I
i
Indian Journal of Marine Sciences
Vol. 10. September 198\, pp. 211-218
Some Physical Characteristics of Andaman Sea Waters during Winter
D V RAMA RAJU, A D GOUVEIA & C S MURTY
National Institute of Oceanography, Dona Paula, Goa 403004
Received
15 December 1980; revised received 27 Api#'1981
Based on the data collected during the cruises of R V Gaveshani in Jan.-Feb. of 1979and 1980around the Andaman Islands,
6 transects are selected, along which the present study on some physical properties of the waters derived from the distributions
of temperature, salinity and ", is made, and on the basis of which the presence of certain water masses are identified and
discussed. The thickness of the surface mixed layer varies, in general, from 25 to 80 m. Persian Gulf water is identified in the
depth range 200-500 m as a high salinity ( - 35.1°/..,)water with temperatures varying from 10-12°Cand 11, from 26.7-27. Below
this, in the depth range 500-900 m, a water mass with a salinity of about 35%., temperature between 7° and 9.5°C and 11,
between 27 and 27.4 shows characteristics of a mixture of Persian Gulf and Red Sea waters. At depths of about l500m and
below the waters on the eastern side of the Andaman Islands are warmer than on the western side. The general characteristics of
the thermocline. halocline and pycnocline in sectional profiles are presented.
The Andaman Sea contains a relatively extensive basin included in this study to understand the hydrographic
with a maximum depth of 4360 m and an uneven conditions in the southern Bay of Bengal compared to
bottom topography. A north-south arc of volcanic those of Andaman Sea. Section V covers the region
islands and sea mounts, including the Barren and from the Ten Degree Channel to the Sunda Shelf at
Narcondam islands in the Andaman Sea, delineates 6°30'N lat. The temperature, salinity and O't
this basin from 2 smaller basins on the north and the distributions along the vertical sections are shown in
south 1 -3. The Andaman Islands which are part of an Figs 2-6. A temperature - salinity scatter diagram is
anticlinal belt passing from Arakan Yoma in Burma presented in Fig. 7 making use of all the data collected
through Andaman and Nicobar Islands and Mentawai during 66 to 68 cruises.
Islands west of Sumatra, separate the Andaman Sea
from the Bay of Bengal except from connections Results
through Preparis Channel on the north with a depth of
Thermal structure-The
sea surface temperature
200 m, the Great Passage on the south which is about varied from 27 to 28SC with an increasing trend from
1800m deep and the Ten Degree Channel in the middle north to south on the western section (I) during 1979
with a depth of about 800 m. This sea is also connected (Fig. 2A). No marked departure from the thermal
.with the South China Sea through the Strait of structure was observed during 1980on the western and
Malacca. Strong tidal currents occur in this strait eastern sections (I and III) and (II and IV). The depth of
which has a depth of 30 m and a width of 35 km. at its the mixed layer here, in the Bay of Bengal side of the
islands, was 75-80m and remained more or less
narrowest part2•
The data for the present investigation were collected constant along section III (Fig. 3A); whereas during
during the cruises 51 and 52 (1979) and 66 to 68 (1980) the previous year it showed a fluctuation from 44 to
of R V Gaveshani in January-February. The aim of this 58 m (Fig. 2A). In the Andaman Sea this thickness
work is to study the distribution of temperature, varied from 50-75 m (Fig. SA). In the region of Ten
salinity and density and to identify the water masses Degree Channel the mixed layer thickness is low
occupying the intermediate levels of the waters of the ('" 40 m). Along sections III and IV, this thickness
Andaman Sea (western and southern region) and the showed a deepening by as much as 30m compared to
that observed along sections I and II during the
adjacent eastern Bay of Bengal.
previous year. Section V showed a relatively higher
Materials and Methods
variation in the temperatures compared to sections III
Temperature and salinity values at standard depths and IV and the mixed layer shallows towards east
were taken from STD record and from conventional (Fig. SA). Presence of a tongue of relatively warm
reversing bottles whenever continuous records were water (28.4°C) was clearly seen between sts 1301 and
not available. In all, 6 transects-2 during Jan.-Feb.
1279 (Fig.4A). Along section VI the surface mixed
1979 and 4 during the same period in 1980-were
layer was around 60m (Fig.6A) and the surface
selected from these cruises (Fig. 1). The zonal section
(VI) from 85°E to 96°30'E long. along 5°45'N lat. is
temperature changed from 27SC to 28SC with
warmer waters in the southern Andaman Sea. Another
211
INDIAN J. MAR. sel., VOL 10 SEPTEMBER 1981
°
86
94°
96°
•
14
..
