June 2008
Journalof the Meteorological Society ofH.
Japan,
G. TAKAHASHI
Vol. 86, No. 3,
andpp.T.429–438,
YASUNARI
2008

429

Decreasing Trend in Rainfall over Indochina during
the Late Summer Monsoon: Impact of Tropical Cyclones
Hiroshi G. TAKAHASHI
Frontier Research Center for Global Change (FRCGC),
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japanand
Tetsuzo YASUNARI
Frontier Research Center for Global Change (FRCGC),
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan
Hydrospheric Atmospheric Research Center (HyARC), Nagoya University, Nagoya, Japan(Manuscript received 14 May 2007, in final form 28 January 2008 )Abstract
We examined the decreasing trend in rainfall during the late summer monsoon season (September)in Thailand from 1951 to 2000 and associated changes in tropical cyclone (TC) activity. Thailand receives
significant rainfall from May to October and experiences two rainy peaks in late May to early June and in
September. A previous study reported a decreasing trend in September rainfall in Thailand and, based ona regional climate model, suggested that the trend was associated with local deforestation. However, thelong-term trend may also be affected by changes in large-scale circulation. Thus, the purpose of this study
was to investigate changes in large-scale circulation associated with the decreasing rainfall trend.
Westward-propagating TCs from the South China Sea and the western North Pacific brought most of
the rainfall over Thailand in September. TCs include tropical depressions, tropical storms, severe tropicalstorms, typhoons, and residual lows. 70% of the rainfall amount in September was estimated to be associated with TCs.
The 50-year time-series of September rainfall over Thailand showed a significant decreasing trend. TC
activity defined by 700-hPa relative vorticity, showed a weakening trend over the Indochina Peninsula. TC
tracks also suggested the weakening of TC activity over this area. The long-term trend in rainfall during
the late summer monsoon season was closely associated with changes in TC activity over the Indochina
Peninsula; these changes were likely caused by changes in the major course of TCs. Concurrent with thechanges in TC tracks, there was a change in the TC steering current around the Philippines archipelago
and Taiwan. This led to the TC activity over the Indochina Peninsula being inactive, probably resulting in
the long-term decrease in rainfall over Thailand.

Corresponding author: Hiroshi G. Takahashi,
Frontier Research Center for Global Change, Japan
Agency for Marine-Earth Science and Technology,
3173-25 Showamachi, Kanazawa-ku, Yokohama City,
Kanagawa 236-0001, Japan.
E-mail: hiroshi3@jamstec.go.jp©2008, Meteorological Society of Japan

1. Introduction
The hydrological cycle is an essential component of the climate system and is crucial for understanding global as well as regional climate change.
In the Indochina Peninsula region, Kanae et al.
(2001) examined the long-term trend in rainfall

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Journal of the Meteorological Society of Japanover Thailand in terms of land-atmosphere interaction and found that September rainfall showed adecreasing trend from 1951 to 1994. Interestingly,
the long-term decreasing trend in rainfall was significant only in September. Based on the resultsof a regional climate model, they suggested that
this decrease may be attributable to changes inland surface conditions and argued that deforestation has selectively decreased evapotranspiration
and rainfall in September. The reason why othermonths did not show the decreasing trend in rainfall may be related to the seasonal march of monsoon circulations. Because the monsoon westerlies
transport much more water vapor to the Indochina
Peninsula during the mature stage of the summer
monsoon season, the effects of land-atmosphereinteraction could be weaker than in September.
Thus, Kanae et al. (2001) concluded that deforestation across northeastern Thailand had probably
caused the long-term decreasing trend in rainfall.
However, long-term climate change at the regional
scale may also be affected by long-term changes
in large-scale circulations.
Over the past decade, numerous studies haveexamined the monsoon over the Indochina Peninsula. Numerous features distinguish the seasonal
march of the summer monsoon over Indochina
from those of the adjacent Indian, South China Sea,
and western North Pacific monsoons (Matsumoto
1997; Zhang et al. 2002; Kiguchi and Matsumoto
2005; Takahashi and Yasunari 2006). Matsumoto(1997) showed two seasonal rainfall peaks in
Thailand during the monsoon period. Takahashi
and Yasunari (2006) reported that the first rainy
peak, in late May to early June, is associated withstrong monsoon southwesterlies, and the second
rainy peak, in mid-August to mid-September, is
probably influenced by tropical cyclone (TC) activity. Previous studies (e.g., Krishnamurti et al.
1977; Saha et al. 1981; Chen and Chen 1993) haveshown that the monsoon depressions over the Bayof Bengal and the Indian subcontinent are closely
related to westward-propagating disturbancesthat cross the Indochina Peninsula and originate
from the South China Sea and western North Pacific. Fudeyasu et al. (2006) reported that Augustand September are the most active months forwestward-propagating TCs around the Indochina
Peninsula. Furthermore, sub-monthly time-scaledisturbances also move westward from the South
China Sea to the Bay of Bengal and are associatedwith convective activity over the Indochina Penin­Vol. 86, No. 3

