Microstructure and Mooring Observations of Enhanced Mixing in the Kerama Gap

Microstructure and mooring observations were conducted around the Kerama Gap, near the midpoint of the Ryukyu Island chain, in December 2016. We observed enhanced mixing near raised seafloor topography with vertical diffusivity from 10−2 to 10−1 m2 s−1 in the bottom water and found that tides were important drivers of enhanced mixing above sills. Two‐dimensional numerical simulations of a vertical cross section of the Kerama Gap indicated that breaking internal tides play an important role in the enhanced mixing behind the sill near the entrance to the gap. In addition, M2 internal tides radiating from the sills at the gap's entrance and exit may correspond to the observed enhanced vertical shear and mixing around the salinity minimum layer of the North Pacific Intermediate Water (NPIW) in the Kerama Gap. Simple quantitative arguments based on the observed turbulence data suggest that the enhanced mixing at the sill may influence the renewal of the bottom water and the NPIW in the Okinawa Trough, about 10% of required fluxes in the Kerama Gap estimated in past studies, indicating its importance on the distributions of water masses and water properties in the North Pacific. Plain Language Summary Turbulence and currents were measured around the Kerama Gap, near the midpoint of the Ryukyu Island chain. We observed enhanced mixing near raised seafloor topography and found that tides were important drivers of enhanced mixing above sills. Since density ranges of the enhanced mixing correspond to those of the North Pacific Intermediate Water and bottom water in the Okinawa Trough, it is suggested that the enhanced mixing in the Kerama Gap may influence the renewal of those water masses. Key Points Microstructure and mooring observations were conducted around the Kerama Gap We observed enhanced mixing near raised seafloor topography with vertical diffusivity from 10−2 to 10−1 m2 s−1 The enhanced mixing behind the sill may influence the renewal of the bottom water and the NPIW in the Okinawa Trough