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Dominant Synoptic Disturbance in the Extreme Rainfall at Cherrapunji,
Northeast India, Based on 104 Years of Rainfall Data (1902–2005)
FUMIE MURATA,a TORU TERAO,b HATSUKI FUJINAMI,c TAIICHI HAYASHI,dHARUHISA ASADA,e JUN MATSUMOTO,f AND HIAMBOK J. SYIEMLIEHg
aFaculty of Science and Technology, Kochi University, Kochi, Japanb
Faculty of Education, Kagawa University, Takamatsu, Japanc
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, JapandCenter for Southeast Asian Studies, Kyoto University, Kyoto, Japane
Nara Women’s University, Nara, Japanf
Tokyo Metropolitan University, Hachioji, and Japan Agency for Marine-Earth Science andTechnology, Yokosuka, Japang
Department of Geography, North-Eastern Hill University, Shillong, India(Manuscript received 5 June 2016, in final form 18 July 2017)
ABSTRACTThe characteristics of active rainfall spells (ARSs) at Cherrapunji, northeast India, where extreme highrainfall is experienced, and their relationships with large-scale dynamics were studied using daily rainfall datafrom 1902 to 2005 and Japanese 55-Year Reanalysis from 1958 to 2005. Extreme high daily rainfalls occur inassociation with ARSs. The extremely large amounts of rainfall in the monsoon season are determined by the
cumulative rainfall during ARSs. ARSs start when anomalous anticyclonic circulation (AAC) at 850 hPa
propagates westward from the South China Sea and western North Pacific, and covers the northern Bay of
Bengal. The AAC propagates farther westward and suppresses convection over central India during ARSs at
Cherrapunji, and continues for 3 to 14 days. Consequently, a northward shift of the monsoon trough during
the ‘‘break’’ in the Indian core region occurs. The westerly wind, which prevails in the northern portion of the
AAC, transports moisture toward northeast India and enhances moisture convergence over northeast India
with southerly moisture transport from the Bay of Bengal, and greatly intensifies the orographic rainfall. In
the upper troposphere, the Tibetan high tends to extend southward with the onset of ARSs. A linear relationship can be seen between the length and total rainfall of an ARS. Longer ARSs tend to result in greater
total rainfall. AACs with a greater zonal scale tend to produce longer and more intense ARSs. This study
provides evidence for the effect of western North Pacific AACs on the Indian summer monsoon.

1. Introduction
The Indian summer monsoon has remarkable intraseasonal variation. Rainfall periods with above and below normal rainfall amounts have been called ‘‘active’’and ‘‘break’’ spells, respectively. A detailed review ofstudies of active and break spells was conducted by
Rajeevan et al. (2010). Traditionally, the active andbreak spells of the Indian summer monsoon have been
based on the convective activity over northwest andcentral India [referred to as the ‘‘Indian core region’’following Rajeevan et al. (2010)]. Negative (positive)
rainfall anomalies over northeast India corresponded toactive (break) spells over northwest and central India.
Corresponding author: Fumie Murata, fumie@kochi-u.ac.jp

The synoptic condition during break spells has been
described as a northward shift of the trough of low
pressure toward the foothills of the Himalayas.
Ohsawa et al. (2000) studied the intraseasonal rainfall
variation during the 1995 summer monsoon season, andfound that rainfall in Bangladesh increased when the
monsoon trough was located at the foot of the Himalayas.
Shrestha et al. (2012) analyzed rainfall over the central
Himalayan region using 11 years’ worth of data fromthe Tropical Rainfall Measuring Mission (TRMM)
Precipitation Radar, and showed that rainfall was heavy
over the southernmost hills of the Himalayas [called the
Siwalik Range or sub-Himalayas; approximately 500–
700 m above mean sea level (MSL)] when the monsoon
trough was lying in the foothills of the Himalayas.
Goswami et al. (2010) analyzed the extreme rainfall over

DOI: 10.1175/JCLI-D-16-0435.1
Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright
Policy (www.ametsoc.org/PUBSReuseLicenses).

