Our study provides new information on the physical characteristics of watermass in the South China Sea. We analyzed the temperature, salinity and density profiles to determine the effect of the NE monsoon on the variability of the physical properties of watermass, in the Gulf of Thailand and the east coast of Peninsular Malaysia. CTD data were obtained from both the M.V. SEAFDEC cruises conducted before (September 1995) and after (April 1996) the northeast (NE) monsoon season. We concluded that the NE monsoon caused the variability of the physical properties of watermasses, in the study area, slightly. We observed the movement of the thermocline, halocline and pycnocline layers from deeper depth to shallower depth, before and after the NE monsoon season, respectively. This movement indicates the possible occurrence of downwelling and upwelling processes in the region

Rojana-anawat, Penjan

Saadon, Mohd. Nasir

Snidvongs, Anond

Aquatic Commons

S1/OG1<SAADON>
-1-
Physical Characteristics of Watermass in the South China Sea, Area I:
Gulf of Thailand and East Coast of Peninsular Malaysia
Mohd. Nasir Saadon1
Penjan Rojana-anawat2
Anond Snidvongs3
1 Universiti Putra Malaysia Terengganu, Mengabang Telipot, 21030 Kuala Terengganu, Malaysia
2 Training Department, P.O. Box 97, Phrasamutchedi, Samut Prakan 10290, Thailand
3 Department of Marine Science, Faculty of Science, Chulalongkorn University,
Bangkok 10330, Thailand
ABSTRACT
Our study provides new information on the physical characteristics of watermass in the South
China Sea.  We analyzed the temperature, salinity and density profiles to determine the effect of the
NE monsoon on the variability of the physical properties of watermass, in the Gulf of Thailand and
the east coast of Peninsular Malaysia.  CTD data were obtained from both the M.V. SEAFDEC cruises
conducted before (September 1995) and after (April 1996) the northeast (NE) monsoon season.
We concluded that the NE monsoon caused the variability of the physical properties of
watermasses, in the study area, slightly.  We observed the movement of the thermocline, halocline
and pycnocline layers from deeper depth to shallower depth, before and after the NE monsoon sea-
son, respectively.  This movement indicates the possible occurrence of downwelling and upwelling
processes in the region.
Key words:  Thermocline, halocline, pycnocline, downwelling and upwelling processes.
Introduction
The climate of the east of Peninsular Malaysia is controlled by the seasonal monsoon winds.
These winds are generated by the difference in atmospheric pressure between the northern (Asian
continent) and the southern (Australia) hemispheres [Nasir and Camerlengo (1997), Nasir and
Marghany (1996)].  During the northern winter (summer), the northeast (southwest) wind prevails
from November to March (May to September) over the South China Sea.
Between these two seasonal monsoon winds, two transitional periods are clearly distinguish-
able.  They last for about four to seven weeks in April and October [Morgan and Valencia (1983)].
The first and second cruises - on board M.V. SEAFDEC - of the SEAFDEC collaborative
research program in the South China Sea, between Malaysia and Thailand, were conducted from 5 to
28 September 1995 (before northeast monsoon season) and from 24 April to 17 May 1996 (after the
NE monsoon season).  The cruises started in the northern coast of the Gulf of Thailand and ended in
the sourhern coast of Johore, Peninsular Malaysia.  The objective of the cruises was to do a compre-
hensive survey of the South China Sea in all fields of oceanography.
Since these two cruises were conducted before and after the NE monsoon season, a study on
the seasonal variations of the physical characteristics of watermass in the Gulf of Thailand and the
east coast of Peninsular Malaysia was conducted.  For this purpose, the temperature, the salinity and
the density profiles, from both M.V. SEAFDEC cruises, were analyzed and compared.
Our results show that there are slight variations in temperature, salinity and density profiles
before and after the NE monsoon season.  Furthermore, salinity and density (temperature) values are
slightly higher (lower) before than those after the NE monsoon season.  These results agree with
earlier findings [Nasir and Camerlengo (1997) and Nasir et al. (1997)].
