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Some Physical Characteristics of Andaman Sea Waters During Winter

Some Physical Characteristics of Andaman Sea Waters During Winter

I i Indian Journal of Marine Sciences Vol. 10. September 198\, pp. 211-218

Some Physical Characteristics of Andaman 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 , 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. 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 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 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 compared to bottom topography. A north-south arc of volcanic those of Andaman Sea. Section V covers the islands and sea mounts, including the Barren and from the to the 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 and Mentawai during 66 to 68 cruises. Islands west of , separate the Andaman Sea from the Bay of Bengal except from connections Results through 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 800m. This sea is also connected (Fig. 2A). No marked departure from the thermal .with the 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 30m and a width of 35km. at its the mixed layer here, in the Bay of Bengal side of the narrowest part2• islands, was 75-80m and remained more or less 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 58m (Fig. 2A). In the Andaman Sea this thickness work is to study the distribution of temperature, varied from 50-75m (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 adjacent eastern Bay of Bengal. that observed along sections I and II during the 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 temperature changed from 27SC to 28SC with (VI) from 85°E to 96°30'E long. along 5°45'N lat. is warmer waters in the southern Andaman Sea. Another

211 INDIAN J. MAR. sel., VOL 10 SEPTEMBER 1981 ° 86 94° 96° • 14

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Fig. I-Location of stations from where data were collected during Jan-Feb 1979 & 1980 feature of importance was the occurrence of inversions varies from 156m to 194m on the western side and ranging from 0.2° to 0.75°C generally in the upper 133m to 194m on the eastern side of the island chain. layers between depths of 30-60m. These inversions In the layers below maximum salinities ( '" 34.86%J were stronger at the southern stations. occur at depths of 300-400m. Low salinities are The thermocline is demarcated by l2°C isotherm at observed below 200m at st 1204 (Fig. 2B) and st 1181 its lower level in the study area. The depth of the lower (Fig. 2D). High salinities are noticed at st 1193 and level along section III (Fig. 3A) remains almost between sts 1295 and 1279. Between sts 1194 and constant at 270 m, whereas along section IV (Fig. 4A) it 1178, within the halocline, inversions are con• shows a variati'on from 240 to 260 m with shallower spicuously seen. Below the halocline from 100to 300m depths towards south Andaman Sea. Along section V waters of low salinities with core value of 34.2°/00are (Fig. 5A) this lower limit is noticed at 280 m along the present (Fig.2D). In the same region during 1980, Ten Degree Channel and found to rise up to 200 m at st similar well defined patterns within the halocline are 1273. not observed. However, the features of low salinity There is a noticeable difference in the temperature of waters (32.8%) exist at st 1298. In the depth range 300• the waters below 1500m on either side of the Island 400 m, intrusion of high salinity water ('" 35.1%) is Chain. On the western side the temperature shows a noticed at st 1301 and west of st 1302 (Fig. 6B). The gradual decrease from 5°C at about 1500m to 3°C at gradients in the halocline layer are steeper on the 1900m while on the eastern side the waters at similar eastern sections compared to the western one. depths present an isothermal tendency with a temperature of 5°C. Sigma-t- The values of at in the mixed layer, in Salinity-The sea surface salinities are in general general are centred around 21 (C in Figs 3-6). The low at the northern ends of the sections I and II, III and depth of the lower limit of the pycnocline coincides IV. The values vary from 31.87 to 32.6°/00(Fig.2B) and approximately with that of thermocline and has a 31.2 to 32%0 (Fig. 2D) and show an increasing trend value of 26.75. At this lower level, in almost all the from north to south on the western side and south to sections examined wavy patterns are observed. Within north on the eastern side. The thickness of the the pycnocline layer, between sts 1280 and 1278 in the isohaline layer varies from 19 to 48m on the western Ten Degree Channel, these can be seen more side of the Andaman Islands and II to 31m on the conspicuously (Fig. 5C). The waters in the Andaman eastern side (Fig. 2B and D). The depth of the sharp Sea and southern Bay of Bengal show strong upper limit of the halocline beneath the isohaline layer stratification.

212 I I RAMA RAJU et at.: PHYSICAL CHARACTERISTICS OF ANDAMAN SEA WATERS

Station Nol.

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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 masks the continuity in this high salinity water mass this region presents a homogenous surface layer with having the characteristics of subtropical' lower water. very low salinities (Fig. 7). From this figure, it can be In depths of around 300m a feeble salinity maximum is seen that the scatter is wide particularly at the surface another feature of importance. The verti~al salinity layers with salinities varying from 32.3 to 35.65%0' profIle at st 1310 near SON, 85°E, represents the water This corresponds to a temperature of 28°C. The mass structure in the southwestern Bay of Bengal. salinity reaches a near maximum value around 75 to Here, a well defined salinity maximum can be seen 100m depths. Occasionally, a salinity minimum is around 300m with a core value of 35.15%0' found to be present within the discontinuity layer. The temperature of 11.2°C, in the (1t ra~e of26.5 to 27. At occurrence of salinity inversions in this layer (st 1181) depths of nearly 600 m another maximum also could

