INTERNATIONAL JOURNAL OF CLIMATOLOGY
Int. J. Climatol. 31: 1192–1204 (2011)
Published online 14 April 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/joc.2146

Characteristic intraseasonal oscillation of rainfall and its
effect on interannual variability over Bangladesh during
boreal summer
Hatsuki Fujinami,a * Daisuke Hatsuzuka,b Tetsuzo Yasunari,a Taiichi Hayashi,c Toru Terao,dFumie Murata,e Masashi Kiguchi,f Yusuke Yamane,g Jun Matsumoto,h Md. Nazrul Islamiand Arjumand Habibja Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japanb Weathernews, Chiba, Japanc Disaster Prevention Research Institute, Kyoto University, Kyoto, Japand Faculty of Education, Kagawa University, Kagawa, Japane Faculty of Science, Kochi University, Kochi, Japanf Institute of Industrial Science, The University of Tokyo, Tokyo, Japang Center for Southeast Asian Studies, Kyoto University, Kyoto, Japanh Department of Geography, Tokyo Metropolitan University, Tokyo, Japani Faculty of Meteorology, King Abdulaziz University, Jeddah, Saudi Arabia
j Bangladesh Meteorological Department, Dhaka, Bangladesh

ABSTRACT: Intraseasonal oscillation (ISO) of rainfall during the summer monsoon season (June–August) and its
plausible effect on interannual variation (IAV) of total summer monsoon rainfall over Bangladesh are examined using daily
rainfall data from 25 rain-gauge stations for 20 years (1981–2000). Submonthly scale (7–25 days) ISO is a dominantmode of summer rainfall fluctuation over Bangladesh. In the active (suppressed) ISO phases, a cyclonic (anticyclonic)
circulation anomaly develops at the low-level troposphere from northeastern India to Bangladesh, and an anticyclonic(cyclonic) circulation anomaly appears over the Bay of Bengal, accompanied by the enhancement of westerly/southwesterly(easterly/southeasterly) moisture flux anomalies over Bangladesh. The IAV of total summer monsoon rainfall is significantly
correlated with that of the rainfall variance, mainly in the submonthly scale, suggesting that the ISO activity controls the IAVof the total summer monsoon rainfall. Moreover, the spatial patterns of atmospheric circulation and convection associatedwith the ISO and the IAV are quite similar to each other around the monsoon trough, especially from northeastern India to
Bangladesh. In wet monsoon years, more frequent strong submonthly scale ISOs and a larger number of rainy days related
to the active ISO phase are evident compared with that of dry monsoon years. Climatologically, Bangladesh is one of themost predominant areas of submonthly scale ISO; however, 30–60-day ISO is not prominent. These characteristic featuresprobably allow the submonthly scale ISO to modulate the total seasonal rainfall and the spatial patterns of circulation andconvection over and near Bangladesh. Copyright  2010 Royal Meteorological Society
KEY WORDS

Asian summer monsoon; Bangladesh; rainfall; intraseasonal oscillation; interannual variation

Received 10 July 2009; Revised 22 February 2010; Accepted 8 March 2010

1.

Introduction

Bangladesh is one of the heaviest rainfall areas in theworld. In the summer monsoon season (June–August),
southwesterly/southerly winds from the ocean bringabundant moisture to Bangladesh. Geographically, thecountry encompasses the flat lowland (less than 10 m
above sea level) confluence zone of the Ganges, Brahmaputra, and Meghna rivers and is surrounded to thenorth, west, and east by high-elevation areas (theHimalaya Mountains/Tibetan Plateau, Chota Nagpur
Plateau, and Chittagong Hill Tracts, respectively), as* Correspondence to: Hatsuki Fujinami, Hydrospheric AtmosphericResearch Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya
464-8601, Japan. E-mail: hatsuki@hyarc.nagoya-u.ac.jp
Copyright  2010 Royal Meteorological Society

shown in Figure 1. Southern Bangladesh faces the Bay of
Bengal, from which southerly moist winds easily intrude
over the country. To the northeast of Bangladesh andsouth of the Himalaya Mountains, the Shillong Plateau
stands at ∼2000 m above sea level, extending east to
west. These geographic features and prevailing southwesterly/southerly winds strongly affect the spatial distribution of seasonal mean rainfall as well as individual precipitation systems over Bangladesh (e.g. Kripalani
et al., 1996; Ohsawa et al., 2000; Ohsawa et al., 2001;
Islam et al., 2005; Kataoka and Satomura, 2005; Teraoet al., 2006; Islam and Uyeda, 2007; Murata et al., 2008;
Terao et al., 2008). Heavy rains often lead to flood disasters over the flat, low-elevation lands of Bangladesh(e.g. Chowdhury, 2003; Murata et al., 2008). Therefore,INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER
Geographic features and rain gauge stations in Bangladesh
Himalaya Mountains

Shillong Plateau

Ganges Plain

India

Bangladeshl Tracts
Chittagong Hil

Chota Nagpur Plateau

Myanmar

Bay of Bengal
Chota Nagpur Plateau(m)Figure 1. Geographic features in and around Bangladesh. Closed circles
denote the locations of the 25 rain-gauge stations used in this study.
Shading denotes areas more than 50 m in surface altitude.

it is important to examine the detailed space–time characteristics of variations in rainfall and convection overBangladesh.
Intraseasonal oscillations (ISOs) of rainfall/convection
and associated atmospheric circulations are prominent
phenomena over the Asian summer monsoon region.
The ISOs can be divided into two types according to timescales: those of 30–60 days, the so-called
Madden–Julian Oscillation (MJO; Madden and Julian,
1972, 1994), and those of ∼6–25 days, or submonthly
timescales (e.g. Vincent et al., 1998; Fukutomi andYasunari, 2002, 2005; Fujinami and Yasunari 2004, 2009;
Hoyos and Webster, 2007). The submonthly timescale
contains the 10–20-day or quasi-biweekly timescales
reported in previous studies (e.g. Krishnamurti andArdanuy, 1980; Chen and Chen, 1993; Chatterjee andGoswami, 2004). The ISOs of the two timescales differ in their spatiotemporal structures; the 30–60-day
ISO is associated with northward propagation of convection from the equatorial Indian Ocean, whereas thesubmonthly scale ISO is a westward propagating featurefrom the western Pacific (e.g. Yasunari, 1979; Krishnamurti and Ardanuy, 1980; Chen and Chen, 1993; Annamalai and Slingo, 2001; Hoyos and Webster, 2007).
Only a few studies have addressed the ISO of rainfall/convection over Bangladesh. Ohsawa et al. (2000)
presented a detailed study on the ISO of rainfall in
Bangladesh for summer 1995 and showed that the submonthly scale ISO was dominant in summer monsoon
rainfall in that year. Rainfall variation in Bangladesh is
closely associated with the north–south oscillation of themonsoon trough. Rainfall increases when the axis of themonsoon trough is located at the foot of the Himalayas,
whereas it decreases when the axis is located to the southof Bangladesh. Murata et al. (2008) showed that submonthly variability in rainfall dominated at Cherrapunjee(located on the southern slope of the Shillong Plateau in
India near the Bangladesh border) in the 2004 monsoon
Copyright  2010 Royal Meteorological Society

