Double diffusion is a distinctive vertical mixing process that arises when water masses with contrasting temperature and salinity meet. This process is expressed primarily as salt fingering—which occurs when warmer, saltier water overlies cooler, fresher water—and as diffusive convection under the opposite configuration. In this study, the presence and characteristics of double diffusion in the East Sea/Sea of Japan (ESSJ) were examined using hydrographic observations collected along the Donghae–Dokdo line in November 2021. Diagnostic metrics—including Turner angle, density ratio, Richardson number, Thorpe scale, and Reynolds number—were applied to identify thermohaline conditions favorable for double-diffusive regimes and to distinguish them from turbulent mixing. The analysis revealed that salt finger-favorable conditions predominantly developed beneath the thermohaline structure influenced by the Ulleung Warm Eddy (UWE), where strong stratification and weak vertical shear provided a dynamically stable environment for salt finger development. Salt finger heat and salt fluxes were estimated following the empirical parameterization of Radko and Smith (2012), yielding downward heat fluxes ranging from approximately −0.34 to −1.24 W m−2 across stations. Despite relatively high density ratios (Rρ ≈ 2–5), elevated buoyancy fluxes were sustained by strong background stratification, consistent with a strongly stratified staircase regime. Stations under direct UWE influence contributed approximately 47.6% of the total vertically integrated buoyancy flux, highlighting the role of the UWE in enhancing double-diffusive activity in the ESSJ. The downward heat and salt fluxes indicate a net transport toward deeper layers, while the accompanying upward buoyancy flux implies a restratifying effect, suggesting that double diffusion may contribute to the maintenance of thermohaline stratification in the ESSJ. These findings underscore the significance of double diffusion as a supplementary vertical mixing mechanism and highlight the potential of mesoscale eddies such as the UWE to regulate fine-scale mixing by modifying local thermohaline stratification.

Double diffusion,Salt finger,Turner angle,Heat flux