213 INDIAN J. MAR. SCI., VOL 10 SEPTEMBER 1981

Station NOl. •• OJ " •Gl •'" • ••• • i I • !! !! !!! !!! !!! !!! !!! !!! o -.- o 50

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1750 2000 :----,'-.----- @ -~ "--"00 .- - - - 5 ~.- - :_--:= 100 o •• .0 " 50 I, JIo' 100 ,, 150

200 .... 250 '000 3'-0 300 ... ~ . '000 ~ 350 )35.0 )35.0 5400 .000 a 450 •• o 0500 '--=-- 11.oo~ : ---=::i:= ••OG : ~ 750 100 ~ : 1400 1000· •00 1250 ______-H-I """r-- - - ••.• 1500 500-1 •.. __ 21.75;---_ - .:_ I.·TS- ••-.- - -,-- .•:-- ---: 17S0 ,..--- -,.... 4 2000 300 o

SO '000 •• .0• 10' 2&0 '4.0 1300 ISO ZO.() '0.0- @ 200 - • zooo 2S0 - - - - - __ 2S.50 .••.- 300 ~----";------....21·7&----:------_------. 3S0 400 --' 450 _".0- 500 7S0 :------i------~_-27.25--;------:------..,.--- . 1000 1250

1&00 1750 2000

Figs 3 and 4-Vertical distribution of A, temperature; B, salinity; C, CT, across section III (3) and section IV (4)

214

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Stotlon Nos. •• •• ... Ii! o· • - i ~ I! !i !! !! i! g ~ 0 = •. a 4004110500 !! !! : o~ • ~ •••.• .c; 0 e 1001102002001000100250400300~3110400450tOO15007501211010001750500150125035017300211011104500 ...•.• ,) : . <2~" . ~S;L: : looj ~ : -; ;; __ .0-;---- •• ~~~~ .. ;:::::::::=- --..:'" : : •••

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Figs 5 and 6-Vertical distribution of A, temperature; B, salinity; C, (It across section V (5) and section VI (6)

be seen. The stations examined within the Andaman Discussion Sea, in contrast, do not present any such conspicuous In the foregoing sections the salient features salinity maximum except-at one location (st 1292)where pertaining to the vertical distribution of temperature, the maximum is situated around 400m having a salinity and (11 have been outlined. A scatter plot ofT-8 salinity of 35%0 and temperature 10°C. At this station, has also been presented in some detail. However, in the in the upper 150m, a very sharp pycnocline associated absence of a clear picture of the circulation pattern with low salinity and high temperature gradient prevailing during this period, no convincing (1.6°CjlOm) is present. conclusions can be arrived at. Besides, the Andaman

21S INDIAN J. MAR. seI., VOL IO SEPTEMBER 1981

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Ci)-1310 I!l- 1302 ....- 10- 1267 v- 1300 .-+- +- 1289 x- 1281 -I-tI t- 12-" Q- 1292 [ .' 10 1 ;dI I_

Fig. 7-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 similar surface water enters the Andaman Sea from the very large and variable quantities of fresh water, is Sunda Sea through the ..It appears partially isolated from the Bay of Bengal by the island that the latter spreads westwards through the Great arcs and is connected, through the Strait of Malacca, Passage into the Bay of Bengal as it does towards the to the south China Sea. During winter, in addition northeast near the Ten Degree Channel. In the area in intense evaporation of surface water occurs in this between, its influence can be seen in the vicinity of the region. The southern parts of this sea are areas of Camorta-Nancowri islands. Part of the mixed surface intense air-sea interaction, being well known for the water ,.reaching the eastern side of the Ten Degree frequent cyclones which originate here. Yet, keeping Channel from the north (Bay of Bengal Water) and the all these in view, it is only the upper layers (above the southwest appears to flow out into the Bay through the pycnocline) of the waters which need careful study. Ten Degree Channel and a little south of it, between This is so because the very sharp pycnocline effectively the Car Nicobar and its southern neighbours. The reduces the vertical exchange of mass, momentum and properties of the surface waters.down to the lower limit heat between the upper and lower layers of the waters. of the pycnocline lend strong evidence to this outflow. This upper layer is perennially subjected to various In the eastern part of section VI surface layer changes in its physical properties by the in- and out• inversions are seen in the isohaline contours which flows from or to the Bay of Bengal and the Sunda Sea. could occur only through differential mixing of the Present findings suggest that, during the period of southern Andaman Sea waters with those coming from study, low density surface water enters the Andaman southeast, through the Strait of Malacca. Sea in the northwestern region through the Preparis In almost all the vertical sections the isolines from Channel. At the same time in the southeastern region temperature, salinity, and 0". show clearly discernible

