ATMOSPHERIC SCIENCE LETTERS
Atmos. Sci. Let. 10: 14–18 (2009)
Published online 11 December 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/asl.203

High-resolution regional climate simulations of thelong-term decrease in September rainfall over Indochina
Hiroshi G. Takahashi,1 * Takao Yoshikane,1 Masayuki Hara1 and Tetsuzo Yasunari1,2
1 Frontier Research Center for Global Change (FRCGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan
2 Hydrospheric Atmospheric Research Center (HyARC), Nagoya University, Nagoya, Japan*Correspondence to:Hiroshi G. Takahashi, 3173-25
Showamachi, Kanazawa-ku,
Yokohama City, Kanagawa
236-0001, Japan.
E-mail: hiroshi3@jamstec.go.jp

Received: 26 June 2008
Revised: 3 September 2008
Accepted: 30 October 2008

Abstract
We address the long-term decrease in September rainfall over the Indochina Peninsula.
Distinct long-term decreases in rainfall along the monsoon trough across the Indochina
Peninsula have been observed. We performed long-term simulations and discuss the effectsof long-term changes in both the local surface conditions and large-scale circulation.
Using a 30-year simulation for September for the period from 1966 to 1995 with landuse conditions fixed at present-day values and neglecting the recorded deforestation, we
successfully simulated the observed long-term decrease in rainfall. We therefore conclude
that the weakening tropical-cyclone activity over the Indochina Peninsula region is probably
responsible for the decrease in rainfall. Copyright  2008 Royal Meteorological Society
Keywords: regional climate change; rainfall; tropical cyclone; deforestation; regional
climate model

1. Introduction
Climate change is an extremely important issue regionally, as well as globally. However, because thehorizontal resolution of global climate models iscoarser than 100 km, these models cannot be used todetermine detailed regional climate changes. Specificapproaches to the analysis of these processes are therefore needed.
In Thailand, on the Indochina Peninsula, Kanae
et al. (2001) (hereafter KA01) found a decreasing
trend in rainfall using rain-gauge observations forthe period from 1951 to 1994. Two explanations
have been proposed for this trend. First, KA01 performed a regional climate-modelling experiment andsuggested that the decrease might have been causedby changes in the land surface conditions, arguingthat deforestation could decrease evapotranspiration
and rainfall in September. Second, Takahashi andYasunari (2006) and Fudeyasu et al. (2006) investigated seasonal conditions in the Indochina Peninsula region. They reported that August and September
were the most active months for westward-moving
tropical-cyclone (TC) activity. Moreover, Takahashi
and Yasunari (2008) (hereafter TA08) highlighted thelarge contribution of TCs to the total rainfall in
September and suggested that the weakening of TC
activity over the Indochina Peninsula region could be
responsible for the observed long-term decrease in
rainfall.
Because KA01 conducted experiments for only
three Septembers (1992, 1993 and 1994), they did notinvestigate the effect of long-term changes in the largescale circulation. The primary motivation for our study
Copyright  2008 Royal Meteorological Society

was the following question: Which mechanisms cancoherently explain the observed long-term decreasein September rainfall over Indochina, TC activity or
deforestation? To address this question, we performed
a long-term (30 years from 1966 to 1995) simulationfor September, excluding the effects of deforestation.

2. Experimental design
We used a high-resolution model to simulate a realistic distribution of rainfall and the local circulations related to complex terrain and land use. The
Advanced Research Weather Research and Forecasting(WRF) modelling system (Skamarock et al., 2005),
which was developed at the National Center for Atmospheric Research (NCAR), was chosen for this purpose. The 40-year reanalysis (ERA-40, Uppala et al.,2005) data set of the European Centre for MediumRange Weather Forecasts (ECMWF) and the Extended
Reconstructed SST V2 (Smith and Reynolds, 2004)
data sets of the National Oceanic and Atmospheric
Administration (NOAA) were used as initial andboundary conditions. Soil moisture and temperature
data from ERA-40 were also used. The model domains
are shown in Figure 1(a). The horizontal grid increment of the coarse domain was 25 km, and that of thetwo-way nested domain was 5 km. Both domains had
31 terrain-following vertical levels. Because preliminary experiments with cumulus convective parameterisation showed a very unrealistic distribution and total
amount of rainfall, we did not apply cumulus convective parameterisation in either domain. The WRF
single-moment six-class microphysics scheme (HongLong-term decrease in rainfall over Indochina

