American Journal of Environmental Science, 2012, 8 (5), 479-488
ISSN: 1553-345X
©2012 Science Publication
doi:10.3844/ajessp.2012.479.488 Published Online 8 (5) 2012 (http://www.thescipub.com/ajes.toc)
Corresponding Author: Syamsul Rizal, Jurusan Ilmu Kelautan, Universitas Syiah Kuala, Darussalam, Banda Aceh 23111, Indonesia

479 Science Publications

AJES
General Circulation in the Malacca Strait and Andaman Sea: A
Numerical Model Study
1
Syamsul Rizal,
2
Peter Damm,
1
Mulyadi A. Wahid,
2
Jurgen Sundermann,

1
Yopi Ilhamsyah,
3
Taufiq Iskandar and
1
Muhammad

1
Jurusan Ilmu Kelautan, Universitas Syiah Kuala, Darussalam, Banda Aceh 23111, Indonesia
2
Institut fur Meereskunde, Universität Hamburg, Bundesstr. 53, 20146 Hamburg, Germany
3
Jurusan Matematika, Universitas Syiah Kuala, Darussalam, Banda Aceh 23111, Indonesia

Received 2012-05-10, Revised 2012-08-11; Accepted 2012-08-11
ABSTRACT
In the Andaman Sea and Malacca Strait, as in other parts of the Indian Ocean, the seasonal change of the
wind plays a most important role: the south-west (hereafter SW) is monsoon active from June through
September and the north-east (hereafter NE) monsoon is active from December through February. During
the NE monsoon the winds are directed from the north and northeast to the south-west, and during the SW
monsoon from the south-west to the north-east. Strong winds between June and September lead to
maximum rainfall over most parts of the Indian subcontinent. These areas are also greatly influenced by the
tides. The circulation in the Andaman Sea and the Malacca Strait is simulated with a three-dimensional
baroclinic primitive equation model. In order to run the model, the HAMSOM model is used. The model is
forced by tides at the open boundaries as well as by wind and heat flux. We use also the NCEP/NCAR data.
The M2-tide amplitudes are bigger in the shallow areas in the northwest part coast of Andaman Sea and in
the Malacca Strait. The phases of M2 tide shows that the M2 tidal wave come from Indian Ocean and
bifurcates to the Andaman Sea and Malacca Strait. The current ellipses of M2-tide are also stronger in the
shallow areas both in the Andaman Sea and Malacca Strait. There are two types of tidal distribution in the
Andaman Sea and Malacca Strait. In the Indian Ocean part and in the middle of the Malacca Strait, the type
is mixed tide prevailing semi diurnal, while in the Andaman Sea and the southern part of the Malacca Strait
the type is semi diurnal tide. Generally, the general circulation caused by tides, heat flux and wind both for
NE and SW monsoon shows the same pattern. These general circulation patterns, vertical structure of
temperature and salinity in the Malacca Strait are compared with the observations carried out by other
researchers. Based on those comparisons, the results of the model are reasonable. It means, the HAMSOM
model can be used for the simulation of the Andaman Sea and Malacca Strait.

Keywords: Tides, Sea Surface Temperature, Seasonal Circulation, Three-Dimensional Model, Malacca
Strait, Andaman Sea

1. INTRODUCTION
The Andaman Sea is located along the north-
eastern side of the Indian Ocean between the Malay
Peninsula to the east and the Andaman-Nicobar
islands chain to the west. It can be considered, to a
certain extent, as a separate sea. The Andaman and
Nicobar islands on the western side of the Andaman
Sea are volcanic in origin. As a result the water depth
in this region changes rapidly from over 3000-4000
meters in the Indian Ocean to approximately 200
meters in the area around the islands, returning to
deeper than 2500 meters in the centre of the Andaman
Sea (Fig. 1).
In the Andaman Sea, the internal wave is commonly
observed. Internal waves occur within subsurface layers
of marine waters that are stratified because of
temperature and salinity variations. Disturbances created

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within the ocean give rise to these waves, which
represent a significant mechanism for the transport of
momentum and energy within the ocean. Disturbances
are often caused by tidal flow passing over shallow
underwater obstacles such as a sill or a shallow ridge
(Osborne and Burch, 1980; Susanto et al., 2005).
The Malacca Strait is located between the east coast
of Sumatra and the west coast of the Malay Peninsula. Its
southern most part is connected to the Sunda Shelf, i.e.,
the southern part of the South China Sea and the western
Java Sea are adjacent waters. Water depth changes
slightly from approximately 30m in the south to 200m at
the line Lhokseumawe (Indonesia)-Phuket (Thailand).
The shelf edge is located to the north of this borderline
and the water depth increases to more than 1000m in the
transition region to the Andaman Sea (Fig. 1).



