By
Diong Jeong Yik, Yip Weng Sang,
Mat Kamaruzaman Mat Adam, Nursalleh K Chang,
Dr. Fariza Yunus and Muhammad Helmi Abdullah
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Perpustakaan Negara Malaysia
Cataloguing-in-Publication Data
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Jalan Sultan,
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Selangor Darul Ehsan
Malaysia
Contents
No.
Subject
Page
1.
Abstract
2.
Introduction
1
3.
Data and Method
4
4.
Result
6
5.
Summary And Discussion
20
6.
Conclusion
22
7.
References
23
The Definitions of the Southwest Monsoon Climatological Onset and
Withdrawal over Malaysian Region
Diong Jeong Yik, Yip Weng Sang, Mat Kamaruzaman Mat Adam, Nursalleh K Chang,
Dr. Fariza Yunus and Muhammad Helmi Abdullah
ABSTRACT
Based on the reanalysis data from JRA-25, NCEP/NCAR and rainfall data obtained from
Malaysian Meteorological Department, the criteria for the climatological onset and
withdrawal of summer monsoon are examined in this study. There are two regions with major
convective centres during the summer monsoon, one over the Bay of Bengal and the other
one in the vicinity of the Philippines-South China Sea region. These two convective centres
act as the atmospheric heat engines that partly drive the monsoon. Therefore the difference of
the zonal wind at the 850 hPa over this region and the Malaysian region is used as an index to
determine the onset and withdrawal of the monsoon objectively. The index or the meridional
shear of the zonal wind at 850 hPa is a good approximation of the low-level shear vorticity
over the whole Malaysian-South China Sea region. Climatological onset date is defined as the
date when the shear vorticity is less than 0 m/s while the withdrawal date is defined as the
date when the shear exceeds 0 m/s. This shear reflects the large-scale rainfall variability in
the Malaysian region. Rainfall is not used as one of the criteria as it is influenced by the
orographic and mesoscale processes, which have more forcing than the synoptic scale
features. The rainfall data is only used as an indicator of the relatively dry period of the
monsoon when compared to the rest of the year. Composites of lower level horizontal wind
indicate that mean onset date coincides with the retreat of low-level subtropical ridge from
Indo-China region to northwest Pacific and at the same time the monsoon trough strengthens
and migrates northward from the Malaysian region into the Indo-China region. This results in
extensive intrusion of southwesterlies into the Malaysian region. The upper-level troposphere
circulations show quasi-stationary anticyclone over the Indo-China region before the onset of
monsoon and slight northwestward movement of the upper tropospheric anticyclone after the
onset. The upper tropospheric circulation also strengthens during the onset.
1. Introduction
The summer monsoon in Malaysian region is not as dramatic as that over the Indian
subcontinent and in East Asian region but it forms an important component of the Asian
monsoon system. The arrival of the monsoon in many regions is viewed with high significance
because it marks the beginning of a gradual process whereby long dry spells give way to long
rainy season. Therefore, monsoons have become synonymous with the rains. In Malaysia, rain
occurs throughout the year. The northern hemisphere winter monsoon in comparison to summer
monsoon is more prominent because of the sudden surge in the rainfall amounts with the onset of
the winter monsoon in November or December over the eastern parts of Peninsular Malaysia.
The summer monsoon or better known as southwest monsoon because of the prevailing
southwesterly during this season in Malaysia, is on the contrary associated with relatively dry
period during the active monsoon months.
Generally, there are several ways of determining the onset and withdrawal of the
monsoon. One of indicators for the arrival and withdrawal used is the abrupt transition from wet
to dry and from dry to wet season. However it is difficult to define the onset and withdrawal of
summer monsoon based on rainfall data alone. Nonetheless, a simple definition to define the
onset and withdrawal of the summer monsoon using rainfall data can be used when comparing it
to the large-scale situation. Ananthakrishnan et al. (1981) for instance, suggested two main ways
of determining the onset dates of summer monsoon rain in India. One method is based on the
long-term averaged daily or pentad rainfall where the onset can be determined by a sudden and
persistent increase in the averaged rainfall. Another method is that the onset can be determined
using frequency distribution of long-term onset dates. Other investigators, for example
Matsumoto (1997) and Tao et al. (1987) defined the onset of the monsoon as the time when rain
exceeds a certain threshold value.
