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Role of Tropical Cyclones along the Monsoon Trough in the 2011 Thai
Flood and Interannual Variability
HIROSHI G. TAKAHASHITokyo Metropolitan University, Hachioji, and Japan Agency for Marine-Earth Science and Technology, Yokohama, JapanHATSUKI FUJINAMI
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, JapanTETSUZO YASUNARI
Research Institute for Humanity and Nature, Kyoto, Japan

JUN MATSUMOTOTokyo Metropolitan University, Hachioji, and Japan Agency for Marine-Earth Science and Technology, Yokohama, JapanSOMCHAI BAIMOUNG
Ministry of Information and Communication Technology, Bangkok, Thailand(Manuscript received 19 February 2014, in final form 23 October 2014)
ABSTRACTThe atmospheric circulation patterns that were responsible for the heavy flooding that occurred in Thailand
in 2011 are examined. This paper also investigates the interannual variation in precipitation over Indochina
over a 33-yr period from 1979–2011, focusing on the role of westward-propagating tropical cyclones (TCs)over the Asian monsoon region. Cyclonic anomalies and more westward-propagating TCs than expected fromthe climatology of the area were observed in 2011 along the monsoon trough from the northern Indian
subcontinent, the Bay of Bengal, Indochina, and the western North Pacific, which contributed significantly to
the 2011 Thai flood. The strength of monsoon westerlies was normal, which implies that the monsoon westerly
was not responsible for the seasonal heavy rainfall in 2011. Similar results were also obtained from the 33-yr
statistical analysis. The 5-month total precipitation over Indochina covaried interannually with that along themonsoon trough. In addition, above-normal precipitation over Indochina was observed when enhanced cyclonic circulation with more westward-propagating TCs along the monsoon trough was observed. Notably, theabove-normal precipitation was not due to the enhanced monsoon westerly over Indochina. Therefore, the2011 Thai flood was caused by the typical atmospheric circulation pattern for an above-normal precipitation
year. It is noteworthy that the effect of sea surface temperature (SST) forcing over the western North Pacificand the Niño-3.4 region on total precipitation during the summer rainy season over Indochina was unclear
over the 33-yr period.

1. Introduction
Denotes Open Access content.

Corresponding author address: Hiroshi G. Takahashi, Tokyo
Metropolitan University and Japan Agency for Marine-Earth Science and Technology, 1-1 Minami-Osawa, Hachioji-shi, Tokyo, 1920397, Japan.
E-mail: hiroshi3@tmu.ac.jp
DOI: 10.1175/JCLI-D-14-00147.1
Ó 2015 American Meteorological Society

In 2011, severe flooding in Thailand during thepostmonsoon season caused substantial human and economic damage. Komori et al. (2012) showed that rainfall
in each month of the rainy season (from May to October)exceeded climatological norms, and that this was a primary cause of the Thai flood. The aim of this study is to1466

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better understand the characteristics of atmospheric circulation during the rainy season of 2011 around Indochina
through an investigation of the interannual variations inprecipitation and related atmospheric circulations overa period of more than 30 yr.
Interannual variations in precipitation for the whole
Asian monsoon and for regional Asian monsoon regions have been investigated in many previous studies.
Indochina is located at the boundary of two monsoon
regions, the South Asian and western North Pacificmonsoons. Wang and Fan (1999) proposed two separate monsoon indices for the South Asian and western
North Pacific monsoons, based on observations indicating that interannual variations in precipitation inthese two regions were not correlated. Furthermore,
Wang et al. (2001) showed that long-term changes orinterdecadal variations in precipitation over these tworegions were also distinct.
Some studies have examined interannual variations in
total precipitation over Indochina. Chen and Yoon (2000)
suggested that high rainfall over Indochina was related toan enhancement of the westward-propagating tropical cyclones (TCs) in the Asian summer monsoon season troughand changes in the sea surface temperatures (SSTs) over
the Niño-3.4 region. Zhang et al. (2002) suggested thata similar mechanism occurred, but only for the onset phaseof the rainy season. They suggested that the above-normal
rainfall in May and June over Indochina was caused by theactivity of westward-propagating TCs, which were associated with cold SST anomalies over the central and western
Pacific. In the peak rainy season, interannual variation in
September precipitation over Indochina has been explained in relation to the activity of TCs (Takahashi andYasunari 2008; Takahashi et al. 2009), although thesestudies focused solely on peak seasonal precipitation.
However, the factors controlling this interannual variationin rainy seasonal precipitation are not yet understood.
Climatologically, the rainy season over the Southeast
Asian monsoon can be characterized as two subseasons
divided by the climatological monsoon break in lateJune (Takahashi and Yasunari 2006). Rainfall in the firsthalf of the rainy season (from May to June) is brought by
typical monsoon southwesterlies, whereas rainfall in thesecond half of the season (from July to September) is
due to active TCs. Takahashi and Yasunari (2008) estimated that 70% of precipitation over Thailand is caused
by TCs in September. In fact, peak precipitation over
Indochina occurs in September (Matsumoto 1997;
Takahashi and Yasunari 2006). Note that Takahashi andYasunari (2008) indicated that weaker TCs, such as tropical depressions (with wind up to 17 m s21), have also
contributed to the peak seasonal precipitation. In thisstudy, the term TC includes these weaker TCs.

