Clim Dyn (2014) 43:2147–2166
DOI 10.1007/s00382-013-2040-1

Dynamics of distinct intraseasonal oscillation in summer monsoon
rainfall over the Meghalaya–Bangladesh–western Myanmar
region: covariability between the tropics and mid-latitudes
Hatsuki Fujinami • Tetsuzo Yasunari •Akihito Morimoto

Received: 14 September 2013 / Accepted: 24 December 2013 / Published online: 25 January 2014
Ó The Author(s) 2014. This article is published with open access at Springerlink.com

Abstract Detailed spatiotemporal structures for the submonthly-scale (7–25 days) intraseasonal oscillation (ISO)in summer monsoon rainfall and atmospheric circulation
were investigated in South Asia using high-quality rainfall
and reanalysis datasets. The Meghalaya–Bangladesh–coastof the western Myanmar (MBWM) region is the predominant area of submonthly-scale ISO in the Asian monsoon
regions. The distinct rainfall ISO is caused by a remarkable
alternation of low-level zonal wind between westerly andeasterly flows around the Gangetic Plain on the sametimescales. In the active ISO phase of the MBWM, a strong
low-level westerly/southwesterly flows around the plain
and a center of cyclonic vorticity appears over Bangladesh.
Hence, a local southerly flows toward the Meghalaya Plateau and there is strong southwesterly flow towards thecoast along southeastern Bangladesh and western Myanmar, resulting in an increase in orographic rainfall. Rainfall
also increases over the lowland area of the MBWM due tothe low-level convergence in the boundary layer under thestrong cyclonic circulation. The submonthly-scale lowlevel wind fluctuation around the MBWM is caused by awestward moving n = 1 equatorial Rossby (ER) wave.
When the anticyclonic (cyclonic) anomaly related to the

H. Fujinami (&)  T. Yasunari
Hydrospheric Atmospheric Research Center, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8601, Japane-mail: hatsuki@hyarc.nagoya-u.ac.jp
Present Address:
T. Yasunari
Research Institute for Humanity and Nature, Kyoto, Japan
A. Morimoto
Mitsubishi Electric Information Network Corporation,
Tokyo, Japan

ER wave approaches the Bay of Bengal from the western
Pacific, humid westerly/southwesterly (easterly/southeasterly) flows enhance around the Gangetic Plain on thenorthern fringe of the anticyclone (cyclone) and in turnpromote (reduce) rainfall in the MBWM. Simultaneously,robust circulation signals are observed over the mid-latitudes. In the active phase, cyclonic anomalies appear over
and around the TP, having barotropic vertical structure andalso contributing to the enhancement of low-level westerly
flow around the Gangetic Plain. In the upper troposphere,
an anticyclonic anomaly is also observed upstream of thecyclonic anomaly over the TP, having wavetrain structure.
The mid-latitude circulation around the TP likely helps toinduce the distinct ISO there in conjunction with theequatorial waves. Thus, the distinct ISO in the MBWM isstrongly enhanced locally (*500 km) by the terrain features, although the atmospheric circulation causing the ISO
has a horizontal scale of *6,000 km or more, extending
across the whole Asian monsoon system from the tropics to
mid-latitudes.
Keywords Intraseasonal oscillation  Orographic effects Rainfall  South Asian monsoon  Tropical-midlatitudeinteraction

1 Introduction
During the summer monsoon season (June–August), theprevailing westerly/southwesterly winds bring abundantmoisture from the ocean to South Asia. A large amount ofprecipitation is generated by the humid monsoon flow.
Land in South Asia possesses complex topographical features (Fig. 1). The interactions between the prevailing flow
and the geographical features of South Asia give rise to a

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Fig. 1 a Geographic features around South Asia and climatological mean (1979–2007) vertically integrated (from the surface to 100 hPa) water
vapor flux vectors for June–August. b As in a, but for the magnified view of the region of interest

high rainfall area near the coastal and inland mountains.
Recent satellite observations, such as those by the TropicalRainfall Measuring Mission (TRMM)-Precipitation Radar(PR), reveal that the mean distribution of the summer
monsoon rainfall is strongly influenced by the terrain distribution (e.g., Hirose et al. 2008; Romatschke and Houze
2011; Biasutti et al. 2012). Narrow mountain ranges (e.g.,
Arakan Mountains and Meghalaya Plateau) are importantin anchoring high rainfall on the windward side. The
monsoon rainfall plays an important role in forming andmaintaining strong monsoon circulation through the releaseof latent heating due to condensation (e.g., Hoskins andRodwell 1995; Ose 1998; Xie et al. 2006).
Monsoon precipitation does not remain constant within
the summer monsoon season but exhibits clear intraseasonal oscillation (ISO) with high and low precipitation
phases, referred to as active and break phases, respectively.
Submonthly-scale (7–25 days)/quasi-biweekly oscillation(which are essentially the same phenomena with a spectral
peak of around 14 days) and 30–60-day oscillation are thetwo main modes of ISOs in the Asian summer monsoon(Krishnamurti and Bhalme 1976; Murakami 1976; Yasunari 1979; Krishnamurti and Ardanuy 1980; Chen andChen 1993; Madden and Julian 1994; Annamalai andSlingo 2001; Wang et al. 2006; Hoyos and Webster 2007;
Yokoi et al. 2007; Fujinami et al. 2011; Moon et al. 2013).
The area around Bangladesh, where a tremendous
amount of rainfall occurs in summer, predominantly
exhibits submonthly-scale ISO, but not 30–60-day ISO(Ohsawa et al. 2000; Murata et al. 2008; Fujinami et al.

1232011). The ISO feature allows the submonthly-scale ISO tomodulate the total seasonal rainfall and the spatial patternsof circulation and convection around Bangladesh (Fujinami
et al. 2011; hereafter referred to as FHY11).
The ISO provides different atmospheric environments inactive and break phases and causes a large difference in the
distribution of rainfall in each phase. However, few studies
have focused on the detailed precipitation distribution
related to the ISO around Bangladesh. Information aboutvariability in the detailed rainfall distribution and precipitation characteristics during active and break phases could
improve understanding of precipitation mechanisms around
Bangladesh, from daily to seasonal timescales. In addition,
the large variation in precipitation area is accompanied bychanges in the location of the atmospheric heat source,
such as diabatic heating due to latent heating from condensation. Therefore, understanding the detailed rainfall
distribution and the processes underlying its variation isimportant for understanding the processes of formation andmaintenance of the ambient atmospheric circulation.
Some previous studies have pointed out that the submonthly-scale/quasi-biweekly ISO in South/Southeast Asia
is associated with a westward propagating n = 1 equatorial
Rossby (ER) wave (Chen and Chen 1993; Annamalai andSlingo 2001; Yokoi and Satomura 2005; Chatterjee andGoswami 2004; Kikuchi and Wang 2009). The ER wave
has periodicity of approximately the submonthly range(Kiladis and Wheeler 1995; Masunaga 2007; Kiladis et al.
2009). However, in the submonthly-scale ISO aroundnortheastern India and Bangladesh, the connection with

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall

equatorial waves is not well known. Additionally, submonthly-scale ISO is also observed over and around the
Tibetan Plateau (TP) during summer (Fujinami and Yasunari 2001, 2004, 2009; Yang et al. 2010). However, the link
between the mid-latitude circulation (e.g., Asian subtropical jet and Tibetan high) and the ISO around Bangladesh isalso not well known.
FHY11 showed the features of low-level atmosphericcirculation and OLR fields related to the ISO in Bangladesh
for a limited area (5–35°N, 65–115°E) over the South
Asian monsoon domain. In this paper, we provide a
detailed picture of the rainfall distribution in the submonthly-scale ISO in relation to topographical features
over South Asia. We also present a broader picture oflarge-scale convection and three-dimensional atmosphericstructures, focusing on intraseasonal linkage between thetropics and mid-latitudes.
Section 2 describes the datasets and analysis methods
used in this study. The detailed topographic features andtheir dependent ISO signals around South Asia are presented in Sect. 3. Section 4 gives the spatiotemporal
structure of large-scale atmospheric circulation and convection extending from the tropics to the mid-latitudes,
related to the distinct submonthly-scale ISO around theMeghalaya (located in northeastern India near the Bangladesh border)–Bangladesh–coast of the western Myanmar region. The dynamics that are likely to cause this
distinct ISO are discussed based on the observational evidence in Sect. 5. Finally, the results are summarized in
Sect. 6.

