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Some Physical Characteristics of Andaman Sea Waters During Winter

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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 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 800m. 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 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 .. 12 8 A Y OF • 1I117 tOO .1I111 '~1I114 8ENGAL IZIZ~OIZII·· • IZIO IZ Tin Ol9rll CAR NICOBAR. ° 6 10 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. 1211 1210 1207_ 111' 11M •• n78 "77 o l!!-~ ~ 1~ , , , , ......, or:.;,-,~-= .. ~ ----- --- -·------a &-- l00~~~ -= ~:- •• 200~~2 8 8 ----7------- IOOO~--8---------7 -_ _ IlIOO _______ 3- __ © 2OOO~,{;- 1'90 2500 ® I, 32'--.'32L . ---:::....---.~ Or ~~ _ ~ I<X»-_34r-~M-2""- _ ~-- ___ a -------..._______34. ",'-.--- __- 3~O ----- ---.------------ »0- @ ® 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.
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