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northeast India, and showed that these extreme eventsoccurred in association with monsoon synoptic events
rather than isolated thunderstorms. In general, studies
that have focused on intraseasonal variation overnortheast India have been limited.
Hartmann and Michelsen (1989) performed spectral
analysis of rain gauge data for 70 years from 3700 stations in India, and noted that several stations along thesouthern flank of the Himalayas had spectral peaks ofnearly 15 days. The 40–50-day spectral peak was not
significant along the southern flank of the Himalayas,
whereas it was dominant at most stations elsewhere in
India. Ohsawa et al. (2000) detected a periodicity ofaround 20 days in an analysis of the 1995 summer
monsoon season. Fujinami et al. (2011) analyzed rain
gauge data for 20 years in Bangladesh, and showed that a
7–25-day spectral peak was dominant, while a 30–60-day
spectral peak was not prominent. Fujinami et al. (2014)
further investigated the spatiotemporal structure of a
7–25-day mode over the Meghalaya–Bangladesh–western
Myanmar region.
A 10–20-day mode has been detected in various elements of the Indian summer monsoon system (e.g.,
Krishnamurti and Bhalme 1976). Past studies suggestthat the 10–20-day mode in the Asian summer monsoon
region originates over the equatorial west Pacific(Annamalai and Slingo 2001), propagates westward(Krishnamurti and Ardanuy 1980) with a double-cellstructure (Chen and Chen 1993), and is a manifestationof the moist equatorial Rossby waves modified by theclimatological mean flow (Kikuchi and Wang 2009).
Fujinami et al. (2014) revealed that a westwardpropagating anticyclonic anomaly related to the equatorial Rossby wave from the western North Pacificpromotes rainfall over the Meghalaya–Bangladesh–western Myanmar region.
This study analyzes active rainfall spells (ARSs) in the
monsoon season at Cherrapunji, which is a town locatedin northeast India. Cherrapunji is a unique station because it holds the world record for the highest rainfall
in a calendar month and for longer time scales than acalendar month (Jennings 1950; Kiguchi and Oki 2010),
as well as the highest daily rainfall records in India(Guhathakurta 2007). Rainfall at Cherrapunji has clear
ARSs, and daily rainfalls with more than several hundred millimeters are common during ARSs (Fig. 3). The
large daily rainfall at Cherrapunji during the ARSs issuitable to detect the synoptic-scale events with highsensitivity because it can minimize other noises causedby few local cumulonimbus clouds.
Cherrapunji is located on the southern slope of the
Meghalaya Plateau (Fig. 1a) with a maximum elevationof around 2000 m above sea level. The plateau is divided

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from the Himalayan ranges by the Brahmaputra Valley.
As the plateau is the first elevated terrain that warm,
moist air from the Bay of Bengal encounters, maximum
precipitation tends to occur (Houze 2012). It is connected to the Indo-Burma Range in the east. The
southern margin of the Meghalaya Plateau is steep andrugged due to the tectonic motion along the Dauki fault
at the southern boundary of the plateau. There areseveral deep valleys that open toward the south with steep
cliffs on both sides (Fig. 1b): the town of Cherrapunji issituated near one such valley. Orographic amplificationis the major plausible cause for the extreme precipitation
at this location (e.g., Romatschke et al. 2010), althoughother factors, such as a local front between the southwest monsoon flow and east–northeast mountain winds
in the Brahmaputra Valley, may also have an influence(Yoshino 1975).
Northeast India receives the heaviest rainfall in India.
As the region includes hilly areas and adjoins the great
Himalayas, orographic effects are important in rainfall
in northeast India (Goswami et al. 2010). Our study reveals the relationship between the local orographic rainfall and large-scale circulation patterns at intraseasonal
time scales. Understanding the rainfall patterns overnortheast India might be crucial in understanding thehydrological processes over the Ganges–Brahmaputra–
Meghna River basin (Hofer and Messerli 2006). For example, heavy rainfall during ARSs at Cherrapunji directly
runs off toward Bangladesh and triggers flash floods.
Also, the high monsoon rainfall years at Cherrapunji
tended to correspond with severe flood years in Bangladesh (Murata et al. 2007).
Causes of the negative correlation between rainfall
anomalies in the Indian core region and northeast India
in the active–break cycle have not been discussed. The
ARSs at Cherrapunji can be representative in northeast
India because the extreme rainfall amount provides
noiseless signal for favorable synoptic-scale events. The
objective of this study is to answer the following questions
by understanding the characteristics of the Cherrapunji
ARSs (CARSs), and discuss the relationship with active–
break cycle in the Indian core region:
1) What is the relationship between CARSs and largescale dynamics?
2) What is the contribution of CARSs to the extreme
rainfall?
Section 2 describes the datasets used in this study.
The definition and characteristics of CARSs are described in section 3. Section 4 gives the spatiotemporal
structure of CARSs. The synoptic condition during
CARSs, the relationship with the Indian core region, andthe contribution to the extreme rainfall are discussed

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FIG. 1. (a) Topographical map of the region around Cherrapunji (shown with the star).
(b) Landscape of the southern cliff of the Meghalaya Plateau nearby Cherrapunji town.

in section 5. Finally, the results are summarized in
section 6.

2. Data
Data on the daily rainfall observed at Cherrapunji
observatory of the India Meteorological Department(IMD) were obtained from the following different
sources: the Research Data Archive at the National
Center for Atmospheric Research (Computational andInformation Systems Laboratory; ds.480.0) for datafrom 1902–70, and the Guwahati regional office of the
IMD for daily rainfall data from 1970 to 2003. Daily
rainfall data for 2003–05 were obtained directly from theCherrapunji Observatory of the IMD. Five years of daily
rainfall data at Cherrapunji are missing completely(1965, 1982–84 and 1987), and 7 years (1946, 1960, 1961,
1963, 1964, 1972, and 1986) have more than 10% missing
data for June–September. To study the relationshipbetween rainfall at Cherrapunji and that at otherregions, the IMD rainfall dataset (Pai et al. 2014) on a
0.258 latitude–longitude grid was used for the period

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FIG. 2. The 92-summer ensemble spectrum of the Cherrapunji
daily rainfall (mm). A red noise spectrum (dashed curve) and 95%
level of significance (solid curve) based on a lag-1 autocorrelation
are also shown.