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season, and the variation strongly affected river levels
around Sylhet (located in northeastern Bangladesh near
the Indian border). The MJO-scale ISO was not dominantduring the 2 years examined in those studies. Hartmann
and Michelsen (1989) investigated the dominant periodicity of intraseasonal rainfall variation over India, using
daily precipitation data from 3700 rain-gauge stationsfor 70 years. They reported that MJO-scale periodicity
predominated over most parts of India south of 23 ° N,
whereas spectral peaks around submonthly timescales
were limited to northern India where the MJO-scale signal did not dominate. These studies suggest that submonthly variation is more common than MJO-scale variation over Bangladesh.
With regard to interannual variation (IAV), althoughmany studies have examined the IAV of summer monsoon rainfall over India, few have addressed the relationship between this parameter and monsoon variability
over Bangladesh (e.g. Goswami et al., 1999; Krishnamurthy and Shukla, 2000). Kripalani et al. (1996) examined the IAV of summer (June–September) rainfall overBangladesh using data from 14 rain-gauge stations from1901 to 1977. They noted that, as a whole, the summer
rainfall over Bangladesh and India showed no relationship. Only the summer rainfall variation over northeastern
India, close to Bangladesh, had statistically significant
positive correlation with that over Bangladesh. They also
found that the summer rain over Bangladesh had no
significant correlation with the El Niño–Southern Oscillation (ENSO) signal, which could act as an external
forcing to influence seasonal mean rainfall over the Asianmonsoon region. Ahmed et al. (1996) examined the relationship between rainfall variation over Bangladesh andthat over El Niño using data from 1950 to 1992 obtainedat four rain-gauge stations in Bangladesh. They reported
that rainfall amount showed a negative or decreasing tendency during El Niño events in pre-monsoon, monsoon,
post-monsoon, and winter seasons. Chowdhury (2003)
also investigated the relationship between the IAV of rainfall over Bangladesh and ENSO for about 40 years (fromthe 1960s to 1990s). He found that although the summer monsoon rainfall in Bangladesh showed a decreasing (increasing) tendency during strong El Niño (LaNiña) years, the quantitative correspondence between thestrength of ENSO and the summer rainfall anomaly overBangladesh was very weak. One reason for the small
number of studies on IAV over Bangladesh may be thelack of reliable rainfall datasets available for climatological research. Cash et al. (2008) pointed out that resultson the IAV of monsoon rainfall in Bangladesh can varydepending on the dataset used [e.g. Global Precipitation
Climatology Project (GPCP), Climate Prediction CenterMerged Analysis of Precipitation (CMAP), or gaugebased gridded precipitation data]. Therefore, long-term
precipitation data from densely situated gauge stations
are needed to represent the IAV, as well as the ISOs, of
summer monsoon rainfall.
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H. FUJINAMI et al.

The relationship between the ISOs and the IAV of thesummer monsoon is an interesting topic in Asian monsoon studies. Krishnamurthy and Shukla (2000) notedthat seasonal mean monsoon rainfall over India consists of a large-scale persistent rainfall anomaly (related
to an external forcing such as sea surface temperature
anomalies) and a fluctuating intraseasonal component (i.e.
internal monsoon variability). The nature of the intraseasonal variability does not differ between wet and dry
years. In contrast, Goswami and Mohan (2001) reported
that higher occurrence frequency of active (break) conditions could result in stronger (weaker) than normal
seasonal mean precipitation over the Indian summer monsoon region. Furthermore, Goswami et al. (2003) showedthat the occurrence frequency of a westward moving
monsoon low-pressure system was ∼3.5 times higher in
the active phase of the monsoon than during the breakphase and that the tracks of these synoptic systems werestrongly spatially clustered along the monsoon troughduring the active phase of the monsoon. Hoyos and Webster (2007) suggested that propagation characteristics of
ISO on 25–80-day and 4–20-day timescales, coupledwith local effects of orography and land–atmosphere
feedbacks, modulate and determine the locations of summer mean precipitation patterns over the Asian monsoon
region. They also noted that, over the Indian Ocean, thecumulative effect of the 25–80-day rainfall variability
was directly connected to a proportion of the interannual
modulation of the summer rainfall, constituting one of
the most important sources of internally generated interannual variability. For Bangladesh, however, no previous
study has addressed how ISOs relate to the IAV of summer rainfall.
The objectives of this study were to (1) reveal a robustfeature of the ISO of rainfall over Bangladesh from June
to August (JJA) based on 20-year (1981–2000) daily
precipitation data from 25 rain-gauge stations; (2) reveal
the typical space–time structure of atmospheric circulation related to the ISO in Bangladesh; and (3) attempt to
explain how the IAV of summer mean rainfall is modulated by the ISO activity over and around Bangladesh.
Section 2 describes the datasets and analysis methods
used in this study. The generalised characteristics of the
ISO over Bangladesh and distinctive space–time structure of atmospheric circulation associated with the ISO
are discussed in Section 3. Section 4 presents the spatial pattern of atmospheric circulation and large-scaleconvection related to the IAV of summer rainfall overBangladesh. Probable effects of the ISOs on the IAV ofsummer rainfall are discussed in Section 5. Finally, theresults are summarised in Section 6.

