined the vertical structures of monsoon lows over theinland and coastal areas of northern India, but his results
did not show an obvious cold core in the lower troposphere for either region. He mentioned that this discrepancy probably reflects the difference between the
monsoon depressions and the relatively weak monsoon
lows. These results indicate that the LPSs related to theextreme active peaks over Bangladesh have a verticalstructure similar to that of the monsoon depressions. In
contrast, a remarkable difference in the vertical structures
between Bangladesh and India is found in the divergence
field. As shown in Fig. 10d, the LPSs over Bangladesh havestronger convergence on the east side of the LPS center inthe lower troposphere, whereas the monsoon depressions
over India, obtained by Godbole (1977), present strong
convergence on the west side of the depression center fromthe surface up to the 400-hPa level. He also mentioned thatthe presence of the pronounced convergence (and rainfall)on the west side contributes to the northwestward propagation of the depressions. Therefore, the difference inthe convergence/rainfall distribution of the LPSs may also
be related to their direction of propagation (i.e., as a suppressive effect on northwestward movement).

c. Genesis and development processes of LPSs overBangladesh
The formation processes of the LPSs are also an important issue because the LPSs provide more rainfall
over the flat lowland of Bangladesh during the extreme
active peak (Fig. 11). The time evolution of the LPS has
been shown in Fig. 6. According to this case study, a lowlevel monsoon westerly seems to flow into Bangladesh
along the Chittagong Hill Tracts and Shillong Plateau
during the formative stage (16–18 June 2003). That is,these orographic features seem to act in the formation

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MONTHLY WEATHER REVIEWVOLUME 142

FIG. 11. Spatial distribution of mean rainfall in APHRODITE dataset (shading) and 925-hPa wind vectors for the(left) extreme and (right) moderate active peaks. (a),(c) LPS case. (b),(d) Non-LPS case.

and development of the vortex over the Bangladesh
region in front of their windward slopes. In contrast, if
the low-level wind flows against these areas of mountainous terrain, forced lifting along their windward
slopes is expected to be more dominant without any
formation of a vortex (i.e., non-LPS case). Hence,
whether a vortex-type LPS is formed over and near
Bangladesh may be controlled by a slight difference inthe low-level wind direction toward the surrounding
regional-scale terrain, such as the Shillong Plateau andChittagong Hill Tracts. Fujinami et al. (2011) reported
that the submonthly rainfall ISO over Bangladesh iscaused by the low-level zonal wind fluctuations over
and around Bangladesh, associated with the westwardpropagating submonthly-scale ISO. In the active ISOphase, an anticyclonic anomaly from the western Pacificis located over the Bay of Bengal (see Fig. 7 of Fujinami
et al. 2011; Fig. 10 of Fujinami et al. 2014). This synoptic
situation enhances the westerly/southwesterly flow over
and around Bangladesh, which provides a favorable
environment for inducing high rainfall within thisregion, together with the surrounding regional mountains. In this study, we found that LPSs bring highrainfall on submonthly time scales over Bangladesh(e.g., Figs. 2 and 6), suggesting that the genesis and development of the LPSs are also controlled by the
submonthly-scale ISO. To illustrate the phase relationship between the submonthly-scale ISO and an LPS fora typical case, Fig. 12 shows the time evolution of theatmospheric circulation related to the ISO during 24–
29 July 1989. The date of 29 July corresponds to theextreme active peak of rainfall over Bangladesh (Fig. 2).
The red circles in Fig. 12 indicate the central positions of
the LPS responsible for the active peak. This LPS
formed over the head of the Bay of Bengal on 26 Julyand disappeared over Bangladesh on 29 July. The horizontal scale of the LPS is estimated to be about 500 km(not shown), consistent with statistical characteristics
shown in this study. Figure 12 shows a remarkablewestward propagation, as seen in previous studies(Fujinami et al. 2011, 2014). On 24 July, an anticyclonic
circulation appears over the westernmost Pacific aroundDECEMBER 2014

HATSUZUKA ET AL.

208N, 1208E and moves westward to the Indian subcontinent by 29 July. On 26 July, when the anticyclonicanomaly moves into the Bay of Bengal, the LPS isgenerated simultaneously with the enhancement ofwesterly flow over the head of the Bay of Bengal. After
26 July, the LPS moves gradually northward under thestrong westerly anomaly because of the anticyclonic
circulation around 158N. Saha et al. (1981) suggested
that most Indian monsoon depressions could be attributed to the redevelopment of westward-propagating
residual lows of typhoons, tropical storms, or othertropical disturbances from the westernmost Pacific. In
Fig. 12, such a westward-propagating low/depression
can be found over western India on 24–26 July, inducing
the break phase over Bangladesh. The LPS over Bangladesh is indeed triggered by the anomalous high (notlow) related to the submonthly-scale ISO that propagates westward in a similar manner. Fujinami et al.
(2011) also revealed that the submonthly-scale ISO is
closely associated with the north–south shift of themonsoon trough. That is, when the anticyclonic circulation anomaly appears over the Bay of Bengal, themonsoon trough axis shifts northward and deepens
along the Ganges Plain. This situation is likely tocreate strong meridional shear of low-level westerlies
and cyclonic vorticity in this region, contributing to
the genesis and development of the LPS. Additionally, as mentioned above, the orographic features
around Bangladesh (i.e., the Shillong Plateau andChittagong Hill Tracts) appear to provide favorable
conditions for the genesis and development of sucha vortex over Bangladesh. Thus, large-scale intraseasonal dynamics for LPS genesis and development
probably differ from Indian monsoon depressions.
Recently, Fujinami et al. (2014) found that robust
circulation signals on the same time scale appear in
midlatitudes. It was reported that in the active phase,a cyclonic anomaly appears over and around the Tibetan Plateau throughout the troposphere, which
contributes to the enhancement of the westerly flow in
conjunction with the anticyclonic anomaly over the
Bay of Bengal. Therefore, using long-term data, weneed to investigate which factors of the submonthlyscale ISO determine LPS or non-LPS cases in active
peaks, and how the subtropical–midlatitude interaction affects the genesis and development of LPSs.

