Contents - Jabatan Meteorologi Malaysia

RESEARCH PUBLICATION NO. 7/2015
Characteristics of the 2015 Southwest Monsoon
in Malaysia
By Yip Weng Sang, Fadila Jasmin binti Fakaruddin,
Fatimah Zahrah binti Salleh, Mat Kamaruzaman bin Mat
Adam, Nursalleh K Chang, Fariza binti Yunus and
Muhammand Helmi bin Abdullah
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Contents
No.
Subject
Page
Abstract
1.0
Introduction
1
2.0
Data
4
3.0
Method
4.0
Observations
2
4.1 Onset
5
4.2 Onset – Index
10
4.3 Withdrawal
13
4.4 Rainfall during Active and Break Periods
20
4.5 Rainfall during the Southwest Monsoon
23
24
6.0
Typhoons and Tropical Storms during the Southwest Monsoon
2015
Discussion and future work
7.0
Acknowledgement
27
8.0
References
28
5.0
26
Characteristics of the 2015 Southwest Monsoon in Malaysia
Yip Weng Sang, Fadila Jasmin binti Fakaruddin, Fatimah Zahrah binti Salleh, Mat
Kamaruzaman bin Mat Adam, Nursalleh K Chang, Fariza binti Yunus and Muhammand
Helmi bin Abdullah
ABSTRACT
This report describes a systematic approach to identify the onset and withdrawal dates of the
southwest monsoon in Malaysia by applying the onset and withdrawal criteria from several
past studies. Two (2) pre-onset criteria, eight (8) onset criteria and five (5) withdrawal
conditions were identified. The onset and withdrawal dates of the southwest monsoon in
Malaysia and the South China Sea are then taken as the dates after which most pre-onset,
onset and withdrawal criteria are fulfilled. The southwest monsoon season of 2015 is then
taken as a case study. Based on the fulfillment of each criteria, the pentad or 5 day mean
centered on 18th of May and the 23rd of October are declared the onset and withdrawal
periods for the southwest monsoon in Malaysia in 2015. In addition to this, the monsoonal
active and break period during the 2015 southwest monsoon in Malaysia has been identified.
The active period corresponds to drier weather in Malaysia in conjunction with enhanced
southwest monsoon winds while a lull in monsoonal winds lead to an increase in precipitation
over Malaysia. This report also describes the total monthly rainfall, ENSO conditions and the
occurrence frequency of tropical storms and typhoons during the 2015 southwest monsoon.
1.0
Introduction
The onset of the southwest monsoon over the South China Sea is associated
with heavy rainfall over the Indochina Peninsular (ICP), China (Meiyu rainfall), Korea
and Japan (Baiyu rainfall) whereas in Malaysia a reduction in precipitation amount is
observed in climatological data followed by an increase in precipitation after the
withdrawal period as reported by Diong (2015). Therefore an objective and consistent
method of identifying the onset and withdrawal dates each year enables policy makers
to prepare themselves for the annual dry season and its withdrawal.
The timeline of the southwest monsoon has been divided into the pre-onset,
onset and finally withdrawal periods by previous studies. In the pre-onset period, He
(2006) and Ding (2001) observed a pair of cyclones (850hPa) straddling the equator,
with one cyclone over Sri Lanka. Meanwhile, Diong (2015), He (2006) and Ding (2001)
recorded the presence of an anticyclone (200hPa) near Indochina (Myanmar) just
before the onset.
The onset features of the southwest monsoon has been presented by various
studies in the past. The Sri Lankan cyclone which was prominent just before the onset
vanished suddenly during the onset according to Diong (2015), He (2006), and Cheang
(1988).
This feature was subsequently replaced by a northeast-southwest oriented
trough along and parallel to the coast of east India. At the same time the subtropical
ridge and anticyclone (850hPa) was found to retreat northeastwards away from the
South China Sea towards the western Pacific Ocean in Diong (2015), He (2006) and
Cheang (1988). The prominent subtropical ridge (850hPa) which extends across the
Indian subcontinent right before the onset was broken or intersected by the northeastsouthwest oriented trough along the coast of east India based on Diong (2015) and He
(2006). Meanwhile, the northern monsoon trough marches steadily north during the
early to middle portion of the southwest monsoon. In the onset it lies along Indochina
according to Diong (2015) and Ding (2001). The northward advance of the trough
towards Indochina is accompanied by a sudden penetration of broad, deep westerlies
(850 to 600hPa) from the Bay of Bengal covering all of Indochina until the western
1
Pacific Ocean. In the upper atmosphere, the southwest monsoon onset is marked by
the sudden northward movement of the 200hPa anticyclone which remained quasistationary a few pentads before the onset.
