Re-evaluation of turbulent mixing vertical structure in the Bussol’ Strait and its impact on water masses in the Okhotsk Sea and the North Pacific

Highlight
• Cross-sectional distribution of mean turbulent vertical mixing intensity is revealed.
• Intense mixing maxima occur at intermediate depths and 300-400 m above bottom.
• Diapycnal velocity is estimated from the vertical profile of the mixing.
• The velocity is consistent with the thickness change of Okhotsk intermediate water.

Distribution of the turbulent kinetic energy dissipation rate ε across the Bussol’ Strait was re-evaluated by applying an improved method using density inversions to tide-resolving density observations. The improved estimates without excessive rejection of density inversions provide about three-times higher mean values of ε (1.45 × 10−7 W/kg), vertical diffusivity Kρ (153 × 10−4 m2/s) and depth-integrated ε (187 mW/m2) in comparison with previous estimates. One-day mean vertical profiles of ε in the potential density co-ordinate allows us to estimate diapycnal velocity at 3 × 10−5 m/s downwards through the 26.85σθ isopycnal surface, and was almost zero through 26.7σθ, suggesting the thinning of Okhotsk Sea Mode Water or of the core of North Pacific Intermediate Water as the water flows through the Bussol’ Strait from the Sea of Okhotsk to the North Pacific. The re-evaluated turbulence and current distributions confirmed following previous knowledge but with quantitative improvements and/or expansion of coverage: (1) All-averaged ε and Kρ show distinctive mid-depth maxima at 800–1600 m, 26.9–27.6σθ and at a height of 300–400 m above the bottom, and enhanced ε within 500 m of the bottom corresponding to the enhanced shear and low-Richardson number at vertical scales of less than 50 m. (2) A significant amount of the ε variance (14%) is explained by the 50 m-scale vertical shear composed of mean and diurnal tidal currents. (3) The diurnal tidal currents are well approximated by six low-mode topographically trapped waves.