6. Summary
The relationship between the LPS activity and thesubmonthly-scale (7–25 days) ISO of rainfall overBangladesh during the summer monsoon season (June–
September) has been investigated using APHRODITE

4771

and TRMM 3B42 rainfall, and JRA-25 reanalysis data.
By detecting and tracking the LPSs formed over the
Indian monsoon region over a period of 29 years (1979–2007), we found that about 59% (62%) of extreme(moderate) active peaks are related to the LPSs. The
characteristics of LPSs for extreme and moderate active
peaks are summarized as follows.
In the extreme active peak, the locations of LPS
centers are clustered significantly over and around
Bangladesh. These LPSs are formed mainly over thehead of the Bay of Bengal and around Bangladesh, andthey tend to remain almost stationary throughout the
lifetime. The composite structures of the LPSs indicatethat the horizontal scale of the systems is about 600 km.
Moreover, as an important characteristic, the maximum
moisture convergence occurs on the southeast sideof the LPSs. This is likely caused by interaction between
the topography to the north and east of Bangladesh andthe prevailing southwesterly winds from the LPSs. In
contrast, in the moderate active peak, the LPS centers
are concentrated over the Ganges Plain around 258N,
858E. Most of the LPSs are formed over the northern tipof the Bay of Bengal and the Ganges Plain, and theytend to move northwestward and remain almost stationary, respectively. The horizontal scale of the LPSs issimilar to that of the extreme active peaks. The maximum moisture convergence occurs around the LPS
center, whereas the intensity of the convergence on thesoutheast side (corresponding to Bangladesh) is relatively
weak. Therefore, we conclude that the modulationof the amplitude of active peaks in the submonthlyscale ISO over Bangladesh is caused by small differences in the locations of the LPS centers. This result
also indicates that these LPSs have significantly different structures from the monsoon depressions
over India.
The vertical structures of the LPSs over Bangladesh
show that the associated cyclonic circulation extends up to
about 9 km (;300 hPa). The temperature fields are characterized by a well-defined cold core in the lower troposphere and a warm core in the upper levels. These verticalfields are similar to those of monsoon depressions. The
most remarkable contrast is found in the field of horizontal
wind divergence. In the extreme active peak, the LPS
has stronger convergence, particularly on the east sidein the lower troposphere, whereas the monsoon depressions show a maximum on the west side. This
contrast seems to be responsible for the difference inthe propagation characteristics compared with themonsoon depression.
The submonthly-scale ISO of rainfall over Bangladesh is dominated by the north–south shift of the monsoon trough; however, this study reveals the important

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MONTHLY WEATHER REVIEWVOLUME 142

FIG. 12. Time evolution of 7–25-day-filtered 850-hPa geopotential height (contour) and wind vectors during 24–29
Jul 1989. The date of 29 Jul corresponds to the extreme active peak over Bangladesh. The red circles indicate thecentral positions of the LPS identified by our method. Solid (dashed) contours represent positive (negative) values.
The contour interval is 10 gpm. Shading denotes areas at an altitude of more than 1500 m.

role that the LPSs have in enhancing the amplitude of
the active peaks. Furthermore, among the extreme active peaks, heavy rainfall over the lowland area of Bangladesh is more likely in the presence of an LPS than in its
absence. Therefore, the prediction of the genesis andtracks of the LPSs is a crucial aspect in the prediction ofseasonal rainfall over Bangladesh, and it is an important
challenge for future research. Because of their small
scale, a numerical simulation by regional and cloudresolving models would be necessary for such a study.

Acknowledgments. The authors thank Drs. T. Hayashi,
T. Kumagai, 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 Grant-in-Aid
for Scientific Research (B-22340137), Grant-in-Aid for
Scientific Research for Young Scientists (B-24740320),and Grant-in-Aid for Scientific Research (C-26400465)
from the Japan Society for the Promotion of Sciences(JSPS). Portions of this study were conducted as part ofDECEMBER 2014

HATSUZUKA ET AL.

a collaborative research program of the Hydrospheric
Atmospheric Research Center, Nagoya University.
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