S1/OG1<SAADON>
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Materials and Methods
Temperature and salinity data were collected, using CTD, in the Gulf of Thailand and the east
coast of Peninsular Malaysia, before and after the NE monsoon season.  The data were gathered from
81 sampling stations (Fig. 1), during the M.V. SEAFDEC first cruise (5 to 28 September 1995) and
second cruise (24 April to 17 May 1996).  For this investigation, only data from selected sampling
stations, were analyzed.
Density data was derived from salinity and temperature data using sigma-t computation tables
[Knauss (1987)].  The temperature, the salinity and the density profiles, before and after the NE
monsoon season, were plotted, compared and analyzed.
Results
Profiles of temperature, salinity and density are shown in the Appendix, respectively, for both
before (thin lines) and after (thick lines) the NE monsoon season.  Each figure is divided into two
groups; A and B.  Group A and B represent data collected from the Gulf of Thailand and the east coast
of Peninsular Malaysia, respectively.  The sampling station numbers are shown in the bottom right
corner of each graphs.
Fig. 1.  Location of 81 sampling station during both of the M.V. SEAFDEC cruises.
1 2 3 4
56789
10 11 12
13141516
17 18 19 20
212223
24 25 26
27282930
31 32 33 34
3536373839
40 41 42 43 44 45 46
4748495051
52 53 54 55 56 57
58596061
62 63
64 65 66 67
686970
71 72 73
747576
77 78
7980
81
99 100 101 102 103 104 105 106 107
Longitude
1
2
3
4
5
6
7
8
9
10
11
12
13
14
La
tit
ud
e
THAILAND
GULF OF
THAILAND
SOUTH
CHINA
SEA
MALAYSIA
L
at
ti
tu
d
e
S1/OG1<SAADON>
-3-
Temberature profiles
Surface water temperatures, at all stations, are slightly higher (about 1oC) after as compared to
before the NE monsoon season, except for stations in the southern tips of Peninsular Malaysia.  The
difference between surface water temperatures, after and before the NE monsoon season, decreases
from the north to the south of the study area.
However, bottom water temperatures, in the Gulf of Thailand and northern region of the east
coast of Peninsular Malaysia, are slightly lower after (about 0.5oC )as compared to before the NE
monsoon season.  In other parts of the east coast of Peninsular Malaysia, bottom water temperatures
are mostly higher after than before the NE monsoon season.
In the Gulf of Thailand, the thermocline layers are more distinct after the NE monsoon season.
Some stations (Stations 32, 34 and 39) have two thermocline layers.  Before the NE monssoon sea-
son, many stations, in the Gulf of Thailand, have no thermocline layer which indicates a very well-
mixed situation.
In the east coast of Peninsular Malaysia, thermocline layers are present both before and after
the NE monsoon season.  However, before the NE monsoon season, thermocline layers are mainly
found at deeper depth as compared to after the NE monsoon season.
Salinity profiles
In the Gulf of Thailand, surface salinity are higher (about 1 psu) before as compare to after the
NE monsoon.  Stations in the northern half of the Gulf have high salinity, in the water column, before
the NE monsoon.  Halocline layers are present at some stations both before and after the NE mon-
soon.  Halocline layers, before (after) the NE monsoon season, are generally found in deeper (shal-
lower) depth.
In the east coast of Peninsular Malaysia, halocline layers are present both before and after the
NE monsoon season.  These layers are observed at deeper depth before the NE monsoon season.
Density profiles
Surface water densities are generally higher before as compared to after the NE monsoon sea-
son, except for areas in the southern part of the east coast of Peninsular Malaysia.  Pcynocline layers
are present in most sampling stations, both before and after the NE monsoon season.  These layers are
found at deeper depth before as compare to after the NE monsoon season.
In the northern region of the Gulf of Thailand (stations 6-20), water densities in the water
column are higher before as compared to after the NE monsoon season.  However, in the sourhern
part of the east coast of Peninsular Malaysia (stations 77-81), water densities
in the water column are lower before as compared to after the NE monsoon season.