For all stations, missing data accounted for less than 2%of the total observation times during the study period.
In this study, the rain-gauge data for each of the 25
stations were simply averaged for each day (defined asthe ‘all Bangladesh daily rainfall’ and abbreviated as
ABDR), similar to the method described by Ohsawa et al.
(2000). Local-scale spatial variations in rainfall series foreach station could be filtered out using this procedure.
These data were used as an index representing temporal variation in rainfall over the whole of Bangladesh.
The space–time structures of the atmospheric circulation fields associated with ISO and IAV in the ABDR
were also investigated using Japanese 25-year reanalysis(JRA25) data on a 1.25° latitude–longitude grid (Onogi
et al., 2007). Streamfunction and moisture flux vectors
integrated from the surface to 100 hPa were mainly usedin this study. Such relatively fine-resolution reanalysis
data are useful for studying Bangladesh because it is asmall country surrounded by areas of complex terrain.
Daily interpolated outgoing longwave radiation (OLR)
data on a 2.5° latitude–longitude grid were also used asa proxy for large-scale convective activity (Liebmann andSmith, 1996).
Daily anomalies of rainfall, OLR, and the reanalysis
data were computed by subtracting the first three harmonics of the annual cycle (about 120 days) for eachyear. Submonthly (7–25 days) perturbations were thencomputed by applying a Lanczos filter (Duchon, 1979) tothe detrended anomalies. As an example, Figure 2 shows
the time series of ABDR in summer 1993 (black bar)and 7–25-day filtered rainfall (solid line). The ABDR
time series for 1993 shows clear intraseasonal variation
on submonthly timescales. The active and break peaks ofthe 7–25-day filtered ABDR series correspond well withthose of the unfiltered ABDR series. Submonthly scale
intraseasonal variance was calculated based on the 7–25day filtered ABDR series and used as the index for the
ISO activity. In this study, an active phase was defined asa period in which the 7–25-day ABDR value exceededzero within one cycle of the ISO and its peak value in thatcycle exceeded more than 1.0 standard deviation of theclimatological mean 7–25-day rainfall variance (shownby shading in Figure 2).
Composites and daily-lag composites were used toinvestigate the temporal-phase relationships of ABDR
fluctuation, circulation fields, and OLR. Regression analysis was also used to examine the spatial pattern ofcirculation and OLR fields associated with the IAV of
Bangladesh monsoon rainfall.

3.
2.

Data and analysis method

Rainfall data observed at 25 rain-gauge stations acrossBangladesh and provided by the Bangladesh Meteorological Department (BMD) were the primary datasets
used to analyse rainfall variation (Figure 1). Daily averaged values were available for 20 years (1981–2000).
Copyright  2010 Royal Meteorological Society

3.1.

Intraseasonal variability over Bangladesh
Characteristic features of ISO over Bangladesh

To show the dominant period of ISO in rainfall for the
20 years, spectrum analysis was performed using the fastFourier transform (FFT) technique. The FFT analyses
used the detrended ABDR data (the first three harmonics
of the annual cycle were removed) between 15 May andInt. J. Climatol. 31: 1192–1204 (2011)

INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER

1195

Time series of rainfall and 7–25-day filtered rainfall anomaly(mm/day)(mm/day)

1.0 std dev

Figure 2. Time series of rainfall (black bars; left axis) and 7- to 25-day filtered rainfall anomaly (solid line; right axis) from 1 June to 31 August
1993. Periods with grey shading are active phases of the ISO in this study. Dotted lines denote the 20-summer (JJA) climatological 1.0 standard
deviation. The dashed line is the zero line for the filtered anomaly. See the text for the definition of the active phase.

1500

Variance (mm2/day2)

Interannual variation of total rainfall and rainfall variances
Bangladesh(25 stations) JJAABDR 15May - 15Sep. ensemble (20 yrs)

Total rainfall (mm)Power spectrum (mm2/day)

(a)

1000

500
(Year)

0
102
(b)

1

JJA total rainfall
7–25-day variance
7–90-day variance
30–60-day variance
3–6-day variance

10Interannual variation of power spectrum

400

Figure 4. Interannual variations of total ABDR (solid line with closed
circles; left axis), 7- to 90-day ABDR variance (dashed line withclosed triangles; right axis. The unit is mm2 day−2 ), 7- to 25-day
ABDR variance (dashed line with closed squares), 30- to 60-day ABDR
variance (dashed line with open squares), and 3- to 6-day ABDR
variance (dashed line with plus signs) for June–August.

400

400

0

40

400

400

400

1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000

400

400

102

101
period (days)

Figure 3. (a) The 20-summer ensemble spectrum of the ABDR timeseries from 15 May to 15 September (124 days). A red noise spectrum(dashed curve) and its 95% level of significance (solid curve) basedon a lag-1 autocorrelation are also shown. (b) Interannual variationof the ABDR spectrum from 1981 to 2000. The contour interval
is 400 mm2 day−1 . The thick solid line shows the 95% level ofsignificance. Areas having a power spectrum >800 (1600) mm2 day−1
are lightly (darkly) shaded.

15 September (124 days). Cosine tapering was applied to
10% of the time series at either end. A 3-point runningmean in the frequency domain was applied to the raw
Copyright  2010 Royal Meteorological Society

spectra to reduce estimate errors. Figure 3 shows the 20summer ensemble spectrum of the ABDR. A pronounced
peak appears at a period of about 14 days, and the peakexceeds the 95% confidence level (Figure 3(a)). A peakaround the 30–40-day period can also be seen but is notstatistically significant. Figure 3(b) presents the IAV of
the ABDR power spectrum for the 20 years. Periods of
7–25 days are common in most of the years, whereas
large IAV appears in peak periodicity of 30–40 days.
Note that a statistically significant peak often appears
at around 10 days in each year, although the ensemble
spectrum shows a peak spectrum at around 14 days.
Thus, submonthly scale ISO is the predominant modeof summer rainfall fluctuation over Bangladesh.
Figure 4 shows the IAV of total ABDR and intraseasonal and higher frequency (3–6-day) rainfall variances
during JJA. Note that the IAV of total summer monsoon
rainfall is well correlated with intraseasonal variance of
the ABDR, suggesting that the ISO activity could control
the IAV of summer monsoon rainfall over Bangladesh.
The relationship of the IAV of summer monsoon rainfall and 7–90-day ABDR variance (i.e. the all-period
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H. FUJINAMI et al.

range of ISOs) has a correlation coefficient of 0.74.
Correlation values larger than 0.56 are statistically significant at the 99% confidence level. This high correlation is not necessarily natural because the seasonal
total rainfall usually consists of a large-scale persistent rainfall anomaly, an intraseasonal component, anda higher frequency component. Furthermore, the 7–25day variance accounts for 60–90% of total intraseasonal variance, except in 1982 and 1990. The correlation
between the IAV of summer monsoon rainfall and the
7–25-day (30–60-day) variance is 0.65 (0.59). Throughout the study period, the 30–60-day ABDR varianceis very small compared with the 7–25-day variance,
except in 1982; the 7–25-day variance is ∼10 times
larger than the 30–60-day variance in the 20-year average. Besides the ISOs, westward moving synoptic-scalemonsoon lows and depressions are main rain-producing
systems with typical timescales of 3–5 days over the
Indian monsoon region. The occurrence frequency of
westward moving synoptic-scale disturbances was nearly
3.5 times higher in the active phase of the monsoon
than that during the break phase (Goswami et al., 2003).
This implies that high-frequency (3–6-day) synopticscale rainfall systems also significantly affect rainfall
variation over Bangladesh and, in turn, contribute to the
IAV of total summer monsoon rainfall there. However, in
(a)