Fewer studies were made to ascertain the withdrawal criteria as compared to the
onset criteria for the southwest monsoon over the South China Sea. Diong (2015)
suggested several withdrawal criteria for the withdrawal of the southwest monsoon. The
withdrawal of the southwest monsoon is marked by the sudden retreat of westerlies
(850hPa) from Indochina to the Bay of Bengal. The westerlies over Indochina before the
withdrawal are completely replaced by easterlies from the western Pacific Ocean
(850hPa). Meanwhile, the northern monsoon trough (850hPa) continues its southward
journey and is now located near 10°N over the Indian Ocean. In the upper atmosphere,
the anticyclone at 200hPa was located in Myanmar (Burma) while two other
anticyclones were observed near the Phillipines or the western Pacific Ocean.
Diong (2015) and Subra (undated) devised indices to determine the onset and
withdrawal dates of the southwest monsoon in a more objective, concise and simple
manner. The index of Diong approximates the meridional shear vorticity (850hPa) over
the whole Malaysian-South China Sea region, whereby the climatological onset date is
defined as the date when the shear vorticity is negative while the withdrawal date is
when the meridional shear vorticity becomes positive. Based on a study by Subra, the
southwest monsoon onset date over Malaysia is deemed to have occurred when the
averaged zonal wind from the 850 to 600 hPa pressure levels are sustained westerlies
over most upper air stations in Malaysia and the withdrawal is said to occur when the
average zonal wind from 850 to 600 hPa are sustained easterlies instead.
Large variations in monsoon wind intensities during the southwest monsoon were
observed by Cheang (1988) and Subra. A weakening of monsoon circulation brings
about increased rainfall in Malaysia while the intensification of monsoonal winds is often
associated with drier conditions in Malaysia. A weaker southwest monsoon circulation is
2
known as a break period while a stronger monsoon circulation is known as an active
period.
This study aims to create and verify a unified procedure to identify the onset and
withdrawal dates of the southwest monsoon in Malaysia using a summary of preonset,onset and withdrawal criteria from past studies on the southwest monsoon over
Malaysia, and also to identify the active and break periods during the southwest
monsoon.
3
2.0
Data
The following table summarizes the data used in this study :
Data
Units
Uses
Resolution
Source
1.25° x 1.25° or
approximately
138 km x 138 km
JRA-55
Raingauge
MetMalaysia
principal stations
Upper air station
Wyoming Upper
Air website
Streamline
JRA-55
reanalysis
wind
(zonal &
meridional)
m/s
Rainfall
mm
Upper air wind
(zonal)
m/s
Niño Indices
°C
-
-
Climate Prediction
Centre (CPC)
Occurences of
typhoons and
tropical
storms
-
-
-
JMA
Wind barbs
Index
(Diong, 2015)
Rainfall graph
Index
(Subra)
Synoptic charts as well as index calculation (Diong, 2015) are derived from the
JRA-55 reanalysis. The JRA-55 dataset are provided by the Japan Meteorological
Agency (JMA) and Central Research Institute of Electric Power Industry (CRIEPI).
Rainfall data were extracted from the MetMalaysia webportal (ecuaca). Upper air
zonal wind were downloaded from the online archive of the Wyoming University. The
00z and 12z vertical winds were vector - averaged to calculate the daily wind.
3.0
Method
This study converts the daily timeseries of reanalysis winds and rainfall into
pentads (5 day means) for analysis to expedite the work process and to better observe
intraseasonal changes of the parameters during the southwest monsoon. However, the
calculation of indices were performed using the original daily values.
4
4.0
Observations
The pre-onset, onset and withdrawal synoptic features for the year 2015 are
described in this section.
4.1
Onset
The following table summarizes the key onset features demonstrated during the
2015 southwest monsoon season.