Discussions
Air mass descends over the cold Asian continent during winter enhancing the formation of a
high atmospheric pressure system.  At the same time, air mass rises over the warm Australian conti-
nent.  A low atmospheric pressure system is formed.  These differences in atmospheric pressure
system in unison with Coriolis effect, generate a NE wind in the Gulf of Thailand and along the east
coast of Peninsular Malaysia (from November to March).  This period is generally known as the NE
monsoon season.
In the northern summer, in the Asian continent, the reciprocal is true.  A high and low atmo-
spheric pressure system over Australia and the Asian continents, respectively, are enhanced.  As a
consequence of this a SW wind prevails over the Gulf of Thailand and Peninsular Malaysia (from
S1/OG1<SAADON>
-4-
May to September).  This period is known as the SW monsoon season.
Two transitional periods between the two monsoon seasons are distinguishable.  These periods
happen in April and October.  They tend to last for about four to seven weeks [Nasir and Marghany
(1996)].
The NE monsoon season brings heavy rain at the east coast of Peninsular Malaysia.  Maximum
precipitation, with values ranging from 600 to 800 mm of rainfall, is recorded in November and
December [Camerlengo et al. (1996a), Camerlengo et al. (1997)].  Minimum precipitation is ob-
served during the SW monsoon, especially in July and August [Camerlengo et al. (1996b)].
The NE monsoon season (especially in November and December) causes a major decrease of
both evaporation and insolation values along the east coast of Peninsular Malaysia [Camerlengo et al.
(1996c)].  Overcast skies prevent solar radiation into the lower atmosphere.  However, highest inso-
lation value is registered in Febreuary and March at the  east coast of Peninsular Malaysia.
The NE monsoon wind is stronger than the SW monsoon wind [Taira et al. (1996)].  This is due
to the fact that the NE monsoon wind is in the same direction as the prevailing NE trade wind of the
northern hemisphere.  During the NE and SW monsoon seasons, average wind speed of 9 m/s and 6
m/s, respectively, have been recorded [Wang et al. (1994)].
Wave heights along the east coast of Peninsular Malaysia are hihger during the NE monsoon
season as compared to the SW monsoon season.  Wave heights of over 3.5 m during November to
January, have been recorded [Malaysian Meteorological Service (1991)]. Larger wave heights, dur-
ing the NE monsoon season, are due to the stronger wind speed and larger wind fetch as compared to
the wind during the SW monsoon season.
Overcast skies, during the NE monsoon season, block the incoming solar radiation.  This re-
duces the sea surface temperature (SST).  However, the SST increases, as the sky clears up, after the
NE monsoon season is over.
Heavy rainfalls, during the NE monsoon season, increase freshwater runoff in the Gulf of
Thailand and along the east coast of Peninsular Malaysia [Nasir et al. (1997)], thus, reducing surface
water salinity and density values at all sampling station.  In the northern part of the Gulf of Thailand,
freshwater runoff even makes the water column less saline and less dense.  In most deep offshore
stations - again due to freshwater runoff - higher salinity and density values in the bottom water have
been recorded, after the NE monsoon season.
The halocline and the pycnocline layers are present, at most stations, both before and after the
NE monsoon season.  However,  these layers are at deeper depth before as compared to after the NE
monsoon season.  This is due to the fact that wave heights are higher, just before the NE monsoon
season.  This enhances mixing throughout the vertical column.
The movement of the thermocline, halocline and pcynocline layers from deeper depth to shal-
lower depth, before and after the NE monsoon season, respectively, could possibly be due to up-
welling and downwelling processes.  Usually, during downwelling, these layers move to a deeper
depth.  However, they will move to a shallower depth during upwelling [Kennish (1994)].
The southern tip of the east coast of Peninsular Malaysia (Stations 77-81), shows a different
variation of physical properties as to the northern part of the study area.  We don’t know what causes
this variation.  Perhaps a further investigation is needed in this particular part of the study area.
Conclusions
The vertical profiles of temperature, salinity and density in the Gulf of Thailand and along the
east coast of Peninsular Malaysia, before and after the NE monsoon season, were analyzed.  Our
results generally agree with early observations [Nasir and Camerlengo (1997)].
