Dynamics of Climatological monsoon breakover the Indochina Peninsula in northern summer.
Hiroshi TAKAHASHI(1) and Tetsuzo YASUNARI(2)(1)Graduate school of Environmental studied, Nagoya University,(2)Hydrospheric Atmospheric Research Center, Nagoya University,Nagoya, Aichi 464-0811, Japan. e-mail: s030111d@mbox.nagoya-u.ac.jp
Abstract
The climatological pentad mean annual cycle derived from 32 stations located inThailand shows a rainfall dip in the middle of rainy season around Julian pentad 34 to36 (June 14-29). The Indochina rainfall system can be divided into two subsystems inthe vicinity of climatological monsoon break (CMB). The rainfall subsystem of theformer is characterized as a monsoon southwesterly system, while that of the latter ischaracterized as a tropical depressions system. The CMB is an important monsoon
singularity event on the monsoon seasonal march over the entire Asian-Pacificregion.
Key words: climatological monsoon break, topographic effect, rainfall system

1. Introduction
In the last decade, there has been asurge of interests in study of the monsoon
over Indochina. The Indochina summer
monsoon exhibits many peculiar featuresthat distinguish it from the adjacentIndian and the western North Pacific(WNP) (including the South China Sea(SCS)) monsoons.
Matsumoto (1997) noted that before
the Thailand monsoon onset in mid-May,there are many significant rainy events,
which are referred to as pre-monsoon
rain. From the pre-monsoon rain to thesummer monsoon onset, the local rainfall
normally increases progressively heavier
so that the onset is more a gradualprocess rather than an abrupt onset fromvery dry conditions such as the ones overIndia or the SCS. Based on analysis of
the rainy season characteristics, Wangand Lin (2002) pointed out that the rainyseason over the Indochina region differs
in many essential aspects from adjacentIndian and the WNP summer monsoons,thus this region might serve as atransition zone (boundary) between thetwo tropical monsoon (the Indian and theWNP) subsystems.
A peculiar feature of the Indochina
monsoon rainy season is the significant
bimodality with a major rainy peak in
September and a minor peak in late May(P28-32) (Fig. 1). Around P35 (late June)
there exists a notable monsoon break inthe climatological annual cycle ofThailand
rainfall(Fig.1).
This
mid-summer break will be referred to asclimatological monsoon break (CMB) in
this study. Examination of each year of
rainfall indicates that the CMB has
strong tendency to occur in late-June.
Thus the CMB is an importantmonsoon singularity event on themonsoon seasonal march. To theauthors’ knowledge, the cause of the CMB
over Indochina has not been studied. The
primary purpose of this study is todocument the circulation changes before,during, and after the CMB and tounderstand the basic processes that areresponsible for the Indochina CMB.

Figure 1: Pentad climatological timeseries of rainfall index for 50 years inunits of mm/day. The index was averaged
rain gauge observations at 32 stations.

2. Data
The rainfall data, which consist of 32
stations in Thailand from 1951 to 2000
compiledby
Thai
MeteorologicalDepartment (TMD) are used for thisstudy. The percentage of missing data foreach station is less than 1 percent.
To analyze large scale rainfall
distribution, CMAP (Climate Prediction
Center (CPC) Merged Analysis ofPrecipitation, Xie and Arkin, 1997) wasused. This dataset covers global domain
and is available for the period from 1979
to 2000 on 2.5 X 2.5 grids.
TheNationalCenterfor
EnvironmentalPrediction(NCEP)/NationalCenterfor
AtmosphericResearch (NCAR) reanalysis data wereused to derive zonal and meridional wind(u, v) and geopotential height (z) at
850hPa at 2.5 x 2.5 each grid.
To describe climatological sub-seasonalvariability, pentad data were calculated
from daily data for each field.
Tropical cyclone tracks archived by theTyphoon Center of JMA for 20 years from1981 to 2000 are also used.

