Keywords:Mixing coefficient, Diapycnal diffusivity, Density ratio, Buoyancy frequency, Buoyancy Reynolds number
On the basis of an accumulation of observational results obtained mostly in the upper ocean, the mixing coefficient Γ in the Osborn's diapycnal diffusivity model is usually treated as a constant, Γ = 0.2. However, it has not been fully addressed whether Γ remains constant throughout the deep ocean. To address this issue, we estimate Γ using deep profiles of the turbulent kinetic energy dissipation rate ε and the temperature variance dissipation rate χT obtained in various regions such as the Izu-Ogasawara Ridge, the Emperor Seamounts, the Aleutian Ridge, and the Southern Ocean. The estimated Γ is surprisingly variable, possibly depending on the density ratio Rρ, the buoyancy frequency Ν, and the buoyancy Reynolds number Reb = ε/(νN2) with ν as the kinematic viscosity. While the estimated Γ remains to be around the conventional value of 0.2 in the temperature-stratified upper ocean with Rρ > 2 or Rρ < −1, Γ tends to increase to ∼1 not only in the salinity-stratified upper ocean with |Rρ| << 1 but also in the deep ocean. The increasing trend of Γ in the deep ocean appears to be related to the decreasing trend of Ν and/or the increasing trend of Reb. This study thus suggests that the diapycnal diffusivity in the deep ocean might be significantly larger than ever thought.