Shipboard current and hydrographic measurements in the Southern Bay of Bengal during 2015 Summer Monsoon

The ocean circulation in the Bay of Bengal is still not well studied specifically based on detailed field measurements. The ADCP data collected onboard of R/V Roger Revelle in August 2015 indicate the presence of north-eastern flow in the upper layer (z < ~ 120m) between 80°E – 84.6°E and 5.62°N - 6.25°N, which is a signature of the Southwest Monsoon Current (SMC). The observations suggest that the SMC flows north-eastward at least up to 8°N and 85.5°E with a predominantly northern velocity component of about 0.6 m/s. Data collected along the zonal transect 8°N, 85.5°E – 88.5°E show mostly eastern flow at the western half of the section. Thereafter, the flow gradually turns to the southwest at the eastern end of the transect. This may indicate the presence of large mesoscale structures in the study area.A 24-hour series of 49 CTD casts were taken at 6.42°N, 85.21°E. The remnant mixed layer (RML) depth was ~ 80 m; the temperature and salinity within RML varied between ~28.68 – 28.73°C and ~ 34.706 - 34.723 psu, respectively. The diurnal mixed layer reached ~20 m with temperature changing from 28.71 to 28.82 °C. A strong vertical shear was observed below the bottom of the mixed layer. In ML, the current was directed predominantly southwest, but in the thermocline the western component intensified and a northern component appeared. Between 200 and 250 m, the current turned again to the south, with negligible zonal component. The analysis also includes shear instability and internal waves in the upper thermocline with respect to the multi-layered current structure.