12
8 A Y
OF
tOO
8ENGAL
• 1I117
IZII··
IZIZ~O
Tin
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IZIO
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IZ
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°
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10
Fig. I-Location
of stations from where data were collected during Jan-Feb 1979 & 1980
feature of importance was the occurrence of inversions
ranging from 0.2° to 0.75°C generally in the upper
layers between depths of 30-60m. These inversions
were stronger at the southern stations.
The thermocline is demarcated by l2°C isotherm at
its lower level in the study area. The depth of the lower
level along section III (Fig. 3A) remains almost
constant at 270 m, whereas along section IV (Fig. 4A) it
shows a variati'on from 240 to 260 m with shallower
depths towards south Andaman Sea. Along section V
(Fig. 5A) this lower limit is noticed at 280 m along the
Ten Degree Channel and found to rise up to 200 m at st
1273.
There is a noticeable difference in the temperature of
the waters below 1500m on either side of the Island
Chain. On the western side the temperature shows a
gradual decrease from 5°C at about 1500m to 3°C at
1900m while on the eastern side the waters at similar
depths present an isothermal tendency with a
temperature of 5°C.
Salinity-The sea surface salinities are in general
low at the northern ends of the sections I and II, III and
IV. The values vary from 31.87 to 32.6°/00(Fig. 2B) and
31.2 to 32%0 (Fig. 2D) and show an increasing trend
from north to south on the western side and south to
north on the eastern side. The thickness of the
isohaline layer varies from 19 to 48 m on the western
side of the Andaman Islands and II to 31 m on the
eastern side (Fig. 2B and D). The depth of the sharp
upper limit of the halocline beneath the isohaline layer
212
varies from 156m to 194m on the western side and
133m to 194m on the eastern side of the island chain.
In the layers below maximum salinities ( '" 34.86%J
occur at depths of 300-400 m. Low salinities are
observed below 200m at st 1204 (Fig. 2B) and st 1181
(Fig. 2D). High salinities are noticed at st 1193 and
between sts 1295 and 1279. Between sts 1194 and
1178, within the halocline, inversions are conspicuously seen. Below the halocline from 100to 300 m
waters of low salinities with core value of 34.2°/00are
present (Fig.2D). In the same region during 1980,
similar well defined patterns within the halocline are
not observed. However, the features of low salinity
waters (32.8%) exist at st 1298. In the depth range 300400 m, intrusion of high salinity water ('" 35.1%) is
noticed at st 1301 and west of st 1302 (Fig. 6B). The
gradients in the halocline layer are steeper on the
eastern sections compared to the western one.
Sigma-t- The values of at in the mixed layer, in
general are centred around 21 (C in Figs 3-6). The
depth of the lower limit of the pycnocline coincides
approximately with that of thermocline and has a
value of 26.75. At this lower level, in almost all the
sections examined wavy patterns are observed. Within
the pycnocline layer, between sts 1280 and 1278 in the
Ten Degree Channel, these can be seen more
conspicuously (Fig. 5C). The waters in the Andaman
Sea and southern Bay of Bengal show strong
stratification.
I
I
RAMA RAJU et at.: PHYSICAL CHARACTERISTICS
OF ANDAMAN SEA WATERS
Station Nol.
1211
1210 1207_
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Fig. 2--Property
distributions along sections I and II [A,C-temperature; B,D-salinity]
Water masses-Vertical structure of the waters in
this region presents a homogenous surface layer with
very low salinities (Fig. 7). From this figure, it can be
seen that the scatter is wide particularly at the surface
layers with salinities varying from 32.3 to 35.65%0'
This corresponds to a temperature of 28°C. The
salinity reaches a near maximum value around 75 to
100m depths. Occasionally, a salinity minimum is
found to be present within the discontinuity layer. The
occurrence of salinity inversions in this layer (st 1181)
masks the continuity in this high salinity water mass
having the characteristics of subtropical' lower water.
In depths of around 300m a feeble salinity maximum is
another feature of importance. The verti~al salinity
profIle at st 1310 near SON, 85°E, represents the water
mass structure in the southwestern Bay of Bengal.