REFERENCES
1.
Diong (2015)
He (2006)
Ding (2001)
REFERENCES
1.
A pair of cyclones (850hPa) on either side of equator
One (1) cyclone over Sri Lanka
ONSET
Sri Lanka cyclonic vortex (850hPa) vanishes
He (2006)
Establishment of northeast-southwest trough parallel to east Indian
coast
Diong (2015)
He (2006)
DATE (2015)
18 Apr – 13
May
An anticyclone (200hPa) over Indochina (Myanmar)
Diong (2015)
Cheang (1988)
2.
PRE-ONSET
Subtropical ridge and High (850hPa) and retreats northeastwards
away from South China Sea to western Pacific Ocean
DATE(2015)
18 May
18 May
Cheang (1988)
3.
Diong (2015)
He (2006)
Subtropical ridge (850hPa) over the Indian subcontinent to Bay of
Bengal is bisected by the northeast-southwest trough along and
parallel to the coast of east India
18 May
4.
Diong (2015)
Cheang (1988)
Northern monsoon trough (850hPa) advances north to southern
Indochina
23 May
5.
Diong (2015)
Ding (2001)
Deep westerlies (850-600hPa) extending from the Bay of Bengal to
Indochina until it reaches the western Pacific Ocean
18 May
6.
Diong (2015)
He (2006)
Ding (2001)
Cheang (1988)
Anticyclone (200hPa) suddenly moves north after remaining quasistationary in Myanmar before the onset.
7.
Diong (2015)
Negative meridional shear (19 consecutive days)
8.
Subra
Easterlies averaged from (850-600hPa) over most upper air
stations
23 May
18 May
13 to 20 May
Based on the onset table the pentad centred on the 18th of May 2015 was
determined to be the date the southwest monsoon begins over Malaysia and the South
China Sea. This falls exactly on the average onset date based on Diong (2015), He
(2006), and Ding (2001). It was also close to the range of typical southwest monsoon
onset dates given by Subra namely from 6 to 15 May 2015.
5
The mosaic below describes points 1 – 4 of the onset criteria table relating to the
low level synoptic scale circulation before, during & after the monsoon onset:
1 Pentad before Onset
Onset Pentad
1 Pentad after Onset
6
Prior to the onset (18th May 2015), a low level cyclone was observed over Sri
Lanka along with another cyclone to the south. They form a cyclone pair straddling the
equator. Strong westerlies were observed in between this cyclone pair. The Sri Lankan
cyclone has been there a month before the onset that is since the 4th pentad of April
(centred on the 18th of April 2015).
One (1) pentad before the onset, the low level subtropical ridge consists of two
components. One component of the ridge extends from the western Pacific to the South
China Sea while the second component forms a continuous line across India and the
Bay of Bengal.
In the period before the southwest monsoon onset, the near equatorial northern
monsoon trough was located to the south of 10°N. It does not reach the Indochina
Peninsular. Meanwhile, easterly-southeasterly winds dominated the South China Sea.
Sharp changes were observed during the onset pentad. At this moment the Sri
Lankan cyclone vanishes. At the same time, a well-formed trough was observed along
the east Indian coastline nearly parallel to the east Indian coast. At this point the
northern subtropical ridge and high (850hPa) has completely retreated from the South
China Sea and is now located in the western Pacific Ocean. In this period the northern
near equatorial trough has shifted abruptly north from the south of the Philippines to the
north of the Philippines with a branch south of Indochina.
One pentad after the onset, the northern near equatorial monsoon trough has
reached the Indochina region.
7
The proceeding mosaic explains the penetration of westerlies before, during and
after the southwest monsoon onset, as stated by point 5 of the onset criteria table:
1 Pentad before Onset
Onset
1 Pentad after Onset
A continuous band of deep westerly winds extending from the 850hPa to 600hPa
pressure levels prevails over the Bay of Bengal, Indochina, and the northwestern Pacific
Ocean during the onset and subsequent periods. Just before the onset the Bay of
Bengal experienced light and variable winds. Red-orange lines depict westerlies while
blue-purple lines show easterlies.