Fig. 1. Distribution of rain-gauge stationsin Thailand.

sula (Fukutomi and Yasunari 1999) and rainfall
in Thailand (Yokoi and Satomura 2005). Over the
Indochina Peninsula, westward-propagating disturbances, which include tropical depressions, tropical storms, and sub-monthly disturbances, may beessential for rainfall variations in September.
We investigated atmospheric circulations associated with rainfall variations over the Indochina
Peninsula in September. The primary purpose
was to address the long-term decreasing trend of
September rainfall over Thailand from the perspective of changes in TC activity and related largescale atmospheric circulations.
Section 2 documents the data used in thisstudy. Section 3 describes the characteristics ofatmospheric circulation associated with rainfall
variation over Thailand in September. Long-termchanges in rainfall and TC activity are examined in
Section 4. Changes in the steering current of TCs
are discussed in Section 5, and a summary is given
in Section 6.

2. Data
The data used in this study were daily precipitation data measured from 1951–2000 in Thailand at
32 stations operated by the Thai Meteorological
Department (TMD), as used by Takahashi andYasunari (2006). We used domain-averaged rainfall.
Figure 1 shows the distribution of rain-gauge stations. Changes in the atmospheric circulation wereJune 2008

H. G. TAKAHASHI and T. YASUNARI

431

investigated using the 40-year Reanalysis (ERA-40,
Uppala et al. 2005) dataset of the European Centre
for Medium-Range Weather Forecasts (ECMWF).
TC track data were provided by the Japan Meteorological Agency (JMA) and the Joint Typhoon
Warning Center (JTWC). Although JTWC trackdata include weak TCs, such as tropical depressions, the quality of weak TC data is not homogeneous over the 50-year study period, making thesedata difficult to use for long-term trend analysis.
Thus, we did not use the weak TC data in the
JTWC TC track dataset. We describe the resultsfrom JMA TC track data; the same results wereobtained from JTWC TC track data. Differencesin TC track data before and after meteorologicalsatellite operation were checked. The numbersof TCs formed over the western North Pacific in
September were 64, 78, and 77 in 1951–1965,
1966–1980, and 1981–1995, respectively. These periods will be explained in Section 4. Before around
1965, TC numbers may have been underestimated;
however, the underestimation would have no practical impact on the results of this paper.

3. TCs and rainfall variations over the Indochina Peninsula during September
3.1 Westward-propagating disturbances associatedwith rainfall variations during the late summer monsoon
To investigate atmospheric circulations associated with rainfall variations in Thailand during
September, Fig. 2 presents a lag-composite analysis of low-level circulation anomalies. The anomalies of low-level circulation are departures from
September climatological means (for 43 years).
When the rainfall amount over Thailand on a given
day exceeded one standard deviation from themean, that day was qualified as a high-rainfall day
and defined as a Lag 0 day in the lag-composite
analysis. Of 1290 days, 190 were qualified as highrainfall days. Lag-composite analysis of low-level
circulation was conducted based on the rainfall
over Thailand. As shown in Fig. 2, cyclonic circulation was located over the Indochina Peninsulaon Lag 0 day, indicating that rainfall variation
over Thailand in September was associated withlarge-scale cyclonic circulation. On Lag -2 day, thecyclonic circulation was located over the South
China Sea. On Lag -1 day, the cyclonic circulation
migrated westward. The cyclonic circulation gradually migrated westward from the South China Sea
to the Bay of Bengal across the Indochina Penin-

Fig. 2. Lag-composite map of daily 850-hPa
wind (vector) and geopotential height(contours) anomalies. The anomalies aredepartures from the 43-year September
mean of each variable. Of 1290 days, 190
were composites. High rainfall days on
which the rainfall amount exceeded one
standard deviation from the mean wereselected. Gray shading indicates thatgeopotential height anomalies were lessthan -8 gpm.

sula. Therefore, rainfall variation over Thailand
during September was strongly affected by westward-propagating disturbances. Note the zonal enlargement of cyclonic circulation, suggesting thatrainfall does not always peak when the cyclonic
circulation is located just over Thailand. Moreover, note that examination of the daily 850-hPa
wind field and TC tracks shows that the westwardpropagating disturbances include several types ofdisturbances (i.e., tropical depressions, tropicalstorms, severe tropical storms, typhoons, and residual lows). Hereafter, the term “TC” is used toindicate all of these types of tropical disturbances
unless otherwise noted.

