The Subsurface Symmetric Instability at a Kuroshio Extension Submesoscale Front

Abstract Submesoscale symmetric instability (SI) is considered to effectively transfer oceanic geostrophic kinetic energy into small‐scale dissipation and enhance the vertical exchange of tracers. While SI is widely reported to be active within the ocean boundary layer, only a few studies have claimed the occurrence of SI in the ocean subsurface layer, and the processes that enable subsurface SI development away from the boundary layer still remain poorly understood. Here, based on high‐resolution in situ observations in the Kuroshio Extension, the anticyclonic potential vorticity (PV) that favors SI is observed in the subsurface layer (the core is at ∼180 m depth in contrast to the surface mixed layer depth ∼40 m) of an intense submesoscale front with a drastic sea surface temperature change of 12°C across 5 km. The analysis results show that the anticyclonic PV preconditioning subsurface SI is generated at the surface layer due to atmospheric‐forced surface buoyancy loss. Subsequently, the diagnosed downward vertical velocity reaches ∼175 m/day, primarily driven by strain‐induced frontogenesis and overturning instabilities. These along‐isopycnal submesoscale processes facilitate the subduction of surface‐origin anticyclonic PV into the subsurface layer. The Lagrangian particle tracking experiments based on model simulation further identify this mechanism. These findings reveal a dynamic pathway linking surface forcing, submesoscale frontogenesis, and the development of subsurface SI, with important implications for the vertical transport of heat, salt, and biogeochemical tracers in frontal regions.