As Southeast Asia (SEA) and in particular Malaysia is located between the large Indian
and East Asia monsoon systems, the large-scale atmospheric circulation over SEA cannot be
isolated from the large-scale atmospheric circulation associated with these two monsoon
1
systems. Therefore the wind field could be used to define the onset of the monsoon. For
example, Orgilll (1967) defined the onset of the summer monsoon to occur over Southeast Asia
when the lower tropospheric equatorial westerlies migrated northward into southern China
during the months of May and June. Examining both the upper and lower level wind field from
1936-1964, he then defined the mean onset of the summer monsoon in Indochina as 17 May with
a range of 33 days. In another example of using the wind field, Cheang and Tan (1988a) and
Cheang and Tan (1988b) defined the onset date as the date after which the winds at 850hPa to
700hPa become westerlies and remain so for at least 20 days after the date.
The onset of the monsoon can be also generally determined by the seasonal change in the
surface winds and local rainfall since the monsoon climate is characterized by the annual reversal
of winds and the contrast between the seasonal rainfalls. Ding et al. (2001), Qian et al. (2000)
used the combination of surface wind or near surface wind and rainfall as the indicator for the
monsoon onset. Forecasters in the Indian Meteorological Department (IMD) also use the
combination of rainfall and lower tropospheric wind condition to define the onset of the summer
monsoon over India (information is available online at http://www.imd.gov.in). Similarly in
Malaysia until recently, forecasters in The Malaysian Meteorological Department use the
combination of surface wind and rainfall to determine the onset of the Northeast Monsoon (see
Cheang, 1987). Although the relations between the change of wind and local rainfall pattern are
not necessary true in some monsoon regions, the rainfall during the summer monsoon in
Malaysia corresponds to the passage of westward propagating monsoon lows and related to
major active and break cycles of the monsoon system (Cheang et al., 1981).
Studies by
Hirasawa et al. (1995), Lau and Yang (1997) and Lau et al. (1998) also showed the convection
centre which is active in the maritime continent during mid-April until mid-May shifted
suddenly and anchored itself over the Southern China and Indochina region during mid-to-late
May. This therefore confirms the relatively dry period in Malaysia during the summer monsoon.
According the Chang et al. (1995), the beginning of the whole Asian Monsoon regime
can be traced back to the onset of summer monsoon over the South China Sea (SCS). One of the
most prominent and unique characteristics of the summer monsoon over South China Sea is that
2
the onset of the monsoon takes place across large latitudinal range from 3° - 22°N (Wang et al.,
2004). This means that the onset marks the arrival of monsoon in East and Southeast Asian
simultaneously. Therefore the study of the SCS monsoon onset has become one of the important
subjects of study in order to fully understand the variability of the Asian Monsoon. However,
defining and finding the onset date can be extremely difficult and subjective as there is no
universally accepted definition of the onset. Nonetheless, the most widely used criteria or
definition for the onset of the monsoon, first proposed by Ramage (1971) are the change of
direction and sustainment of the prevailing wind. The general consensus among the numerous
studies is that the monsoon onset over the SEA and SCS region falls in the 4th pentad of May.
While finding the onset definition of the monsoon can be difficult, finding the definition
for the withdrawal of the monsoon is equally tricky. Fong and Wang (2001) pointed out that by
the 5-9 August pentad, the monsoon starts to retreat from the East Asia region. The retreat of the
southwesterlies from northern to southern China is rapid, about a month or less and completely
disappear in the region to the south of Yangtze basin two pentads later. The monsoon moves
further south and stays stationary in the South China Sea and Indochina region. These monsoon
westerlies stay over this region until late October when the monsoon fully ends over Southeast
Asia (Matsumoto, 1997).
This paper attempts to find useful variables which best represent the onset and
withdrawal of the monsoon over Malaysian region. Although Cheang and Tan (1988b) proposed
a useful method of defining the onset of the southwest monsoon, the onset date can only be
confirmed after at least 20 days. Therefore a new and simple method needs to be devised in order
to determine the onset of the summer monsoon for operational forecasting use. In view of the
complex nature of the monsoon system, one way of doing so is to investigate first the
climatological mean onset features of the monsoon over this region and this will be the primary
objective of this study. Our objective is further extended to explore the evolution of the large
circulation in the region just before, during and after the onset of the monsoon in this region.