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Journal of the Meteorological Society of Japan

3.2 Detection of the TCs using reanalysis
As discussed in the previous subsection, daily
rainfall variation of Thailand during September
was closely associated with westward-propagating
TCs. As also noted above, “TC” includes not only
tropical storms, which would be recorded in TC
track data, but also weaker TCs, such as tropicaldepressions and residual lows. To estimate the effect of TCs on rainfall, a new TC index was necessary and is defined in this subsection.
September corresponds to the withdrawal of
the Indochina monsoon (Wang and LinHo 2002)
and thus a dramatic seasonal change in winds andgeopotential heights. Because of filter noise, highpass and band-pass filtered winds and geopotential
heights cannot detect TCs when the mean state
drastically changes. To identify the TCs aroundthe Indochina domain during September, 700-hPa
relative vorticity was used, following the methodof Fudeyasu et al. (2006). Relative vorticity of 700
hPa was selected because 850-hPa relative vorticity is affected by the friction in the planetary
boundary layer, while 700-hPa relative vorticity is
not strongly affected. To detect most TCs, including those weaker than a tropical storm, the criterion was set to 3.0×10–5 s–1. Fudeyasu et al. (2006)
set their criterion value to 5.0×10–5 s–1. However,that value could not detect weaker TCs. A criterionof 4.0×10–5 s–1, did not detect weaker westwardpropagating TCs, while at a criterion of 2.0×10–5 s–1,
TCs were difficult to distinguish from noise. Aftermuch trial and error, the criterion of 3.0×10–5 s–1
was determined. In addition, we confirmed that
the TCs defined by 700-hPa relative vorticity basically corresponded to convective activity estimatedfrom outgoing longwave radiation (not shown). If
any grid of the 700-hPa relative vorticity exceeded
3.0×10–5 s–1 within the Indochina domain (97.5–
107.5°E, 12.5–22.5°N), a TC was considered to exist over the domain. A day on which a TC existed
within the Indochina domain is referred to as a
TC-day.
As an example of the relationship between TCdays and daily rainfall variation, Fig. 3 shows thetime-series of TC-days and daily rainfall variationfor September 1993. The star symbols in the left
panel, which shows the TC-day index defined by
700-hPa relative vorticity, indicate the existenceof a TC in the Indochina domain. The right panel
shows the daily variation in rainfall over Thailand.
September 1993 was a normal year in terms ofrainfall amount over Thailand and TC activity overVol. 86, No. 3

Fig. 3. TC index (left panel), defined by
the relative vorticity at 700 hPa, as explained in the text, and time-series ofdaily rainfall over Thailand (right panel).
Panels are for September of a typicalyear (1993).

the Indochina Peninsula. As shown in Fig. 3, TCdays had good agreement with daily rainfall variation, which again confirms that the daily rainfall
over Thailand is strongly affected by TCs.
As an example of TC detection, Fig. 4 shows thetime sequence of the total fields of 700-hPa relative
vorticity (shaded), 850-hPa winds, and geopotential heights in September 1993. Strong cyclonic circulation was observed over the Indochina Peninsula from 2 to 4 September. The TC corresponded
to the rainy peak on 2 September in Fig. 3. From 5
to 10 September, a clear cyclonic vortex migrated
northwestward from the South China Sea to the
Bay of Bengal (Fig. 4), which also corresponded
to the rainy peak on 8 and 9 September (Fig. 3).
The weak cyclonic circulation went across theJune 2008

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Fig. 4. Time-sequence of 700-hPa relative vorticity (shaded) and 850-hPa winds and geopotential
heights (contours) in September 1993. Shading indicates that relative vorticity at 700 hPa exceeds 3.0–5 –1
×10 s .

southern tip of the Indochina Peninsula on 14 and
15 September (Fig. 4), when rainfall was observed(Fig. 3). From 18 to 20 September, the larger cyclonic circulation passed through the Indochina

Peninsula (Fig. 4). The rainy peak on 19 and 20
September coincided with the passage (Fig. 3). In
this manner, when the criterion for 700-hPa relative vorticity was set 3.0×10–5 s–1, most TCs were434