Fig. 1. The topography of Andaman Sea and Malacca Strait,
depths are given in meters


(a) (b)

Fig. 2. Wind field during (a) Northeast monsoon season and
(b) Southwest monsoon season
In the Andaman Sea and Malacca Strait, as in other
parts of the Indian Ocean, the seasonal change of the
wind plays a most important role: the south-west
(hereafter SW) is monsoon active from June through
September and the north-east (hereafter NE) monsoon is
active from December through February.
Figure 2 shows the NE and SW monsoon conditions
in the Andaman Sea, derived from long-term (1985-
2003) averages of NCEP/NCAR reanalysis data. During
the NE monsoon the winds are directed from the north
and northeast to the south-west, and during the SW
monsoon from the south-west to the north-east. Strong
winds between June and September lead to maximum
rainfall over most parts of the Indian subcontinent.
This background, together with the complex
topography, makes the Andaman Sea and also the
Malacca Strait a great challenge for hydrodynamic-
numerical modelling. The Andaman Sea and the Malacca
Strait are relatively poorly studied and available data for
these areas are sparse.
The objective of this investigation is to study the
hydrodynamics of the Andaman Sea and Malacca Strait,
with a high resolution simulation of the baroclinic
circulation using the HAMburg Shelf Ocean Model
(HAMSOM). The M2-tide amplitudes, phases, M2-tidal
ellipses and tidal type are presented. The surface
circulation pattern, sea surface salinity and sea surface
temperature are analysed and compared with other
results. In order to validate the model results, the vertical
structure of salinity and temperature in the Malacca
Strait obtained by HAMSOM are also compared with the
observation carried out by Keller and Richards (1967).
2. MATERIALS AND METHODS
The model covers the region 90.5 E to 103.5E and
1.5 N to 17.5 N (Fig. 1). In this investigation, the model
area is discretized with a horizontal mesh size of ∆x = ∆y
= 10 angular minutes. In the vertical direction, the model
has 19 layers, i.e., 0-10, 10-20, 20-30, 30-50, 50-75, 75-
100, 100-125, 125-150, 150-200, 200-250, 250-300, 300-
400, 400-600, 600-800, 800-1000, 1000-1500, 1500-
2000, 2000-4000, and greater than 4000 m.
The time-step is ∆t = 300 s. At the open boundaries,
amplitudes and phases of the five major tidal constituents
(M2, S2, N2, K1, O1) are prescribed from a global tidal
model (Zahel et al., 2000) and T, S from the climatological
data of Levitus (1982). The atmospheric forcing, i.e. winds
and surface heat fluxes are derived from the NCEP/NCAR
reanalysis data (Kalnay et al., 1996).
In order to run the model, we use the HAMSOM
Model. HAMSOM is a three-dimensional baroclinic
primitive equation model. The underlying differential

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equations are as follows, x-component momentum
equation:

2
H V x
u u u u
u v w fv
t x y z
1 p u
A u A F
x z z
∂ ∂ ∂ ∂
+ + + − =
∂ ∂ ∂ ∂
∂ ∂ ∂  
− + ∇ + +
 
ρ ∂ ∂ ∂  


y-component momentum equation:

2
H V y
v v v v
u v w fu
t x y z
1 p v
A u A F
y z z
∂ ∂ ∂ ∂
+ + + + =
∂ ∂ ∂ ∂
∂ ∂ ∂  
− + ∇ + +
 
ρ ∂ ∂ ∂  


The variables are three components of the velocity
u, v, and w, pressure p, density ρ, three space variables,
i.e., x (positive in the east direction), y (positive in the
north direction), z (positive upwards), time t, and
Coriolis acceleration f. The variables AH and Av are the
horizontal and vertical coefficients of turbulent viscosity,
respectively, while FX and FY are the components of the
horizontal exterior forces.
Continuity equation:

u v w
0
x y z
∂ ∂ ∂
+ + =
∂ ∂ ∂


Hydrostatic equation:

p
g
z
∂
= −ρ
∂


where g is the gravity acceleration.
Heat transport equation:

T 2 T
H V T
T T T T
u v w
t x y z
T
K T K S
z z
∂ ∂ ∂ ∂
+ + + =
∂ ∂ ∂ ∂
∂ ∂ 
∇ + +
 
∂ ∂ 


and salt transport equation:

S 2 S
H V S
S S S S
u v w
t x y z
S
K S K S
z z
∂ ∂ ∂ ∂
+ + + =
∂ ∂ ∂ ∂
∂ ∂ 
∇ + +
 
∂ ∂ 


where:
KH and KV = The horizontal and vertical coefficients of
turbulent diffusion, respectively
ST and SS = Sources of heat and salinity, respectively

At the surface, the kinematic boundary condition is
used:

w
t
∂ζ
=
∂


where ζ is the water level height.
The differential equations are integrated over the
vertical extent of the model layer to arrive at differential
equations for the layer-averaged fields of transports (U,
V), temperature T and salinity S. The deduction of the
layer averaged equations of motion can be found in
Pohlmann (1996). These latter equations are transformed
into finite-difference representations on the staggered
Arakawa c-grid (Arakawa and Lamb, 1977).
For the discretization of the time domain a two-time
level scheme is introduced. The prognostic variables ς,
U, V, T, S which enter the implicit algorithm, are defined
at staggered time-levels. In order to eliminate the
stability limitation imposed by the CFL criterion in the
hydrodynamic equations, semi-implicit algorithms for
sea surface height in the horizontal direction and vertical
shear stress in the vertical direction are applied.
In the equations of motion, a stable second-order
approximation is introduced to the Coriolis terms, in
order to avoid linear numerical instability arising from
the forward-in-time approximation (Backhaus, 1985).
3. RESULTS AND DISCUSSION
Figure 3 shows the simulated amplitudes and phases
of the M2-tide in the model region. In the Malacca Strait,
the results agree well with those of Rizal and
Sundermann (1994) and Rizal (2000). There is no real
amphidromic point in the Malacca Strait and Andaman
Sea, because Malacca Strait and Andaman Sea are
located in low latitude, see the explanation in Rizal
(2002). The M2 amplitudes are large in the areas of
shallow continental waters (< 100m) along the west coast
of Myanmar and in the southern part of the Malacca Strait
(Fig. 3a). Its waves propagate from the Indian Ocean to
the Andaman Sea and bifurcate towards the Gulf of
Martaban and the Malacca Strait, indicated by increasing
phase change on the shallow shelf (Fig. 3b).
The M2 current ellipses and their contours of semi
major corresponding axes at the surface are shown in
Fig. 4. Distinct ellipses occur on the shallow shelf in the
north-east of the Andaman Sea. There, the orientation of
the semi major axes is normal to the coastline, i.e., the
strong tidal currents are directed normal to the coastline.

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(a) (b)

Fig. 3. The distribution of M2-tide based on HAMSOM (a) Amplitudes in cm, (b) Phases in degree


(a) (b)

Fig. 4. (a) Current ellipses of M2-tide in cm/s and (b) its Semi Major in cm/s, both figures based on HAMSOM

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Also considerable M2 current ellipses are seen in the
narrow part of the Malacca Strait. The amplitude of these
ellipses decreases toward the Andaman Sea. Their major
axes are perpendicular to the main axis of the Malacca
Strait, and turn slightly north in the north-east corner of the
transition zone from the Malacca Strait to the Andaman Sea
(Fig. 4a). The semi major surface currents are weak (< 5
cm/s) in the deep area of the Andaman Sea and stronger
than 10 cm/s on the shelf (Fig. 4b). The locations of large
current gradient contours are due to the shelf edge along the
north-east Andaman Sea, the entry to the Malacca Strait and
around the shallow off shore area north of Sumatra.


