Enhanced double diffusive convection at the Oyashio-TWC front

At the northwestern boundary of the Pacific Ocean, off the southeastern coast of Hokkaido Island, Japan, two current systems – the Oyashio Current (OC) and the Tsugaru Warm Current (TWC) – merge, mix and form a prominent oceanic front while meandering downstream. In order to understand the detailed mixing processes between the OC and the TWC waters along the front, we conducted multiple zigzag surveys across the front. Surveys were conducted in summer, 2017, using ship-based observations from R/V Wakataka-maru (692t equipped with a 38 kHz ADCP, a turbulence profiler, and an underway-CTD system) and a fully autonomous underwater glider (Slocum G2 with a turbulence sensor and a 600 kHz ADCP). The observed frontal structure clearly highlighted the subduction and intrusion processes of the cold and fresh OC water between the warm and salty TWC water and the cold and salty Cold Layer (CL) along the layer between 26.5 to 26.8 sigma-theta. Our results also show conditions were favorable for salt finger convection at the upper interface along the 26.5 sigma-theta and diffusive convection at the lower interface along the 26.8 sigma-theta. Vertical profiles of temperature and salinity illustrated favorable conditions for double diffusive convections, with enhanced vertical mixing of K_t up to O(10^-2 m²/s) especially evident at the sharp interfaces where Turner angle (Tu) ranges were 90° > Tu > 75° and -90° < Tu <-75°.