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Westward-propagating TCs coupled with convective
activity in the rainy season over tropical Asian monsoon
regions have been reported in previous studies (e.g., Chen
and Chen 1993; Chen and Yoon 2000; Fukutomi andYasunari 1999; Yokoi and Satomura 2005). Yokoi et al.
(2007) showed that the dominant time scales of precipitation in the first and second halves of the rainyseason of the Southeast Asian monsoon were 30–60 and
10–20 days, respectively. Hence, these studies support
the idea that the basic rainfall system over Indochina
can be characterized as westward-propagating TCs. It istherefore possible that the activity of the westwardpropagating TCs is a key factor in determining interannual variation in rainfall over Indochina.
This study describes the atmospheric circulation patternthat caused the seasonal heavy precipitation and subsequent flooding in Thailand in 2011. To understand theheavy seasonal precipitation in 2011, we statistically investigated the interannual variations in total precipitationin the summer rainy season over Indochina over a 33-yr
period. We focused on the activity of westwardpropagating TCs. In addition, we considered whether
the 2011 Thai flood was caused by an untypical or a commonly occurring atmospheric circulation pattern in theabove-normal precipitation years. Furthermore, the roleof SSTs over the Niño-3.4 region on the interannual variation in total precipitation of the summer rainy season
over Indochina is discussed.
Section 2 describes the data used in this study. Section 3
presents our results regarding the 2011 Thai flood andinterannual variation in seasonal precipitation, atmospheric circulation, and TC activity. Finally, the impactof SSTs on the interannual variation in seasonal precipitation is discussed in section 4, and our conclusions
are presented in section 5.

2. Data
Komori et al. (2012) reported above-average precipitation in 2011 at 15 weather stations. To better understand the spatial and temporal distributions ofprecipitation in 2011, we used daily rainfall data from 56
stations in Thailand (Fig. 1), which were compiled by theThai Meteorological Department. The data spanned
1979–2000 and included 2011. Climatological means
were calculated over 23 yr, from 1979 to 2000 and again
in 2011. To produce precipitation time series for Thailand (north of 128N), we averaged data from 53 stations(Fig. 1). The average precipitation index was referred toas the ‘‘all-Thailand rainfall’’ (ATR).
To analyze large-scale rainfall on a daily time-scale
across Indochina, the Global Precipitation Climatology
Project 18 Daily Precipitation (GPCP-1DD; Huffman

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FIG. 1. Spatial distribution of 2011 precipitation expressed as thepercentage of climatological precipitation (1979–2000) that fell
from May to September.