2 Data and methods
We used the APHRODITE rainfall dataset to examine
rainfall fluctuations and spatial distributions around South
Asia from 1979 to 2007 (29 years). The APHRODITE
dataset is a state-of-the-art daily precipitation dataset withhigh resolution (0.25° 9 0.25°) grids for Asia covering theland only (Yatagai et al. 2009, 2012). The datasets are
created primarily with data obtained from a dense raingauge observation network with orographic effects taken
into account during interpolation. To support the rainfall
distribution based on APHRODITE and further elucidate
the detailed relationship between terrain features andrainfall distribution including over the ocean area, we also
used TRMM PR 2A25 version 7 data (Iguchi et al. 2000)
from 1998 to 2010 (13 years). We compiled the TRMM PR
data into gridded data with a resolution of 0.05° 9 0.05°.
We used the near-surface rain rate to examine the spatial
distribution of rainfall. Rainfall frequency and intensity
were also calculated using the near-surface rain rate. Daily
interpolated outgoing longwave radiation (OLR) data on a2149

2.5° latitude–longitude grid were used as a proxy for largescale convective activity from 1979 to 2010 (32 years)(Liebmann and Smith 1996). We also used the Japanese
25-year (1979–2004) reanalysis (JRA25)/Japan Meteorological Agency (JMA) Climate Data Assimilation system(JCDAS) datasets on a 1.25° latitude–longitude grid to
explore large-scale atmospheric circulation associated withthe rainfall ISO (Onogi et al. 2007). The JRA25 dataset
covers the period from 1979 to 2004. Since 2005 to thepresent, JCDAS has provided data of the same quality as
JRA25 using the same assimilation system as JRA25. Thus,
we can conduct a diagnostic study with a homogeneous
dataset covering the period from 1979 to the present. These
reanalysis data with high spatial resolution can provide
detailed information on atmospheric structures in the ISO
evolution around South Asia, which cannot be discerned incoarse reanalysis data. The dataset variables included
horizontal wind (u and v), specific humidity (q) andstreamfunction (w). Vertically integrated water vapor flux
from the surface to 100 hPa was also calculated. The data
were daily-averaged before analysis.
Most of the data processing and analysis methods
employed here are similar to those used by FHY11. Daily
anomalies of rainfall, OLR, and the reanalysis data werecomputed by subtracting the first three harmonics of theannual cycle (about 120 days) for each year. To extract thesubmonthly (7–25 days) variability signal, a Lanczos bandpass filter was applied to the detrended daily anomalies(Duchon 1979). The power spectrum of the unfiltereddetrended rainfall time series for the 29-year APHRODITE
rainfall around Bangladesh had dominant power in thesubmonthly range with a peak at *14 days (not shown),
similar to the findings of FHY11 (Fig. 3a of FHY11).
Hence we used the frequency range (i.e., 7–25 days) fortime filtering.
A composite method similar to that of FHY11 was used
to describe the mean behavior of the ISO. We chose areference time series of APHRODITE precipitation averaged over the area including a part of northeastern India,
Bangladesh, and the coast of western Myanmar (20–26°N,
88–93°E), where submonthly signals are strongest in South
Asia (Fig. 3b, c). On the basis of the 7–25-day rainfall
anomalies, positive and negative extremes that exceeded
the 29-summer climatological 1.0 standard deviation(*7.5 mm day-1) were selected as active and break peaks
for the composites in atmospheric circulation, APHRODITE precipitation, and OLR fields. In total, 110 peak
active and 105 peak break days were identified during the
29 summer monsoon seasons (June to August; JJA). Daily
lag composites of various fields were also constructed
relative to these peak active days, denoted by day 0. The
statistical significance of the composite differences andanomaly fields at each grid point was estimated using

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Dynamics of distinct intraseasonal oscillation in summer monsoon rainfallb Fig. 2 Time series of rainfall averaged over the rectangular domainof 20°–26°N, 88°–93°E from 1979 to 2007 (black bars; left axis) and
7–25-day filtered rainfall (solid line; right axis). Dotted lines in eachfigure denote the 29-summers (JJA) climatological 1.0 standard
deviation. Closed circles and cross marks near the filtered time series
denote selected active and break peaks, respectively

Student’s t test. In the composite of TRMM-PR data, a
7–25 day filtered area-averaged (20–25°N, 88.5–92.5°E)
OLR time series was used as a reference because the
APHRODITE rainfall dataset ended at 2007. The correlation value between the reference time series of the APHRODITE dataset and the filtered OLR anomalies for the 29
summers (2,668 days) was *0.7 in Bangladesh (figure notshown), which exceeded the 99 % significance level.
Therefore, it is reasonable to use the filtered OLR anomalies instead of APHRODITE rainfall anomalies for thecomposites of TRMM-PR. We confirmed that nearly
identical distributions of OLR and atmospheric circulation
fields were seen in the composites using both of the references. The method used to choose active and break peaks
was similar to that used for APHRODITE rainfall, and 76
peak active and 67 peak break days were identified during
the 13 summer monsoon seasons. In order to present arobust rainfall distribution associated with active and breakphases of the ISO from TRMM-PR, we used three consecutive days centered on the peak days as active phase andbreak phases for the composites. In short, totals of 228
active days and 201 break days could be used to make thecomposites. Some previous studies successfully showed
clear composite differences in rainfall distribution associated with active and break phases related to ISOs using
TRMM-PR data (Singh and Nakamura 2010; Shrestha
et al. 2012).

