Hydrography and circulation of the Bay of Bengal during withdrawal phase of the southwest monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two tran sects; one at 18 degrees N and the other at 90 degrees E. The data were use d to examine the thermohaline, circulation and chemical properties of the B ay of Bengal during the withdrawal phase of the southwest monsoon. The surf ace salinity exhibited wide spatial variability with values as low as 25.78 at 18 degrees N / 87 degrees E and as high as 34.79 at 8 degrees N / 90 de grees E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90 degrees E transect. The wide scatter in T-S values between 10 0 and 200 m depth was attributed to the presence of the Arabian Sea High Sa linity (ASHS) water mass. Though the warm and low salinity conditions at th e sea surface were conducive to a rise in the sea surface topography at 18 degrees N / 87 degrees 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. Re latively stronger and mutually opposite currents were noticed around 25 m d epth across the 18 degrees N transect with velocity slightly in excess of 3 0 cm s(-1). Similar high velocity (> 40 cm s(-1)) pockets were also noticed to extend up to 30 m depths in the southern region of the 90 degrees E tra nsect. However, the currents below 250 m were weak and in general < 5 cm s( -1). The net geostrophic volume transports were found to be of the order of 1.5 x 10(6) m(3) s(-1) towards the north and of 6 x 10(6) m(3) s(-1) towar ds west across the 18 degrees N and 90 degrees E transects respectively. Th e surface circulation-patterns were also investigated using the trajectorie s of drifting buoys deployed in the eastern Indian Ocean around the same ob servation period. Poleward movement of the drifting buoy with the arrival o f the Indian Monsoon Current (IMC) at about 12 degrees 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 I MC along the southern periphery of the Bay of Bengal were also evident in t he drifting buoy data. (C) 1999 Ifremer / CNRS / IRD / Editions scientifiqu es et medicales Elsevier SAS.