1902–2005, and the APHRODITE V1101 rainfall dataset (Yatagai et al. 2009, 2012) on a 0.258 latitude–
longitude grid was used for the period 1958–2005. The
streamfunction, horizontal wind components and vertically integrated water vapor flux components of theJapanese 55-Year Reanalysis (JRA-55) (Ebita et al.
2011; Kobayashi et al. 2015) on a 1.258 latitude–longitude
grid were used to analyze the features of circulation
patterns and moisture transport during CARSs. Daily
interpolated outgoing longwave radiation (OLR) dataon a 2.58 latitude–longitude grid were used to show largescale convective activity, including ocean areas from1979 to 2005 (Liebmann and Smith 1996). In the composite analysis, the statistical significance of the differences from climatological values at each grid point wasestimated using the Student’s t test.

3. Features of ARSs at Cherrapunji
In this section CARSs are defined and described.
Figure 2 shows the 92-summer ensemble spectrum of theCherrapunji daily rainfall. The years for which morethan 10% of the June–September daily data are missing
were excluded; in total, 92 years were used, the first
three harmonics of the annual cycle were removed, andthe fast Fourier transform technique was used to calculate the spectra. A 5-point running mean in the frequency domain was applied to the raw spectra to reduce
estimate errors. Pronounced peaks appear around a
10–30-day period and a peak of about 11 days exceeds
the 95% confidence level. The spectrum is similar to thatof all Bangladesh daily rainfall (Fujinami et al. 2011)
and represents the dominance of the 10–20-day mode.

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Fujinami et al. (2011, 2014) examined the characteristicsof the 10–20-day mode over the Meghalaya–Bangladesh–western Myanmar region. This study focused on the timingof onset, length, and total rainfall amount of CARSs because they are the important hydrological characteristics of this region.
Figure 3 shows examples of the daily rainfall from
21 May to 10 October in the years 1915 and 1974. The
annual rainfall in 1915 was almost mean value and included four active spells. The annual rainfall in 1974 was
the highest between 1902 and 2005. The solid curve illustrates the daily climatological rainfall: it shows theaverage daily rainfall in each year, and smoothing by the
15-day running mean twice. The daily climatologicalrainfall reaches a maximum in late June, which wasmore than 1 month earlier than the rainfall over the
Indian core region (Rajeevan et al. 2010; Pai et al. 2016).
Heavy rainfall days (HRDs) are days with more than
1.5 times the daily climatological rainfall between Juneand September. CARSs are defined as consecutive
HRDs on which the total rainfall exceeds 500 mm. For
HRD periods that include 1 June or 30 September, thenumber of consecutive HRDs extended into May or
October. There were only three (two) cases of 1-day(2-day) intervals between the two adjacent CARSs.
Definitions of active–break spells can vary (Rajeevan
et al. 2010). Fujinami et al. (2014) defined active andbreak monsoon spells according to filtered data. In thisstudy unfiltered data are used to define ARSs, with noassumption regarding frequency. The shaded periods in
Fig. 3 represent ARSs in this study. There were nine
CARSs in 1974, which was the maximum number recorded during the analysis period. The CARSs repeatedperiodically at intervals of 10–20 days, which is consistent with the result of Fig. 2.
Table 1 shows the relationship between the total
rainfall on consecutive HRDs and the duration of consecutive HRDs. The numbers in the column in Table 1
indicate the periods of consecutive HRDs between 1902
and 2005. The number of periods decreases as the total
rainfall increases. Regarding the length of a period,
there was a greater percentage of consecutive 3-day
HRDs. A linear relationship is observed between thenumber of consecutive HRDs and the total rainfall.
Greater rainfall is observed in the periods with moreconsecutive HRDs. There are some extreme cases when
the total rainfall was between 3500 and 5000 mm during
7- or 9-day consecutive HRDs. In this study, CARSs aredefined as events with more than 500 mm total rainfall,
and the minimum length of a CARS is 3 days.
Variation between intra- and interannual scales has
been considered. For example, Gadgil and Joseph(2003) showed that the interannual variation in all-India

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FIG. 3. Daily rainfall (histogram; mm) for 21 May–10 October in (a) 1915 and (b) 1974, and the climatological daily
rainfall (solid curve; mm). The gray-shaded areas represent Cherrapunji active rainfall spells (CARSs).

summer monsoon rainfall is related to the number ofdays of rain breaks and active spells. Fujinami et al. (2011)
showed that more frequent and strong submonthly-scale
oscillations, and a greater number of rainy days, wererelated to the wet monsoon years in Bangladesh. Figure 4
shows that the cumulative rainfall of CARSs is proportional to total monsoon rainfall amount from June to
September. The cumulative rainfall during the period
without CARSs ranges from 2000 to 6000 mm and has anegative tendency with monsoon total rainfall because
the number of days without CARSs decreases when
CARSs increases.