3 Regional-scale rainfall distribution related
to submonthly-scale ISO and topography
3.1 Climatological features
Our target area in this study is around northeastern India,
Bangladesh, and the coast of western Myanmar (Fig. 1).
The Gangetic Plain is characterized by an area of east–west
elongated flat lowland, south of the Himalayas/TP. The
eastern part of the Gangetic Plain faces the Bay of Bengal,
from which southerly moist winds can intrude into theplain. In the easternmost part of the Gangetic Plain, theMeghalaya Plateau stands at *2,000 m above sea level
extending east to west, and the Chittagong Hill Tracts andthe Arakan Mountains extend north to south. These
regional-scale topographical features enhance the localized
area of high rainfall in the windward regions (Fig. 3a).
Previous papers have noted the localized high rainfall areas

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arising from regional-scale topography in a climatologicalsense using high-resolution TRMM-PR data (Hirose et al.
2008; Nesbitt and Anders 2009; Romatschke and Houze
2011; Biasutti et al. 2012).
Figure 2 presents the daily rainfall time series averaged
over the region 20–26°N, 88–93°E for the 29 summers. A
distinct submonthly-scale ISO appears in almost all of the
29 summers even in the unfiltered time series. The mean
spectrum for the 29-year rainfall time series shows that apronounced peak appears at a period of *14 days thatexceeds the 95 % confidence level. A peak around the
30–40-day period can also be found but is not statisticallysignificant (not shown). The 7–25-day filtered time series isalso shown in Fig. 2. The active and break peaks of thefiltered time series represent those of the unfiltered timeseries well. Thus, these results justify our choice of frequency range for time filtering.
Figure 3b presents the percentage of the total varianceof rainfall explained by the 7- to 25-day band in summer.
Localized high values (C40 %) are observed around theMeghalaya Plateau, the Chittagong Hill Tracts, and thecoast of western Myanmar. Furthermore, the area withvalues greater than 35 % covers almost all of Bangladesh.
Thus, the Meghalaya–Bangladesh–coast of western
Myanmar (hereafter abbreviated to MBWM) region is themost dominant area of the submonthly-scale ISO among
the onshore Asian monsoon regions. High percentage values extend westward from the MBWM along the Gangetic
Plain. Relatively high values are also observed aroundsoutheastern China, the east coast of the Indochina Peninsula, the southern TP, and the Western Ghats. The percentage of total variance explained by 7–25 day OLR
anomalies reveals a comprehensive picture of the submonthly-scale ISO, including regions over the oceans(Fig. 3c), although a regional-scale structure cannot be
detected because of the coarse grid of the OLR data. Large
submonthly variances appear over the Bay of Bengal andSouth China Sea, while large percentages of varianceappear around the MBWM, northeastern coast of Vietnam,
and the western TP. The highly localized strong signals of
the submonthly-scale ISO suggest the presence of uniquedynamics for enhancing the submonthly-scale ISO preferentially around the MBWM.
Low-level wind variation is an important element needed to induce rainfall fluctuation. To indicate the preferred
locations of low-level horizontal wind variation, 925-hPa
perturbation kinetic energy (PKE) on the 7–25-day timescale is shown in Fig. 3d. PKE is defined by
1  02PKE ¼
u þ v02
ð1Þ
2
where the overbar denotes the time mean, and u0 and v0 are
the 7–25 day filtered zonal and meridional winds,

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Fig. 3 Climatological(1979–2007) seasonal (June–
August) mean distribution ofa rainfall from APHRODITE
datasets, b the percentage of thetotal variance (with the seasonal
cycle of rainfall removed)
explained by the 7–25-day band
obtained from APHRODITE.
Only areas where climatologicalseasonal mean rainfall is morethan 1.5 mm day-1 are shaded,c the variance of 7–25-day
filtered OLR (solid line) and thepercentage of the total variance(with the seasonal cycle of OLR
removed) explained by the
7–25-day band (shading), andd 7–25-day PKE at 925 hPa.
a–c The topographic contour for
3,000 m is shown as a thicksolid line. d The topographic
contours for 1,500 and 3,000 m
are shown as dashed lines

respectively, following the method used in Matthews andKiladis (1999). A local maximum extends zonally along
the Gangetic Plain, south of the TP. The zonal wind perturbation (u0 ) mainly contributes to form the PKE maximum over the plain. The enhanced zonal wind variation
provides a favorable environment for distinct submonthlyscale ISO in rainfall around the MBWM.
3.2 Detailed rainfall distribution and its time evolution
To show in detail the spatial distribution of rainfall and its
relationship with topography, the rainfall distribution andintegrated water vapor flux vectors in active and breakphases of the MBWM are shown in Fig. 4. In the peak
active phase (Fig. 4a), high rainfall areas spread around the
MBWM, the southern slope of Himalayas, and the Western
Ghats. Westerly/southwesterly moisture flow prevails
around the MBWM. Note that a localized center ofcyclonic relative vorticity appears at 925 hPa over western
Bangladesh. Therefore, wind components normal to thewindward slopes of the Meghalaya Plateau, the Chittagong
Hill Tracts, and the western coast of Myanmar areenhanced, and then high rainfall (C50 mm day-1) isobserved over these regions probably due to orographic lifting of moist air. Relatively high rainfall

123(C20 mm day-1) is also observed in the lowland area ofthe MBWM away from the regional-scale mountains,resulting from frictional convergence in the boundary layer
under strong low-level cyclonic circulation. In contrast, thedirection of moisture flow changes such that easterly/
southeasterly flow prevails along the Gangetic Plain in thepeak break phase (Fig. 4b). The easterly/southeasterly flow
can travel along the Gangetic Plain from northwestern
Bangladesh without orographic barriers. Thus, precipitation decreases significantly along the Gangetic Plain on
peak break days. The composite difference highlights thesedifferences mentioned above (Fig. 4c). A significant positive difference in rainfall also appears along the southernslope of the Himalayas and the southeastern TP, where
significant moisture flux vectors are also shown. Figure 4d–f from TRMM-PR datasets highlight and reinforce
the relationship between the locations of rainfall maxima
and regional-scale topographic features including over theocean. The composites from TRMM-PR can capture
overall features of those from APHRODITE datasets well,although the absolute values of rainfall in TRMM-PR are
smaller than those in APHRODITE over land. In the active
phase (Fig. 4d), the southern slope of the Meghalaya Plateau has a localized precipitation maximum. In contrast,
around the coast of western Myanmar, the high rainfallDynamics of distinct intraseasonal oscillation in summer monsoon rainfall

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Fig. 4 Composites of a rainfall(APHRODITE) and verticallyintegrated (from surface to
100 hPa) water vapor flux
vectors in the peak active phaseof the 7–25-day ISO over the
MBWM. The contours of
925-hPa positive (i.e., cyclonic)
vorticity more than
2.5 9 10-5 s-1 are shown by
dotted lines. The contour
interval of vorticity is
0.5 9 10-5 s-1. The unit ofwater vapor flux is kg m-1 s-1.
The topographic contours of
500 and 3,000 m are also shownas thin solid lines. b As in a, but
for peak break phase. c As in a,
but for the difference between
peak active and peak breakphases. The thick solid line
indicates areas with 99 %statistically significant
difference in rainfall. Only
water vapor flux vectors thatshow 99 % statisticallysignificant difference areplotted. d Composite of rainfall
obtained from TRMM-PR in theactive phase. e As in d, but forthe break phase. f As in d, but
for the difference between theactive and break phases

area extends to the Bay of Bengal with highest rainfall
along the coastline. The break phase is characterized by theincrease in rainfall over the Bay of Bengal and central
India. The scattered high rainfall area spreads widely overthe Bay of Bengal (Fig. 4e). The difference shows a north–south shift in the high rainfall area associated with thesubmonthly-scale ISO (Fig. 4f).