8
The next figure describes the upper level synoptic flow just before, after, and
during the onset itself (point 6 of the onset criteria table):
2 Pentad before Onset
1 Pentad before Onset
Onset
1 Pentad after Onset
Two (2) pentads before the onset, an anticyclone in the Indian Ocean to the
south of India moved to Indochina and remained stationary in Indochina (Myanmar) until
the onset pentad. This anticyclone shifted northward suddenly to southern China
immediately after the onset pentad.
9
4.2
Onset – Index
A southwest index (SWI) developed by Diong (2015) is used to obtain an onset
date. The SWI estimates the low level meridional shear vorticity over the entire
Malaysian-South China Sea region. It is calculated using the difference in 850hPa zonal
wind over the region between the Bay of Bengal, Indochina and the Phillipines (WF1)
and the Malaysian west coast region (PM1). The usage of SWI as an objective indicator
of monsoon onset and withdrawal over the Malaysian region is justified by the fact that it
includes the two major convective centres during the southwest monsoon, namely the
Bay of Bengal and the Phillipines-South China Sea region (region WF1). Furthermore
the climatological SWI has been found to correlate well with the long-term averaged
rainfall over the west coast of Peninsular Malaysia as well as east Malaysia.
The figure below illustrates the regions used in the calculation of SWI:
SWI (meridional shear vorticity) in the Peninsular = PM1 – WF1
SWI (meridional shear vorticity) in East Malaysia = B – WF1
WF1, PM1, and B denotes the area averaged zonal (u-component) wind at the
850hPa pressure level in their respective boxes as shown in the figure above.
10
The next graph shows the SWI index computed from 01 April to 31 October 2015
over the Peninsular Malaysia region:
It can be inferred that the southwest monsoon onset in Malaysia falls on the 18th
of May 2015. This was based on the fact that 19 consecutive days of negative
meridional shear occurred starting from 18th May. This date is also in agreement with
the synoptic conditions required for the monsoon onset as discussed earlier.
Another index was also computed for further verification of the onset date. Subra
developed an index calculated from the zonal wind averaged from the 850hPa to
600hPa pressure level using upper air stations. The surface plot below illustrates
Subra’s index for each upper air stations in Malaysia (white means missing upper air
data).
11
It can be clearly seen that the monsoon onset date falls on the week between the
th
13 and 20th of May 2015 because nearly all upper air stations recorded westerly winds
when averaged between 850 to 600hPa. Hence, the southwest index based on Subra
gives an onset date between the 13th and 20th of May 2015.
Both Diong and Subra did not state the number of days over which the indices
must exceed a certain threshold to define the southwest monsoon onset. This is
because the southwest monsoon onset has strong year-to-year variation. Hence
synoptic features need to be interpreted together with the indices to confirm the proper
onset date.
12
4.3
Withdrawal
This table summarizes the withdrawal features displayed by the 2015 southwest
monsoon.
WITHDRAWAL
DATE
(2015)
1.
Easterlies (850hPa) completely cover Indochina while westerlies
retreat to the Bay of Bengal
13
October
2.
Northern monsoon trough (850hPa) is located near 10°N at the
Indian Ocean.
18
October
3.
Anticyclone (200hPa) was located in Myanmar (Burma) with 2 other
anticyclones near the Phillipines or the western Pacific Ocean.
3-8
October
4.
Positive meridional shear
23
October
Deep easterlies (850-600hPa average) over most upper air stations
21 – 28
October
REFERENCES
Diong (2015)
5.
Subra
These analyses support a withdrawal date of 23rd October for the 2015
Malaysian southwest monsoon.
13
The following figure illustrate the synoptic scale low level flow before, during and
after the withdrawal of the southwest monsoon over the South China Sea (point 1 of the
withdrawal criteria table):
14
Starting from the pentad centred on the 13th of October the Indochina Peninsular
is completely dominated by easterlies. The withdrawal is dated after or on the 13th of
October on the basis that the westerlies have completely left Indochina by that time.
Red regions indicate areas where westerlies prevail while blue regions illustrate
easterlies.
The next figure illustrates the southward progression of the monsoon trough as
stated in point 2 of the withdrawal criteria table:
15
The monsoon troughs over the Indian Ocean has retreated southward to below
10°N over the Indian Ocean after the 18th of October. According to a study by Diong
(2015) on the climatological features of the southwest monsoon, this is another indicator
that the withdrawal of the southwest monsoon falls after or on the 18th of October.