OLR,

Bangladesh, statistically significant peaks are not found in
the high-frequency period range in more than half of the
20 years (Figure 3(b)), implying that synoptic-scale lows
and depressions on the 3–6-day period are not common
rain-producing systems in Bangladesh, unlike in India.
In addition, the correlation between the IAV of summer
monsoon rainfall and the 3–6-day variance is low, with acorrelation coefficient of 0.36 (cf Figure 4). These results
suggest that 7–25-day ISO activity is essential to modulate total summer rainfall and to understand the IAV ofsummer monsoon rainfall over Bangladesh. Nearly simultaneous increases in the variances of both timescale bands
in wet monsoon years suggest that the amplitude of the
7–25-day ISO may be modulated by the envelope of
convection in 30–60-day bands, with greater amplitude
during the active phase of the 30–60-day ISO. That is,a possible dynamical process may exist by which submonthly scale ISO can be superimposed on the active
phase of the 30–60-day ISO, whereas inactive 30–60-day
ISO phases do not suppress the submonthly ISO activity
significantly.
3.2. Space–time structure of atmospheric circulationand convection fields
Our focus here is to understand the space–time structureof atmospheric circulation and large-scale convection(c)and moisture flux

OLR,

and moisture flux

Active(W m−2)

(b)

Break

(Kg m−1 s−1)

Moisture flux and its divergence(W m−2)(d)

Moisture flux and its divergence

Break

Active(1.0 x 10−4 Kg m−2 s−1)

(Kg m−1 s−1)

(Kg m−1 s−1)(1.0 x 10−4 Kg m−2 s−1)

(Kg m−1 s−1)

Figure 5. Composites of the (a) total OLR (shadings), 925-hPa streamfunction (ψ), and vertically integrated (from the surface to 100 hPa)moisture flux vectors in a peak active phase of 7- to 25-day rainfall variation over Bangladesh. The contour interval for ψ is 2 × 106 m2 s−1 .
The topographic contour lines for 1000 and 3000 m are also shown (thick dashed lines). (b) As in (a), but for vertically integrated moisture fluxand its divergence (shading). Convergence areas less than −0.5 × 10−4 Kg m s−1 are shaded. (c), (d) As in (a) and (b), but for a peak breakphase. Thick solid lines in (a) and (c) indicate the axis of the monsoon trough from India to Bangladesh.
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INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER

associated with submonthly scale rainfall variation overBangladesh using composite analyses. On the basis of
the 7–25-day ABDR anomalies, positive and negative
extremes that exceeded the 20-summer climatological
1.0 standard deviation (∼10 mm day−1 ) were selected
as active and break peaks for the composites. In total,
78 peak active and 84 peak break phases were identified
during the 20-summer monsoon seasons. This represents
a sufficient number of samples from multi-year data toreveal generalised and robust features of the space–timestructure associated with ISO over Bangladesh. The
composite active and break peaks of total ABDR showed
rainfall values of 46.6 and 6.9 mm day−1 , respectively.
Figure 5 shows composites of the total atmosphericcirculation fields and large-scale convective activity forthe peak active and break phases. In the active phase(Figure 5(a)), the monsoon trough is located from northwestern India to Bangladesh in the low-level troposphere, providing westerly/southwesterly moisture flow
over Bangladesh. In this synoptic situation, the low-levelsouthwesterlies blow towards the high mountain regions
to the northeast and east of Bangladesh (Figure 1). The
southwesterlies seem to stagnate in windward areas suchas Bangladesh and northeastern India, forming a remarkable convergence field there. The centre of active convection is found over Bangladesh and the head of the Bayof Bengal, corresponding to the centre of large moisture
flux convergence (Figure 5(b)). The centre of 925-hPa
positive vorticity is also located over Bangladesh (notshown). In contrast, in the break period (Figure 5(c)),
the monsoon trough shifts southward and extends from
central Pakistan to the head of the Bay of Bengal. The
wind direction over Bangladesh is southerly/southeasterly
rather than westerly/southwesterly due to the shift of themonsoon trough. In this case, the incoming southerly
moisture flow from the Bay of Bengal can easily flow out
from the flat lowland of northwestern Bangladesh overthe Ganges Plain. In fact, the easterly flow dominatesalong the Ganges Plain. Thus, this circulation change
induces a significant decrease in moisture flux convergence over Bangladesh. A maximum area of cloudinessmoves to the Bay of Bengal along 15 ° N, accompanied
by an increase in moisture flux convergence. These atmospheric features of the monsoon trough and associatedwind fields are very similar to those found for summer
1995 by Ohsawa et al. (2000).
Figure 6 shows the composite differences between theactive and break peaks. The statistical significance ofthe difference at each grid point was estimated using
Student’s t-tests. In OLR and moisture flux divergence
fields, shaded areas are nearly in accordance with theareas of 99% confidence (Figure 6(a) and (b)). Associated with the submonthly rainfall ISO over Bangladesh,convection anomalies show a negative sign (i.e. enhanced
convection) over northeastern India, all of Bangladesh,and northwestern Myanmar, with a positive sign (i.e. suppressed convection) over the southern Bay of Bengal. The
active rainfall/convection is associated with the deepening of the monsoon trough from the northern fringe of
Copyright  2010 Royal Meteorological Society

(a)

OLR,

and moisture flux
active-break(W m−2)(b)(Kg m−1 s−1)

Moisture flux and its divergence
active-break(1.0 x 10−4 Kg m−2 s−1)

(Kg m−1 s−1)

Figure 6. Composite differences of (a) OLR (shadings), 925-hPa
streamfunction (ψ), and vertically integrated (from the surface to
100 hPa) moisture flux vectors between peak active and break phasesof 7- to 25-day rainfall variation over Bangladesh. The contour intervalfor ψ is 5 × 105 m2 s−1 . The contour interval for OLR is 10 W m−2 ,and the zero contours are omitted. Only moisture flux vectors that show
99% statistically significant difference are plotted. The topographic
contour lines for 1000 and 3000 m are also shown (thick dashedlines). (b) As in (a), but for vertically integrated moisture flux and its
divergence (shading). Contours for moisture flux divergence are shownat −3.5, −2.5, −0.5, 0.5, 2.5, and 3.5. The unit is 1 × 10−4 kg m s−1 .