et al. 2001, version 1.2) dataset was also used. This
provides daily, global 18 3 18 gridded fields for total
precipitation from October 1996 to December 2011.
This study also used satellite-derived rainfall data fromthe Climate Prediction Center (CPC) Merged Analysis
of Precipitation (CMAP; Xie and Arkin 1997) over 33 yr
from 1979 to 2011 for statistical analysis of the interannual variation in precipitation with monthly, global
2.58 3 2.58 gridded fields.
To investigate the differences in precipitation activity
between wet and dry years, 2011 and 1998 were selected
as the highest and lowest precipitation years, respectively, because of the limitations of the daily global
precipitation dataset. We show the daily time series of
ATR (Fig. 2) and the longitude–time sections of GPCP1DD precipitation along 128–208N (Fig. 3) in both years.
To examine the atmospheric circulation that caused theheavy precipitation and subsequent severe flooding andinterannual variation in precipitation over Thailand, weused the Japanese 55-year Reanalysis (JRA-55; Ebita et al.
2011) dataset, including zonal and meridional winds (u, y)and streamfunction (c) at 850 and 200 hPa. The Joint
Typhoon Warning Center (JTWC) TC track dataset wasalso used to examine the interannual variation in TC activity. The JTWC TC track datasets include tropical storms(with winds of 18–32 m s21) and some tropical depressions(with winds up to 17 m s21). We also used Extended Reconstructed SST (ERSST) version 3 (Smith et al. 2008)
datasets, which have a spatial resolution of 28, to discuss
the impact of SST on precipitation over Indochina.

FIG. 2. Time series of daily all-Thailand rainfall (ATR) in(a) 2011 and (b) 1998. ATR data were generated by averaging
precipitation data from 53 stations north of 128N (Fig. 1). Black line
indicates 5-day running mean. Star symbols indicate precipitation
events. The letter B shows longer monsoon breaks.

3. Resultsa. 2011 Thai flood
1) HIGH PRECIPITATION IN 2011
Figure 1 shows a spatial map of rates of precipitationin 2011 to the climatological precipitation. High precipitation was observed over Thailand from May to
September at most stations. The seasonal mean precipitation was particularly high over the northwesternpart of Thailand. The rates of monthly precipitation(expressed as the ATR) were 105.6%, 100.5%, 124.1%,
121.3%, and 127.5% of the climatological monthly
precipitation for May, June, July, August, and September, respectively. As demonstrated, precipitation was
higher than climatological mean data for all months, butparticularly so in the second half of the summer rainyseason. To understand whether this high precipitation
over Thailand in 2011 was caused by an atypical atmospheric circulation pattern or not, we investigate the33-yr interannual variation in precipitation over Indochina in sections 3b and 4a.

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FIG. 3. Time–longitude cross section generated from the GPCP-1DD daily precipitation (mm) dataset in (a) 2011
and (b) 1998 along the latitudinal band between 128 and 208N. The target region located around 1008E is shown by
a vertical dashed line. Precipitation events in Fig. 2 are also indicated by horizontal dashed lines.

2) WESTWARD-PROPAGATING PRECIPITATIONSYSTEMSThe ATR time series in 2011 showed frequent precipitation events throughout the five months from May to
September (Fig. 2a). Three or four precipitation events
per month were observed from June to September in
2011, indicating that precipitation events occurred frequently during the rainy season in 2011. However, twolong monsoon breaks were observed in mid-June and Julyin 1998 (Fig. 2b). In addition, the major climatologicalrain peak in September was unclear in 1998. These longer
monsoon breaks and the below-normal rainy peak in
September were characteristics of the 1998 rainy season.
Figure 3 shows that many westward-propagating
precipitation signals were found in both 2011 and 1998.
Many westward-propagating precipitation signals wereobserved over Thailand (approximately 1008E) from
May to September. These signals originated from thewestern North Pacific or over the east of Indochina(Fig. 3). The timing of arrival or approach of theseprecipitation activities basically corresponded with thehigh precipitation of the ATR (see the horizontal
dashed lines in Fig. 3).
In 2011, many westward-propagating precipitation
signals were consistently observed from June to

September, which corresponded with the precipitation
events observed in the ATR. Compared to the westwardpropagating precipitation signals in 2011, fewer precipitation signals were observed in 1998 during the longer
monsoon breaks in mid-June and July and September.
Climatologically, the major precipitation peak, which is
caused by westward-propagating TCs, tends to occur
in September (Takahashi and Yasunari 2006). In conjunction with the long monsoon breaks in mid-Juneand July and the below-normal precipitation peak in
September, the westward-propagating rainfall activity
over Indochina experienced a temporary lull. Over
Indochina, the frequency of westward-propagating precipitation systems, which are probably the westwardpropagating TCs, differed greatly between the summersof 1998 and 2011.