Our results show that the variability of the physical properties, in the Gulf of Thailand and
along the east coast of Peninsular Malaysia, is due to the monsoon season.  Profiles of temperature,
salinity and density show slight different in these physical values, before and after the NE monsoon
season, are recorded.  In some stations (especially in the northern Gulf of Thailand), temperature,
S1/OG1<SAADON>
-5-
salinity and density values are the same throughout the water column, indicating a well-mixed condi-
tion, before the onset of the NE monsoon.
Thermocline, halocline and pycnocline layers are present, both before and after the NE mon-
soon season.  However, these layers are at deeper depth before the onset of the NE monsoon and
move to a shallower depth after the NE monsoon.  This movement of physical layers, from deeper to
shallower depth, can be linked to the downwelling and upwelling processes in the region.
In southern part of the east coast of Peninsular Malaysia (particularly the southern tip), the
variability of the physical properties of the watermass is difference as compared to the rest of the
study area.  We do not have enough data to explain this difference.
We feel that more physical oceanographic cruises, to study the variability of the physical prop-
erties during both SW and NE monsoon seasons, are needed.  The physical oceanographic data in the
South China Sea (especially in the east coast of Peninsular Malaysia), are too scarce to make a good
comparison of the variability of physical properties during both monsoon seasons.
Acknowledgment
The authors would like to extend their thanks to Universiti Putra Malaysia Terengganu for
cooperating in this research.
References
Camerlengo, A.L., M.O. Rosni and M.S. Nasir (1996a).  Monthly distribution of precipitation in Peninsular
Malaysia.  Submitted for publication to the Pertanika J. of Sci. & Tech.
Camerlengo, A.L., M.R. Hisham and M.S. Nasir (1996b).  On the passage of the intertropical convergence zone
in Peninsular Malaysia.  Malaysian Journal of Physics, 17(4).
Camerlengo, A.L., M.S. Nasir, M.O. Rosni and M.R. Hisham (1996c).  On evaporation and insolation in Penin-
sular Malaysia. J. of Phys. Sci., Vol. 8.
Camerlengo, A.L., M.S. Nasir and B.M. Ismail (1997).  Precipitation-evaporation difference in Peninsular
Malaysia.  Accepted for publication to ASEAN J. of Sci. & Tech. for Development.
Kennish, M.J. (1994).  Practical Handbook of Marine Science.  CRC Press, London, p. 566.
Knauss J.A. (1987).  Introduction to Physical Oceanography.  Prentice-Hall, New Jersey, p. 338.
Malaysian Meteorological Services (1991).  Monthly summary of marine meteorological observation - Year
1991.  Issued under the authority of the Director General, Malaysian Meteorological Service, P. Jaya,
Malaysia.
Morgan, J.R. and M.J. Valencia (1983).  The natural environmental setting.  In: Atlas for marine policy in
Southeast Asian Seas. J.R. Morgan and M.J. Valencia (eds.).  Univ. of California Press. pp. 4-17.
Nasir, M.S., A.L. Camerlengo and A. Supirman (1997).  Salt intrusion in the Terengganu. J. of Phys. Sci., Vol. 8.
Nasir, M.S. and A.L. Camerlengo (1997).  Response of the ocean mixed layer, off the east coast of Peninsular
Malaysia, during the northeast and southwest monsoons.  Geoacta, 22, 134-143.
Nasir, M.S., A.L. Camerlengo and F.A.A. Hasan (1997).  Seasonal variations of physical properties in the east
coast of Peninsular Malaysia. J. of Science, Vol. 6, 1.
Nasir, M.S. and M.M. Marghany (1996).  Surface circulation off Kuala Terengganu in the transitional period
between the northeast and southwest monsoons.  Pertanika J. Sci. & Technol. 4(1), 141-148.
Taira, K., M.S. Nasir, S. Kitagawa and T. Yanagi (1996).  J. of Oceanography. Vol. 52, 251-257.
Wang, W., Q. Huang, Y.S. Li and Z.W. Li (1994).  Three-dimensional numerical modelling of the water circula-
tion in South China Sea.  In:  Oceanology of China Sea, Vol. 1.  D. Zhou, Y.B. Lian and C.K. Zeng
(eds.).  Kluwer Academic Publishers, p. 344.