Figure 5 shows the vertical latitude/longitude sectionsof horizontal wind speed, specific humidity, and horizontal
water vapor flux. Rainfall, rain frequency, and rain intensity derived from TRMM-PR are also displayed in thisfigure. Selected latitudes and longitudes include highrainfall areas over the Meghalaya Plateau (Fig. 5a, b, e, f)
and the coast ranging from southeastern Bangladesh to

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(a) WVF, q & wind speed 91E-92E Active

0.2

8
6

6

(c) WVF, q & wind speed 20.5N-21.5N Active(g) WVF,q & wind speed 20.5N-21.5N Break

0.2

0.2

8
6

6

[mm/hr][mm/day,%](h) Rainfall, rain frequency and intensity[mm/hr](d) Rainfall, rain frequency and intensity

[m/s]

10

[hPa]

8

[m/s]

10

[hPa]

[mm/hr][mm/day,%](f) Rainfall, rain frequency and intensity[mm/hr][mm/day,%](b) Rainfall, rain frequency and intensity

[m/s]

10

[hPa]

8

[m/s]

[hPa]

10

western Myanmar, respectively, where the Chittagong Hill
Tracts and Arakan mountains are located to the east of thecoastline (Fig. 5c, d, g, h). In the active phase, high rainfall
appears around 25–26°N and its peak (C60 mm day-1)
corresponds only to the southern slope of the Meghalaya
Plateau (Fig. 5a, b). The rainfall is characterized by highrainfall frequency (*40 %) and relatively high rainfall
intensity (*7 mm h-1) (Fig. 5b). The rainfall peak around
21°N is a part of the rainfall distribution near the coastfrom southeastern Bangladesh to western Myanmar. A
strong and humid low-level southwesterly/southerly flows
toward the Meghalaya Plateau with its maximum windspeed around 900 hPa. In contrast, the low-level windspeed toward the plateau in the break phase becomes significantly weakened (Fig. 5e). Rainfall frequency reduces
greatly (B20 %), and in turn the rainfall amount decreases
around the Meghalaya Plateau (Fig. 4f). The difference inthe low-level specific humidity between active and breakphases is not so large; the value averaged from 1,000 to123(e) WVF, q & wind speed 91E-92E Break

0.2[mm/day,%]

Fig. 5 a Vertical-latitudesection of horizontal wind speed(m s-1; color shadings),
horizontal water vapor flux
vectors (m s-1 g kg-1), andspecific humidity (q kg-1; black
contours) averaged between 91°
and 92°E in the peak active
phase over the MBWM.
Horizontal wind speed of morethan 6 m s-1 is shaded. The
contour interval for horizontal
wind speed is 1 m s-1 (white
contours). The unit of water
vapor flux is g g-1 m s-1.
Black shading denotes the
topography; the peak from 25°
to 26°N is on the Meghalaya
Plateau. b Rainfall (mm day-1;
black bar; left axis), rain
frequency (%; red line; leftaxis), and rain intensity(mm h-1; green line; right axis)
averaged between 25° and 26°N
in the active phase from
TRMM-PR. c As in a but forvertical longitude section
averaged between 20.5° and
21.5°N. The peak around 94°E
is over the Arakan Mountains;the coastline is located at*92°E, and the Chittagong Hill
Tracts is in between. d As inb but for averaged between
20.5° and 21.5°N. e, f As ina and b but for the peak breakphase in the MBWM. g, h As inc and d but for the break phase

H. Fujinami et al.

850 hPa at the same longitudes is less than 0.5 g kg-1. Inshort, the change in the low-level wind speed mainly
contributes to the large difference in water vapor flux
between the active and break phases. The vertical stratification in the peak active and break days (defined as thevertical difference of equivalent potential temperature
between 1,000 hPa and 600 hPa) is 15.2 and 16.3 K,
respectively, at 22.5°N, 90°E. Thus, low-level atmosphere
exhibits slightly stronger convective instability on peak
break days than on peak active days. Recently, Sato (2013)
pointed out that the intensity of southerly/southwesterly
low-level wind around 900 hPa is a crucial factor in heavy
precipitation around the Meghalaya Plateau during theactive period of submonthly-scale ISO because stronger
low-level wind is sufficient to overcome the verticalstratification barrier and can reach the lifting condensationlevel. At latitudes of 20.5–21.5°N, a strong southwesterly
low-level wind is dominant over the head of the Bay of
Bengal in the active phase (Fig. 5c), whereas a weakDynamics of distinct intraseasonal oscillation in summer monsoon rainfall

southerly is observed in the break phase (Fig. 5g), indicating that the wind component of the direction normal tothe coastline and mountains is enhanced in the active phaseand significantly decreases in the break phase. The coastline under examination here is located at *92°E, and the
Arakan Mountains are found around 94°E. The Chittagong
Hill Tracts are in between. In the active phase, the lowlevel wind speed decreases eastward, indicative of the lowlevel convergence around the coast (Fig. 5c). An area ofrainfall of more than 20 mm day-1 spreads from 90.5 to
93.8°E with its peak (C40 mm day-1) over the coastline(i.e., *92°E), which is broader than that around theMeghalaya Plateau. In this region, the high rainfall is a
result of both high rainfall frequency and intensity(Fig. 5d). A sharp decrease in rainfall appears on the leeward side of the Arakan Mountains (i.e., the rain shadow
effect of mountains). In the break phase, both rainfall frequency and intensity become weakened, and in turn therainfall amount decreases greatly (Fig. 5h).
To illustrate the phase relationship between rainfall andtopographical features, Fig. 6 presents daily-lag compositesof APHRODITE rainfall and vertically integrated moisture
flux vectors from day -5 to day 0. Active rainfall peaks over
MBWM are referred to as day 0 phases. The evolution fromday -5 to day 0 nearly completes a half-cycle because thepattern at day 0 is similar to that at day -5, with opposite
signs. The next life cycle is almost a mirror image of theaforementioned life cycle. At day -5, a negative rainfall
anomaly extends over the MBWM with minima in thesouthern Meghalaya Plateau and the coastal areas of southeastern Bangladesh and western Myanmar. Southeasterly/
easterly moisture flux anomalies prevail along the Gangetic
Plain. In contrast, a positive rainfall anomaly appears overcentral India where cyclonic circulation is observed. After
day -3, an anti-cyclonic circulation anomaly moves westward from the Indochina Peninsula to the Bay of Bengal
along *15N°. Southerly/southwesterly humid flow begins
to become enhanced in the MBWM, and in turn the rainfall
anomaly changes sign quickly from negative to positive
between day -3 and day -2. By day 0, the anticyclonic
circulation moves farther westward and reaches at 90°E overthe Bay of Bengal. Then, the westerly/southwesterly moisture current is further enhanced over the MBWM. Wind
components toward the windward slopes of the Meghalaya
Plateau, the Chittagong Hill Tracts and the coast of western
Myanmar reach a maximum. The time sequence reveals thatthe areas of large rainfall anomaly around the MBWM
remain stationary, although the anticyclonic circulation overthe Bay of Bengal that brings the westerly anomaly leading
to the active ISO phase moves westward. The stationary
feature of the rainfall anomaly suggests that the topographical features are essential to enhance rainfall and anchor it ata fixed location in the MBWM.