15

22°N

(a) Model Domains & Terrain [m](b) Observed Rainfall [mm/month]

20°N
600

20°N

18°N

500
400

16°N

16°N

300
200

14°N

100

12°N

12°N

8°N

10°N

4°N

22°N

92°E

96°E

100°E

104°E

108°E

112°E

96°E 98°E 100°E 102°E 104°E 106°E 108°E 110°E(c) Simulated Rainfall [mm/month]

20°N
600

100 200 400 800 1200 2000

18°N
16°N

30

300

500

0

0

50

400
300
200

14°N

100
12°N
10°N
96°E 98°E 100°E 102°E 104°E 106°E 108°E 110°E

Figure 1. Model domains are shown in (a). The inner thick box showed nested domain. Right panels show total amounts of
September rainfall over the Indochina Peninsula. (b) The observed September rainfall averaged over 30 years (1966–1995) in
September, and (c) the equivalent simulated rainfall. The black box in (b) corresponds to the display range of (c). Less than 10% of
the rain-gauge station data (b) were missing. The units of rainfall are in millimetres per month.

and Lim, 2006) and the Noah land-surface model(Chen and Dudhia, 2001) were also used.
To investigate the long-term changes in rainfall,
we defined two periods: Term 1 (T1), from 1966 to
1980; and Term 2 (T2), from 1981 to 1995. They
were chosen because TA08 showed a decrease in
rainfall from T1 to T2. These periods were thus
appropriate for investigating the long-term changes
in rainfall. The simulation with current land-use andvegetation was performed 30 times from 30 August
to 30 September from 1966 to 1995. For the durationof the simulation, the land use was fixed. The current
land use was taken from 1992 data that were obtained
from the United States Geological Survey (USGS).
In the past, Thailand was highly forested, whereas
the 1992 data categorised the majority of Thailand as
under cultivation. Thus, if the effect of deforestation
was the most significant factor, a long-term decreasein rainfall would not be apparent in the model.
We used a 30-year period of gauge-observed rainfall data that were collected from 1966 to 1995 as partof the Global Energy and Water Cycle Experiment(GEWEX), Asian Monsoon Experiment (GAME)
and Monsoon Asian Hydro-Atmosphere Scientific
Research and Prediction Initiative (MAHASRI). The
previous studies of changes in rainfall used rainfall
data only from Thailand. In contrast, we used rainfall data from Vietnam, Laos, Thailand and Myanmar.
Copyright  2008 Royal Meteorological Society

The rain-gauge observations demonstrated the extent
of the long-term change in rainfall over the Indochina
Peninsula.

3. Long-term change in rainfall
Before analysing the long-term change, we compared
the simulated rainfall with the observed climatology.
Abundant rainfall was observed along the coast of the
Bay of Bengal, the Gulf of Thailand and the eastern
coast of northern Vietnam (Figure 1(b)). The peaksof the simulated rainfall were closely correlated withthe peaks indicated by rain-gauge observations. In
addition, the total simulated amount of rainfall was
generally very close to that observed using rain gauges.
Because the rainfall along the eastern coast of northern
Vietnam was very likely associated with the westwardmoving TCs, the peak in the simulated rainfall there
suggests that the model can simulate both TCs and therainfall pattern caused by TCs. Thus, the simulation
captured the climatology of both the amount anddistribution of rainfall in the Indochina Peninsula. The
high resolution of the simulation also allowed the veryfine structure of the rainfall distribution to be captured.
To evaluate the interannual variation and long-termchange in September rainfall, we plotted a ten-point
Atmos. Sci. Let. 10: 14–18 (2009)
DOI: 10.1002/asl

16

H. G. Takahashi et al.

450
400[mm/month]

350
300
250
200
150
100
50

T2

T1

0
1970

1975

1980

1985

1990

1995

Figure 2. Time series of the observed September rainfall
ten-point average (black) and the simulated September rainfall
domain average (blue). The ten stations were located within
an area at 101–105 ◦ E, 15–18◦ N. The simulated rainfall was
domain-averaged within the box. The units of rainfall are in
millimetres per month. The blue (black) dashed lines denote
15-year averages of simulated (observed) rainfall for each term.
Copyright  2008 Royal Meteorological Society

20°N
80
40
20
-20
-40
-80
-120
-160

18°N
16°N
14°N
12°N
10°N
96°E

98°E 100°E 102°E 104°E 106°E 108°E 110°E(b) Difference in Simulated Rainfall[mm/month]