3) ENHANCED CYCLONIC CIRCULATION AND TC
ACTIVITY ALONG THE MONSOON TROUGH
IN 2011To investigate the characteristics of atmospheric circulation and TC activity over and around Indochina in
2011, 5-month mean anomalies in the 850-hPa streamfunction were investigated based on climatological values
from 1979 to 2011, and TC tracks from May to September

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explained by the typical atmospheric circulation patternin high precipitation years, we statistically investigated
the interannual variation in precipitation and atmospheric circulation over a 33-yr period.

b. Interannual variation of rainy seasonal
precipitation over a 33-yr period
1) A ZONALLY EXPANDED CYCLONICCIRCULATION SIGNAL ALONG THE MONSOONTROUGH

FIG. 4. Five-month mean anomalies in streamfunction at 850 hPa
from May to September (a) 2011 and (b) 1998, based on climatological data from the 33-yr period from 1979 to 2011. Only contours
below 0 m2 s21 are plotted (cyclonic anomalies) and the interval is
5 3 105 m2 s21. JTWC TC tracks from May to September are alsoplotted in black and the green lines outline land areas.

in 2011 and 1998 (Fig. 4). Negative streamfunction
anomalies were observed along the monsoon troughfrom the northern Indian subcontinent, the head of the
Bay of Bengal, Indochina, and the western North Pacific. In addition, more TC tracks were observed along
the monsoon trough in 2011 than in 1998. However,positive streamfunction anomalies were found along themonsoon trough in 1998 (Fig. 4b). Note that there wereclearly fewer TCs in 1998 than in 2011, particularly on
the eastern coast of Indochina. These results imply that
cyclonic anomalies with many TCs along the monsoon
trough were a major contributing factor to the heavy
precipitation over Thailand in 2011.
We also confirmed that the TCs along the monsoon
trough in Fig. 4 corresponded to the westward-propagating
precipitation systems, which were observed in Fig. 3 (notshown). However, not all of the westward-propagating
TCs were categorized as tropical storms, severe tropicalstorms, or typhoons while over the western North Pacific.
Hence, weaker TCs, such as tropical depressions,contributed to the total precipitation, as reported by
Takahashi and Yasunari (2008). In addition, it is difficult
to understand the changes in TC activity from a comparison of two specific years, given the random nature ofTCs. To determine if the 2011 Thai flood could be

To statistically investigate the atmospheric circulationthat controlled interannual variation in total precipitation from May to September over and around
Indochina, we generated regression maps showing interannual variation in precipitation and circulation in
the lower troposphere with area-averaged precipitation.
To obtain a robust result, we conducted a regression
analysis over 33 yr from 1979 to 2011, based on CMAP
data over Indochina (12.58–208N, 97.58–107.58E), because the observations by rain gauges were limited to
23 yr. This is referred to as the reference region of precipitation. Based on CMAP data, the time series of interannual variation in precipitation over Thailand from
May to September showed that 2011 was one of theheaviest precipitation years (Fig. 5a). This was consistent with our results for the ATR analysis based on rain
gauge observations (not shown). The interannual correlation coefficient between the two precipitation datasets over 23 yr from 1979/2000 and 2011 was 0.83. Similar
results were obtained when we used the area-averaged
53-gauge precipitation dataset instead of the CMAP
dataset.
Figure 5b shows the regression of the 5-month mean
precipitation from May to September and low-level
circulation at all grid points in the Asian monsoon
region against the normalized CMAP dataset over
Indochina. High correlations and highly regressed precipitation values are observed across the northern
Indian subcontinent, the head of the Bay of Bengal,
Indochina, and the western North Pacific. This indicatesthat interannual variation in total precipitation during
the rainy season over Indochina was synchronized withthat over the zonally expanded area. This zonally expanded area corresponded to the monsoon trough. Interannual variation in total precipitation during therainy season showed a broad spatial structure along themonsoon trough over the Asian monsoon region. No
significant correlation was observed over central or
southern India.
The regression coefficient in the 850-hPa winds andstreamfunction showed zonally expanded signals along
the monsoon trough (Fig. 5c). Negative streamfunction

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FIG. 5. (a) Precipitation time series generated from the CMAP dataset for the rainy season (May–September) over
the reference region of Indochina (12.58–208N, 97.58–107.58E) from 1979–2011. The reference region is used for the
regression analysis in (b),(c) and is indicated by a solid rectangle in these panels. (b) Regression of CMAP data during
the rainy season against the normalized data (mm day21) shown in (a) from 1979 to 2011. (c) As in (b), but for
the 850-hPa zonal and meridional winds and streamfunction (colors) during the rainy season. Areas with colorsin (b) and plotted vectors (winds; m s21) and contours and colors (streamfunction; 106 m2 s21) in (c) are statisticallysignificant at the 90% level, as determined by correlation coefficients based on 31 degrees of freedom (df).