Physical characteristics of watermass in the South China Sea, Area I: Gulf of Thailand and east coast of Peninsular Malaysia

Our study provides new information on the physical characteristics of watermass in the South China Sea. We analyzed the temperature, salinity and density profiles to determine the effect of the NE monsoon on the variability of the physical properties of watermass, in the Gulf of Thailand and the east coast of Peninsular Malaysia. CTD data were obtained from both the M.V. SEAFDEC cruises conducted before (September 1995) and after (April 1996) the northeast (NE) monsoon season.
We concluded that the NE monsoon caused the variability of the physical properties of watermasses, in the study area, slightly. We observed the movement of the thermocline, halocline and pycnocline layers from deeper depth to shallower depth, before and after the NE monsoon season, respectively. This movement indicates the possible occurrence of downwelling and upwelling processes in the region

Southeast Asian Fisheries Development Center, Training Department Institutional Repository

S1/OG1<SAADON>-1-Physical Characteristics of Watermass in the South China Sea, Area I:Gulf of Thailand and East Coast of Peninsular MalaysiaMohd. Nasir Saadon1Penjan Rojana-anawat2Anond Snidvongs31 Universiti Putra Malaysia Terengganu, Mengabang Telipot, 21030 Kuala Terengganu, Malaysia2 Training Department, P.O. Box 97, Phrasamutchedi, Samut Prakan 10290, Thailand3 Department of Marine Science, Faculty of Science, Chulalongkorn University,Bangkok 10330, ThailandABSTRACTOur study provides new information on the physical characteristics of watermass in the SouthChina Sea.  We analyzed the temperature, salinity and density profiles to determine the effect of theNE monsoon on the variability of the physical properties of watermass, in the Gulf of Thailand andthe east coast of Peninsular Malaysia.  CTD data were obtained from both the M.V. SEAFDEC cruisesconducted before (September 1995) and after (April 1996) the northeast (NE) monsoon season.We concluded that the NE monsoon caused the variability of the physical properties ofwatermasses, in the study area, slightly.  We observed the movement of the thermocline, haloclineand pycnocline layers from deeper depth to shallower depth, before and after the NE monsoon sea-son, respectively.  This movement indicates the possible occurrence of downwelling and upwellingprocesses in the region.Key words:  Thermocline, halocline, pycnocline, downwelling and upwelling processes.IntroductionThe climate of the east of Peninsular Malaysia is controlled by the seasonal monsoon winds.These winds are generated by the difference in atmospheric pressure between the northern (Asiancontinent) and the southern (Australia) hemispheres [Nasir and Camerlengo (1997), Nasir andMarghany (1996)].  During the northern winter (summer), the northeast (southwest) wind prevailsfrom November to March (May to September) over the South China Sea.Between these two seasonal monsoon winds, two transitional periods are clearly distinguish-able.  They last for about four to seven weeks in April and October [Morgan and Valencia (1983)].The first and second cruises - on board M.V. SEAFDEC - of the SEAFDEC collaborativeresearch program in the South China Sea, between Malaysia and Thailand, were conducted from 5 to28 September 1995 (before northeast monsoon season) and from 24 April to 17 May 1996 (after theNE monsoon season).  The cruises started in the northern coast of the Gulf of Thailand and ended inthe sourhern coast of Johore, Peninsular Malaysia.  The objective of the cruises was to do a compre-hensive survey of the South China Sea in all fields of oceanography.Since these two cruises were conducted before and after the NE monsoon season, a study onthe seasonal variations of the physical characteristics of watermass in the Gulf of Thailand and theeast coast of Peninsular Malaysia was conducted.  For this purpose, the temperature, the salinity andthe density profiles, from both M.V. SEAFDEC cruises, were analyzed and compared.Our results show that there are slight variations in temperature, salinity and density profilesbefore and after the NE monsoon season.  Furthermore, salinity and density (temperature) values areslightly higher (lower) before than those after the NE monsoon season.  