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4 Large-scale atmospheric circulation and convection
related to the submonthly-scale ISO
4.1 Three-dimensional structure of atmosphericcirculation in the active and break phases
In this subsection, we present the three-dimensional
structure of large-scale atmospheric circulation and the
distribution of convection with reference to the submonthly-scale rainfall ISO in the MBWM region to showa large-scale process that causes the distinct ISO in thewhole Asian summer monsoon system. Figure 7 shows
atmospheric circulation and OLR fields in the active andbreak phases. In the peak active phase, a trough (i.e.,monsoon trough) deepens over the Gangetic Plain, and acenter of convection is located over the MBWM where an
850 hPa westerly/southwesterly prevails (Fig. 7c). The
enhancement of the westerly/southwesterly corresponds tothe northward extension of large-scale monsoon westerly
flow. In the middle troposphere, the main axis of thewesterly jet is located around 40°N (Fig. 7b). A deep
trough is observed around the southern TP; a dominantwesterly blows around the MBWM, similar to that in the
low-level troposphere. Note that a part of the mid-latitude
westerly intrudes into South Asia along the southwestern
periphery of the TP, related to the deep trough. The
westerly intrusion from mid-latitudes and the westerly
over the MBWM are clearly observed in a layer from 700
up to 400 hPa in the active phase (figure not shown). The
upper-level troposphere is characterized by the Tibetan
high and its center is located to the west of 80°E (Fig. 7a).
A ridge is also observed from 40° to 60°E along the Asiansubtropical jet at *40°N. In contrast, the monsoon troughshifts southward in the break phase, and the center ofconvection appears over the Bay of Bengal (Fig. 7f). The
strong low-level westerly appears up to 20°N, while aweak easterly is observed over the MBWM. In the midtroposphere, there is a remarkable easterly over the
MBWM south of the TP (Fig. 7e). The center of theupper-level Tibetan high is located around 25°N, 80°E,
and a stronger easterly is observed over the MBWM
compared to the active phases (Fig. 7d). Thus, the easterly
dominates in all layers of the troposphere over the
MBWM.
Figure 8 highlights the difference in streamfunction (w)and wind vectors between the active and break phases at
each level. A striking feature in the 850 hPa difference is apair of anticyclonic anomalies along 85°E with centersaround 15°N and 5°S (Fig. 8c). The other important difference is the cyclonic circulation anomaly around the TP.
The strong low-level westerly wind, causing high rainfall
around the MBWM, is induced by the pressure gradient
between the anti-cyclonic anomaly and cyclonic anomaly

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Fig. 6 Composites of 7–25-day
filtered rainfall (APHRODITE;
shadings) and vertical
integrated moisture flux
anomalies from day -5 to day 0
based on 7–25-day rainfall
variation in the MBWM. Day 0
corresponds to the active
rainfall peak in the MBWM.
Only 99 % statisticallysignificant areas of rainfall are
shaded. Only 99 % statisticallysignificant moisture flux vectors
are plotted. The topographic
contours of 500 and 3,000 m areshown as thin solid lines

over the Bay of Bengal and areas to the south of TP,
respectively. Thus, there is a north–south tripole structureof w anomalies ranging from the tropics to the mid-latitudes. At 500 hPa, the horizontal structure of w anomalies
is nearly identical to that at 850 hPa (Fig. 8b). A cyclonicanomaly becomes apparent over the TP. The north–southpressure gradient to the south of the TP results in the strong123

westerly anomalies from the western to southern peripheryof the TP. At 200 hPa (Fig. 8a), the anti-cyclonic anomaly
is located centered on 35°N, 50°E and extends to the
Indochina Peninsula zonally, as if to avoid the TP. A
cyclonic circulation anomaly is observed over the TP on
the downstream side of the center of the anticyclonicanomaly.

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall
Fig. 7 Composites ofa 200 hPa streamfunction (w)and wind vectors in the peak
active phase. The contour
interval for w is
1.0 9 107 m2 s-1. Shading
denotes areas at an altitude ofmore than 3,000 m. b As ina but for 500 hPa. Shading
denotes areas of westerly flow
(i.e., u [ 0). The contour
interval for w is
2.0 9 106 m2 s-1. Topographic
contour for 3,000 m is shown asa thick solid line. c As in a, but
for 850 hPa. OLR values\220 Wm-2 are shaded. The
contour interval for w is
3.0 9 106 m2 s-1. The
topographic contour for
3,000 m is shown as a thicksolid line. d–f As in a–c but forthe peak break days(a) 200-hPa

and u,v

2157

Active phase(d) 200-hPa

and u,v

Break phase(m s-1)(b) 500-hPa

and u,v

Active phase(m s-1)(m s-1)(c) 850-hPa ,u,v and OLR

Active phase(W m-2)4.2 Covariability between the tropics and mid-latitudes
To show the extent of important anomalies linked to thesubmonthly-scale rainfall ISO over the MBWM, the variance in the composite life cycle from day –5 to day ?5 isshown in Fig. 9. Almost all grids in the shaded area ofFig. 9 passed the 99 % level of significance in the daily
lag-composites at least once during the life cycle. The
distribution of OLR variance exhibits the two preferredbands in convection along the latitudes of 15–30°N and
5–15°N from 50 to 140°E (Fig. 9d). The northern band
extends from southern China to Pakistan via the MBWM,
while the southern one extends from the western North
Pacific/the South China Sea to western Indian Ocean
through the Bay of Bengal. The w variances reveal interesting signals that range widely from the tropics to the midlatitudes of the Northern Hemisphere across the Asiansummer monsoon region (Fig. 9a–c). At 850 hPa, an east–(m s-1)(m s-1)(e) 500-hPa

and u,v

Break phase(m s-1)(m s-1)(f) 850-hPa ,u,v and OLR

Break phase(W m-2)(m s-1)west elongated area of large variance is evident along thelatitudes of 10–20°N from 50 to 160°E (Fig. 9c). To thesouth of this area, along the latitudinal band from *10°S
to the equator, there is a zonally elongated area of moderatevariance. Thus, the two bands of large variance areremarkable in exhibiting a line-symmetric shape withrespect to *5°N. As shown later, the two bands correspondto the pathways of the westward propagating twin vortices.
As in the tropics, large variance is also observed in thesubtropics and the mid-latitudes, surrounding the TP except
in the eastern/southeastern area. At 500 hPa, the two zonal
bands of high variance also appear in the tropics (Fig. 9b).
A large variance maximum appears centered on the western TP. The 200 hPa w variance shows local maxima at
50°E, 80°E and 110°E along *40°N along the Asiansubtropical jet (Fig. 9a). A local center of large variance isalso found over the head of Bay of Bengal, extending
northwestward and connecting the large-variance area

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Fig. 8 Composite difference of a 200-hPa streamfunction (w) andwind vectors between peak active and peak break days. Shading
denotes statistically significant areas at the 99 % level in the w
difference. The topographic contour for 3,000 m is shown as a thickdashed line. The contour interval for w is 1.5 9 106 m2 s-1. b As ina but for 500 hPa. The contour interval for w is 1.0 9 106 m2 s-1.
c As in a but for 850 hPa. The contour interval for w is
0.5 9 106 m2 s-1

Fig. 9 a Variance in the 200-hPa streamfunction (shaded) in thecomposite life cycle (from day -5 to day ?5). Topographic contours
for 1,500 and 3,000 m are shown as a thick solid line. b As in a but
for 500 hPa. c As in a but for 850 hPa. d As in a but for OLR

around the Asian jet. No robust signals appeared in the
mid-latitudes of the Southern Hemisphere (figure notshown).