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signals along the monsoon trough with above-normal
precipitation over Indochina were observed. Thus, when
above-normal precipitation occurred over Indochina,
cyclonic circulation strengthened over the monsoon
trough region. This suggests that interannual variation in
total precipitation during the rainy season over Indochina
is controlled by the mean seasonal strength of cyclonic
circulation along the zonally expanded monsoon trough.
Westerlies were stronger south of Indochina, while
easterly anomalies were observed over the northern
Indochina. Thus, no significantly enhanced monsoon
westerly was observed over the reference region,
which probably indicates that the above-normal precipitation over Indochina was not directly induced by
the enhanced monsoon westerlies over Indochina. In
addition, the zonally expanded precipitation and lowlevel cyclonic circulation along the monsoon troughwere consistent with the primary route of westwardpropagating TCs, originating from the western North
Pacific to the Bay of Bengal, which will be examined in
the next section.

2) ASSOCIATION BETWEEN CYCLONICANOMALIES AND WESTWARD-PROPAGATINGTROPICAL CYCLONESIn 2011, many westward-propagating precipitation
systems were observed along the zonal band of 128–208N(Fig. 3), which basically corresponds to the monsoon
trough. In addition, cyclonic circulation anomalies withmany TCs were found along the monsoon trough in 2011
(Fig. 4). To investigate the robust relationship between
cyclonic circulation and TC activity along the monsoon
trough from the Indian subcontinent, the head of the Bayof Bengal, Indochina, and the western North Pacific, we
produced composite maps of TC tracks and frequency
for the five wettest years and driest years (Fig. 6). The fivewettest years were 1980, 1994, 2002, 2005, and 2011, andthe five driest years were 1979, 1982, 1989, 1993, and1998, based on Fig. 5a. The TC frequency was calculated
from the 6-h TC track dataset. TC frequency was defined
as how often TCs were observed at each grid point,
which was expressed as the number of days across theentire 5-month period from May to September.
High TC frequencies were observed along the monsoon trough and over the western North Pacific in the
climatology (Fig. 6d) and even in the driest years(Fig. 6b). This indicated that many TCs occur along themonsoon trough during the summer monsoon season.
The climatological correspondence between the TC
activity and cyclonic circulation of the monsoon troughshows their close linkage.
Higher TC frequencies east of Indochina and over
the western North Pacific were observed in the five

FIG. 6. JTWC TC tracks and the TC frequency [days (5-month)21]
from May to September across a given bin (58 increments) for (a) the
five wettest years (1980, 1994, 2002, 2005, and 2011) and (b) the fivedriest years (1979, 1982, 1989, 1993, and 1998). (c) Difference in TC
frequency between the five wettest and five driest years [(a) minus(b)]. (d) Climatology of TC frequency from 1979 to 2011. The unit is
the number of days per rainy season (from May to September).

wettest years compared with lower TC frequencies in
the five driest years. Figure 6c shows the differences in
TC frequency, and clearly indicates higher frequencies
over the monsoon trough in the wettest years. Thesehigher TC frequencies east of Indochina, and over the1472

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western North Pacific, were consistent with the lowlevel cyclonic circulation signal (Fig. 5c). This supports
the notion that enhanced TC activity (Fig. 6c) andthe 5-month-averaged cyclonic signal (Fig. 5c) along themonsoon trough are linked. However, lower TC frequencies were observed over the northern Indian
Ocean in the wetter years. The relationship between
the weaker TC activity in the wettest years over the
Arabian Sea and the eastern Indian subcontinentand the atmospheric circulation anomalies was notclear, although there are far fewer TCs over thenorthern Indian Ocean than over the eastern side ofIndochina.