Here, a well defined salinity maximum can be seen
around 300m with a core value of 35.15%0'
temperature of 11.2°C, in the (1t ra~e of26.5 to 27. At
depths of nearly 600 m another maximum also could
213
INDIAN J. MAR. SCI., VOL 10 SEPTEMBER 1981
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1250
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Figs 3 and 4-Vertical
distribution of A, temperature; B, salinity; C,
across section III (3) and section IV (4)
CT,
214
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I
RAMA RAJU et al.: PHYSICAL CHARACTERISTICS
a
0
4110
400
500
e 110
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1250
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100
1000
500
150
300
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12110
200
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Figs 5 and 6-Vertical
distribution of A, temperature; B, salinity; C,
be seen. The stations examined within the Andaman
Sea, in contrast, do not present any such conspicuous
salinity maximum except-at one location (st 1292)where
the maximum is situated around 400m having a
salinity of 35%0 and temperature 10°C. At this station,
in the upper 150m, a very sharp pycnocline associated
with low salinity and high temperature gradient
(1.6°CjlOm) is present.
(It
across section V (5) and section VI (6)
Discussion
In the foregoing sections the salient features
pertaining to the vertical distribution of temperature,
salinity and (11 have been outlined. A scatter plot ofT-8
has also been presented in some detail. However, in the
absence of a clear picture of the circulation pattern
prevailing during this period, no convincing
conclusions can be arrived at. Besides, the Andaman
21S
INDIAN J. MAR. seI., VOL IO SEPTEMBER 1981
S A L I NIT
Y
~oo
35·0
10
,.-t-r::J::P
I :.::::F"""
-1[;W
I
I
Ci)-1310
I!l- 1302
10- 1267
1300
v+x-
t-
Q-
-I-tI
1289
1281
.-+-
....-
12-"
1292
[
.'
10
;d
I
1
I_
7-
Fig.
Temperature-Salinity (T-S) scatter diagram for Andaman
Sea, Eastern Bay of Bengal and along lat. 5°30'N-6°N and long.
78°E-98SE
Sea has very uneven bottom topography, receives
very large and variable quantities of fresh water, is
partially isolated from the Bay of Bengal by the island
arcs and is connected, through the Strait of Malacca,
to the south China Sea. During winter, in addition
intense evaporation of surface water occurs in this
region. The southern parts of this sea are areas of
intense air-sea interaction, being well known for the
frequent cyclones which originate here. Yet, keeping
all these in view, it is only the upper layers (above the
pycnocline) of the waters which need careful study.
This is so because the very sharp pycnocline effectively
reduces the vertical exchange of mass, momentum and
heat between the upper and lower layers of the waters.
This upper layer is perennially subjected to various
changes in its physical properties by the in- and outflows from or to the Bay of Bengal and the Sunda Sea.
Present findings suggest that, during the period of
study, low density surface water enters the Andaman
Sea in the northwestern region through the Preparis
Channel. At the same time in the southeastern region
similar surface water enters the Andaman Sea from the
Sunda Sea through the Strait of Malacca ..It appears
that the latter spreads westwards through the Great
Passage into the Bay of Bengal as it does towards the
northeast near the Ten Degree Channel. In the area in
between, its influence can be seen in the vicinity of the
Camorta-Nancowri islands. Part of the mixed surface
water ,.reaching the eastern side of the Ten Degree
Channel from the north (Bay of Bengal Water) and the
southwest appears to flow out into the Bay through the
Ten Degree Channel and a little south of it, between
the Car Nicobar and its southern neighbours. The
properties of the surface waters.down to the lower limit
of the pycnocline lend strong evidence to this outflow.
In the eastern part of section VI surface layer
inversions are seen in the isohaline contours which
could occur only through differential mixing of the
southern Andaman Sea waters with those coming from
southeast, through the Strait of Malacca.
In almost all the vertical sections the isolines from
temperature, salinity, and 0". show clearly discernible
216
"" ,
RAMA RAJU
el al.:
PHYSICAL CHARACTERISTICS
wavy patterns which may probably be produced by
internal waves. At this stage, it is rather difficult to say
whether they are due to internal waves, solitons or
otherwise or are due to some other factors although
with low density waters lying over denser sub-surface
layers it is possible to visualize Kelvin-Helmholtz type
instabilities.
Apart from all these, it is also of importance to note
the presence of inversions in the surface layers both in
temperature and salinity structures. These are,
obviously, indications of nonhomogeneous mixing
::>ccurringin these regions. With due precautions one
may say that locally intense surface evaporation
coupled with incursion of low density surface water
may give rise to such phenomena. At the same time one
can visualize breaking of internal waves as another
causative factor.