These results agree withearlier findings [Nasir and Camerlengo (1997) and Nasir et al. (1997)].S1/OG1<SAADON>-2-Materials and MethodsTemperature and salinity data were collected, using CTD, in the Gulf of Thailand and the eastcoast of Peninsular Malaysia, before and after the NE monsoon season.  The data were gathered from81 sampling stations (Fig. 1), during the M.V. SEAFDEC first cruise (5 to 28 September 1995) andsecond cruise (24 April to 17 May 1996).  For this investigation, only data from selected samplingstations, were analyzed.Density data was derived from salinity and temperature data using sigma-t computation tables[Knauss (1987)].  The temperature, the salinity and the density profiles, before and after the NEmonsoon season, were plotted, compared and analyzed.ResultsProfiles of temperature, salinity and density are shown in the Appendix, respectively, for bothbefore (thin lines) and after (thick lines) the NE monsoon season.  Each figure is divided into twogroups; A and B.  Group A and B represent data collected from the Gulf of Thailand and the east coastof Peninsular Malaysia, respectively.  The sampling station numbers are shown in the bottom rightcorner of each graphs.Fig. 1.  Location of 81 sampling station during both of the M.V. SEAFDEC cruises.1 2 3 45678910 11 121314151617 18 19 2021222324 25 262728293031 32 33 34353637383940 41 42 43 44 45 46474849505152 53 54 55 56 575859606162 6364 65 66 6768697071 72 7374757677 7879808199 100 101 102 103 104 105 106 107Longitude1234567891011121314LatitudeTHAILANDGULF OFTHAILANDSOUTHCHINASEAMALAYSIALattitudeS1/OG1<SAADON>-3-Temberature profilesSurface water temperatures, at all stations, are slightly higher (about 1oC) after as compared tobefore the NE monsoon season, except for stations in the southern tips of Peninsular Malaysia.  Thedifference between surface water temperatures, after and before the NE monsoon season, decreasesfrom the north to the south of the study area.However, bottom water temperatures, in the Gulf of Thailand and northern region of the eastcoast of Peninsular Malaysia, are slightly lower after (about 0.5oC )as compared to before the NEmonsoon season.  In other parts of the east coast of Peninsular Malaysia, bottom water temperaturesare mostly higher after than before the NE monsoon season.In the Gulf of Thailand, the thermocline layers are more distinct after the NE monsoon season.Some stations (Stations 32, 34 and 39) have two thermocline layers.  Before the NE monssoon sea-son, many stations, in the Gulf of Thailand, have no thermocline layer which indicates a very well-mixed situation.In the east coast of Peninsular Malaysia, thermocline layers are present both before and afterthe NE monsoon season.  However, before the NE monsoon season, thermocline layers are mainlyfound at deeper depth as compared to after the NE monsoon season.Salinity profilesIn the Gulf of Thailand, surface salinity are higher (about 1 psu) before as compare to after theNE monsoon.  Stations in the northern half of the Gulf have high salinity, in the water column, beforethe NE monsoon.  Halocline layers are present at some stations both before and after the NE mon-soon.  Halocline layers, before (after) the NE monsoon season, are generally found in deeper (shal-lower) depth.In the east coast of Peninsular Malaysia, halocline layers are present both before and after theNE monsoon season.  These layers are observed at deeper depth before the NE monsoon season.Density profilesSurface water densities are generally higher before as compared to after the NE monsoon sea-son, except for areas in the southern part of the east coast of Peninsular Malaysia.  Pcynocline layersare present in most sampling stations, both before and after the NE monsoon season.  These layers arefound at deeper depth before as compare to after the NE monsoon season.In the northern region of the Gulf of Thailand (stations 6-20), water densities in the watercolumn are higher before as compared to after the NE monsoon season.  However, in the sourhernpart of the east coast of Peninsular Malaysia (stations 77-81), water densitiesin the water column are lower before as compared to after the NE monsoon season.DiscussionsAir mass descends over the cold Asian continent during winter enhancing the formation of ahigh atmospheric pressure system.  At the same time, air mass rises over the warm Australian conti-nent.  