The data and methods used in this study are described in Section 2. The results of the
climatological mean onset of the monsoon and its characteristic are first presented in Section 3
3
and this is followed by features of the seasonal transition characteristic of the large scale
circulation. These results are then summarized and discussed in Section 4. Section 5 provides the
conclusion of the study and some recommendations for further studies.
2. Data and Method
In the present analysis, the climatology of the large-scale atmospheric circulation of the
summer monsoon is used as a means to depict the summer monsoon over the whole Asian region
and in particular the Malaysian region. The 5-day mean data used to depict the regional-scale
circulation are derived from a 30 years (1981-2010) Japanese 55-year Reanalysis (JRA-55;
Kobayashi et al., 2015; Ebita et al., 2011; Onogi et al., 2007) dataset provided by Japan
Meteorological Agency (JMA) and Central Research Institute of Electric Power Industry
(CRIEPI). This daily climatology mean wind data is also used to calculate the index to define the
onset of the monsoon.
The outgoing longwave radiation (OLR) provides a good approximation of large-scale
convective variability on monthly to seasonal timescales. Therefore the OLR is used in this study
to depict the large-scale convective variability before and during the onset of the monsoon. The
daily mean OLR on a 2.5° x 2.5° grid data is obtained from the National Centers for
Environmental Prediction-National Center for Atmospheric Research (NCAR-NCEP) Reanalysis
(Liebmann et al., 1996).
4
Figure 1: Regions and stations used to compute the onset date of the Southwest Monsoon.
To define the climatological mean onset and withdrawal date objectively, we modified
the method proposed by Wang and Fan (2004) (hereafter WF) based on the theoretical
considerations and empirical relationships to define the onset. WF found that there are two main
convection centres exist during the summer monsoon; one which is located in the vicinity of Bay
of Bengal and the other in the vicinity of the Philippines. A study by Yanai and Tomita (1998)
found that these convection centres are the atmospheric heat sources which act similarly to those
proposed by Gill (1980) except that in this case the heat sources are centred at 15°N. Based on
the finding of WF that : (1) the South China Sea Monsoon (SCSM) commences simultaneously
across large latitudinal range from 3° - 22°N, (2) the westerly shear which best correlates with
the convection in the Philippines is located in the Southeast Asia region (0°N-10°N, 90°E130°E), (3) good correlation between convection in Philippine and u850 (850-hPa zonal wind)
over the Malaysian region and (4) that the 850-hPa westerlies are strong in the narrow latitudinal
belt between 5°N and 15°N, we constructed two boxes (as shown in Figure 1) in addition to the
one box (5°N-15°N,90°E -130°E ) suggested by WF (hereafter WF1). The values over these
boxes are used to calculate the onset and withdrawal dates of the southwest monsoon. The area
bounded between 1.75°N to 4.25°N and 100.75°E to 103.25°E (hereafter Box PM) is used to
5
calculate the average value of the zonal wind over Peninsular Malaysia, while area bounded
between 1.75°N to 4.25°N and 110.75°E to 113.25°E (hereafter Box B) is used to calculate the
average value of the zonal wind over Borneo.