(a) (b)

Fig. 5. Type of tidal distribution in Andaman Sea and Malacca Strait based on HAMSOM, (a) ratio of (K1 + O1)/(M2 + S2), (b)
Tidal types distribution


(a) (b)

Fig. 6. The surface currents caused by tides, wind and heat flux derived from long-term (1985-2003) (a) December through
February average (NE Monsoon) and (b) June through September average (SW Monsoon) based on HAMSOM. Contour
values mean magnitude of velocity in cm/s

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The tidal type is defined by the amplitude ratio of
the major diurnal to semidiurnal constituents:

1 1
2 2
K O
F
M S
+
=
+


Two major types are found in the Andaman Sea as
well as in the Malacca Strait, (1) 0 - 0.25: Semidiurnal
tide. Two high waters and two low waters daily, of
almost equal amplitude, (2) 0.25-1.5: Mixed tide,
prevailing semidiurnal. Daily, two high waters and two
low waters, but divergent in high and high water time.
The results of the hydrodynamic-numerical
simulation with HAMSOM reveal a tidal type of less than
0.25 for the whole Andaman Sea, i.e., semidiurnal tidal
conditions with two high and two low waters daily of
almost equally high (Fig. 5a and b).


(a) (b)

Fig. 7. Surface circulation pattern in the Bay of Bengal and Andaman Sea during (a) NE monsoon and (b) SW monsoon,
reproduced after Varkey et al. (1996)


(a) (b)

Fig. 8. The climatology surface currents caused by tides, wind and heat flux derived from long-term (1985-2003) (a) February and
(b) August average based on HAMSOM. Contour values mean magnitude of velocity in cm/s

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These results agree well with those of Wyrtki (1961)
except in the middle of the Malacca Strait, where Wyrtki
also observed semidiurnal tides. However, the tidal types
in Belawan and Kuala Tanjung (see station 1 and 2 in Fig.
5b) have values 0.27 and 0.42, respectively, after
Dishidros (2002).
Surface currents due to tides, wind and heat flux
are shown in Fig. 6. Figure 6a represents the long-
term (1985-2003) December through February (NE
monsoon) average, and Fig. 6b the long-term (1985-
2003) June through September (SW monsoon)
average. During the NE monsoon, surface water
masses from the north of domain enter the Andaman
Sea. These water masses move to the south and leave
the Andaman Sea in the wide area between south of
the Andaman Islands and Sumatra to the Indian Ocean.
A second surface water masses enter the Andaman Sea
from the north-east side of the Malacca Strait. It spreads
to the borderline between Thailand and Myanmar. An
anticlockwise gyre is located north of Sumatra, in the
Malacca Strait. This gyre blocks the outflow of the
Malacca Strait over a part of its whole breadth.
During the SW monsoon, the surface water masses
in the north enter the Andaman Sea over a long section
from Cape Negrais to the north of the Nicobar Islands.
The outflow of the Andaman Sea surface water is
concentrated between the south of the Nicobar Islands
and Sumatra. The local anticlockwise gyre north of
Sumatra vanishes. A recirculation regime is generated,
with water masses coming from the Andaman Sea
recirculating along the north coast of Sumatra to the
Indian Ocean. The second entry of surface water masses
into the Andaman Sea still occurs. However, it is closer
to the coast of Malay Peninsula and flows towards the
island Phuket.
In the Malacca Strait, the surface flow is always
directed north-westward towards the Andaman Sea, for
both SW and NE monsoon situations, since the sea
surface elevation in the south-east domain (South China
Sea) is always higher than in the Andaman Sea during
both SW and NE monsoons (Wyrtki, 1961). For the NE
monsoon, a water mass with high salinity from the South
China Sea flows into the Malacca Strait. The SW
monsoon drives a water mass with low salinity from the
Java Sea into the Malacca Strait.
In general, circulation patterns shown in Fig. 6
are not similar to that of Varkey et al. (1996). They
found that there is one main gyre for each season in
the centre of the Andaman Sea, i.e. a clockwise gyre
during the NE monsoon and an anticlockwise gyre
during the SW monsoon, see Fig. 7. However, the
investigation of Varkey et al. is mainly focused on the
Bay of Bengal.
The climatological surface circulations for the
months February and August obtained by averaging over
the period 1985-2003 are shown in Fig. 8. These are
similar to our results of seasonal circulation patterns and
also to the work of Wyrtki (1961). In particular, in his
report no gyre in the centre of the Andaman Sea is
documented (Fig. 9).