A low atmospheric pressure system is formed.  These differences in atmospheric pressuresystem in unison with Coriolis effect, generate a NE wind in the Gulf of Thailand and along the eastcoast of Peninsular Malaysia (from November to March).  This period is generally known as the NEmonsoon season.In the northern summer, in the Asian continent, the reciprocal is true.  A high and low atmo-spheric pressure system over Australia and the Asian continents, respectively, are enhanced.  As aconsequence of this a SW wind prevails over the Gulf of Thailand and Peninsular Malaysia (fromS1/OG1<SAADON>-4-May to September).  This period is known as the SW monsoon season.Two transitional periods between the two monsoon seasons are distinguishable.  These periodshappen in April and October.  They tend to last for about four to seven weeks [Nasir and Marghany(1996)].The NE monsoon season brings heavy rain at the east coast of Peninsular Malaysia.  Maximumprecipitation, with values ranging from 600 to 800 mm of rainfall, is recorded in November andDecember [Camerlengo et al. (1996a), Camerlengo et al. (1997)].  Minimum precipitation is ob-served during the SW monsoon, especially in July and August [Camerlengo et al. (1996b)].The NE monsoon season (especially in November and December) causes a major decrease ofboth evaporation and insolation values along the east coast of Peninsular Malaysia [Camerlengo et al.(1996c)].  Overcast skies prevent solar radiation into the lower atmosphere.  However, highest inso-lation value is registered in Febreuary and March at the  east coast of Peninsular Malaysia.The NE monsoon wind is stronger than the SW monsoon wind [Taira et al. (1996)].  This is dueto the fact that the NE monsoon wind is in the same direction as the prevailing NE trade wind of thenorthern hemisphere.  During the NE and SW monsoon seasons, average wind speed of 9 m/s and 6m/s, respectively, have been recorded [Wang et al. (1994)].Wave heights along the east coast of Peninsular Malaysia are hihger during the NE monsoonseason as compared to the SW monsoon season.  Wave heights of over 3.5 m during November toJanuary, have been recorded [Malaysian Meteorological Service (1991)]. Larger wave heights, dur-ing the NE monsoon season, are due to the stronger wind speed and larger wind fetch as compared tothe wind during the SW monsoon season.Overcast skies, during the NE monsoon season, block the incoming solar radiation.  This re-duces the sea surface temperature (SST).  However, the SST increases, as the sky clears up, after theNE monsoon season is over.Heavy rainfalls, during the NE monsoon season, increase freshwater runoff in the Gulf ofThailand and along the east coast of Peninsular Malaysia [Nasir et al. (1997)], thus, reducing surfacewater salinity and density values at all sampling station.  In the northern part of the Gulf of Thailand,freshwater runoff even makes the water column less saline and less dense.  In most deep offshorestations - again due to freshwater runoff - higher salinity and density values in the bottom water havebeen recorded, after the NE monsoon season.The halocline and the pycnocline layers are present, at most stations, both before and after theNE monsoon season.  However,  these layers are at deeper depth before as compared to after the NEmonsoon season.  This is due to the fact that wave heights are higher, just before the NE monsoonseason.  This enhances mixing throughout the vertical column.The movement of the thermocline, halocline and pcynocline layers from deeper depth to shal-lower depth, before and after the NE monsoon season, respectively, could possibly be due to up-welling and downwelling processes.  Usually, during downwelling, these layers move to a deeperdepth.  However, they will move to a shallower depth during upwelling [Kennish (1994)].The southern tip of the east coast of Peninsular Malaysia (Stations 77-81), shows a differentvariation of physical properties as to the northern part of the study area.  We don’t know what causesthis variation.  Perhaps a further investigation is needed in this particular part of the study area.ConclusionsThe vertical profiles of temperature, salinity and density in the Gulf of Thailand and along theeast coast of Peninsular Malaysia, before and after the NE monsoon season, were analyzed.  