Figure 5 shows the relationship between the cumulative
number of CARSs and the cumulative rainfall amounts
during CARSs over one year. As in the analysis of Fig. 2,
data for 92 years were used. The bottom and top of
the box are the first and third quartiles, respectively. The
thick band inside the box is the median. The ends of thewhiskers show the minimum and maximum. The number
of CARSs ranges from one to nine per year. The years
with four to nine CARSs over one year compose approximately half of the total sample years. In the range of
one to four spells per year, there is a tendency for total
rainfall to increase with the number of CARSs. However,

TABLE 1. Relationship between the total rainfall of consecutive heavy rainfall days (HRDs; horizontal line) and the length of consecutive
HRDs (vertical line). Asterisks denote cells with more than 10 periods.
14-day13-day12-day11-day10-day
9-day
8-day
7-day
6-day
5-day
4-day
3-day
2-day
1-day
Total

1
0
3
0
5
15
13
28
57
90
103
167
82
68
632
Total

1
1

1
1
2
*15
*103
*82
*68
272
, 500

1
3
*10
*46
*70
*64

194
, 1000

1
5
3
*13
*30
*28
*18

3
5
4
*14
*11

98
, 150038, 2000

2
3
1
3
3
2
3

1
5
1
2

15
11
, 2500, 3000Total rainfall (mm)11
, 35001

1

1, 40001, 4500

1
,5000

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FIG. 4. Scatterplots of monsoon rainfall (total rainfall amountfrom June to September; mm) and the cumulative rainfall with andwithout CARSs (solid and open circles, respectively; mm). The
regression line is shown for within CARSs.

there is no clear trend toward an increase in rainfall when
there are more than four CARSs per year. It is not always
the case that years with a greater number of CARSs are
wet monsoon years. It implies that CARSs with greater
total rainfalls contribute to wet monsoon years in addition
to a number of CARSs.
Figure 6 shows a composite of the anomalous rainfall
from daily climatological rainfall over India during
CARSs. This map was produced by using the IMD grid
rainfall data for the period 1902 to 2005. Positive andnegative values show regions of excess and less rainfall
during CARSs, respectively. The hatched regions are
significant at the 95% confidence level. While excess
rainfall is observed over northeast India during CARSs,
less rainfall is observed over central India and theWestern Ghats. Usually, the active and break spells of
the Indian monsoon are defined based on the rainfall
over central India. Thus, it is important to note that
northeast India generally has the opposite tendency tothat of central India: the spatial distribution is very
similar to that of the break monsoon composition of the
Indian core region (e.g., Pai et al. 2016).

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FIG. 5. The dependence of the cumulative total rainfall amounts(mm) during CARSs over 1 yr on the number of CARSs. The
bottom and top of the box are the first and third quartiles. The thickband inside the box is the median. The ends of the whiskers show
the minimum and maximum values. The data in parentheses next
to the number of CARSs on the horizontal axis show the number of
periods.

and north of the relatively low rainfall area. The area ofrelatively low rainfall is consistent with the location of themonsoon trough or continental tropical convergence zone
in past studies (Gadgil 2003). Usually, typhoons and othercyclonic disturbances, such as depressions or lows, move
westward along the monsoon trough zone and produce
rainfall over the monsoon trough area.
To explain the anomalous rainfall distribution during
CARSs (Fig. 7), data from composite anomalous streamfunction and composite anomalous wind (Figs. 8a,b) and

4. Relationship with large-scale circulation field
Figure 7 is the same as Fig. 6, but the APHRODITE
dataset is used to provide an extended view over the Asianmonsoon region. Although the analysis period (1958–
2005) is shorter than that in Fig. 6, the distribution of highand low rainfall areas over India is similar. The region withsignificantly lower rainfall during CARSs extends longitudinally over Pakistan, central India, the Indochina
Peninsula, and the northern Philippines. The region withsignificantly excessive rainfall during CARSs lies south

FIG. 6. Composite of anomalous rainfall (mm) from daily climatological rainfall during CARSs over India. The India Meteorological Department (IMD) grid dataset was used (1902–2005).
Hatched areas are statistically significant at the 95% level.

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FIG. 7. Composite of anomalous rainfall (mm) from daily climatological rainfall during CARSs over the Asian monsoon region. The APHRODITE dataset was used (1958–2005).

composite vertically integrated anomalous moisture fluxand its divergence (Fig. 8d) were produced by using JRA55 reanalysis data. The analysis period was 1958–2005. The
composite of anomalous OLR is also shown (Fig. 8c) forcomparison with the convective activity, including ocean
areas. The analysis period is 1979–2005, due to the limited
satellite data. Anomalous anticyclonic circulation (AAC)
appears over central India and the northern Bay of Bengal,
at both 200- and 850-hPa levels. Anomalous cyclonic circulations appear over the western equatorial Indian Ocean
and the Tibetan Plateau at the 200-hPa level. The positive
OLR anomaly (Fig. 8c) and anomalous moisture divergence (Fig. 8d) show intense convective suppression
over the western Indian subcontinent. In the lower troposphere (Fig. 8b), the AAC is extended farther east over
the South China Sea; this location is consistent with thearea of relatively low rainfall in Fig. 7 and the positive
OLR anomaly over the South China Sea and the Philippines in Fig. 8c. The convergence of the anomalous
moisture flux over northeast India (Fig. 8d) corresponds
well with active convection over northeast India during
CARSs (Figs. 6 and 7), although negative OLR (Fig. 8c)does not represent well the convective activity overnortheast India. The anomalous negative streamfunction is
located over the central equatorial Indian Ocean at the
850-hPa level, and represents a double-cell structure.
To further investigate how synoptic conditions produce moisture convergence over northeast India, acomposite of vertically integrated moisture flux and divergence during CARSs (Fig. 9b) is compared with acomposite of climatological means throughout June to
September (Fig. 9a). CARSs occupy only 8.5% of total