India to northern Bangladesh and the enhancement of
anticyclonic circulation over the Bay of Bengal. The circulation features enhance westerly moisture flux anomalies around 20° –25 ° N and moisture flux convergenceover and near Bangladesh.
To further illustrate the time evolution, Figure 7 shows
daily-lag composites of 925-hPa streamfunction, moistureflux, and OLR fields from day −4 to day +3 based on
the 7–25-day rainfall variation over Bangladesh. Active
rainfall peaks over Bangladesh are referred to as day-0
phases, whereas day −5 and day +5 correspond to breakpeaks, indicating that one cycle of the composite variation
is 10 days. This is because, as seen in Figure 3(b), statistically significant peaks often appear at around 10 daysin each year, and therefore, a large number of composite samples have an ∼10-day period. In each phase, 78
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H. FUJINAMI et al.
7–25-day OLR, 925-hPa

and moisture flux anomalies(a) Day -4

(e)

Day 0

(b) Day -3(f)

Day +1(c) Day -2(g) Day +2(d) Day -1(h) Day +3(W m−2)(Kg m−1 s−1)

(active peak)(W m−2)(Kg m−1 s−1)

Figure 7. Composites of 7- to 25-day filtered OLR, 925-hPa streamfunction (ψ), and vertical integrated moisture flux anomalies from day −4
to day +3 based on the ABDR variation. Day 0 corresponds to the active peak of 7- to 25-day ABDR variation. OLR anomalies less than
−6 (6) W m2 are darkly (lightly) shaded. The contour interval for OLR is 6 W m−2 (white contours). Solid black (dashed) contours represent
positive (negative) ψ values. The contour interval is 2 × 105 m2 s−1 . Only 99% statistically significant vectors are plotted.

samples were used to construct the daily-lag composites.
Shaded areas of OLR nearly correspond to areas of 99%
confidence. On day −4, suppressed convection is shownover Bangladesh and northeastern India, accompanied
by southeasterly moisture flux anomalies. Bangladesh is
located under the northeastern part of a cyclonic anomaly
centred on the western coast of India and under thesouthwestern part of a local anticyclonic circulation overnorthern Bangladesh. An anticyclonic circulation appears
over the South China Sea around 115 ° E, 15 ° N from day
Copyright  2010 Royal Meteorological Society−4 and moves westward to the west of India by day +3
via the Indochina Peninsula and the Bay of Bengal. These
atmospheric structures are very similar to the so-calledsynoptic-scale 10–20-day ISO (e.g. Chen and Chen,
1993; Annamalai and Slingo, 2001; Hoyos and Webster,
2007; Yokoi and Satomura, 2008). In addition, a negative
streamfunction anomaly is centred on the equator, southof the anticyclonic anomaly over the Bay of Bengal (notshown). The negative streamfunction anomaly moves
westward in synchrony with the anticyclonic anomaly
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INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER

around 15 ° N, as also noted by Chen and Chen (1993)
and Chatterjee and Goswami (2004). On day −2, when
the anticyclonic anomaly moves into the eastern partof the Bay of Bengal from the Indochina Peninsula, acyclonic anomaly starts to develop around Bangladesh.
Simultaneously, the active convection anomaly appears
over Bangladesh, northeastern India, and western Myanmar due to the enhancement of southwesterly/westerly
moisture inflow. The anticyclonic anomaly moves westward, with its centre reaching the western part of the
Indian subcontinent by day +2. On day +1, the anticyclonic circulation seems to split to the east and west of the
Deccan Plateau, a large plateau located to the southwestof the Chota Nagpur Plateau. This split is indicative of theeffect of the plateau on the shape of the anticyclonic circulation anomaly around India. Interestingly, the centre of
the cyclonic circulation in and near Bangladesh remains
at almost the same location from day −2 to +2. Like
the cyclonic circulation, the convection anomaly overBangladesh remains almost stationary from day −2 to+2. The standing feature of the cyclonic circulation andconvection likely suggests that the terrain features around
Bangladesh, especially to its northeast and east, areessential to enhance cyclonic circulation and convection.
If the southwesterly flow, which varies on submonthly
timescales, is affected by the mountain ranges and thenconverges over Bangladesh, the convergence area (i.e.
the cyclonic circulation) does not necessarily move during the southwesterly wind phase. In contrast, suppressed
convection anomalies over the Bay of Bengal, locatedaround the south/southwestern part of the anticyclonic
circulation where easterly anomalies dominate, move
westward from day −3 to −1 and reach the southwestern
Bay of Bengal. On day +2, a cyclonic anomaly emerges
over the South China Sea and moves westward acrossthe Indochina Peninsula. After day +4, as the cyclonic
circulation anomaly moves westward along 15 ° N, theeasterly moisture flux component becomes dominant overBangladesh, northeastern India, and northwestern Myanmar. Suppressed convection is observed in these regions
under the atmospheric condition.
Ohsawa et al. (2000) noted that ∼20-day variation
over Bangladesh in summer 1995 was associated with
the northward propagation of synoptic-scale 10–20-day
ISO. However, the northward propagation signal is notevident in our results for both convection and circulation
anomalies. The northward propagation of the 10–20-day
mode seemed to be remarkable, especially in a case withheavy rainfall from 7 June to 5 July 1995. However,in other cases in 1995, the northward movement was
not evident [Figures 7 and 13 of Ohsawa et al. (2000)].
The latitude–time cross sections of OLR along 90 ° E
from 0 to 30 ° N in JJA for the other years of theperiod 1981–2000 show that well-organised northward
propagation is very rare on submonthly timescales (notshown). In contrast, the 30–60-day signal often exhibits
well-organised northward propagation from the equator
to around 20 ° N (figures not shown, e.g. Yasunari, 1979;
Wang et al., 2006).
Copyright  2010 Royal Meteorological Society

4.