Journal of the Meteorological Society of Japan

Vol. 86, No. 3

detected over the Indochina domain. Note that the
westward-propagating TCs were almost consistent
with rainfall variations. It is more noteworthy thatthe weak cyclonic circulations, which were not categorized as tropical storms in TC track data, wereassociated with large amount of rainfalls, implyingthat they are also quite important for the rainfall
variation over Thailand.
3.3 Contribution of TCs to total rainfall
To quantify the contribution of westward-propagating TCs to rainfall over Thailand, TC-rain was
defined as the proportion of rainfall amount fromTCs to the total rainfall amount for Thailand. TCrain was defined according to TC-day; that is, TCrain indicates the rainfall amount associated with
TC activity. TC-rain was estimated as the total rainfall over Thailand on all TC-days.
The climatological mean (43 years) TC-rain was
5.97 mm day–1, which was about 68.5% of the total
rainfall. In the study period, 730 TC-days werequalified, comprising 56.6% of the total analyzed
days. Approximately seven-tenths of September
rainfall was brought by the TCs, indicating that therainfall associated with TCs is quantitatively essential to the rainfall in September over Thailand.
Long-term changes of TC activity and atmosphericcirculations will be investigated in the following
section.

Fig. 5. Interannual variation of September
rainfall over Thailand for 1951–2000. The
dashed (solid) line shows the originaltime-series (5-year running mean) ofrainfall. P1 (1951–1965), P2 (1966–1980),
and P3 (1981–1995) are shown in thepanel. Unit is mm day–1.

4. Long-term changes in rainfall and TC
activity
Figure 5 shows interannual variations in September rainfall from 1951 to 2000. A long-term decreasing trend is prominent, as reported by Kanae
et al. (2001). The nonparametric Mann-Kendall
test (Press et al. 1992) was applied to the timeseries of the rainfall data, following the methods ofKanae et al. (2001), and the test reconfirmed thesignificant decrease in rainfall. The 50-year study
period was divided into three periods (1951–1965,
1966–1980, and 1981–1995, referred to as P1, P2,
and P3, respectively) to facilitate examination of
TC activity changes associated with the long-term
trend in rainfall over Thailand and to discuss thedecreasing trend noted by Kanae et al. (2001). Periods P1, P2, and P3 had mean rainfall amounts of
9.73, 9.02, and 7.85 mm day–1, respectively.
To investigate the relationship between therainfall amount over Thailand and TC activity overthe Indochina Peninsula, Fig. 6 shows a scatterplot of September rainfall over Thailand and TC-

Fig. 6. Scatterplot between September rain­
fall over Thailand and TC day definedby 700-hPa relative vorticity. Rectangle,
circle, and star symbols indicate the relationship in each year during P1, P2, andP3, respectively. The crosses labeled P1,
P2, and P3 show the mean relationship ofeach period. There were fewer samples
for P1 than for the other two periods
because of limitations of the ERA-40 dataset.

day. Rectangle, circle, and star symbols show the
relationship in each year during P1, P2, and P3,
respectively. Crosses with P1, P2, and P3 denotethe mean relationships of each period. The sample
number for P1 was smaller than those of the othertwo periods because of limitations of the ERA-40

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Fig. 7. TDistribution of TC-appearance frequencies during P1, P2, and P3 and the difference between
P2 and P3 (i.e., P3 minus P2). As in Fig. 6, P1 had fewer samples than the other two periods. In the
difference map, the shading shows the area with values less than -1. Unit is times per month. The contour interval for P1, P2, and P3 is one time per month. The contour interval of the right lower panel is 0.5
times per month.

dataset. The correlation coefficient between
rainfall and TC-day for the 43 years from 1958
to 2000 was 0.56, which is significant at the 99%
confidence limit. This result implies that TC-day,
which is basically associated with TC activity overthe Indochina Peninsula, is closely related to rainfall variation in Thailand on an interannual timescale. Chen and Yoon (2000) pointed out that the
number of TCs was correlated with rainfall during
the whole summer rainy season on an interannual
time-scale, which is consistent with our result.
Figure 6 shows the crosses moving toward lower
rainfall with less frequent TC-days over time, implying that rainfall and TC activity have decreased
over the Indochina Peninsula. Therefore, this result clearly demonstrates that changes in TC activity over Thailand are consistent with the long-term
decreasing trend of rainfall.
Figure 7 shows the spatial distribution of thefrequencies of TC appearance. As for TC-day, a