216

"" , RAMA RAJU el al.: PHYSICAL CHARACTERISTICS OF ANDAMAN SEA WATERS wavy patterns which may probably be produced by internal waves. At this stage, it is rather difficult to say Table I-Earlier Observations on Water Masses whether they are due to internal waves, solitons or Persian Gulf Red Sea otherwise or are due to some other factors although Water Water with low density waters lying over denser sub-surface . Rochford7 layers it is possible to visualize Kelvin-Helmholtz type D 300-400 500-600 instabilities. Varadachari el al.8 (a) Apart from all these, it is also of importance to note D 200-350 400-900 the presence of inversions in the surface layers both in S 35.03-35.35 35-35.33 temperature and salinity structures. These are, at 26.2-26.9 26.95-27.4 obviously, indications of nonhomogeneous mixing Wyrtki9(b) ::>ccurringin these . With due precautions one D 300-370 - 600 may say that locally intense surface evaporation S 35.02-35.1 > 34.8 coupled with incursion of low density surface water T 8-14 7-9 may give rise to such phenomena. At the same time one 26.5-27 27.2-27.4 can visualize breaking of internal waves as another causative factor. D = depth (m), S = salinity (°/00) and T = temperature('C) The choice of 4 meridional sections close to the a = Results obtained in southwestern Bay of Bengal b = The values against Red Sea water are those quoted eastern and western sides of the Andaman Islands near the equator 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 The wide scatter in the surface layer which rapidly much broader scale for the southernmost parts of these narrows down and trails of within a very narrow zone 2 bodies of water. An examination of these vertical suggests the possible presence of a series of water profiles along with those of section VI for temperature, masses characterised by their T-S relation and the salinity and 6-, clearly brings out the contrasting depths at which they are found. features wherein the western and southern Andaman While discussingtillise water masses one is definite in Sea waters can clearly be seen as those belonging to this the case of southern Bay of Bengal which is not exactly semi-enclosed tropical basin-both in the surface the case with the Andaman Sea. The Andaman Sea is a layers as well as in the depths below 1500m. In general, semi-enclosed basin and exchange of water with the mixed layer and the lower reaches of the adjacent is obviously restricted by the sill depths thernlOcline, halocline and pycnocline are distinctly of the main channels. Within the scope of the above deeper on the Bay of Bengal side of the Andaman and the general characteristics of the T-S relationships Islands compared to the western Andaman Sea waters, mentioned above we now delineate the following water as also the surface temperatures are higher in the latter masses with their 0', values: than in the former. On the southernmost parts, the (a) Warm, relatively less saline surface water, 20.5 to differences in the surface layers are more pronounced 22; (b) transition water, 22 to 26.5; (c) Persian Gulf with warmer waters associated with shallower surface water, 26.7 to 27; (d) Red Sea water, 27 to 27.4; and mixed layer and wavy iso-surfaces (T, S, O'J being (e) deep water, > 27.4. conspicuously present in the southern Andaman Sea. Observations made by earlier workers are given in In the deeper layers, the most important finding is that Table 1 for comparison. The deep water (e) has below 1,500m the temperature gradually falls from 50 characteristics similar to Antarctic Bottom Water and to 3°e in the Bay of Bengal whereas in the Southern hence it isnot attempted here to make another separate Andaman Sea this layer of deeper water retains an classification. It would be pertinent to mention that the almost isothermal character centered around 5°C. above classifications are more apt in the case of Bay Earlier works4 -6 suggest that the latter near-neutral and Bengal waters than Andaman Sea waters where water column is a product of the appreciably large the sub-surface waters present nearly homogeneous amount of heat flow from the sea floor. Moreover, this conditions with no marked salinity maximum. is not an isolated phenomenon observed in the southern regions alone, and is manifest in the central Acknowledgement Andaman Sea with more prominence. The authors express their thanks to Dr S.Z. Qasim, A more comprehensive idea about the physical Director, for his interest and to Dr V.V.R. Varadachari, characteristics of these waters during winter could be Deputy Director for his guidance. Thanks are also due gained from the T-S relationship and Fig. 7 very to Shri P.K. Das for his help and to Dr J.S. Sastry and clearly shows many of the features discussed above. Shri M.J. Varkey for their suggestions.

217 INDIAN J. MAR. SCI., VOL 10 SEPTEMBER 1981

References 5- Sewell R B S, Mem Asiat Soc Bengal, 9 (No. 6) (1932) 357, I Wyrtki K, Naga report 2 (University of California. La Jolla, 6 Gangadhara Rao L V & Jayaraman R, Bull natn Inst Sci , 38 California) 1961. (1968) 123. 2 Rodolfo K S, Encyclopedia of oceanography (Van Nostrand 7 Rochfo1d D J, Aust J mar Freshwat Res, 15 (1964) I. Reinhold Company, New York) 1966. 8 Varadachari V V R, Murty C S & Gangadhara Reddy C V, Bull 3 Curray J R & Moore D G, Geology of the continental margins natn Inst Sci India, 38 (1968) 338. (Springer - Verlog Publication) 1974. 9 Wyrtki K, Oceanographic atlas of the IIOE(Nat. Sci. Foundation 4 Ramesh Babu V & Sastry J S, Ind J mar Sci, 5 (1966) 179. Washington D C) 1971.

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