Ourresults generally agree with early observations [Nasir and Camerlengo (1997)].Our results show that the variability of the physical properties, in the Gulf of Thailand andalong the east coast of Peninsular Malaysia, is due to the monsoon season.  Profiles of temperature,salinity and density show slight different in these physical values, before and after the NE monsoonseason, are recorded.  In some stations (especially in the northern Gulf of Thailand), temperature,S1/OG1<SAADON>-5-salinity and density values are the same throughout the water column, indicating a well-mixed condi-tion, before the onset of the NE monsoon.Thermocline, halocline and pycnocline layers are present, both before and after the NE mon-soon season.  However, these layers are at deeper depth before the onset of the NE monsoon andmove to a shallower depth after the NE monsoon.  This movement of physical layers, from deeper toshallower depth, can be linked to the downwelling and upwelling processes in the region.In southern part of the east coast of Peninsular Malaysia (particularly the southern tip), thevariability of the physical properties of the watermass is difference as compared to the rest of thestudy area.  We do not have enough data to explain this difference.We feel that more physical oceanographic cruises, to study the variability of the physical prop-erties during both SW and NE monsoon seasons, are needed.  The physical oceanographic data in theSouth China Sea (especially in the east coast of Peninsular Malaysia), are too scarce to make a goodcomparison of the variability of physical properties during both monsoon seasons.AcknowledgmentThe authors would like to extend their thanks to Universiti Putra Malaysia Terengganu forcooperating in this research.ReferencesCamerlengo, A.L., M.O. Rosni and M.S. Nasir (1996a).  Monthly distribution of precipitation in PeninsularMalaysia.  Submitted for publication to the Pertanika J. of Sci. & Tech.Camerlengo, A.L., M.R. Hisham and M.S. Nasir (1996b).  On the passage of the intertropical convergence zonein Peninsular Malaysia.  Malaysian Journal of Physics, 17(4).Camerlengo, A.L., M.S. Nasir, M.O. Rosni and M.R. Hisham (1996c).  On evaporation and insolation in Penin-sular Malaysia. J. of Phys. Sci., Vol. 8.Camerlengo, A.L., M.S. Nasir and B.M. Ismail (1997).  Precipitation-evaporation difference in PeninsularMalaysia.  Accepted for publication to ASEAN J. of Sci. & Tech. for Development.Kennish, M.J. (1994).  Practical Handbook of Marine Science.  CRC Press, London, p. 566.Knauss J.A. (1987).  Introduction to Physical Oceanography.  Prentice-Hall, New Jersey, p. 338.Malaysian Meteorological Services (1991).  Monthly summary of marine meteorological observation - Year1991.  Issued under the authority of the Director General, Malaysian Meteorological Service, P. Jaya,Malaysia.Morgan, J.R. and M.J. Valencia (1983).  The natural environmental setting.  In: Atlas for marine policy inSoutheast Asian Seas. J.R. Morgan and M.J. Valencia (eds.).  Univ. of California Press. pp. 4-17.Nasir, M.S., A.L. Camerlengo and A. Supirman (1997).  Salt intrusion in the Terengganu. J. of Phys. Sci., Vol. 8.Nasir, M.S. and A.L. Camerlengo (1997).  Response of the ocean mixed layer, off the east coast of PeninsularMalaysia, during the northeast and southwest monsoons.  Geoacta, 22, 134-143.Nasir, M.S., A.L. Camerlengo and F.A.A. Hasan (1997).  Seasonal variations of physical properties in the eastcoast of Peninsular Malaysia. J. of Science, Vol. 6, 1.Nasir, M.S. and M.M. Marghany (1996).  Surface circulation off Kuala Terengganu in the transitional periodbetween the northeast and southwest monsoons.  Pertanika J. Sci. & Technol. 4(1), 141-148.Taira, K., M.S. Nasir, S. Kitagawa and T. Yanagi (1996).  J. of Oceanography. Vol. 52, 251-257.Wang, W., Q. Huang, Y.S. Li and Z.W. Li (1994).  Three-dimensional numerical modelling of the water circula-tion in South China Sea.  In:  Oceanology of China Sea, Vol. 1.  D. Zhou, Y.B. Lian and C.K. Zeng(eds.).  Kluwer Academic Publishers, p. 344.

Physical Characteristics of Watermass in the South China Sea, Area I: Gulf of Thailand and East Coast of Peninsular Malaysia

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Physical characteristics of watermass in the South China Sea, Area I: Gulf of Thailand and east coast of Peninsular Malaysia

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