The choice of 4 meridional sections close to the
eastern and western sides of the Andaman Islands
provide important grounds for a comparative study of
the waters of the Bay of Bengal and the western
Andaman Sea. Section VI provides a similar scope on a
much broader scale for the southernmost parts of these
2 bodies of water. An examination of these vertical
profiles along with those of section VI for temperature,
salinity and 6-, clearly brings out the contrasting
features wherein the western and southern Andaman
Sea waters can clearly be seen as those belonging to this
semi-enclosed tropical basin-both
in the surface
layers as well as in the depths below 1500m. In general,
the mixed layer and the lower reaches of the
thernlOcline, halocline and pycnocline are distinctly
deeper on the Bay of Bengal side of the Andaman
Islands compared to the western Andaman Sea waters,
as also the surface temperatures are higher in the latter
than in the former. On the southernmost parts, the
differences in the surface layers are more pronounced
with warmer waters associated with shallower surface
mixed layer and wavy iso-surfaces (T, S, O'J being
conspicuously present in the southern Andaman Sea.
In the deeper layers, the most important finding is that
below 1,500m the temperature gradually falls from 50
to 3°e in the Bay of Bengal whereas in the Southern
Andaman Sea this layer of deeper water retains an
almost isothermal character centered around 5°C.
Earlier works4 -6 suggest that the latter near-neutral
water column is a product of the appreciably large
amount of heat flow from the sea floor. Moreover, this
is not an isolated phenomenon observed in the
southern regions alone, and is manifest in the central
Andaman Sea with more prominence.
A more comprehensive idea about the physical
characteristics of these waters during winter could be
gained from the T-S relationship and Fig. 7 very
clearly shows many of the features discussed above.
OF ANDAMAN SEA WATERS
Table I-Earlier
D
D
S
at
D
S
T
Observations on Water Masses
Persian Gulf
Water
Red Sea
Water
. Rochford7
300-400
500-600
Varadachari el al.8 (a)
200-350
400-900
35.03-35.35
35-35.33
26.2-26.9
26.95-27.4
Wyrtki9(b)
300-370
- 600
35.02-35.1
> 34.8
8-14
26.5-27
7-9
27.2-27.4
D = depth (m), S = salinity (°/00) and T = temperature('C)
a = Results obtained in southwestern Bay of Bengal
b = The values against Red Sea water are those quoted
near the equator
The wide scatter in the surface layer which rapidly
narrows down and trails of within a very narrow zone
suggests the possible presence of a series of water
masses characterised by their T-S relation and the
depths at which they are found.
While discussingtillise water masses one is definite in
the case of southern Bay of Bengal which is not exactly
the case with the Andaman Sea. The Andaman Sea is a
semi-enclosed basin and exchange of water with
adjacent seas is obviously restricted by the sill depths
of the main channels. Within the scope of the above
and the general characteristics of the T-S relationships
mentioned above we now delineate the following water
masses with their 0', values:
(a) Warm, relatively less saline surface water, 20.5 to
22; (b) transition water, 22 to 26.5; (c) Persian Gulf
water, 26.7 to 27; (d) Red Sea water, 27 to 27.4; and
(e) deep water, > 27.4.
Observations made by earlier workers are given in
Table 1 for comparison. The deep water (e) has
characteristics similar to Antarctic Bottom Water and
hence it is not attempted here to make another separate
classification. It would be pertinent to mention that the
above classifications are more apt in the case of Bay
and Bengal waters than Andaman Sea waters where
the sub-surface waters present nearly homogeneous
conditions with no marked salinity maximum.
Acknowledgement
The authors express their thanks to Dr S.Z. Qasim,
Director, for his interest and to Dr V.V.R. Varadachari,
Deputy Director for his guidance. Thanks are also due
to Shri P.K. Das for his help and to Dr J.S. Sastry and
Shri M.J. Varkey for their suggestions.
217
INDIAN J. MAR. SCI., VOL 10 SEPTEMBER 1981
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I Wyrtki K, Naga report 2 (University of California. La Jolla,
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2 Rodolfo K S, Encyclopedia of oceanography (Van Nostrand
Reinhold Company, New York) 1966.
3 Curray J R & Moore D G, Geology of the continental margins
(Springer - Verlog Publication) 1974.
4 Ramesh Babu V & Sastry J S, Ind J mar Sci, 5 (1966) 179.
6 Gangadhara Rao L V & Jayaraman R, Bull natn Inst Sci India, 38
(1968) 123.
7 Rochfo1d D J, Aust J mar Freshwat Res, 15 (1964) I.
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9 Wyrtki K, Oceanographic atlas of the IIOE(Nat. Sci. Foundation
Washington D C) 1971.
218
II~ I'
IH
I1I1