4. Discussion
a. 2011 Thai flood
In the previous section, we investigated the characteristics of precipitation and atmospheric circulation in2011 and 1998, and performed a statistical analysis fora 33-yr period. These results demonstrated that interannual variation in atmospheric patterns, including
TC activity, explains the precipitation over Indochina.
One of the aims of this study was to determine if the 2011
Thai flood was caused by a typical atmospheric circulation pattern for a high precipitation year. From both thecase study for 2011 and the statistical analysis of a 33-yr
period, an increase in precipitation over Indochina wascommonly determined to be responsible for the enhanced cyclonic circulation with many TCs along themonsoon trough. We also confirmed that similar results
were obtained when we conducted the same analyses for
Figs. 5 and 6 even excluding the 2011 rainy season. The
correspondence between the two analyses implies that
our mechanism for the typical interannual variation isapplicable to the 2011 Thai flood. Hence, the 2011 Thai
flood was not caused by an atypical atmospheric circulation pattern.
We should also examine whether these severe floods
are likely to increase in future. Multiple global climate
models have projected a decrease in the number of TCs
around Indochina, which may be due to the decrease in
the total number of TCs over the western North Pacific(Yokoi et al. 2013). However, the number of weakerTCs, such as tropical depressions, may also influence
total precipitation over Indochina. Thus, further investigations are necessary to generate future projections
for precipitation over the Southeast Asian monsoon
regions.

b. ENSO forcing of precipitation over Indochina
Many previous studies have suggested that the interannual variation in precipitation over the overall AsianVOLUME 28

monsoon area and individual Asian regional monsoon
areas is correlated with SSTs over the Niño-3.4 region(e.g., Webster and Yang 1992; Webster et al. 1998; Wanget al. 2001). However, this relationship remains unclear
over Indochina because the relationship varies greatly
from region to region and seasonally. The influence of
SSTs over the Niño-3.4 region on the total precipitation
over Indochina during the rainy season is not understood.
Here, we show the regression coefﬁcients in SSTs against
normalized CMAP data for May–September (Fig. 5a)over 33 yr (Fig. 7).
No significant SST signals were observed over the Philippine Sea or the central or eastern Pacific. Significant SST
signals were observed only over the western North Pacificnear the date line. The spatial pattern of the SST signals
did not resemble typical SST anomalies caused by El
Niño–Southern Oscillation (ENSO), and the statistical
relationship between ENSO and total precipitation during
the rainy season over Indochina remains unclear. Although the relationship between SSTs over the Niño-3.4
region and precipitation over Indochina was somewhat
clearer in May and October (not shown), the SSTs over the
Niño-3.4 region contributed little to the total precipitation
over Indochina during the rainy season on an interannual
time scale.
To confirm the impact of SSTs over the Niño-3.4
region on the Asian monsoon, Fig. 7b shows the regression of total precipitation data during the rainyseason (May–September) and low-level circulation at
all grid points in the Asian monsoon region against SST
data over the Niño-3.4 region. High correlations andhighly regressed precipitation values are seen acrossthe central equatorial Paciﬁc, western equatorial Paciﬁc, and the Maritime Continent, indicating that precipitation and low-level circulation vary with ENSO.
However, the impact is unclear over the South andSoutheast Asian monsoon regions. This also indicatesthat the impact of ENSO on the Asian monsoon islimited, particularly over Indochina and the Indian
subcontinent.
Goswami et al. (1999) showed a weak correlation
between SSTs over the eastern Pacific, namely ENSO,
and precipitation variations in the Asian monsoon region, including Indochina. In addition, Kripalani et al.
(1995) used detailed statistical analyses to show thatrainfall variation over Thailand was correlated withthat over India on interannual time scales. Our results
are consistent with these previous studies, implyingthat remote SST forcing from the Niño-3.4 region on
precipitation over Indochina is weak. However, Chen
and Yoon (2000) claimed that SSTs over the Niño-3.4
region impacts Indochina through changes in TC activity. These discrepancies may be explained by long-term

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FIG. 7. (a) Regression of rainy season SST data onto the normalized data (K) shown in Fig. 5a from 1979 to
2011; pink indicates positive and blue negative values. (b) Regression of the rainy season 850-hPa zonal andmeridional winds (vectors; m s21), streamfunction (contours; 106 m2 s–1), and precipitation (color; mm day21)
onto the normalized SST data for the Niño-3.4 region from 1979 to 2011. Colors in (a) and plotted values in(b) are statistically signiﬁcant at the 90% level, as determined by correlation coefﬁcients based on 31 degrees of
freedom (df).