Interannual variability over Bangladesh

This section presents the spatial structure of atmosphericcirculation and convection associated with the IAV of the
ABDR. As shown in Figure 4, the IAV of ABDR corresponds well with the submonthly scale variance of the
ABDR, suggesting that the submonthly scale ISO activity
could modify the spatial pattern of IAV over and around
Bangladesh. Linear regression coefficients for estimating the streamfunction, moisture flux, and OLR values
at each grid point from the interannual ABDR series
are shown in Figure 8. A value equal to 1.0 standard
deviation of the interannual ABDR series was input tothe regression equation to produce the grid-point values.
The overall pattern of anomalies in Figure 8 is characterised by a cyclonic anomaly from northern India to
Bangladesh and an anticyclonic anomaly centred on the
Bay of Bengal. A striking feature is the statistically significant westerly/southwesterly moisture flux anomalies
over northeastern India, Bangladesh, and northwestern
Myanmar, accompanied by active convection anomalies.
The spatial pattern around Bangladesh is very similar
to that of submonthly scale ISO. Statistically significant convection and moisture flux signals in the IAV
are also observed over northwestern India, Pakistan,and southern China. No statistically significant convection anomalies are shown over India south of 20 ° N,consistent with the findings of Kripalani et al. (1996).
A weak relationship is also found in the correlation
between ABDR and ‘all India monsoon rainfall’ (AIMR,
available at ftp://www.tropmet.res.in/pub/data/rain/iitmregionrf.txt) (Table I). These results indicate that the IAVof summer rainfall over Bangladesh has little relationshipto the IAV of the Indian summer monsoon.
Regression coefficient of OLR, y and moisture flux

(W m−2)(Kg m−1 s−1)

Figure 8. Regressions of OLR, 925-hPa streamfunction, and verticallyintegrated moisture flux vector in JJA for 1980–2000 against thestandardised interannual time series of JJA total ABDR. OLR values
less (greater) than −2 (2) W m−2 are darkly (lightly) shaded. Opencircles indicate locally statistically significant grids for OLR at the 95%
significance level. The contour interval for OLR is 1 W m−2 (white
contours). Only 95% statistically significant moisture flux vectors areplotted. The contour interval for ψ is 6 × 104 m2 s−1 . The topographic
contour lines for 1000 and 3000 m are also shown (thick dashed line).
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H. FUJINAMI et al.

Table I. Simultaneous correlation of interannual variations
between all Bangladesh daily rainfall (ABDR) and the all
India monsoon rainfall (AIMR), meridional thermal gradient(MTG), Niño-3 SST, and Indian Ocean dipole (DMI) modeindices for June–August. Correlation values larger than 0.56
are statistically significant at the 99% confidence level.
MTG

Niño-3 SST

DMI

−0.22

−0.01

−0.16

0.33

In the IAV of the total ABDR series (Figure 4), 3to 5-year variation is observed for the period from1981 to 1991 and after 1996, whereas a 2- to 3-year
signal appears from 1991 to 1996. The 2- to 5-year
period of variation is common in ENSO signals. However, Bangladesh summer monsoon rainfall has almost
no correlation with the Niño-3 SST index (available
at http://www.cpc.noaa.gov/data/indices/sstoi.indices),
which represents ENSO variability, for JJA. As in
the Niño-3 SST index, no relationship exists between
Bangladesh summer monsoon rainfall and the meridional thermal gradient (MTG) index, a large-scale monsoon index defined by Kawamura (1998), for JJA. The
correlation coefficient is −0.16 in the Niño-3 SST
and almost zero in the MTG (Table I). The MTG,
defined as the difference in area-averaged upper tropospheric (200–500 hPa) thickness between the Tibetan
Plateau (50° –100 ° E, 20° –40 ° N) and the Indian Ocean(50° –100 ° E, 0° –20 ° N) regions, has shown good correlation with ENSO and another large-scale monsoon
index proposed by Webster and Yang (1992) (Kawamura, 1998; Kawamura et al., 2005). The correlation
coefficient between the MTG and Niño-3 SST indicesduring the 20 years of our study is 0.68, which exceeds
the 99% confidence level. Moreover, an Indian Ocean
dipole event can be an effective external forcing of
the summer Asian monsoon as well as ENSO (e.g.
Saji et al., 1999; Ashok et al., 2001, 2004). However,the simultaneous relationship between ABDR and theIndian Ocean dipole mode index (DMI, available at
http://www.jamstec.go.jp/frsgc/research/d1/iod/) is also
weak (Table I). These results likely suggest that totalsummer monsoon rainfall in Bangladesh is modulated byinternal monsoon variability, such as intraseasonal variation, rather than by large-scale persistent anomalies due
to external forcing such as by SST anomalies.
However, further detailed study using long-term data isneeded to clarify why the IAV of rainfall in Bangladesh
has little simultaneous relationship with the Indian summer monsoon, large-scale Asian monsoon circulation, andexternal forcing induced by tropical SST anomalies suchas ENSO and Indian Ocean dipole events.

5.

Discussion

The similarity of the spatial patterns between the IAV
and ISO indicates that more frequent occurrence of strong
Copyright  2010 Royal Meteorological Society

Black bar: Wet monsoon years
Grey bar : Dry monsoon years(Times/JJA)

AIMR

Frequency of rainfall peaks

+0.5σ

+1.0σ

+1.5σ

(Criteria)

Figure 9. Average occurrence frequency of rainfall peaks of 7- to
25-day variation during JJA for six wet monsoon years (black bars)
and five dry monsoon years (grey bars) based on three different criteria (+0.5σ , +1.0σ , and +1.5σ of climatological 7- to 25-day ABDR
variance). The frequency was counted when a rainfall peak exceeded
each criterion in wet and dry monsoon years, respectively. The 20-summer (JJA) climatological 1.0 standard deviation (∼10 mm day−1 ) of 7to 25-day variation is +1.0σ .

active conditions of the ISO produces a wet monsoon year
over Bangladesh. Figure 9 shows the average frequencyof occurrence of rainfall peaks in the submonthly ISO in
JJA for six wet monsoon years (1982, 1984, 1987, 1988,
1993, and 1999) and five dry monsoon years (1985, 1986,
1989, 1992, and 1994). The IAV of total summer rainfallof these years exceed 0.7σ , except for 1985, in which the
IAV failed to reach the value by only a narrow margin.
The frequencies were counted based on three separate
criteria (i.e. +0.5σ , +1.0σ , and +1.5σ of climatological
7–25-day ABDR variance) to ascertain the differences in
the properties of rainfall peaks in wet and dry monsoon
years. The value of +1.0σ corresponds to ∼10 mmday−1 . The frequencies did not differ largely between
wet and dry monsoon years when small amplitude peaks(>+0.5σ ) were included. However, the difference inthe frequency of the rainfall peaks became larger as
the amplitude increased. The differences for +1.0σ and+1.5σ are statistically significant at the 95% level. Thus,
high-amplitude active peaks of the ISO occur more
frequently in wet monsoon years than in dry monsoon
years. In addition, on average, 29 active-phase days
were observed in the wet monsoon years, and 19 activephase days were found in the dry monsoon years (e.g.
the periods shown by grey shading in Figure 2); thisdifference in the number of days is also statisticallysignificant at the 99% level. In contrast, no statisticallysignificant difference exists in the average frequency of
break peaks (defined as peaks that exceed −1.0σ ofclimatological 7–25-day ABDR variance) between the
wet and dry monsoon years (not shown). These results
indicate that the characteristic spatial pattern of the active
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INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER
(a)