22°N
20°N

0

00

September Rain (101-105E, 15-18N)
500(a) Difference in Observed Rainfall[mm/month]
22°N

-2

average of rain-gauge rainfall and area-averaged simulated rainfall over the Khorat Plateau (Figure 2).
The reference area chosen was an area (101◦ –105 ◦ E,
15◦ –18◦ N) in which rainfall decreased significantly(KA01). Both time series showed clear interannual
variation, with close agreement. The year-to-year correlation was 0.70, which was significant at the 99%
confidence limit. The total amount of simulated rainfall in T1 and T2 over the reference area was 343.8 and
293.0 mm month−1 , respectively. The observed rainfall in T1 and T2 was 273.7 and 214.8 mm month−1 ,respectively. These differences were statistically significant at a 90% confidence limit, as determined using
Student’s t-test. The 15% decrease in simulated rainfall was close to the 21% decrease in the observed
rainfall. Thus, the model well simulated the long-termchange in rainfall. The simulated rainfall amount waslarger than observational one, and the simulated longterm changes are somewhat less than the observation.
The simulated rainfall was the area average, whereas
observations are point measurements located at plain
field or valley, which could explain the difference.
We examined the spatial distribution of the longterm change in rainfall between T1 and T2 (Figure 3).
Over the Khorat Plateau, rainfall was decreased in
T2 (Figure 3(a)), which was consistent with KA01.
Decreases were observed along the latitudinal band
between 15◦ and 22◦ N of the Indochina Peninsula,showing that a long-term decrease was observed
not only over inland Thailand, but also across the
Indochina Peninsula. This decrease was observed
along the major route of the westward-moving TCs.
The simulated rainfall was in close agreement withthe observations in both spatial distribution and quantity (Figure 3). The regional long-term climate change
was simulated without inclusion of the recorded deforestation. In addition, a significant decrease in rainfall

-80

18°N

-4

0

16°N
0

-4

14°N

80
40
20
-20
-40
-80
-120
-160

12°N
10°N
96°E

98°E 100°E 102°E 104°E 106°E 108°E 110°E

Figure 3. The same as Figure 1, but for differences in the(a) observed and (b) simulated September rainfall between T1
and T2 (i.e. T2 – T1).

was simulated along the eastern coast of northern Vietnam and offshore. The long-term decrease in rainfall
exceeded 150 mm month−1 .
Finally, we examined the long-term changes inatmospheric circulation associated with the decrease in
rainfall. Climatologically, the monsoon trough appears
along the 15◦ –20◦ N zonal band because of TC activity (e.g. Takahashi and Yasunari, 2006). The trough in
T2 was much weaker than that in T1 (Figure 4(a) and(b)). The difference clearly shows the anti-cyclonic
circulation over northern Vietnam, which indicates aweakening of the monsoon trough (Figure 4(c)). The
vertically integrated water vapour fluxes showed asimilar pattern to the winds at 850 hPa (not shown).
Weakening of the water vapour convergence over thenorthern Vietnam region presumably reduced the rainfall there. To examine the weaker monsoon troughformation in T2, the frequency of TC appearance
was calculated. Because the anti-cyclonic circulation appeared over northern Vietnam (Figure 4), thefrequency was examined there. The frequency wasdefined from the 925-hPa wind speed offshore ofnorthern Vietnam (106–109 ◦ E, 17–20◦ N). If any grid
point in the region at 925 hPa had a wind speed
greater than 12 m s−1 , a TC was defined as occurring on that day (Table I). We tested the effects ofAtmos. Sci. Let. 10: 14–18 (2009)
DOI: 10.1002/asl