TC was considered to exist at the grid when the
700-hPa relative vorticity exceeded 3.0×10–5 s–1 forthat grid. Also as in Fig. 6, P1 had less samples
than P2 or P3. All three periods showed high frequencies of TC appearance over the Bay of Bengal,
South China Sea, and western North Pacific (Philippine Sea). Note that the high frequencies of TC
appearance over the Bay of Bengal and western
North Pacific are consistent with the monsoon
depressions and TC activity in those respective
areas. In P1 and P2, areas of high-frequency TC
appearance expanded westward from the South
China Sea to the Indochina Peninsula. TC frequency was higher in P1 than in P2 over the Indochina
Peninsula. Distributions were similar around the
Indochina Peninsula, but high-frequency TC appearance was not observed over the Indochina
Peninsula in P3. The difference map shows thatTC-appearance frequencies over the Indochina
Peninsula clearly became lower, suggesting that

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Journal of the Meteorological Society of Japan

westward-propagating TCs decreased over the
Indochina Peninsula from P2 to P3. On the other
hand, higher frequencies of TC appearance wereobserved over the southern South China Sea andPhilippine Sea to China, which probably indicatesincreases in TCs there from P2 to P3. In thismanner, TC activity over the Indochina Peninsula
weakened from P1 to P2 and from P2 to P3. In addition, we also checked the weaker criteria for TCappearance of 2.0×10–5 s–1 and 1.0×10–5 s–1. The
difference of TC-appearance of the weaker criteria
showed the same tendency. Thus, we concluded
that the number of the westward propagating TC
have decreased over the Indochina Peninsula.
Figure 8 shows TC tracks in each period, illustrating changes in the motion of TCs. For thisfigure, JMA TC track data in September in eachperiod were plotted. The TC track data do notinclude TCs weaker than a tropical storm. In P1
and P2, major clusters of TC passage over theSouth China Sea and western North Pacific wereobserved as southeast–northwest-oriented bands.
Over the South China Sea, most TCs propagatedwestward or west-northwestward. Some landed
over the Indochina Peninsula and some approached the peninsula. Interestingly, the TC passage during P3 over and around the South China
Sea was clearly different from that in P1 and P2.
Westward or west-northwestward-propagating TCs
over the South China Sea were not dominant during P3. In P3, TCs moved more northward or took
a weaving course over the South China Sea. In
addition, more TCs were observed over southern
China in P3. Consequently, only a small numberof TCs made landfall or approached the Indochina
Peninsula in P3.
Furthermore, it is noteworthy that the long-termchanges in JMA TC tracks (Fig. 8) are consistent
with the long-term changes in the frequency of
TC appearance (Fig. 7), derived from ERA-40 data.
The two independent parameters, namely TC appearance derived from ERA-40 and JMA TC tracks,
yielded a similar result, implying that TC activity
over the Indochina Peninsula very likely became
increasingly inactive from P1 to P2 and from P2
to P3. From P1 to P2, the frequency of TC appearance simply became lower over the Indo­china
Peninsula. From P2 to P3, the major course ofTCs changed over the Philippines archipelago andSouth China Sea, indicating that a small numberof TCs made landfall or approached the Indochina
Peninsula in P3. Thus, the weakening trend of

Vol. 86, No. 3

Fig. 8. JMA TC tracks in September during P1, P2, and P3. Some tracks startfrom the midstream of the lifetime of aTC, while other tracks end in the midstream. Only TC tracks in September
were plotted. JMA TC tracks do not include TCs weaker than a tropical storm.

TC activity over the Indochina Peninsula is likely
responsible for the long-term trend of rainfall overThailand.

5. Large-scale circulation
The previous section discussed the likely association between changes in TC activity (Figs.
6 and 7) and changes in the major course of TCs
from P2 to P3 (Fig. 8). Mid-tropospheric flow (700hPa–500 hPa) can be considered the steering current of TC motion (e.g., Chan and Gray 1982).
Experience shows that the best altitude of TC
motion is around 4–5 km (Emanuel 2005). To
discuss differences in the changes of TC tracks,
Fig. 9 shows winds and geopotential height at
600 hPa. However, before discussing these differences, the quality of ERA-40 data should be noted.

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437

were very weak over the Philippines and Taiwan
during P3, which could suggest that the westward
steering current for TC motion was weaker than in
the other periods. This result corresponded to theobserved small number of TCs propagating westward to the Indochina Peninsula and the weavingcourse of TCs over the South China Sea (Fig. 8).
The changes in the mid-tropospheric circulations
were not inconsistent with the changes in TC
activity. Therefore, the weakening of TC activity
from P2 to P3 over the Indochina Peninsula can be
explained by changes in the courses of TCs, whichwere probably consistent with the changes in midtropospheric large-scale circulation.