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analysis period, and the composite that excludes CARSs
is almost same as the climatological mean. There is alarge difference in the moisture flux field over 208–308N,
while there is almost no difference south of 208N, exceptthat the moisture flux is somewhat smaller during
CARSs. Climatologically, cyclonic circulation due to a
monsoon trough appears over central India, and moisture transport is limited over this area (Fig. 9a). In
contrast, a strong westerly moisture transport appears
over central India, which intensifies moisture convergence over northeast India during CARSs (Fig. 9b). The
location of moisture convergence over the foot of thecentral Himalayas is shifted slightly northward and intensifies during CARSs.
Figure 10 is the same as Fig. 8, but shows the composites of the CARS onset days. In the lower troposphere (Fig. 10b), AAC is distributed over the northern
Bay of Bengal, the Indochina Peninsula, the South
China Sea, and the western North Pacific along 108–
208N. Compared with Fig. 8b, the AAC does not cover
the Indian subcontinent. The location of the AAC isalmost consistent with the convective suppression represented by positive OLR anomalies (Fig. 10c) and thedivergence of the anomalous moisture flux (Fig. 10d).
The anomalous negative streamfunction is located tothe south of the AAC over the eastern Indian Ocean,and shows the double cell structure. Another anomalous
negative streamfunction is located around Japan. In
the upper troposphere (Fig. 10a), the AAC is locatedjust over northeast India where convection is active(Fig. 10c). There are two other anomalous negative
streamfunctions situated in the northeast and south of
the AAC. The negative OLR anomaly (Fig. 10c) and theconvergence of the anomalous moisture flux (Fig. 10d)
are distributed not only in northeast India but also extend to southern China along the 208–308N zone; thislocation is consistent with the northern edge of the AAC
at the 850-hPa level (Fig. 10b).
Table 1 shows that greater total rainfall tends to beobserved during longer CARSs. The data were thendivided by the length of CARSs, and compared withthose in Fig. 11. The composite numbers of periods were21, 40, and 9 for 3-, 5-, and 7-day CARSs, respectively.
Figure 11 shows the composite of anomalous streamfunction and anomalous wind data at 850 hPa for
3- (Figs. 11a,d,g), 5- (Figs. 11b,e,h), and 7-day CARSs(Figs. 11c,f,i), respectively. The time evolution is also
shown from day 22 to day 12. On day 22 (Figs. 11a–c),
an AAC forms over the South China Sea. This AAC
propagates westward, and the onset of CARSs(Figs. 11d–f) occurs when the AAC covers the northern
Bay of Bengal. The AAC moves farther westward on
day 12 (Figs. 11g–i). The westward propagation speed is

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FIG. 8. Composite of anomalous streamfunction (shading; 106 m2 s21) and anomalous wind (vector; m s21) at(a) 200- and (b) 850-hPa levels from daily climatological values during CARSs. The JRA-55 dataset was used(1958–2005). (c) As in (a) and (b), but for outgoing longwave radiation (OLR) (shading; W m22). (d) As in (a) and(b), but for vertically integrated moisture flux (vector; kg m21 s21) and its divergence (shading; 1024 kg m22 s21).
Hatched areas are statistically significant at the 95% confidence level. Only anomalous wind vectors that showed
a 95% statistically significant difference are plotted.

in the range of 5–10 m s21. On day 22, the AAC extends
more zonally toward the western North Pacific in 7-day
periods than in 3-day periods. Furthermore, the AACs
are more intense in 7-day periods than in 3-day periods.
On day 12, the westerly anomalies still remain over thewestern North Pacific along 208N in the 5- and 7-day
periods. In the 7-day periods, the center of the AAC isstill located in the western North Pacific.
The time evolution of the vertical structure was analyzed. The composites of the anomalous streamfunction
and anomalous wind at 500 hPa were almost the sameas in Fig. 11 (figures not shown). Figure 12 shows
the composite of the anomalous streamfunction andanomalous wind at 200 hPa in 5-day periods on day 22(Fig. 12a), the onset of CARSs (Fig. 12c), and day 12(Fig. 12e). They are compared with anomalous OLR on
day 22 (Fig. 12b), the onset of CARSs (Fig. 12d), andday 12 (Fig. 12f) to examine the effect of diabatic
heating on the circulation field in the upper troposphere.