Figure 10 demonstrates the temporal sequence of thecomposites for the 850 hPa streamfunction (w850), windvectors, and OLR fields, based on the submonthly-scale

123

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall
Fig. 10 a–f Composites of the
850-hPa streamfunction (w850;
contours), wind vectors, andOLR (shaded) anomalies fromday -5 to day 0 with reference
to the 7–25-day rainfall
anomalies over the MBWM.
OLR anomalies less (greater)
than -5 (5) Wm-2 are shaded;
Shaded areas of OLR nearlycorrespond to areas of 99 %confidence. Only wind vectors
statistically significant at the
99 % level are plotted. The
contours of topographic
contours for 1,500 and 3,000 m
are shown as dashed lines. The
contour interval for w850 is
3.0 9 105 m2 s-1. Zero
contours in w850 are omitted

2159

850-hPa ψ, wind vector and OLR anomalies(a) Day -5(d) Day -2(b) Day -4(e) Day -1(c) Day -3(f) Day 0 (peak active phase)(W m-2)

rainfall ISO over the MBWM. A striking feature of the
850 hPa atmospheric circulation is the westward propagating twin vortices (i.e., twin lows or highs) with centersaround *15°N and *5°S (Fig. 10). In day -5, suppressed
convection appears over the MBWM where the easterly/
southeasterly anomaly dominates in the northern peripheryof a cyclonic anomaly over the east coast of central India.
Another cyclonic circulation is observed with a centeraround 5°S, 80°E. Thus, the anomaly distribution exhibits atwin-low structure. At day -4, the next twin-vortex
structure emerges from the South China Sea to the areaaround the maritime continent. The easterly anomaly is
accelerated over the South China Sea along 5°–7°N
between the two anti-cyclonic vortices, and in turn convection is suppressed there. From day -4 to day -2, thenorthern vortex moves westward from the South China Sea
to the Bay of Bengal. On day -2, a strong low-level(m s-1)(W m-2)(m s-1)southwesterly flows into the MBWM and convection
becomes enhanced there. Note that a northerly intrudes
from the mid-latitudes through a lowland area around
30°N, 60°E between the western TP and the Zagros
Mountains (thick dashed line from 40°N, 40°E to 30°N,
55°E) from day -1 to day 0. The enhanced northerly flows
into the northernmost part of the Arabian Sea. Then thenortherly changes direction into a westerly, merging withthe southwesterly along the northwestern periphery of the
anticyclone over the Bay of Bengal. On day 0, whenconvection/precipitation reaches a maximum, the anticyclonic anomaly is centered on the Bay of Bengal. A narrowcyclonic circulation deepens just south of the TP (Fig. 10f).
The anomaly structure forms a strong westerly/southwesterly anomaly around the MBWM.
The half zonal wavelength of these vortices is*3,000–3,500 km(Figs. 8c,
10f),
approximately

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equivalent to zonal wavenumber 6 around the equator. The
wave has westward phase speed of about 6.5 m s-1 aroundthe equator from the time sequence. The horizontal structure and propagation characteristics are consistent withthose found by Chen and Chen (1993), Yokoi and Satomura (2005), and Chatterjee and Goswami (2004) ofn = 1 equatorial Rossby waves with submonthly period.
The two vortices are not symmetric about the equator, but
appear to be shifted northward by *5–7°N, consistent withthe results of Chatterjee and Goswami (2004). Figure 11
shows time–longitude sections of the OLR anomaly (withonly the seasonal cycle of OLR removed) and Rossbywave filtered OLR anomaly averaged from 10–20°N during
June to August 1988. The Rossby-wave filter is a bandpass
filter in the frequency-wavenumber domain to detect n = 1
equatorial Rossby (ER) waves (e.g., Masunaga 2007; Kiladis et al. 2009). Here, we adopted the ER wave filter usedby Masunaga (2007). The filter is defined for -10 B k \ 0(k is the horizontal zonal wavenumber) and two dispersion
relations with an equivalent depth of 5 and 100 m. Notethat remarkable westward OLR anomalies were observedin the summer of 1988 and these anomalies correspondwell with the ER-wave-filtered anomalies. When largeamplitude ER waves with a positive (negative) OLR
anomaly pass over the area around 90°E from June to midJuly, rainfall increases (decreases) remarkably over the
MBWM (Fig. 11b), consistent with the composites shownin Fig. 10. Eastward shifts of large amplitude ER wave
filtered anomalies can also be observed during the same
period, which may be an eastward energy dispersion of the
ER wave (i.e., group velocity) (e.g., Kiladis and Wheeler
1995). After mid-July, the ER wave signals passing over
*90°E weaken and the amplitude of the ISO in rainfall
over MBWM becomes small but the same tendency can befound even after mid-July.
The sequence of 500-hPa circulation anomalies showsthat the spatial structure and its evolution are quite similar
to that at 850 hPa in the tropics, and w anomalies become
prominent in the mid-latitudes (Fig. 12). On day -4, ananomalous low emerges centered on 40°N, 120°E. Simultaneously, the north–south twin-high structure becomes
obvious along approximately 115°E. From days -4 to -2,
although the twin-high anomalies in the tropics move
westward, the cyclonic anomaly to the east of the TP
remains almost stationary and contributes to form a westerly anomaly over southern China. On day -1, the cyclonic
circulation appears to move into the TP rapidly and theninduces a strong westerly anomaly over the MBWM to thesouth of the TP, in conjunction with the anti-cyclonicanomaly over the Bay of Bengal. On day 0, the cyclonic
circulation becomes further strengthened over the TP.
In the upper troposphere, a notable feature of the w200
anomaly is a wavetrain structure along 40°N where the axis

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Fig. 11 a Time-longitude section of the OLR anomaly (shaded) andRossby-wave-filtered OLR anomaly (contours) averaged between 10°
and 20°N in 1988. b Time series of summer rainfall in 1988 in the
MBWM region. OLR anomaly is calculated by subtracting theseasonal cycle from the raw time series. Contours of the filtered OLR
anomaly are shown from -3 and 3 Wm-2 at 6 Wm-2 intervals. Zero
contours are omitted. See the text for details of the Rossby wave filterof the Asian jet is located (Fig. 13). From days -5 to -3, thew200 anomalies along the Asian jet move slowly westward.
An eastward development of the anomalies is also observed(Fig. 13a–c). By day -2, the anticyclonic w200 anomaly over
the TP seen on day -5 moves to 40°N, 55°E and develops inalmost same position by day 0. From day -3 to day 0, thecyclonic circulation to the east of the TP moves westward
slowly to the TP, accompanied with active convection signals to its south. The waves along the Asian jet (*40°N)
have a wavenumber of *4.5, with a phase speed of *2.7 m s-1, as estimated from the composite sequence. The
zonal phase speed of the mid-latitude barotropic Rossby
wave relative to the background flow is given asbc  u ¼  2;
k þ l2ð2Þ

where c is the zonal phase speed, u is the background zonal
wind speed, k and l are the zonal and meridional wave

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall

2161

500-hPa ψ, wind vector and OLR anomalies(a) Day -5(d) Day -2(b) Day -4(e) Day -1(c) Day -3(f) Day 0 (peak active phase)(W m-2)(m s-1)(W m-2)(m s-1)

Fig. 12 As in Fig. 10 but for 500 hPa. The contour interval for w500 is 4.0 9 105 m2 s-1

numbers, respectively, and b is the meridional gradient of the
Coriolis parameter. We can assume that the zonal andmeridional wavenumbers are of similar scale (k & l) (Holton1992). The value of b is 1.75 9 10-11 m-1 s-1 at 40°N. The
zonal wavenumber is *4.5. Hence, the Rossby wave speed
relative to the background flow calculated from Eq. (2) is 10.6 m s-1. Here, the background zonal wind for the barotropic Rossby waves is defined as the vertical mean zonal
flow in summer and is *8.0 m s-1 (Fujinami and Yasunari2009). Hence, the phase speed of the waves relative to theground is *-2.6 m s-1, consistent with the observed
phase speed of the waves along the Asian jet. To the southof the TP, a weak anticyclonic circulation covers the active
convection area over the MBWM. The anti-cyclonic circulation is strengthened as the convection becomes stronger in the MBWM. The upper-level convergence to thesouth of the anti-cyclone covers the area of suppressed
convection over the Bay of Bengal. After day 0, an anticyclonic anomaly develops rapidly to the east of the TP,
indicative of downstream development of the anomaly, andthen wholly moves westward (not shown).
The w anomaly over the Bay of Bengal at the peak phasehas a nearly barotropic vertical structure throughout thetroposphere, while the large w anomaly at 5°S is confined
to below 500 hPa (Figs. 10f, 12f, 13f). This feature issimilar to those reported by Chen and Chen (1993) and
Chatterjee and Goswami (2004). The vertical structure of
the w anomaly around the TP also displays a near barotropic nature. Note that the vertical baroclinic structure isevident in the w anomaly just over the MBWM, probably
associated with diabatic heating due to the high rainfall
there. However, the zonal wind anomaly shows vertical

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Fig. 13 As in Fig. 10 but for
200 hPa. The contour interval
for w200 is 6.0 9 105 m2 s-1

H. Fujinami et al.

200-hPa ψ, wind vector and OLR anomalies(a) Day -5(d) Day -2(b) Day -4(e) Day -1(c) Day -3(f) Day 0 (peak active phase)(W m-2)in-phase structure over the MBWM throughout the troposphere (not shown).