16
The subsequent figure demonstrates the southeastward retreat of the upper level
anticyclone and subtropical ridge as in point 3 of the withdrawal criteria table:
Prior to the withdrawal of the 2015 southwest monsoon, the upper anticyclone
has retreated south to Myanmar on the pentads centred at 03 and 08 October 2015. On
the pentads centred on the 18th and 23rd of October 2015 the anticyclonic vortices on
17
land has vanished. This is in agreement with Diong (2015) concerning the synoptic
conditions at the upper level just after the climatological withdrawal of the southwest
monsoon. The withdrawal month (October, 2015) saw a sudden and significant
southward retreat of the upper subtropical ridge from 20°N in the beginning of the month
to 10°N by the end of the month.
The meridional shear index (Diong, 2015) and average wind (850-600hPa) from
Subra were computed to pin-point the withdrawal date. The following figures illustrate
these indices:
18
The former figure is the meridional shear over the Peninsular Malaysia devised
by Diong. The latter figure is the 850hPa to 600hPa average zonal wind from all upper
air stations as used by Subra.
The meridional shear was found to be consistently positive and steadily
increasing from the 23rd of October onwards. Intense easterlies averaged from 850600hPa were recorded from the period of 21-28 October onwards by most upper air
stations.
Hence, the withdrawal date of the 2015 southwest monsoon over Malaysia and
the South China Sea lies within the pentad centred on the 23rd of October 2015. This
was a late withdrawal date. It was more than 2 weeks later than the average withdrawal
19
date given by Diong (2015). The average withdrawal date falls on the 8th of October as
observed by Diong (2015).
4.4
Rainfall during Active and Break Periods
The following table details the rainfall time series during the period from May to
October 2015 which covers the southwest monsoon over the South China Sea in 2015.
A peak in rainfall was observed from pentads centred on 03 – 13th June 2015
while a low-point (minimum) was seen from the pentads centred on 18th June to the 3rd
of July 2015.
The synoptic winds at the lower and upper atmospheric levels, corresponding to
850hPa and 200hPa are compared and analyzed between the break (13th June) and
active (23rd June) periods in the following diagram.
20
BREAK
ACTIVE
In the break period both low level and upper level circulations represented by the
850 and 200hPa pressure levels exhibit weaker than usual wind speeds. This was
characterized by 2.5 ms-1 southwesterlies at the 850hPa level and northeasterlies
between -5 to -10 ms-1 at the 200hPa levels over the Malaysia – South China Sea
region.
21
At the active period the monsoon circulation is strengthened in the upper and
lower levels. The Malaysia – South China Sea region experienced strong
southwesterlies at the 850hPa pressure level with speed 7.5 ms-1 and intense
northeasterlies at the 200hPa pressure level with speeds of up to 15 ms-1.
This observed lull in typical southwest monsoon synoptic scale wind circulation
occuring in conjunction with enhanced precipitation followed by the intensification of the
southwest monsoon circulation and drier weather in Malaysia was described as a form
of intra-seasonal oscillation by Cheang (1988) and was also noted by Subra.
22
4.5
Rainfall during the Southwest Monsoon
The onset of the southwest monsoon was said to begin from the pentad centred
on the 18th of May and the withdrawal of the southwest monsoon from Malaysia and the
South China Sea was determined to be on the pentad centred on the 23rd of October
2015 based on the analyses performed in this report. This section describes the
monthly rainfall pattern from May to October 2015 which are the months corresponding
to the southwest monsoon over Malaysia and the South China Sea. The rainfall data
were taken from the monthly accumulated rainfall of principal stations maintained by the
Malaysian Meteorological Department.
The Peninsular generally received average rainfall throughout the southwest
monsoon in 2015, while Sarawak and especially Sabah had mostly below normal
rainfall. The abnormally dry southwest monsoon season in Sarawak and Sabah, 2015
can be attributed to an ongoing and intense El Niño episode as shown by the positive
monthly Niño indices that are above 0.5°C from May to October 2015.
The following graph and table describes the average monthly rainfall over the
Peninsular Malaysia, Sarawak and Sabah and the Niño indices from May to October
2015.