In this study, the daily rainfall will not be used as one of the forecasting parameters but as
an indicator to show the arrival and withdrawal of the dry season. In Box PM, climatological
mean daily rainfall from 4 stations, namely Subang, Petaling Jaya, Temerloh and Melaka are
used. Rainfall recorded from Sibu and Bintulu are used as averaged rainfall over Box B. This
rainfall data from 1981-2010 period are obtained from the Malaysian Meteorological Department
(MMD) stations. In order to avoid stations with higher rainfall dominating the average rainfall
within the boxes and to show the seasonal variation in the climatological rainfall, the daily
climatological mean is first deducted with the annual climatological mean of each individual
stations’ rainfall data. This rainfall anomaly is then divided by the standardized rainfall recorded
at the respective rain gauge station. In order to get the average standardized rainfall in the box,
the standardized rainfall at each station located in the boxes is then averaged to get the
standardized rainfall index (SRI). This index is a simple way of assessing the wetness or dryness
of the season. The season is considered wet if the SRI is positive and vice versa
3. Result
The meridional shear between Box PM and WF1 of the zonal wind at 850hPa (u850) and
the rainfall distribution (represented by SRI in this study) over Box PM are shown in Figure 2
(a). From the figure, it can be seen that there are two peaks in the rainfall; the first one falls
between April to May and the second one falls between October to early December and a
noticeable dry period from mid-May to early October. During the dry period, the rainfall is
observed to decrease steadily by 13 May and the rainfall is seen to increase steadily by 9 October
which signals the end of the dry period. The commencement (withdrawal) of the negative
meridional shear of u850 seem to more or less coincide with the period of decreasing
(increasing) rainfall. The wind regime’s transition from easterly to westerly at the 850hPa level
over Peninsular Malaysia is gradual, unlike the sudden retreats of easterly and abrupt
6
accelerations of westerly over the WF1 region as shown in Figure 2(b). Westerly wind speed
over Peninsular Malaysia stays more or less the same throughout the summer monsoon season
and is weak (not more than 3 m/s), while the westerly over WF1 is strong and peaks during the
height of the summer monsoon. The weak westerly wind over Peninsular Malaysia at 850hPa
does not retreat until early December. In contrast, the westerly fully withdraws from the WF1
region by the 18 October and at the same time the negative meridional shear turn positive by 8
October. The obvious life cycle of the westerly wind, from bursting in mid-May, gradually
becoming stronger during the peak of the monsoon, weakening and retreating in the early
October in WF1 region shows that signal for the change in the monsoonal wind in this region is
strong as opposed to those found in Box PM.
Figure 3 shows the zonal difference of the u850 between Box B and WF1 and the
rainfall distribution over Box B. The rainfall over Borneo follows a different regime from that
found in Peninsular Malaysia. There is only one peak in the rainfall distribution. From midFebruary until mid-September, there is very little rainfall with a minimum located somewhere
between July and August which is the peak of the summer monsoon. The maximum rainfall in
this region occurs during the winter monsoon. There is not much difference in the meridional
shear of u850 between this region and the Peninsular Malaysia region. In fact the
commencement and withdrawal of the negative meridional shear of u850 are similar to those
found in the Peninsular Malaysia region. Unlike the westerly wind over Peninsular which is
generally weak throughout the monsoon, the westerly wind in this region is found to be a little
stronger. The length of days with westerly at 850hPa in this region is longer by about 14 days
than the length of westerly day in the Peninsular Malaysia region.
7
u850 (m/s)
-2.00
1
14
27
40
53
66
79
92
105
118
131
144
157
170
183
196
209
222
235
248
261
274
287
300
313
326
339
352
365
-2.00
1-Apr
8-Apr
15-Apr
22-Apr
29-Apr
6-May
13-May
20-May
27-May
3-Jun
10-Jun
17-Jun
24-Jun
1-Jul
8-Jul
15-Jul
22-Jul
29-Jul
5-Aug
12-Aug
19-Aug
26-Aug
2-Sep
9-Sep
16-Sep
23-Sep
30-Sep
7-Oct
14-Oct
21-Oct
28-Oct
u850 (m/s)
8.00
3.50
6.00
2.50
4.00
1.50
2.00
0.00
0.50
-4.00
-6.00
-8.00
Julian Day
Rain
-4.00
-6.00
Date
PM
WF1
8
PM-WF1
Figure 2(a): Meridional shear of u850 for Box PM and WF1.
(b) u850 in each boxes and the difference.