5 Discussion
5.1 The dynamics of distinct submonthly-scale ISO
over the MBWM
The rainfall distribution and rainfall amount, related to theISO, are highly controlled by regional-scale topographyaround the MBWM such as the Meghalaya Plateau and thecoastline from southeastern Bangladesh to western Myanmar, just to the west of the Chittagong Hill Tracts andArakan Mountains (Figs. 4, 5). These essential terrain
features lie on the easternmost part of the Gangetic Plain,
which is an east–west flat lowland area extending to the123(m s-1)(W m-2)(m s-1)south of the Himalayas/TP. The plain is also surrounded by
the mountainous regions of India (e.g., Chota Nagpur
Plateau) to its south (Fig. 1). Thus, the plain and the surrounding topography likely help to enhance low-level
zonal wind fluctuations along the plain, behaving like aduct for the low-level atmospheric circulation (Fig. 3d).
Strong westerly/southwesterly flow enhances the orographic lifting of low-level moisture in the windward areaof the mountains, which results in a large increase in precipitation in the area. A region away from mountainous
area of the MBWM also gets high rainfall probably due tothe low-level convergence, associated with the large
cyclonic vorticity over Bangladesh. A recent study
revealed that a closed low pressure system of the horizontal
scale of *600 km often appears centered on Bangladesh in
the active ISO phase over the MBWM and produces large
amounts of rainfall in both the plain and mountainous areas

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall(Hatsuzuka et al. 2013, submitted). The center of cyclonicvorticity over Bangladesh seen in Fig. 4a reflects theeffects of the lows. The narrow cyclonic circulation
reaching a height of up to *7,000 m just over the Gangetic
Plain in the active phase, along with the shape of the surrounding regional mountains, likely provide favorable
environmental conditions for the genesis of such a mesoscale vortex or low around the MBWM (Figs. 4a, 7b, c).
Thus, the MBWM region has environmental conditions
that concentrate a low-level moisture flux convergence
there in the active phase.
The n = 1 equatorial Rossby (ER) wave is a key agent
that regulates the low-level zonal wind on submonthly
timescales around the Gangetic Plain to the south of the TP
and controls the periodicity of the ISO around the MBWM.
Note that the rainfall over the MBWM is not directly
induced by the convection coupled with the ER wave, but
the westerly/southwesterly flow in the northern peripheryof the anticyclonic vortex over the Bay of Bengal provides
a suitable condition for enhancing precipitation around theMBWM. In contrast, over the Bay of Bengal and central
India, the westward moving convection of the ER wave
brings rain. The twin-vortex structure of the ER waves
shifts northward by *5°N relative to the equator. This
northward shift probably allows the northern part of thenorthern vortex to interact with the regional-scale topography around the MBWM efficiently. Chatterjee andGoswami (2004) explained the reason for the northward
shift of the ER waves in the Indian Ocean in the Northern
Hemisphere summer using the ‘dynamic equator’ concept.
The mean flow over the Indian Ocean at low levels showsthat the ‘dynamic equator,’ where ambient absolute vorticity is zero, shifts to around 5°N during summer andresults in a shift of the ER waves towards the north byabout 5°. The ambient absolute vorticity fields are controlled by the large-scale mean Asian summer monsoon
flow, which is induced by the north–south heat contrast
between the Eurasian continent and the Indian Ocean(Fig. 7c, f). Thus, the strong Asian summer monsoon circulation itself could help to displace the ER waves northward. The shifted ER waves can effectively induce strong
horizontal fluctuations on submonthly timescales to thesouth of the TP, and in turn rainfall increases distinctly on
the same timescales over the MBWM. Additionally, large
diabatic heating due to the high precipitation around the
MBWM contributes to maintain strong Asian monsoon
circulation.
Meanwhile, the mid-latitude atmospheric circulation
also seems to play an active role in enhancing the zonal
wind fluctuation to the south of the TP. The w200 anomaly
along the Asian jet (*40°N) has almost vertically barotropic structure around the TP (Figs. 10, 12, 13). The
negative w200 anomaly to the east of the TP moves

2163

westward to the TP from days -2 to -1 (Fig. 13d, e).
Concurrently, a negative w500 anomaly appears over the TP(Fig. 12e). The w500 anomaly increases an east–west
pressure gradient along the western periphery of the TP anda north–south pressure gradient along the southern
periphery. A similar time sequence is also observed at
850 hPa; cyclonic circulation is strengthened from thewestern through to the southern periphery of the TP(Fig. 10e). This means that the monsoon trough just to thesouth of the TP deepens. Thus, the westward moving
vertically barotropic cyclonic circulation enhances the low
and mid-level northerly to the west of the TP and thewesterly to the south of the TP (Figs. 10e, 12e).
In short, the equatorial wave and the mid-latitude wave
probably enhance the submonthly-scale low-level zonal
wind fluctuation to the south of the TP synchronously, andthe interaction between the zonal wind fluctuation and theregional-scale topography causes a distinct rainfall ISO on
the same timescale over the MBWM.
5.2 Covariability of atmospheric circulation
between the tropics and mid-latitudes
The results demonstrate a robust connection between thetropics and the mid-latitudes, associated with the distinct
rainfall ISO over the MBWM region. However, themechanism of this linkage remains unclear. A possible
interpretation is that diabatic heating related to the submonthly-scale ISO may induce a Rossby-wave response
along the Asian jet in the mid-latitudes. Note that theconvection and low-level circulation anomalies over the
MBWM remain in almost the same location from theinitiation to termination of the active ISO phase (Figs. 6,10), whereas convection and circulation anomalies overthe Bay of Bengal and central India clearly move westward from the South China Sea to northwestern India(Fig. 10). The anomalous active convection/rainfall overcentral–northwestern India could excite or reinforce theupper-level high to its north over the Asian jet throughRossby response from days -5 to -3 (Figs. 6a–c, 13a–c)(e.g., Ambrizzi and Hoskins 1997; Krishnan and Sugi
2001; Ding and Wang 2007). The enhanced anticyclone
induces the downstream development of a wave train due
to eastward Rossby wave dispersion along the Asian jet(Enomoto et al. 2003; Fujinami and Yasunari 2004).
Therefore, the cyclonic anomaly with barotropic structure
likely forms downstream of the anticyclonic anomaly as apart of the wave train by day -4. The anticyclonicanomaly over the western TP is elongated westward andenhances rapidly at around 40°N, 55°E from day -3 today -2. Then, the cyclonic anomaly to the east of the TP
moves westward over the TP, accompanied by an active
convection band to its south. Meanwhile, the existence of