or decadal changes in the ENSO–monsoon system, basedon previous work by Goswami et al. (1999) showing that
this relationship has been changing. Indeed, over the last
33 yr, high precipitation years over Indochina occurred
primarily in the 2000s (Fig. 6a). The results of Chen andYoon (2000) may be significant only for the decadesthat they studied.
Our statistical analysis showed that the impact of
ENSO was not a major determinant of interannual
variation in precipitation during the rainy season over
Indochina. Also note that the SST anomalies over thewestern North Pacific and over the Niño-3.4 region
during the 2011 rainy season were small and not typicalof the ENSO pattern (not shown).

c. Seasonal mean monsoon trough and tropical
cyclone activity
Our results show that interannual variation in precipitation over Indochina is primarily controlled by
5-month mean enhanced cyclonic anomalies withfrequent TC activity along the monsoon trough. On aninterannual time scale, the 5-month mean cyclonicanomalies along the monsoon trough corresponded tofrequent TC activity there. This idea is consistent withprevious work showing that the spatial pattern of thedominant mode of intraseasonal variability of low-levelwinds is similar to that of interannual variability ofseasonal mean winds over the Indian monsoon region(Goswami and Mohan 2001). The authors concluded
that the similarity between spatial patterns in the twoparameters (variability) indicates that the higher frequency of active monsoon conditions would resultin a stronger than normal seasonal mean. In addition,
Fujinami et al. (2011) showed that the activity of intraseasonal disturbances controlled seasonal mean precipitation over Bangladesh. However, the physicalmechanism that controls the activities of TCs or the
intraseasonal variations of atmospheric circulation
over the Asian monsoon region is still unclear. The
mechanism generating this interannual variation in

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FIG. 8. Regression of the rainy season 200-hPa zonal and meridional winds (vectors; m s–1) and streamfunction(contours; 106 m2 s–1) and CMAP precipitation (color; mm day–1) onto the normalized data in Fig. 5a from 1979 to
2011. The plotted values are statistically significant at the 90% level, as determined by correlation coefficients basedon 31 degrees of freedom (df).

westward-propagating TCs along the monsoon troughwill be the subject of additional studies.

d. Regional differences in interannual variability
This subsection investigates the relationship amonginterannual variations in the regional components of the
Asian monsoon, such as over the Indian subcontinent,
over the western North Pacific, and over the monsoon
trough. Wang et al. (2001) demonstrated differences ininterannual variability in precipitation in the Indian andwestern North Pacific monsoon regions. Interannual
variation in the western North Pacific monsoon was associated with that of the East Asian monsoon. The
western North Pacific monsoon modulated the subtropical high over the western North Pacific, the eastern
Asian subtropical front, the Okhotsk high, and precipitation over the Great Plains of the United States.
The authors concluded Indochina should be included
within the western North Pacific monsoon regions, although their results were not statistically significant.
However, our results show that the interannual variability in Indochina differs from that in the western
North Pacific, particularly in terms of the effect of
ENSO. In addition, the spatial structures of the variation
in interannual precipitation over Indochina, which weresynchronized with those along the monsoon trough, arevery different from those of both the Indian and western
Pacific monsoons.
Figure 8 shows the regression coefficients for 200-hPa
winds and streamfunction onto the normalized CMAP
data (Fig. 5a) over 33 yr (May–September). This allows
investigation of the atmospheric response in the upper

troposphere to the interannual variations in seasonalmean precipitation totals along the monsoon trough.
Enhancement of anticyclonic circulation was observed
over the Tibetan Plateau. This may have been the resultof increased precipitation along the monsoon trough,which can be explained by the Rossby wave response to
the enhanced latent heating along the monsoon troughdue to vigorous TC activity. No significant atmospheric
response to the enhanced latent heating along themonsoon trough was observed over the western North
Pacific or North America. This was significantly different from the atmospheric response of the convective
activity over the western North Pacific, as shown by
Wang et al. (2001). However, the atmospheric responseto the enhanced latent heating along the monsoon
trough was somewhat similar to the atmospheric response of the convective activity over the Indian monsoon region, as shown by Wang et al. (2001). This may
indicate that the interannual variability in precipitation
along the monsoon trough or over Indochina is moresimilar to that of the Indian monsoon than to that of thewestern North Pacific monsoon, although the spatial
structures of interannual precipitation variation weredifferent. Of course, because Indochina is located between the Indian subcontinent and the western North
Pacific, the interannual variability in seasonal precipitation over the monsoon trough is possibly similar toboth regions. These results suggest the possibility thatthe interannual variation in precipitation along themonsoon trough is independent of the interannual variability in seasonal precipitation over the Asian monsoon region.