Long-term decrease in rainfall over Indochina(a) 850 Wind (T1)17

4. Discussion

4 [m/s]20°N

In this section, we discuss the effect of deforestation.
We emphasise that the regional long-term climatechanges were simulated without consideration of theeffect of the recorded deforestation. On the other
hand, KA01 proposed that deforestation has caused
long-term decreases in rainfall. However, signals were
simulated only over the deforested region (KA01).
Even if deforestation can reduce local rainfall, theeffects on the Indochina Peninsula in September werelikely to be spatially limited. Consequently, the effects
of deforestation cannot explain the observed long-termchange of September rainfall in the Indochina region.
Because of the prominent seasonal evolution of
the Indochina monsoon (e.g. Takahashi and Yasunari,2006), the effects of deforestation may show seasonality over the Indochina Peninsula. Sen et al. (2004)
investigated local and remote effects of reforestationof the Indochina Peninsula in June, July and Auguston the East Asian climate. They showed increasein rainfall downstream of monsoon westerlies. Theirresults were not consistent with KA01. The difference between KA01 and Sen et al. (2004) would bepossibly explained by the seasonal march of largescale circulations, such as strength of the monsoon
westerlies. Because a few numerical experiments on
the effects of long-term changes in surface condition
were conducted over Southeast Asia, a large numberof high-resolution experiments are needed. The effectof deforestation over the Indochina Peninsula in theother season, using high-resolution models, should be
the subject of further study.

16°N

12°N
100°E

104°E

(b) 850 Wind (T2)108°E
4 [m/s]

20°N

16°N

12°N
100°E

104°E

108°E

(c) Difference (T2-T1) 1 [m/s]
20°N

16°N

12°N
100°E

104°E

108°E

Figure 4. The 15-year mean 850-hPa winds in September
during (a) T1 and (b) T2, and (c) the difference between the
two terms (T2 – T1). The units of wind speed are metres per
second.

using other thresholds such as 10 or 15 m s−1 andfound no significant change in the results. The frequencies of appearance at T1 and T2 were 13.7 and11.8 days month−1 , respectively, which was consistent with the observed tendency of TC activity (TA08).
Therefore, the weakening of the monsoon trough wasprobably associated with the decrease in TC frequency
there.

5. Conclusion
We addressed the long-term decrease in September
rainfall in the Indochina Peninsula. Two explanations
for the phenomenon were proposed: long-term change
in local surface conditions and long-term change in
TC activity.
The long-term high-resolution simulation accurately
captured the climatological rainfall distribution andquantity. A 30-year run simulated the interannual
variation and long-term change in rainfall, both ofwhich were in close agreement with observations. The
spatial distribution of the long-term change in rainfall was also found to be in close agreement with

Table I. Observed amounts of September rainfall averaged over ten stations. The ten stations are located within an area at
101–105 ◦ E, 15–18 ◦ N. The simulated rainfall is area-averaged within this area. The definition of simulated tropical-cyclone (TC)appearance is given in the text. The observed TC-days were counted by TA08, although the definition of TA08 was different fromthat of this study.

Category
T1 (1966–1980)
T2 (1981–1995)

observedrain(mm month−1 )Simulatedrain(mm month−1 )Simulated
TC appearance(days month−1 )Observed
TC-day from
TA08 (days month−1 )

273.7
214.8

343.8
293.0

13.7
11.8

16.6
13.9

Copyright  2008 Royal Meteorological Society

Atmos. Sci. Let. 10: 14–18 (2009)
DOI: 10.1002/asl

18

observations. A long-term decrease in rainfall wasobserved along the major route of the westwardmoving TCs, including the eastern coast of northern Vietnam, rather than just over Thailand. Finally,
the simulated monsoon trough weakened from T1
to T2, which again was consistent with observations. All of these long-term changes in rainfall wererealistically simulated without including the effects
of deforestation in the model. The effects of deforestation over the Indochina Peninsula in September were likely spatially limited and hence are not
likely to explain the observed decrease in rainfall.
The weakening of TC activity over and around the
Indochina Peninsula could explain the spatial distribution of the observed decrease in rainfall. Therefore,
we conclude that the observed long-term decrease in
September rainfall has been caused by the change in
TC activity, rather than by the changes in the local
surface conditions.

Acknowledgements
The rain-gauge data were kindly provided by the Departmentof Meteorology and Hydrology in Myanmar, the Thai Meteorological Department, the Department of Meteorology andHydrology in Laos, the Department of Meteorology in Cambodia and the National Hydrometeorological Service of Vietnam.
These data were collected under the GAME-T and MAHASRI
projects. We thank the agencies and people involved in the
projects. We thank Prof. F. Kimura of University of Tsukuba forvaluable comments and suggestions. We also thank two anonymous reviewers for their helpful suggestions and comments.
This work was partly supported by the Global Environment
Research Fund (B-061) of the Ministry of the Environment,
Japan.

Copyright  2008 Royal Meteorological Society

H. G. Takahashi et al.

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Atmos. Sci. Let. 10: 14–18 (2009)
DOI: 10.1002/asl