-0.50
-1.50
Standard Rainfall Index
(a)
Peninsular Malaysia
-2.50
-3.50
u850
(b)
Peninsular Malaysia
8.00
6.00
4.00
2.00
0.00
u850 (m/s)
-2.00
-4.00
1
14
27
40
53
66
79
92
105
118
131
144
157
170
183
196
209
222
235
248
261
274
287
300
313
326
339
352
365
-2.00
1-Apr
8-Apr
15-Apr
22-Apr
29-Apr
6-May
13-May
20-May
27-May
3-Jun
10-Jun
17-Jun
24-Jun
1-Jul
8-Jul
15-Jul
22-Jul
29-Jul
5-Aug
12-Aug
19-Aug
26-Aug
2-Sep
9-Sep
16-Sep
23-Sep
30-Sep
7-Oct
14-Oct
21-Oct
28-Oct
u850 (m/s)
10.00
4.00
8.00
6.00
3.00
4.00
2.00
2.00
1.00
0.00
0.00
-4.00
-6.00
-8.00
-10.00
Julian Day
Rain
-6.00
Date
B
WF1
9
B-WF1
Figure 3(a): Meridional shear of u850 for Box B and WF Box1.
(b) u850 in each boxes and the difference.
-1.00
-2.00
-3.00
Standardized Rainfall Index
(a)
Borneo
-4.00
u850
(b)
Borneo
8.00
6.00
4.00
2.00
0.00
Criteria for the onset and withdrawal
The weak westerly wind in the Peninsular Malaysia and Borneo regions which is
established firmly by the end of April and early May alone could not provide any crucial
information as to
the onset date of the summer monsoon. Similarly, the westerly wind which
continues well into December and mid-December clearly could be used as a criterion for the
withdrawal of the monsoon. We therefore take the prominent strong westerly feature found in
WF1 and the decrease and increase in the rainfall over Peninsular Malaysia as the indicator to
define the onset and withdrawal of the monsoon over the Malaysian region. The criterion for the
onset (withdrawal) of the monsoon over the Malaysian region is deemed to have been satisfied
when the difference of the 850-hPa wind between the box in the respective regions and the WF1
is less (more) than 0 m/s. Therefore using the above criteria, the climatological onset date for the
Malaysian region falls on 19 May and the withdrawal of the monsoon falls on 8 October.
Regional Circulation and features- Onset
The regional pentad wind patterns at 850 hPa level are shown in Figure 4. In April and
early May, there is a strong near-equatorial westerlies/southwesterlies wind over the equatorial
Eastern Indian Ocean at around 80°E to 90°E. This southwesterly expands northeastward rapidly
towards Indo-China region during the pentad centred on 13 May and the pentads onward. The
wind condition over the Malaysian region is weak westerlies/southwesterlies. During the onset of
the monsoon, which falls in the pentad centred on 18 May, this westerlies/southwesterlies have
already extended upward to 700hPa (figure not shown) in most parts of the Indo-China region,
the Peninsular Malaysia region and the South China Sea. Over the South China Sea and Malaysia
region, westerlies and southwesterlies flow strengthen.
The northern hemisphere monsoon trough which is located near the central region of
Peninsular Malaysia remains quasi-stationary since mid-April to late-April. The trough then
migrates north by early May and during the fourth pentad in May where the onset of monsoon is
taking place, this trough has already migrated north and is located at the southern part of the
10
Indochina region. By end of May (figure not shown), this trough moves further north and the
axis of the trough runs parallel with the coast of the northern Indochina region. At the same time
with the northward progression of the monsoon trough, the subtropical ridge begins to weaken
and retreats from the South China Sea region and moves toward western Pacific Ocean. During
this time the trough over Bay of Bengal becomes well established and extends southward,
signalling the beginning of the Indian summer monsoon. This monsoon trough breaks the
continuous subtropical ridge around the Bay of Bengal region.
11
Figure 4 : Climatology of the large-scale circulation in Southeast Asia at 850 hPa before and during onset.
12
Figure 5 : Climatology of the large-scale circulation in Southeast Asia at 200 hPa before and during onset.
13
Figure 5 depicts the regional circulation at 200 hPa. Two main features of the 200 hPa
wind patterns during the onset of the monsoon are strengthening of the northeasterlies and the
northwestward movement of the anticyclone circulation over the Indochina region. This quasistationary and weak anticyclone is located at the southern part of the Indochina region just before
the onset of the monsoon. It intensifies and moves northwestward right after the onset. A month
before the onset, the divergent centre (Figure 6) is located at the equatorial western Pacific
Ocean. During the pentad of 16-20 May when the onset of the monsoon takes place, the
divergent centre is located at around 10-15° latitudinal belt over the central, southern South
China Sea and Indochina region. The upper level divergence coupled with the lower level
convergence clearly enhance convective activities over this region.