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the Himalayas/TP massif may transform the Rossby
response in the active phase of the MBWM because lowand mid-level flow is highly restricted by topography. The
enhancement of the westerly in the low/middle troposphere to the south of the TP could be interpreted as a
response to diabatic heating due to high rainfall undersuch restricted conditions. In the active ISO phase over the
MBWM, a trough lies just to the south of the Himalayas/
TP from the low level to 400 hPa (i.e., *7,500 m). From
700 to 400 hPa, the airflow from the mid-latitudes directly
intrudes into the MBWM along the southwestern/southern
periphery of the TP (Fig. 7b). In contrast, the air flow of
mid-latitude origin in the break ISO cannot intrude into
the MBWM, as it is inhibited by a strong easterly to thesouth of the TP (Fig. 7c). Thus, the mid-latitude circulation can interact physically with the tropical circulation to
the south of the TP in the active phase of ISO in the
MBWM. The circulation anomalies between the areaaround the TP and the Bay of Bengal are reflected by suchinteractions (Fig. 8b). In any case, the horizontal structureof the atmospheric circulation anomaly seems to be greatly
affected by the TP topography. The existence of the TP
may link submonthly-scale ISO between the tropics andthe mid-latitudes through the enhancement in verticallybarotropic circulation. Numerical modeling research isneeded to investigate the cause of the linkage between thetropics and the mid-latitudes associated with distinct
submonthly-scale ISO over MBWM and the effects of theHimalayas/TP on forming submonthly-scale circulation
signals around the TP.

6 Summary
The present study investigated the detailed rainfall distribution related to the submonthly (7–25-day)-scale ISO
around the MBWM region. The associated three-dimensional atmospheric circulation and time-evolution features
were investigated for the whole Asian summer monsoon
system from the tropics to the mid-latitudes using APHRODITE, TRMM-PR, and JRA25 reanalysis datasets.
The MBWM region, where a tremendous amount of
rainfall is observed in summer, is the most dominant areaof submonthly-scale ISO among the Asian summer monsoon regions. A distinct rainfall ISO is caused by the lowlevel zonal wind fluctuations and its interaction with theregional-scale topographical features around the MBWM,
with a remarkable alternation between westerly and easterly flow. In the active phase of the ISO over the MBWM,
a low-level westerly/southwesterly prevails over the
MBWM and a center of cyclonic vorticity appears overBangladesh. Therefore, the wind components normal to thewindward slopes of the Meghalaya Plateau, the Chittagong

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H. Fujinami et al.

Hill Tracts, and the western coast of Myanmar areenhanced; high rainfall (C50 mm day-1) is then observed
over these regions due to the orographic lifting of moist air.
Relatively high rainfall (C20 mm day-1) in the lowland
area of the MBWM away from the regional-scale mountains is due to frictional convergence in the boundary layer
under strong low-level cyclonic circulation.
The distinct alternation of the low-level zonal windaround the MBWM is caused by the westward propagating
vortex pair with zonal wavenumber *6 and westward
phase speed of *6.5 m s-1 in the tropics, which is consistent with the n = 1 ER wave. The centers of the vortices
propagate westward along *15°N and 5°S. Thus, the twinvortex structure of the ER wave shifts northward by 5°,
likely due to the large north–south asymmetric nature of
the ambient Asian summer monsoon circulation in thetropics. The northern vortex can affect the MBWM region.
As the anticyclonic vortex moves into the Bay of Bengal,
the westerly/southwesterly becomes strengthened aroundthe MBWM, which is covered by the northern fringe of the
anticyclonic vortex. Additionally, the MBWM region is
located in the easternmost part of the Gangetic Plain, an
east–west flat lowland that is surrounded to the north andsouth by high-elevation areas (i.e., the Himalayas/TP and
the Chota Nagpur Plateau). The topographic features of theGangetic Plain enhance the zonal wind fluctuation, andwesterly/southwesterly flow brings high rainfall on submonthly timescales around the MBWM.
Additionally, robust circulation signals relevant to the
ISO appear over the mid-latitudes over the MBWM. In theupper troposphere, a wavetrain structure is evident along
the Asian subtropical jet. The waves have a Rossby wavelike structure with a zonal wavenumber of *4.5 and awestward phase speed relative to the ground of*2.7 m s-1, consistent with the theory of mid-latitude
barotropic Rossby waves in terms of the zonal phase speed
relative to the mean background wind. In addition to theslow westward propagation, the eastward development of
the new center of action is observed along the Asian jet,indicative of Rossby wave energy dispersion along the
waveguide of the Asian jet. The waves have barotropic
vertical structure around the TP. In the active phase of the
ISO over the MBWM, the cyclonic anomaly dominates
throughout the troposphere around the TP. This is associated with the deepening of the monsoon trough just to thesouth of the TP. Thus, the cyclonic anomaly to the south of
the TP enhances the north–south pressure gradient and inturn enhances the westerly flow in conjunction with theanticyclonic anomaly over the Bay of Bengal.
The robust circulation and convection signals relevant to
the ISO extend across the whole Asian summer monsoon
system from the tropics to the mid-latitudes over the
MBWM region. The submonthly-scale ISO in rainfallDynamics of distinct intraseasonal oscillation in summer monsoon rainfall

around the MBWM becomes obvious on the local scale(*500 km), although the atmospheric circulation causing
the ISO has a horizontal scale of *6,000 km or more
extending across the whole Asian monsoon system.
Diurnal variation is also a dominant fluctuation in
rainfall/convection around the MBWM (Ohsawa et al.
2001; Hirose et al. 2008; Takahashi et al. 2010; Biasutti
et al. 2012). The relationship between the submonthlyscale ISO and diurnal variation has been presented inprevious studies that analyzed a single year (e.g., Ohsawa
et al. 2000; Terao et al. 2006; Sato 2013). Sato (2013)
pointed out the importance of the nocturnal low-level jet(LLJ) appearing around 900 hPa for enhancing orographic
rainfall around the Meghalaya Plateau. The direction of
the LLJ varies with the diurnal cycle, which accelerates
the pre-existing southwesterly flow (e.g., Fig. 5 a)
between 18 and 06 local time in the active ISO phase.
Thus, the diurnal variation in the LLJ causes the evening
to early morning maximum of precipitation. The change
in atmospheric circulation related to submonthly-scale
ISO around the MBWM provides different atmosphericenvironments for the diurnal variation and meso-scaledisturbances. Further statistical analysis is needed toclarify the multi-timescale interaction with the submonthly-scale ISO. Establishing why the 30–60-day ISO
is so weak around the MBWM is also an important areafor future research.
Acknowledgments We thank the anonymous reviewers for theirconstructive comments. We also thank Drs. T. Hayashi, R. Kawamura, M. Satoh, H. Masunaga, T. Sato, S. Yokoi, Y. Fukutomi, and H.
Kanamori for their helpful discussions. This study was conductedwith support from a Grant-in-Aid for Scientific Research (B22340137) and a Grant-in-Aid for Scientific Research for Young
Scientists (B-24740320) from the Japan Society for the Promotion of
Sciences (JSPS). Portions of this study were conducted as part of acollaborative research program of the Hydrospheric AtmosphericResearch Center, Nagoya University. The APHRODITE dataset was
obtained from the project web site (http://www.chikyu.ac.jp/precip/).
Open Access This article is distributed under the terms of theCreative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original
author(s) and the source are credited.

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