A significant AAC over the Tibetan Plateau tends tomove southward with the onset of CARSs, and lies overthe convectively active area. The comparison between
the onset day of the CARSs and day 12 shows a westward propagation. The composite of 3- and 7-day
CARSs showed similar AAC movement toward thesouth, but they did not reach significance (figures notshown), possibly due to the relatively weak signal for
3-day CARSs and insufficient number of samples for
7-day CARSs.

5. Discussion
a. Synoptic conditions during CARSs
The composite of the CARS onset days implies that
the CARSs begin when the AAC in the lower troposphere is over the Bay of Bengal (Fig. 10b). This AAC
signal formed in the South China Sea and the western15 OCTOBER 2017

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FIG. 9. Composite of vertically integrated moisture flux (vector; kg m21 s21) and its divergence (shading;1024 kg m22 s21) for (a) June–September climatology and (b) during CARSs.

North Pacific propagates westward along 108–208N(Fig. 11). An anomalous negative streamfunction issituated to the south of the AAC, and makes a doublecell structure (Fig. 10b). During the CARSs, the AAC

propagates farther westward over central India (Fig. 8b),and causes convective suppression (Fig. 8c). The AAC
over central India has a barotropic structure (Fig. 8a).
These features are consistent with the results of Fujinami

FIG. 10. As in Fig. 8, but for the onset day composites of each CARS.

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FIG. 11. As in Fig. 8b, but for the composites for 2 days before the onset of (a) 3-, (b) 5-, and (c) 7-day CARSs, respectively. (d)–(f) As in(a)–(c), but for the composites of the onset day of CARSs. (g)–(i) As in (a)–(c), but for the composites of the 2 days after the onset
of CARSs.

et al. (2014) and other studies in the 10–20-day mode
analysis. The AAC signal in the upper tropospheremoved southward from the Tibetan Plateau and covered
over central and northeast India (Fig. 12).
During CARSs, suppressed convective anomalies are
distributed over a zonally elongated band from southern
Pakistan, via central India, the northern Bay of Bengal,
the Indochina Peninsula, and the Philippines, to thewestern North Pacific (Figs. 7 and 10c). A similar elongated band, but with opposite sign, was shown by Moonet al. (2013) during the active phase of the Indian summer monsoon, and by Takahashi et al. (2015) during
above-normal precipitation over the Indochina Peninsula. The zonally elongated band is formed due to thewestward propagation of AAC.
Wang and Xie (1997) simulated the similar elongatedprecipitation band over northwest India toward theequatorial central Pacific. They used the extension of

the model developed by Wang and Li (1994). It is anequatorial beta-plane channel model with a model
atmosphere consisting of a two-layer free troposphereand a well-mixed boundary layer, and includes surface evaporation in the moisture budget. The instability generated by convection–frictional convergenceproduced a low-frequency multiscale convective complex in which the condensation heating coupled moist
Kelvin wave and moist Rossby wave with the gravest
meridional structure (n 5 1 mode). Wang and Xie(1997) introduced the climatological July mean flow at
200- and 850-hPa levels and July mean surface specifichumidity over Asian monsoon region as the model’s
basic state.
The path, propagating speed, and horizontal scale ofthe observed westward propagating AACs in this study(Fig. 11) are similar to that of the simulated northern cellof the n 5 1 equatorial Rossby waves in Wang and Xie

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8247

FIG. 12. As in Figs. 11b,e,h, but for 200 hPa for (a)–(c). (d)–(f) As in (a)–(c), but for the composite for OLR (shading; W m22).

(1997). The structure of the simulated Rossby wave was
highly asymmetric about the equator, featuring the intensified northern cell propagated from the South China
Sea to the Arabian Sea and the much weaker southerncell that was located much closer to the equator over the
Indian Ocean sector. This happens because the verticaleasterly shear and high specific humidity are confined to
the Northern Hemisphere. Xie and Wang (1996) showedthat an easterly shear can effectively destabilize Rossby
waves and the convective heating can further amplify
waves with the most unstable wavelength (about
4000 km) for reasonably realistic parameters.
While the composite during CARSs shows the barotropic structure over central India (Figs. 8a,b), the time
evolution of the 200-hPa AAC (Figs. 12a–c) shows that
the AAC in the upper troposphere moves not westward
but southward from the Tibetan Plateau on the onset of
CARSs. It corresponds to a southward extension of the
Tibetan high. Wang and Xie (1996) showed that thevertical easterly shear tended to trap Rossby waves in
the lower troposphere. A plausible cause of the barotropic structure over central India is that the upper
tropospheric AAC generated as a response to the diabatic heating over northeast India is superimposed on
the lower-tropospheric AAC that propagated from thewestern North Pacific. It cannot be produced in themodel of Wang and Xie (1997) because topography was
not included in their model. The barotropic structure
over central India during CARSs has been described