(c)

OLR (7–25-day)

925-hPa zonal wind (7–25-day)

(%)

(%)

(b)

(d)

OLR (30–60-day)(%)

925-hPa zonal wind (30–60-day)(%)

Figure 10. (a) Variance of 7- to 25-day filtered OLR for JJA (solid line). Shading represents the percentage of the total variance (with theseasonal cycle of OLR removed) explained by the 7- to 25-day band. The contour interval is 150 W2 m−4 . The topographic contour lines for1000 and 3000 m are also shown (thick dashed line). (b) As in (a), but for 30- to 60-day filtered OLR. The contour interval is 30 W2 m−4 . (c),(d) As in (a) and (b), but for the 925-hPa zonal wind component (u). The contour intervals are 2 and 1 m2 s−2 for (c) and (d), respectively.

phase (Figures 6 and 7) appears more frequently in wet
monsoon years, and in turn, contributes to modulating the
spatial pattern of the IAV over and near Bangladesh.
The 7–25-day ISO variance is more than ∼10 times
larger than the 30–60-day ISO variance in the 20-year
average. This is a peculiar feature of the ISO overBangladesh. Hartmann and Michelsen (1989) showed that
the MJO-scale (30–60-day) period was predominant overmost parts of India, except in a few areas of northern India. Whereas the 30–60-day ISO is important for
defining the observed mean seasonal rainfall and circulation pattern over India and the Bay of Bengal (e.g.
Goswami and Mohan, 2001; Hoyos and Webster, 2007),the submonthly scale ISO also contributes to the meanmonsoon circulation and rainfall/convection pattern. In
Myanmar, the 30–60-day variance of rainfall fluctuation
dominates in most of the coastal region during summer,
whereas the 10–20-day ISO has larger variance in theinland region (Yokoi et al., 2007; Yokoi and Satomura,2008). Compared with these results, Bangladesh is probably one of the most dominant regions of the submonthly
scale ISO of rainfall in the South/Southeast Asian monsoon region. This suggestion is confirmed by OLR andlow-level zonal wind fields. Figure 10 shows the spatial distributions of filtered OLR and zonal wind (u)variances on intraseasonal timescales and the percentage
of total variances explained by the filtered variances. A
local maximum of OLR variance is observed over the
Bay of Bengal in both the 7–25-day and the 30–60day bands (Figure 10(a) and (b)). Large percentages ofthe explained variance in both bands are also found in
Copyright  2010 Royal Meteorological Society

this region. Notably, just over Bangladesh, a local maximum of the 7–25-day explained variance (≥45%) anda local minimum (≤6%) of the 30–60-day explained
variance are shown. In the variance of 925-hPa zonal
wind, a zonally oriented maximum of the 7–25-day filtered u variance extends from northern India to western
Bangladesh, south of the Himalayas, where the explained
variance exceeds 45% (Figure 10(c)). The 30–60-day
explained variance of 925-hPa u is relatively larger tothe south of 20 ° N compared with the 7–25-day variance.
The spatially localised feature of the large percentage
variance of 7–25-day filtered OLR may be related togeographic features of Bangladesh, as shown in Figure 1.
The common signals spread into northeastern India close
to Bangladesh. Thus, these results strongly support thatthe activity of submonthly scale ISO modulates the meanseasonal rainfall and circulation pattern only in the area
over and near Bangladesh.
The southeasternmost portion of the monsoon troughis situated over Bangladesh in the active phase of theISO, and hence southwesterly moisture flow dominates
over Bangladesh (Figure 5(a)). When the southwesterly
flow reaches high-elevation regions such as the Shillong
Plateau and Chittagong Hill Tracts, orographical lifting
induces rainfall on the windward slopes of these geographic features. Moreover, the westerly/southwesterly
flow stagnates in windward areas such as Bangladesh andnortheastern India, forming a remarkable convergence
field. Thus, the orographical barrier effect of large-scale
high-elevation regions to the north and east of Bangladesh
also seems to spread to windward areas. Additionally,
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H. FUJINAMI et al.

the southeastern part of the monsoon trough is accompanied by a local maximum of cyclonic vorticity overBangladesh. The strong meridional cyclonic shear andabundant moisture could create a favourable environment
for inducing precipitation system formation over all of
Bangladesh. Although we did not examine what processenlarges the amplitude of ISO-related active peaks in thisstudy, this issue is important for understanding the process of IAV in seasonal mean rainfall over Bangladesh.
This process likely depends on which precipitation system dominates in the active phase of the ISOs. Ohsawa
et al. (2001) pointed out that, in the active phase of thesubmonthly scale ISO in 1995, convective activity overBangladesh presented clear diurnal variation with its peakduring late night–early morning hours, providing rainfall
over the whole of Bangladesh. Using a non-hydrostatic
cloud-resolving model, Kataoka and Satomura (2005)
found that late night–early morning precipitation maxima
in northeastern Bangladesh on 15–18 June 1995 wereassociated with squall-line precipitation systems that were
triggered near the southern foot of the Shillong Plateau
and moved southward. Rafiuddin et al. (2010) reported
detailed statistics of summer monsoon precipitation systems over Bangladesh using 6-year radar data. They classified the precipitation systems into three types according
to shape: line type, arc type, and scattered type. Interestingly, the scattered-type systems, composed of groupsof poorly organised small individual systems, dominatedover Bangladesh during the summer monsoon season.
About half of the systems were of the scattered type withwide areal coverage [referred to as SWAC in Rafiuddin
et al.’s (2010) study]. In a SWAC, small echoes grew
and disappeared continuously. Almost all SWACs developed during the monsoon period and contributed greatly
to monsoon rainfall in this region, suggesting again thatsynoptic-scale disturbances such as a monsoon low are
not common rain-producing systems over Bangladesh.
The mesoscale disturbances may be contained within theintraseasonal envelope of convection in Bangladesh.
In addition to the features of mesoscale precipitation
systems in active ISO phases, we should also examine
the dynamical relationship between the submonthly scale
ISO and the 30–60-day ISO, and how external forcings(i.e. ENSO and Indian Ocean dipole mode) can affectprecipitation systems and ISOs over Bangladesh. We
also need to investigate why the submonthly scale ISO
exhibits such a strong signal over Bangladesh.