May-15
Jun-15
Jul-15
Aug-15
Sep-15
Oct-15
NINO1+2
2.43
2.54
2.87
2.24
2.57
2.52
ANOMALY
NINO3 NINO4
1.66
1.09
2.17
1.09
2.34
1.00
2.63
0.98
2.66
1.04
2.92
1.12
23
NINO3.4
1.03
1.32
1.60
2.07
2.28
2.46
5.0
Typhoons and Tropical Storms during the Southwest Monsoon 2015.
The following table shows the number of tropical disturbances reaching tropical
storm intensity from May to October in 2015.
MONTH
NAME
NUMBER
NOUL
MAY
2
DOLPHIN
KUJIRA
JUNE
2
CHAN-HOM
CHAN-HOM
LINFA
JULY
4
NANGKA
HALOLA
SOUDELOR
MOLAVE
AUGUST
4
GONI
ATSANI
KILO
ETAU
SEPTEMBER
VAMCO
5
KROVANH
DUJUAN
MUJIGAE
CHOI-WAN
OCTOBER
4
KOPPU
CHAMPI
24
The next table compares the 2015 May to October monthly number of tropical
disturbances reaching tropical storm strength to the 1981-2010 average.
MONTH
2015
AVERAGE
MAY
2
1.1
JUN
2
1.7
JUL
4
3.6
AUG
4
5.8
SEP
5
4.9
OCT
4
3.6
TOTAL
21
20.7
The number of tropical disturbances reaching tropical storm strength over the
western Pacific Ocean in 2015 was near average.
25
6.0
Discussion and future work
A systematic procedure to determine the onset of the southwest monsoon over
Malaysia has been developed. This procedure was based on synoptic analyses and two
indices to objectively determine the onset and withdrawal dates of the southwest
monsoon. The southwest monsoon over Malaysia has large interannual variability
according to Subra. Therefore, this study needs to be repeated for at least 30 years or
more to establish the consistency of the criteria on a year to year basis. The indices of
Diong and Subra also needs to be applied and studied for at least 30 years in order to
determine the proper threshold to declare an onset or withdrawal.
Indices that could identify the break and active periods during the southwest
monsoon could also be developed. The following figure was created by Cheang (1988) :
26
The preceeding figure hints at a clear correlation between the zonal winds at
both the 850hPa and 200hPa pressure levels and the total rainfall over Peninsular
Malaysia and East Malaysia. An active and break index based on these parameters can
be identified to quantify and study the active and break periods of 30 years of southwest
monsoon and see their relation to large scale atmospheric cycles such as the ENSO,
IOD, PDO, QBO, MJO and others.
7.0
Acknowledgement
The author would like to thank Diong Jeong Yik for his helpful advice in this work
and the use of his earlier paper as a basis paper. The author is aware that all ideas are
taken from other papers, and this work merely applies those ideas for 2015. Finally, the
author would like to thank all the staff of R&D Section of MetMalaysia for their kind
assistance and support throughout this study.
27
8.0
References
Cheang Boon Kheang, 1988 : A summary of the results of studies on southwest
monsoon in Malaysia conducted during 1972 – 1988 . Technical Note No. 31,
December 1988, Malaysian Meteorological Service.
Ding Yihui and Liu Yanju, 2001 : Onset and the evolution of the Summer Monsoon
over the South China Sea during SCSMEX Field Experiment in 1998. J. Meteor. Soc.
Japan, 79, 255-276.
Diong Jeong Yik, Yip Weng Sang, Mat Kamaruzaman Mat Adam, Nur Salleh K. Chang,
Fariza Yunus, and Muhammad Helmi Abdullah, 2015 : The Definitions of the Southwest
Monsoon Climatological Onset and Withdrawal over Malaysian Region. Research
Publication No. 3, 2015, Malaysian Meteorological Department.
He Jinhai, Wen Min, Wang Lijuan, and Xu Haiming, 2006 : Characteristics of the Onset
of the Asian Summer Monsoon and the Importance of Asian-Australian “Land Bridge”.
Advances in Atmospheric Sciences, Vol 23, No. 6, 951-963.
Subramaniam Moten : Onset of the Asian Summer Monsoon over the Malaysian region.
Malaysian Meteorological Service (Unpublished).
28