during the break in the Indian summer monsoon(Ramaswamy 1962). There are studies that consideredbarotropic instability as a mechanism to control a revivalof the Indian summer monsoon after the break events(Rao 1971; Govardhan et al. 2017). Figures 8a and 10ashow a structure similar to a stationary Rossby wave
pattern (Hoskins and Karoly 1981). However, the meaneasterly flow in the upper troposphere over the south of
the Tibetan Plateau will not form a stationary Rossby
wave train.
Why do CARSs occur when the AAC is located southof northeast India? Figures 8d and 10d show theanomalous moisture convergence at the northern portion of the AAC. It can be seen that the area is consistent
with the excessive rainfall area (Figs. 6 and 7). Figure 9b
suggests that the westerly moisture transport over central India enhances water vapor convergence overnortheast India together with the moisture transport
from the Bay of Bengal during CARSs. In the climatological condition, the westerly moisture transport does
not exist due to convergence over the Indian monsoon
trough that extends from the northern Bay of Bengal tonorthwest India (Fig. 9a).
Murata et al. (2008) compared radiosonde soundings
at Dhaka, Bangladesh, with daily rainfall at Cherrapunji
during the monsoon in 2004, and showed that westerly
wind dominated over the Bengal plain during the
CARSs there. Figure 13 uses JRA-55 grid data corresponding to the location of Dhaka during 1958–2005,

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FIG. 13. Scatterplots of the dependence of daily rainfall (mm) on (a) zonal and (b) meridional winds (m s21) at the
925-hPa level JRA-55 grid data at 22.58N, 90.08E. The solid lines show the cumulative frequency distribution (%).

and shows the correlation between the 925-hPa level
horizontal wind and daily rainfall at Cherrapunji. The
curve of the cumulative frequency distribution showsthat nearly half of the period was an easterly phase. Thissituation corresponds with typical monsoon conditions
when the Indian monsoon trough is intensified along theGanges plain (Fig. 9a). However, the heavy daily rainfall
was concentrated in the westerly phase of 5–10 m s21,
and occupies less than 20% of the overall period. For
the meridional wind component, the southerly windcomponent was dominant throughout the monsoon period, and the heavy daily rainfall appeared in thesoutherly phase of 5–15 m s21. This result implies thatthe westerly phase or southwesterly wind is a favorable
condition for CARSs. As a small piece of additional
evidence, there is a short story in which a boatman livingin the Bengal plain recognizes a change in wind direction
from easterly to westerly as a sign of an ARS.1

b. Relationship with the break spell of the Indian coreregionThe composite of the ARSs at Cherrapunji (Fig. 6)
shows the opposite tendency to that of the composite of
the ARSs over the Indian core region (e.g., Pai et al.
2016). The AAC at the 850-hPa level (Fig. 8b) overcentral India during CARSs weakens the airflow toward
the Western Ghats, as well as the monsoon trough overthe Ganges plain and the northern Bay of Bengal(Fig. 9), and suppresses the generation of monsoon depressions or lows, which are the main contributor to

1

The short story ‘‘Boatman Tarini’’ written by Tarashankar
Bandyopadhyay, a twentieth-century Bengali novelist.

rainfall over the Indian core region. This perspective
regarding the Indian monsoon break is similar to that of
Krishnamurti and Ardanuy (1980), and implies that theonset of CARSs has the potential to predict break spells
over the Indian core region. The CARSs overlap with
61% of break spells and only 2% of active spells in the
Indian core region in the list shown in Pai et al. (2016).
However, Hartmann and Michelsen (1989) showsthat a 30–60-day periodicity is dominant over the Indiancore region, although years with a dominance of a
10–20-day periodicity are also included (Kulkarni et al.
2011). The 30–60-day mode exhibits remarkable northward propagation from the equatorial Indian Ocean tocentral India (Sikka and Gadgil 1980; Yasunari 1980).
Recently, Hatsuzuka and Fujinami (2017) investigatedthe condition that low pressure systems (LPSs) with awavelength of around 6000 km develop over Bangladesh.
The LPSs tended to occur when an anomalous cyclonic
circulation in 30–60-day mode was located over central
India and the northern Bay of Bengal, while an AAC in
10–20-day mode was located over the northern Bay of
Bengal. The westward propagation of the AAC from thenorthern Bay of Bengal toward central India was unclear
in the LPS cases. Their result implies that combination of
10–20-day and 30–60-day modes produces the differencein the dominant frequencies between northeast and central India. CARSs may include more non-LPS cases as
they showed that convection over Meghalaya Plateau ismore active in the non-LPS cases.

c. Contribution of CARSs to extreme rainfall
Northeast India corresponds to the maximum rainfall area in the Indian subcontinent including not only
the Meghalaya Plateau but also the adjoining areas of15 OCTOBER 2017

MURATA ET AL.