(a) (b)

Fig. 9. Surface currents during (a) February and (b) August, according to Wyrtki (1961)

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(a) (b)

Fig. 10. SSS based on HAMSOM during (a) NE Monsoon (b) SW Monsoon


(a) (b)

Fig. 11. SST based on HAMSOM in (a) February (b) August 1996


(a) (b)

Fig. 12. SST in (a) February and (b) August 1996 according to UKMO

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Fig. 13. The salinity cross section in April 1961 in the Malacca Strait as show in inset for the depth of 0-120 m based on HAMSOM



Fig. 14. The temperature cross section in April 1961 in the Malacca Strait as shown in inset for the depth of 0 to 120 m based on
HAMSOM

Sea surface salinity for the NE and SW monsoons
obtained by averaging over the period 1985-2003 for the
months December through February and June through
September, respectively, is shown in Fig. 10. In the Gulf
of Martaban and in the south-western part of the Malacca
Strait, the sea surface salinity is extremely low during the
SW monsoon. As has been stated above, strong winds
from the SW lead to maximum rainfall over most parts
of the Indian subcontinent from June to September with
corresponding maximum river runoff to the Gulf of
Martaban, whereas NE monsoon winds during
December-February bring heavy rainfall only to south-
eastern India (Ramage, 1971; Unger et al., 2003). The
low salinity in the south-western part of the Malacca
Strait is caused by inflow of a water mass from the Java
Sea, which is driven by the SW monsoon.
Figure 11 shows simulated SST distributions during
February 1996 and August 1996, representing NE and
SW monsoons, respectively. Maximum temperatures for
both cases occur in the Malacca Strait and the Indian
Ocean near Sumatra Island. The same distribution can be
found in Fig. 12 which shows the distribution of SST
according to United Kingdom Meteorological Office
(UKMO), see
http://badc.nerc.ac.uk/data/hadisst/file_format.html#sst.
Another validation of the model is provided by
comparison with observations in April 1961 made by
Keller and Richards (1967). Figure 13 and 14 can
reproduce the same results as has been observed by
Keller and Richards (1967). Figure 13 shows a salinity
cross section in April 1961 in the Malacca Strait
produced by HAMSOM. It is seen that only in the south-
eastern part of the Malacca Strait, the salinity is well
mixed. In other areas, salinity ranges from 31.5 at the
surface and 34.5 at the bottom. This pattern agrees well
with that of Keller and Richards (1967).

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Figure 14 shows the same cross section of model
results as mentioned above, but now for vertical
temperature distribution in April 1961. The temperature
pattern also exhibits a vertical gradient with values of
29°C at the surface and 19°C at the bottom. This pattern
is also very similar with the observations in April 1961
which were carried out by Keller and Richards (1967).
4. CONCLUSION
From the results described above, it can be
concluded that the semidiurnal tides, especially the M2-
tide, are very dominant in the domain of investigation.
The M2 current magnitude is very large northwest part
of Andaman Sea and in the Malacca Strait. Surface
current distributions in the Andaman Sea and the
northern part of the Malacca Strait change seasonally,
depending on the two different monsoon situations. The
good agreement of the general circulation pattern from
Wyrtki (1961) with the simulation results confirms
Wyrtki’s work. The high resolution of the model reveals
more details of the current distribution and therefore a
deeper understanding of the current dynamic and its
water mass transport is possible. This demonstrates the
importance of using hydrodynamic-numerical models for
investigations concerning water mass processes in the ocean
as well as on the shelf. Salinity and temperature cross
sections are in good agreement with those observed by
Keller and Richards (1967). There are vertical gradients of
salinity and temperature in the Malacca Strait with surface
values of 31.5 and 29°C for salinity and temperature,
respectively and bottom values of 34 and 19°C.
5. ACKNOWLEDGEMENT
The researchers would like to thank Dr. Mutiara R.
Putri of Institut Teknologi Bandung, Indonesia, Dagmar
Hainbucher, Kieran O’Driscoll, Udo Hubner and Dr.
Thomas Pohlmann of Institut fur Meereskunde der
Universität Hamburg, Germany, for fruitful discussions.
This research is funded by German Federal Ministry of
Education and Research (BMBF, IDN 01/007) and
Ministry of National Education, Republic of Indonesia
(Contract: 096/H11-P2T/A.01/2009/Riset Unggulan
Strategi Nasional).
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