6.

Summary

The ISO of rainfall during the summer monsoon season(June–August) and its possible effect on the IAV of totalsummer monsoon rainfall over Bangladesh were examined using daily rainfall data from 25 rain-gauge stationsfor 20 years (1981–2000). The space–time structures ofatmospheric circulation and large-scale convection fields
related to the ISO and the IAV of the ABDR were also
investigated using Japanese 25-year reanalysis (JRA25)
Copyright  2010 Royal Meteorological Society

and interpolated OLR data. The analysis using multiyear rain-gauge data from a dense observation network
revealed robust and characteristic features of the ISO
climatology in Bangladesh. Results are summarised asfollows:
1. An ISO of 7–25 days (submonthly scale) appears in
the ABDR time series in almost all of the 20 years(1981–2000). The variance shows a large IAV. In
contrast, the activity of 30–60-day ISO is weak andspectral peaks occasionally appear in this period range.
The 7–25-day filtered ABDR variance is always
very large compared with the 30–60-day variance.
Thus, submonthly scale ISO can be regarded as adominant intraseasonal mode of rainfall fluctuation
over Bangladesh during the summer monsoon season.
2. The IAV of the intraseasonal rainfall variances shows
significant correlation with the IAV of total summer
monsoon rainfall, suggesting that the ISO activity
could modulate total summer monsoon rainfall. In wet
monsoon years, the ISO activities on both intraseasonal periods tend to become enhanced simultaneously. The activity of high-frequency variation (3–6day) has little relationship to the IAV of the intraseasonal variance and total summer monsoon rainfall.
3. Submonthly timescale ISO in ABDR is closely associated with the north–south shift of the monsoon trough.
In the peak active (break) phase of the 7–25-day ISO
over Bangladesh, total atmospheric circulation fields
show the monsoon trough to be located from northern India to Bangladesh (from northwestern India to
the northern part of the Bay of Bengal) in the lowlevel troposphere, enhancing westerly/southwesterly(southerly/southeasterly) moisture flow and increasing(decreasing) its convergence over Bangladesh. Hence,
the active (suppressed) convection centre is observed
over Bangladesh and the head of the Bay of Bengal.
4. The time evolution of atmospheric circulation andconvention anomalies associated with submonthly
scale ABDR ISO reveals that the changes in atmospheric circulation over the Bay of Bengal are associated with the so-called synoptic-scale 10–20-day ISO
that originates around the western Pacific and thenmoves westward to India. Over and near Bangladesh,the active convection anomaly shows sudden enhancement when the westward moving anticyclonic circulation anomaly appears over the eastern Bay of Bengal around 15 ° N and southwesterly moisture inflow
begins to be enhanced over Bangladesh. Then, convection/precipitation is further enhanced and the cyclonic
circulation anomaly deepens over Bangladesh. In thepeak active phase, active convection anomalies appear
over northeastern India, all of Bangladesh, and western Myanmar, whereas suppressed convection anomalies occur over the southern Bay of Bengal, accompanied by a cyclonic anomaly from northeastern
India to northwestern Myanmar, centred on northern
Bangladesh, and an anticyclonic circulation anomaly
over the central Bay of Bengal. Note that despite
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INTRASEASONAL OSCILLATION OVER BANGLADESH DURING BOREAL SUMMER

the continuous westward migration of convection andcirculation anomalies over the Bay of Bengal, theconvection anomalies and cyclonic anomalies overBangladesh remain in almost the same location fromthe initiation to termination of the active ISO phase.
5. The spatial pattern of atmospheric circulation and convection related to the IAV of total summer ABDR
is characterised by a cyclonic anomaly from northern India to Bangladesh and an anticyclonic anomaly
centred on the Bay of Bengal, accompanied by anactive convection anomaly from northeastern India
to all of Bangladesh. Westerly/southwesterly moisture flux anomalies are evident under such circulation
fields. Note that the spatial pattern is quite similar to that of submonthly scale ISO over and near
Bangladesh. Also, interestingly, no or weak correlations were found with the IAV of some climate indicessuch as all India monsoon rainfall, a large-scale monsoon index, Niño-3 SST, and the Indian Ocean DMI.
6. The similarity of the spatial patterns of the IAV and
ISO indicates that the occurrence of the strong active
ISO condition results in a wet monsoon year overBangladesh. A higher occurrence frequency of strong
submonthly scale ISOs and larger number of rainydays related to the active ISO phase are evident in wet
monsoon years compared with dry monsoon years. In
addition, Bangladesh is characterised by a local maximum of 7–25-day explained variance (i.e. the percentage of total variance explained by the filtered variance)
and a local minimum of 30–60-day explained variance in the South/Southeast Asian monsoon region.
The characteristic features of ISOs over Bangladesh
likely allow the submonthly scale ISO to modify theseasonal mean rainfall and spatial pattern of circulation and convection over and near Bangladesh.
In the Asian monsoon region, ISOs are importantphenomena in the regional water cycle. To the north of
Bangladesh, convective activity also exhibits remarkablesubmonthly scale ISO over the Tibetan Plateau andits leeward areas (e.g. Fujinami and Yasunari, 2004,2009). Mid-latitude waves such as Rossby waves regulate
southerly moisture inflow to the Tibetan Plateau area,and hence, a remarkable ISO appears. Interestingly,
statistically significant moisture flux anomalies related to
the ISO of rainfall over Bangladesh can also be foundover the Tibetan Plateau (Figures 6 and 7). How these
ISOs interact between the mid-latitudes and subtropics,
and how ISOs regulate the water cycle of the Himalayas
and Tibetan Plateau are also important problems for
future research.
Acknowledgements
The rain-gauge data for Bangladesh used in this study
were provided by the Bangladesh Meteorological Department (BMD). The two anonymous reviewers are thanked
for their comments, which have improved this work.
This study was partially supported by the Japan Earth
Copyright  2010 Royal Meteorological Society

1203

Observation System Promotion Program (No. 2-2, leader:
Jun Matsumoto) and by a Grant-in-Aid for Scientific
Research (B-12574020) and a Grant-in-Aid for Young
Scientists (B-20740268) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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