A recent compilation of global observational data, including from the Argo floats array, has promoted understanding of the spatiotemporal variability of vertical mixing intensity. Vertical mixing is found to be enhanced near the seafloor, where the bottom topography is rough and/or where the external force such as tide and wind is strong. This article reviews observational data on the vertical mixing intensity at mixing hotspots in the North Pacific: the Hawaiian ridge, the Izu-Ogasawara ridge, the Kuroshio/Kuroshio Extension, the East China Sea, the Luzon Strait and the South China Sea, the Kuril Straits, the Oyashio and the Mixed Water region, the Aleutian Passes, the storm track region, the Equatorial area, and the Indonesian Archipelago. In future research, more efficient methods of measuring turbulence by autonomous platforms, in addition to conventional shipboard observation, would facilitate quantification of vertical mixing intensity and elucidation of mixing processes in North Pacific mixing hotspots.

Turbulent mixing in the deep ocean plays a crucial role in controlling the strength and pattern of the global thermohaline circulation. The energy available for turbulent mixing is initially supplied by internal tides and internal lee waves generated, respectively, by tidal and geostrophic flows over rough topography, and by near-inertial internal waves generated by wind stress forcing of atmospheric disturbances. The large-scale internal wave energy thus generated is cascaded across the internal wave spectrum down to small dissipation scales through nonlinear wave-wave interactions causing turbulent mixing in the deep ocean. This article focuses on the generation processes of oceanic internal waves and reviews the recent progress in studies of the global distributions of topographic- and wind-generated internal waves. Outstanding problems for clarifying the global distribution of turbulent mixing in the deep ocean are also discussed.

This article reviews observational studies of circulation in intermediate and deep layers in the North Pacific. While meridional overturn in the North Pacific is relatively weak in comparison to other basins, deep layer observation studies show that the vertical diffusion inferred from mass or volume conservation differs. Large differences are present north of the North Pacific midlatitudes. The differences could be due to large mooring observations capturing high frequency transport changes and influences of long-term small changes in water properties, however, estimates of overturn have not sufficiently accounted for variations in transport and water properties. In addition, although many previous studies have successfully captured the processes relating to tracer distributions and circulation structures in intermediate layers, quantitative evaluation of the processes is not sufficient to explain the small changes in deep layers. As mixing in the ocean is assumed to be negligible or spatially and temporally constant in many observational studies, evaluation of ocean mixing may be one of the keys for better understanding North Pacific circulations.

The treatment of vertical mixing in ocean general circulation models is reviewed and its future direction discussed, especially concerning the reproducibility of the Pacific deep circulation. Internal tides are thought to be the principal source of mixing in the deep ocean, and there have been many studies focusing on generation of internal tides over rough bathymetry and their dissipation in the vicinity of generation. A great challenge currently is how to adequately parameterize the dissipation process of far-propagating internal tides.

Ocean-mixing plays an essential role in ocean currents, particularly meridional overturning. In conjunction with increased observations, there has been a focus on synthesis of ocean-mixing data. This paper discusses current ocean state estimation and possible synthesis of ocean-mixing observations.

Near-inertial internal gravity waves radiating from the surface mixed layer propagate into the deep ocean and ultimately dissipate. In this review (Part 1), I introduce how those processes are formulated, focusing on an excitation of inertial oscillation and a propagation of internal waves without background flow.

In this review (Part 2), I further introduce how the propagation of near-inertial internal gravity waves in a background flow is formulated, focusing on dispersion relations, spatial scales of waves and background flow fields and time evolutions of waves.

Recent findings on turbulent mixing processes in the ocean surface boundary layer are reviewed. Based on their kinetic energy source, turbulences are categorized as wind-induced (shear-driven), convective, and wave-induced (Langmuir). The mechanism and consequent mixing associated with different turbulences are described. Finally the framework of parameterization for these mixing processes is shown.