Observations of Eddy-Modulated Turbulent Mixing in the Southern Bay of Bengal

Long-term measurements of turbulent kinetic-energy dissipation rate ( ε ), and turbulent temperature-variance dissipation rate ( χ T ) in the thermocline, along with currents, temperature, and salinity were made at two subsurface moorings in the southern Bay of Bengal (BoB). This is a part of a major international program, conducted between July 2018 and June 2019, for investigating the role of the BoB on the monsoon intraseasonal oscillations. One mooring was located on the typical path of the Southwest Monsoon Current (SMC), and the other was in a region where the Sri Lanka Dome is typically found during the summer monsoon. Microstructure and finescale estimates of vertical diffusivity revealed the long-term subthermocline mixing patterns in the southern BoB. Enhanced turbulence and large eddy diffusivities were observed within the SMC during the passage of a subsurface-intensified anticyclonic eddy. During this time, background shear and strain appeared to influence high-frequency motions such as near-inertial waves and internal tides, leading to increased mixing. Near the Sri Lanka Dome, enhanced dissipation occurred at the margins of the cyclonic feature. Turbulent mixing was enhanced with the passage of Rossby waves and eddies. During these events, values of χ T exceeding 10 −4 °C 2 s −1 were recorded concurrently with ε values exceeding 10 −5 W kg −1 . Inferred diffusivity peaked well above background values of 10 −6 m 2 s −1 , leading to an annually-averaged diffusivity near 10 −4 m 2 s −1 . Turbulence appeared low throughout much of the deployment period. Most of the mixing occurred in spurts during isolated events.