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e. Relationship between the seasonal march ofprecipitation and interannual variation
Our results showed that interannual variation in precipitation over Indochina is primarily explained by
westward-propagating TC activity along the monsoon
trough that appears from the Arabian Sea, the Indian
subcontinent, the head of the Bay of Bengal, Indochina,
and the equatorial Philippine Sea. This pattern can beobserved in the first and second halves of the Asiansummer rainy season, although Takahashi and Yasunari(2006) showed the climatological atmospheric patternchanges from the first half of the Asian summer rainyseason (May–June) to the second half (July–September).
Our previous studies showed that water vapor transport by the monsoon southwesterly was important in
the first half of the rainy season, while in the second
half was associated with TC activity. Interannual variation in precipitation over Thailand was controlled byzonally expanded cyclonic circulation along the monsoon
trough, namely the frequent occurrence of westwardpropagating TCs, throughout the rainy season. Interestingly, the atmospheric patterns that control total
precipitation during the rainy season can differ fromthose of the climatological atmospheric pattern in thefirst half of the rainy season.

5. Conclusions
This study examined the characteristics of atmospheric circulation patterns, which induced heavy precipitation throughout the rainy season, and thesubsequent severe flooding that occurred in Thailand in
2011. To determine whether the 2011 Thai flood was an
unusual event or an occasionally occurring heavy rainfall event, we also investigated interannual variation inprecipitation over Indochina for the 33-yr period from
1979–2011. This analysis was carried out using groundand satellite-based rainfall observations, atmosphericreanalysis datasets, and TC tracks.
The average precipitation recorded by 53 stations inThailand throughout the rainy season from May to
September in 2011 was higher than in any other year in
the 22-yr period from 1979–2000. Westward-propagating
precipitation systems (i.e., westward-propagating TCs)
from the western North Pacific to Indochina were periodically observed from mid-May to September. Whenthese TCs approached Thailand, heavy precipitation wasobserved. In 2011, enhanced cyclonic circulation withmany TCs along the monsoon trough was observed.
Interannual variation in total precipitation from
May to September over Indochina was synchronizedwith that over the zonally expanded area from thenorthern Indian subcontinent, the head of the Bay of

Bengal, Indochina, and the Philippine Sea. The zonallyexpanded area corresponded to the monsoon trough.
Interannual variation in total precipitation during therainy season over Indochina was explained by cyclonic
circulation anomalies with the activity of westwardpropagating TCs along the monsoon trough. Enhanced
cyclonic anomalies corresponded to the vigorous TC
activity along the monsoon trough. Note that the monsoon westerlies did not strengthen with increased precipitation over Indochina, indicating that they were notresponsible for the interannual variation in that region.
In addition, the interannual variation in total precipitation during the rainy season over Indochina
showed no statistical correlation with SSTs over thewestern North Pacific and Niño-3.4 regions, which indicates that SSTs in these regions did not control theheavy precipitation over Indochina.
The correspondence between the case study in 2011
and the 33-yr statistical analysis implies that the 2011
Thai flood and the occasional above-normal precipitation events were also explained by the same mechanism. Therefore, the frequent TCs along the monsoon
trough were a major contributing factor to the heavy
precipitation over Thailand in 2011. In addition, theinterannual modulation of TCs along the monsoon
trough controls the interannual variation in precipitation over the Southeast Asian monsoon region. The
above-normal precipitation over Indochina due to theenhanced monsoon westerlies was not applicable to thisregion.
Acknowledgments. We thank the two anonymous reviewers of this manuscript for their helpful suggestions
and comments. This work was partly supported by theGreen Network of Excellence (GRENE) program of
the Ministry of Education, Culture, Sports, Science, andTechnology (MEXT), Japan, the 7th Japan Aerospace
Exploration Agency (JAXA) Precipitation Measuring
Mission (PMM) Project 306, and KAMANHI (26220202)
from the Japan Society for the Promotion of Science.
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