Figure 6 : Climatology of the 200 hPa velocity potential and divergent wind before and during onset.
14
Based on the pentad mean
OLR field as shown in Figure 7, the
Maritime Continent is covered with
convection in the fourth pentad of
April (16-20 April). The area
around South China Sea shows
higher OLR values which indicates
that this area is cloud free. During
the fourth pentad of May (16-20
May) when the onset of monsoon
has taken place, the convection has
now shifted to the north and
anchored over the Indochina region.
The zone of higher OLR values
over South China Sea and western
Pacific Ocean and over the Indian
sub-continent
have
retreated
towards the western Pacific Ocean
and east of Indian sub-continent
respectively during this time. The
low OLR band over the equatorial
Figure 7 : Climatology of the outgoing longwave radiation (OLR) before
and during onset.
East Indian Ocean merges with the
low OLR band coming from the mainland of China. The movement of the low OLR band
correspondes with the northward movement of monsoon trough at the lower tropospheric level
and divergence at the upper level. The merging of low OLR band in East Indian Ocean and
mainland China correspondes with the breaking of the subtropical ridge over Bay of Bengal
before and during the onset of the monsoon.
15
Regional Circulation and features – Withdrawal
After the onset of the monsoon, the monsoon trough advances further north, up until the
Yangtze River Basin (figure not shown). This advancement is consistent with the timing and
location of the Meiyu rainy season in this region. The monsoon begins to retreat rapidly from
this region starting in August. By mid-September, the monsoon trough is now located around the
southern Indochina region (Figure 8). The strong westerlies originating from the equatorial East
Indian Ocean still dominate the southern Indochina region. These westerlies begin to weaken by
the first pentad in October (3-7 October) and are replaced by easterlies at the withdrawal pentad
(8-12 October). The monsoon trough now is located at around 10°N and stretches from Western
Pacific to the Indian subcontinent and is quasi-stationary over the Indochina region since
September. Over the Malaysian region weak southwesterlies predominate throughout the period.
16
Figure 8 : Climatology of the large-scale circulation in Southeast Asia at 850 hPa before and during withdrawal.
As the summer monsoon withdraws to the south, the westerlies in the upper level also
shift southward accordingly (Figure 9). As the monsoon begins to retreat southward, the
anticyclonic circulation over the Tibetan Plateau also retreats southeastwardly at the same time.
By the withdrawal pentad, the anticyclonic vortex is located in the Burmese region with two
other anticylonic vortices forming over the SCS near the Philippines and western Pacific Ocean.
A pentad later, the anticylonic vortex over the landmass disappears leaving only the vortices over
the ocean (figure not shown).
17
Figure 9 : Climatology of the large-scale circulation in Southeast Asia at 200 hPa before and during withdrawal.
18
Figure 10 shows the pentad
mean OLR before the withdrawal and
during the withdrawal of the monsoon.
A month before the withdrawal of the
monsoon, there is a belt of active
convection located around the 15°- 20°
N latitudinal band spanning from the
western Pacific Ocean to the east coast
of
Indian
subcontinent.
Over
the
Maritime Continent, pockets of active
convection can be seen during this time.
During the withdrawal pentad, the low
OLR band breaks into two, one forming
an active convection belt that moves
southward and dominates the whole of
Maritime Continent and the other one
moves northward toward the Tibetan
Plateau. In the Indian subcontinent,
active convection can still be seen over
the
southwestern
subcontinent.
This
part
of
this
signals
the
withdrawal of the Indian Summer
Figure 10 : Climatology of the outgoing longwave radiation (OLR)
before and during withdrawal.
monsoon.
19
4. Summary and Discussion
The majority of the Asian summer monsoon onset definitions use a combination of low
level equatorial westerlies and rainfall as the criteria. Nevertheless, using rainfall as one of the
criteria is impractical because of the nature of the convective environment in the Malaysian
region during the pre-onset phase. The rainfall in Malaysia is also heavily influenced by
mesoscale and orographic processes compared to synoptic scale features. Hence in order to show
that its rainfall is relatively less during the summer monsoon, the rainfall has been standardized
and normalized in this study. This standardized and normalized rainfall is then used as one of the
indicators of the arrival of monsoon.