the Himalayan range. Xie et al. (2006) pointed out theimportance of narrow mountain ranges in the Indian
subcontinent for the Indian monsoon circulation. In
general, diabatic heating due to convective activity is
an important factor in intensifying the monsoon
circulation.
Daily rainfall equaling the record-breaking levels at
Mumbai (Jenamani et al. 2006) is sometimes observed at
Cherrapunji (Figs. 3 and 13; Guhathakurta 2007). Interestingly, however, such extreme daily rainfall exceeding 500 mm does not occur during on 1 or 2
consecutive HRDs and is included in longer CARSs(Table 1). As in the findings of Goswami et al. (2010),
the fact implies that synoptic conditions during ARSs
are important for heavy rainfall at Cherrapunji. The
influence of the AACs on moisture flux convergenceseems to produce persistent orographic lifting and result
in large cumulative rainfall (Fig. 9). However, thecomposite of OLR does not represent well the heavy
rainfall during CARSs (Fig. 8c). The orographic heavy
rainfall may be difficult to be detect by OLR. Hamada
et al. (2015) found that the extreme rainfall events withmaximum near-surface rainfall rates had relatively low
echo-top heights in the statistical analysis of TRMM
observations, and pointed out the weak linkage between
the heaviest rainfall and tallest storms.
The cumulative rainfall during CARSs has high correlation with the total monsoon rainfall (Fig. 4). The
world record precipitation from 3 to 15 days was observed at Réunion Island in the southwestern Indian
Ocean, and the records from 10 to 15 days were causedby the three approaches of Cyclone ‘‘Hyacinthe’’ inJanuary 1980 (Chaggar 1984; Kiguchi and Oki 2010). On
the other hand, the world record precipitation for morethan 1-month time scales at Cherrapunji is based on the
climatological long monsoon season during June–
September including high-frequency CARSs with thedominance of 10–20-day periodicity.
Both total rainfalls during a CARS and frequency of
CARSs contribute to the cumulative rainfall (Fig. 5).
There is a linear relationship between the length andtotal rainfall of an ARS (Table 1). Figure 11 shows that
AACs with a greater zonal scale tend to produce longer
and more intense CARS. The AACs over the western
North Pacific have been identified as an importantcomponent because they exert a great influence on the
dynamics of East Asia monsoon through the production of a Pacific–Japan pattern (Nitta 1987). Longer
persistent AACs, with a greater zonal scale, have been
observed as a specific feature in post–El Niño summers,
and various hypotheses have been proposed to explain
the dynamics, such as wind–evaporation–SST feedback(Wang et al. 2000) or an Indian Ocean capacitor effect8249(Xie et al. 2009). The findings of this study provide evidence for the effect of AACs in the western North
Pacific on the Indian summer monsoon. Some severe
floods in Bangladesh (Hofer and Messerli 2006) occurred in post–El Niño summers. Terao et al. (2013)
investigated the relationship between the rainfall overBangladesh and western North Pacific monsoon, andfocused on the post–El Niño summers of 1983, 1988, and1998. Further investigations of the relationship between
the AACs over the western North Pacific and CARSs
are necessary for better forecasting of intense CARSs.

6. Conclusions
The synoptic conditions of active rainfall spells (ARSs)
at Cherrapunji (CARSs), northeast India, were investigated. The town is famous for the extreme high rainfall that occurs over time scales greater than 1 month. The
data that were predominantly used in this study were thedaily rainfall data from 1902–2005 and the JRA-55 reanalysis data from 1958–2005. The CARSs correlated withthe monsoon break spells over the Indian core region.
Although several studies described excess rainfall in thenorthern part of India during the Indian monsoon break,research focusing on the intraseasonal variation overnortheast India have been limited.
The CARSs start when an AAC is over the Bay of
Bengal. The orographic rainfall at Cherrapunji intensifies when a westerly wind prevails in the northern
portion of the AAC and enhances moisture flux convergence over northeast India. The AAC propagates
westward from the South China Sea, and the same AAC
suppresses convection over central India and theWestern Ghats. In the upper troposphere, the Tibetan
high tends to extend southward with the onset of
CARSs. While the synoptic conditions during the Indian
monsoon break have been described in terms of anorthward shift of the Indian monsoon trough, thewestward-propagating synoptic system plays an important role in the occurrence of CARSs: this implies thepossibility of forecasting CARSs by monitoring the
AAC over the South China Sea and the western North
Pacific. A linear relationship was found between thelength of a CARS and the total rainfall during the spell.
Longer and heavier CARSs occurred when zonallylarger AAC existed over the South China Sea and thewestern North Pacific. These findings provide evidence
that AACs in the western North Pacific control theseverity of CARSs and have an effect on the Indiansummer monsoon.
Cherrapunji is a calcium-rich area geologically, andthere are several limestone caves that have a potential toreconstruct the paleo monsoon climate. For example,

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Myers et al. (2015) compared the stalagmite record ofrecent 50 years at Mawmluh cave in Cherrapunji withindices that represent Pacific decadal variabilities. The
fact that the rainfall at Cherrapunji occurs during the
break in the Indian core region may improve the interpretation, although it is still uncertain whether thesame mechanism can be applicable on longer (interannual to centennial) time scales.
Acknowledgments. The authors are grateful to the editor (Dr. Jason Evans) and anonymous reviewers for appropriate and thoughtful comments and suggestions. The
authors thank to Drs. Ajit Tyagi, B. N. Goswami, G. S.
Bhat, and Someshwar Das for their supports to the authors’ study in northeast India. Drs. Kazuo Ando and
Masayuki Onishi provided useful information about life in
the Bengal plain. This study was conducted with support
from a Grant-in-Aid for Scientific Research (S26220202
and A23241057) from the Japan Society for the Promotionof Sciences (JSPS). Portions of this study were conductedby the joint research program of the Institute for SpaceEarth Environmental Research, Nagoya University.

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