OCEANOLOGICA ACTA ” VOL. 22 - N” 5 Hydrography and Circulation of the Bay of Bengal during withdrawal phase: of the southwest Monsoon Y.V.B. SARMA, E.P. RAMA RAO, P.K. SAJI, V.V.S.S. SARMA National Institute of Oceanography, Dona Paula, Goa 403 004, India (Received 4 June 1998, revised 23 March 1999, accepted 6 May 1999) Abstract - Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18” N and the other at 90” E. The data w’ere used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18” N / 87” E and as high as 34.79 at 8” N / 90” E. Two high salinity cells (S > 35.2 ) were noticed around 100 m depth along the 90” E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the pres- ence of the Arabian ;SeaHigh Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18” N I 87” E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18” N transect with velocity slightly in excess of 30 cm s-l. Similar high velocity (> 40 cm s-l) pockets were also noticed to extend up to 30 m depths in the southern region of the 90” E transect. However, the currents below 250 m were weak and in general < 5 cm SK’.The net geostrophic volume transports were found to be of the order of 1.5 x lo6 m3 s-l towards the north and of 6 x 1O6 m3 SK’ towards west across the 18” N and 90” E transects respectively. The surface cir- culation-patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Cur- rent (IMC) at about 12” N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data. 0 1999 Ifremer / CNRS / IRD / editions scientifiques et medicales Elsevier SAS hydrography / watermass I circulation I Monsoon I Bay of Bengal RCsumC - Hydrologie et circulation dans le golfe du Bengale B la renverse de la mousson du sud-ouest. La circulation thermohaline et les proprietes chimiques du golfe du Bengale sont CtudiCes pendant la phase de renversement de la mous- son du sud-ouest ; les donnees hydrologiques ont Cte collect&es du 3 au 10 septembre 1996 sur deux radiales, l’une a 18” N, l’autre suivant 90” E. La salinite de surface prtsente une grande variabilite spatiale, de 25,78 (par 18” N / 87” E) jusqu’a 34,79 (par 8’” N / 90” E), avec deux maxima (plus de 35,2) vers 100 m de profondeur sur la radiale 90” E. La forte dispersion des temperatures et des salinites, observte entre 100 et 200 m de profondeur, est attribuee a l’eau t&s salee en provenance de lamer d’ Arabie (ASHS). Bien que les eaux superlicielles chaudes et peu salees Cl&vent la topographie de la surface par 18” N / 87” E, la hauteur dynamique presente une baisse de 0,2 m dyn, attribuee ici a la remontee de la ther- mocline. Les flux geostrophiques sont altemes a travers les deux radiales. Des courants relativement plus forts (dtpassant 30 cm s-l) et opposCs sont observes vers 25 m de profondeur a travers la radiale 118”N. Des poches similaires de fort courant (plus de 40 cm s-l) depassent 30 m de profondeur dans la partie sud de la radiale 90” E. Cependant, au-dessous de 250 m de profondeur, les courants sont faibles (moins de 5 cm s-l). Les flux geostrophiques nets sont respectivement de l’ordre de 1,5 x lo6 m3 s-l vers le nord et 6 x lo6 m3 s-l vers l’ouest a travers les radiales 18” N et 90” E. Les schemas de la Oceanologica Acta (1999) 22, 5, 453-471 453 0 1999 lfremer I CNRS I IRD / Editions scientifiques et medicales Elsevier SAS. Tous droits rbservbs circulation superficielle sont &ablis B partir des trajectoires de boukes dCrivantes dCployCes dans l’est de l’ocCan Jndien pendant la m$me p&ode. Le mouvement est dirigC vers le p6le au dCbut du mois d’octobre, B 1’arrivCe du Courant Yndien de Mousson (IMC) le long du bord oriental du golfe du Bengale, vers 12” N. La dCrive des bouCes met en evidence Paprb- sence d’un tourbillon au large de la c&e sud-est de 1’Inde et le Courant Indien de Mousson 3 la p&iphCrie du golfe du Ben- gale. 0 1999 Ifremer / CNRS / IRD / hditions scientifiques et mtdicales Elsevier SAS hydrologie / masse d’eau i circulation I mousson J golfe du Bengale 1. INTRODUCTION and September and winter monsoon (known as northeast monsoon) during November, December, January and The northern Indian Ocean as a whole is characterized by February [26]. The Bay of Bengal forms the northeastern the reversing monsoons; summer monsoon (also known part of the Indian Ocean and is characterized by complex locally as southwest monsoon) during June, July, August current systems that vary in strength and location both Figure 1. Area of study along with CTD iocations (prepared using GMT software). HYDROGRAPHY AND CIRCULATION IN THE BAY OF BENGAL seasonally and inter-annually [5, 8, 13, 21, 25, 281. It is a distribution of the Bay of Bengal [ 181. The flow pattern semi-enclosed water body extending from 6” N to 24” N becomes disorganised and breaks into a multitude of and from 80” E to 92” E. The continental shelf along the mesoscale gyres in the Bay of Bengal during the south- east coast of India is very narrow in the south and gradu- west monsoon season [4, 7, 121. During this season the ally widens towards north with an average width of about Bay of Bengal receives excessive precipitation and fresh- 40 km. The coastline: itself has interesting geomorpholog- water runoff from several major and minor rivers all ical features such as, a northeast-oriented coastline from along the east coast of India. The effect of these factors the southern end of the peninsula to 10” N which on the distribution of physical and chemical properties abruptly, turns to east-west and then orients in rather a and the circulation of the Bay is expected to be reflected north-south direction from 10” N to 13.5” N. Beyond it, by September when the withdrawal of the southwest rather abruptly, the coa.stline orientation changes to monsoon system begins from this region. The hydro- southwest-northeast direction. This feature presumably graphic observations along 90” E were planned to com- plays a critical role in the circulation along the east coast plement the WOCE hydrographic data along section I-5 of India as was evident from the selective upwelling in the eastern Indian Ocean further north. zones, northward coastal current and variable offshore The weather conditions during the cruise period were Ekman transport along the northwestern Bay during the characterized by easterlies of about 5 m s-‘north of 13” N southwest monsoon period [23]. The summer monsoon is and strong southwesterlies of about 10 m s-’ with speeds the single largest event that brings about large scale occasionally exceeding 15 m s-l in the south cfigure 2) changes in circulation [3, 16, 17, 19, 241 and heat content which is the characteristic feature of the southwest Figure 2. Distribution of wind (m s-l) vector along the cruise track. 455 V.V.B. SARMA et al. monsoon trough. In general, weather was cIear in the with 15 L Niskin samplers mounted on the CTD rosette northern Bay but cloudy towards south of 13” N. The and analysed using a calibrated salinometer (Guildline, rainfall recorded around 1 lo N / 90” E over the sea region Autosai model: 84OOA). The salinity values obtained on September 7 was as high as 57 mm. The wind patterns both by CTD and tbe Autosal were regressed to verify the indicate the beginning of southwest monsoon withdrawal drift in the CTD salinity measurements. We found that from the northern Bay of Bengal. both CTD and the Autosal salinity values correlated In this article, we present an analysis of the results of excellently (Y - 0.998) with a constant salinity difference (0.03) between these values with the CTD salinity values hydrographic observations and discuss the water masses marginally on the lower side. The geostrophic currents and circulation patterns along with associated volume and the volume transport estimates were computed rela- fransport structure in the Bay during the withdrawal phase of the southwest monsoon. tive to 1000 m depth. The heat content was computed with respect to a 13 “C isotherm which is considered as the base of the thermocline. In this process the heat con- tent changes due to vertical motions caused by diver- 2.