The analysis of the large scale circulation reveals the seasonal march of the monsoon
follows the northward movement of the monsoon trough. Right after the withdrawal of winter
monsoon from the Malaysian region in March, the monsoon trough is located close to equator
and extends up to 700 hPa. This trough then moves to the central Malaysian region in April. By
this time in the equatorial western Indian Ocean, westerlies start appearing at the 850 hPa and
700 hPa levels. In early May, the trough advances further north and this establishes the
southwesterly flow over the Malaysian region below the 700 hPa. By end of May, the summer
monsoon is well established with deep zonal westerlies prevailing over the Malaysian and the
Southeast Asia region. Right after the onset, the westerlies strengthen and extend upward to 600
hPa while at the same time the easterlies in the upper troposphere strengthen. This form a very
strong vertical shear which could inhibit convective activities.
The method used in this study to derive the index for the onset and withdrawal of the
monsoon may appear too simple for a large and complex system of the monsoon. However, the
index which is basically the meridional shear of the 850 hPa westerly can be used to describe the
variability of the large scale low-level shear vorticity over the Malaysian region. The date of
climatological onset is defined as the first day when the meridional shear dips below 0 m/s
(anticyclonic shear vorticity) and the climatological withdrawal is defined as the first day when
the meridional shear exceeds 0 m/s (cyclonic shear vorticity). This shear realistically reflects the
20
large-scale rainfall variability and the lack of deep convection over the region during the summer
monsoon. Although most indices require certain threshold values to be sustained for a number of
days, we could not establish such criterion in this study as the intraseasonal oscillation (ISO) has
been removed from the data in this study. However for our climatological study of the summer
monsoon over Malaysian region, our definition of the onset and withdrawal can be seen as a
good representation of rainfall variability in the Malaysian region and large-scale changes in the
regional circulation.
Based on this study, we can summarize that the climatological onset of the summer
monsoon which falls on the 19 May over the Malaysian region must fulfill the following
conditions:
(1) The meridional shear of the u850 must be less than 0 m/s.
(2) The monsoon trough at the 850 hPa must be seen in the vicinity of the Indochina
region.
(3) Concurrently, the subtropical ridge must retreat from the South China Sea towards
western Pacific Ocean.
(4) The anticyclonic vortex in the upper troposphere moves northwestward right after the
onset and the flow at this level also strengthens.
As for the withdrawal of the monsoon, the following conditions must be fulfilled:
(1) The meridional shear of the u850 must be greater than 0m/s.
(2) The monsoon trough at the 850 hPa is located within the latitudinal band of equator to
10°N.
(3) The anticyclonic vortex in the upper troposphere moves southeastward.
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5. Conclusion
In this study, the meridional difference of the climatological 850 hPa zonal wind, the
large scale circulation patterns and features were examined to determine the climatological onset
and withdrawal of the monsoon. The climatological onset and withdrawal based on the 19812010 mean pentad reanalysis data shows that the meridional difference in the low level wind
must be negative for the onset to take place and positive for the withdrawal to take place. This is
further substantiated with the relativly dry period, the movement of the trough, subtropical ridge
and the active convective region.
This study shows only the climatological state of the monsoon over the Malaysian region.
Further studies need to be carried out to investigate individual cases of the monsoon onset and
withdrawal as previous studies have shown the effects of the ISO on the onset and withdrawal
processes of the monsoon. The summer monsoon is also the peak of the tropical storm and
typhoon season over the western Pacific Ocean which could complicate the determination of the
monsoon onset and withdrawal dates.
Besides the ISO, there is also a large interannual
variability in the onset date of the summer monsoon over Malaysia and the Southeast Asia region
which needs to be examined. All these need to be addressed in future studies. Discrepancy
analysis on the index needs to be carried out in each individual year to examine the robustness of
this monsoon onset and withdrawal definition.
Acknowledgements: The authors would like to thank all the staff of the R&D Section of
Malaysian Meteorological Department (MMD) for their kind assistance and support throughout this
study. The first author acknowledges that the idea of this study was first mooted by Mr. Subramaniam
Moten, former Head of the R&D Section, MMD.
22
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