3. Atmospheric circulation associatedwith Indochina rainfall
3.1 CMBTo investigate the atmosphericcirculation fields associated with theCMB, difference in 850hPa wind andgeopotential fields (upper panel), andrainfall (lower panel) between before
CMB (P31-33) and during CMB(P34-36) are shown in figure2,respectively. In general, the monsoon
westerly is dominant in this period (notshown). In the transition period toCMB, the monsoon westerly isenhanced along the latitudinal belt
centered on 15N-20N, especially overIndianmonsoonsection.
An
anti-cyclonic ridge is located over thewest of Indochina Peninsula (Bay ofBengal, BOB) and the western
Indochina Peninsula. In the lower
troposphere the anti-cyclonic ridge isclearly fixed with the mountain ranges.
The anti-cyclone coincides with thedecreased rainfall over the western
Indochina Peninsula. At the same time,a cyclonic circulation and a cyclonictrough appear over the SCS to the WNP.
In that area, rainfall reasonablyincreases. Thus the enhancement of thelow-level monsoon westerly along
15N-20N is likely responsible for theCMB, because the intensified monsoon
westerly flows over the north-southoriented Indochina mountain range,generating topographic Rossby waves.
The increased rainfall over the SCS isconsidered as the downstream impact
ofthestationary-Rossby
waves.
Therefore the CMB is not a local
phenomenon, it is significant formonsoonseasonal
march.

Figure 4 shows the tracks of tropical
cyclones over the WNP. Figure 4 clearlyshows that the 1st peak has little tropicaldepressions, whereas the 2nd peak has
numerous tropical depressions.

Figure 2: Difference in 850hPa winds andgeopotential height (upper panel) andprecipitation rate (lower panel) between
P31-33(before CMB) and P34-36 (CMB).
Contours show geopotential height andprecipitation rate at an interval of 2gpm
and 1mm/day, respectively. In the lowerpanel, light shading denotes rainfall
decreased and heavy shading denotes
rainfall increased.
3.2 The difference in rainfall system
between before and after the CMB
Figure 3 shows the difference in
850hPa winds and geopotential height
between P28-32 (the 1st peak) andP47-P51 (the 2nd peak). The monsoon
westerly is dominant during both theperiods. The cyclonic circulation isobserved over the SCS and the WNP,
which suggest that the rainfall system of
2nd peak is characterized as tropicaldepressions. During the 1st peak, theincrease rainfall is likely to be caused by
the intensification of the monsoon
southwesterly. Thus the rainfall systemof the 1st peak is characterized as themonsoon southwesterly from BOB.

Figure 3: Difference in 850hPa winds andgeopotential height between P28-32 (the1st peak) and P47-51 (the 2nd peak).
Contours show geopotential height at aninterval of 4gpm.

Figure 4: Tracks of tropical depressions(including typhoons) from 1981 to 2000
during the 1st peak (upper panel) and the
2nd peak (lower panel).

4. Conclusions and remarks
Major findings obtained in the study
are summarized as follows;
1) Climatological 50-years mean pentad
values of rainfall clearly show distinct
climatological monsoon break (CMB)
occurring over Thailand in late-June. It
is found that CMB occurs in p34-36.
The CMB has strong tendency to occur
in late-June, which indicates the CMB
is an important monsoon singularity.
2) The decreasing rainfall is associatedwith an anti-cyclonic circulation overthe Bay of Bengal with decent northerly
wind into the Indochina Peninsula.
When the monsoon westerly isenhanced along the band centered on
15N-20N, the anti-cyclonic circulationis enhanced over the west of Indochina
Peninsula.
Theenhancementofmonsoon
westerlyisprobably
associated with northward expansion of
the monsoon westerly, as a part ofintraseasonal variation.
3) CMB corresponds to the anti-cyclonic
circulation in the lower troposphere onjust the western mountain range overthe Indochina Peninsula. We suggestthat the formation of anti-cyclonicanomaly over the Indochina Peninsula
is affected by the topographic effect,
which is like stationary-Rossby wave
caused by a coupling of the monsoon
westerly and topographic effect. At thesame time, the cyclonic circulation isenhanced over the South China Sea
and Philippine Sea, which corresponds
to the onset of rainy season overPhilippine Sea and the drastic increaseof the rainfall over the South ChinaSea.
4) We can identify that the Indochina
rainfall system can be divided into twosubsystems in the vicinity of CMB. The
former has a peak rainfall in late-May toearly-June that is caused by theintensification of monsoon southwesterly
along 10N. While the latter that has apeak rainfall in early-September that iscaused by the appearance of active
tropical depressions. Moreover the latteris also associated with CMB, because theCMB propagates eastward active rainfall
area to SCS and the WNP. The CMB
plays a significant role as bridge totransfer active rainfall area to the SCS
and the WNP regions. Moreover
Indochina rainfall is affected from thetransferred active rainfall area as
tropical depressions.

Reference
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