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

Abstract

Hydrographic data were collected from 3 to 10 September 1996 along two transects: one at 18 degrees N and the other at 90 degrees E. The data were 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 degrees N /87 degrees E and as high as 34.79 at 8 degrees N/90 degrees E. Two high salinity cells (S greater than 35.2) were noticed around 100 m depth along the 90 degrees E transect. The wide scatter in T-S values between 100 m depth was attributed to the presence of the Arabian Sea High 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 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. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18 degrees N transect with velocity slightly in excess of 30 cm s sup(-1). The surface circulation 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 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 IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data