6. Summar y

Fig. 9. Monthly mean geopotential heights(contours) and winds (vectors) at 600
hPa in each period. There were fewer
samples for P1 than for the other two periods. The shading indicates that easterly
winds were stronger than 2 m s–1.

Some satellite data were available after 1973, withsatellites in full-scale operation after 1979. Thus,
gaps are likely in ERA-40 around 1973 and 1979.
Because the time-series of the 600-hPa geopotential height and winds over the South China Sea andwestern North Pacific showed no strong gaps (notshown), ERA-40 can be used to show long-termchanges in mid-tropospheric circulation aroundthe South China Sea and western North Pacific.
Again, the sample number for P1 was smaller than
the sample numbers for the other two periods.
As shown in Fig. 9, easterlies along the periphery of the subtropical high were dominant over
the western North Pacific and South China Sea
during P1 and P2. Easterlies penetrated from thewestern North Pacific to the Indochina Peninsula.
The region of easterlies (Fig. 9) was consistent
with the region where the westward propagationof TCs was dominant in P1 and P2 (Figs. 7 and 8).
On the other hand, the mid-tropospheric easterlies

We investigated the long-term decreasing trend
in rainfall over Thailand in the late summer monsoon season, using daily rainfall from Thailand,
JMA and JTWC TC tracks, and the ERA-40 Reanalysis dataset. Thailand usually experiences arainy peak in September. September rainfall overThailand was strongly influenced by TCs propagating westward from the South China Sea andwestern North Pacific to the Indochina Peninsula.
The westward-propagating TCs included several
types of westward-propagating tropical disturbances, such as tropical depressions, tropical storms,
severe tropical storms, typhoons, and residual
lows. When TCs were defined by 700-hPa relative
vorticity, about 70% of rainfall over Thailand wasbrought by TCs in September.
The 50-yr time-series of September rainfall
showed a long-term decreasing trend. The frequency of TC-days over the Indochina Peninsula,derived from 700-hPa relative vorticity, decreased
from P1 to P2 and from P2 to P3 and was significantly correlated with rainfall variation over Thailand. JMA TC tracks showed that westward- andwest-northwestward-propagating TCs were dominant in P1 and P2. However, P3 had fewer westward-propagating TCs over the Indochina Penin­sula compared to P1 and P2. Two independent
parameters, namely TC appearance derived from
ERA-40 and JMA TC tracks, showed weakeningof TC activity over the Indochina Peninsula. From
P1 to P2, the frequency of TCs simply decreased.
Changes in TC activity from P2 to P3 over the
Indochina Peninsula were caused by changes in
the major course of TCs over the South China Sea.
Changes in the major course of TCs were likely
associated with changes in steering currents over

438

Journal of the Meteorological Society of Japanthe Philippines archipelago and Taiwan. Therefore, we concluded that the long-term decreasing
trend in September rainfall over Thailand has beencaused by changes in TC activity over the Indochina Peninsula and South China Sea.
Our results have demonstrated that the weakening of TC activity over the Indochina Peninsula
is probably a significant factor in the long-term
decrease in September rainfall over Thailand.
Kanae et al. (2001) proposed that deforestation
over northern Thailand has caused long-term decreases in rainfall through a decrease of surface
evapotranspiration. Further study will include
comprehensive comparison of changes in land
surface effects and TC activity, using a regional
climate model.

Acknowledgments
Rainfall data were kindly provided by the Thai
Meteorological Department through GEWEX(Global Energy and Water Cycle Experiment)
Asian Monsoon Experiment (GAME)-Tropics(GAME-T). We would like to acknowledge twoanonymous reviewers for helpful suggestions
and comments. We would like to thank Dr. H.
Fujinami, HyARC, Nagoya University for gratefulcomments and continuous encouragements. We
thank Dr. N. Endo of IORGC, JAMSTEC for valuable comments and suggestions. We are gratefulto Prof. J. Matsumoto of Tokyo Metropolitan University for valuable comments. Some figures weremade with the Generic Mapping Tools (GMT)
graphics system developed by Wessel and Smith
in 1995. This work was partly supported by the
Global Environment Research Fund (B-061) of the
Ministry of the Environment, and Ministry of Education, Culture, Sports, Science and Technology(MEXT), Japan.

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