Microstructure measurements were made in the Mixed Water Region of the Oyashio/Kuroshio/Tsugaru currents system where both turbulence and double diffusion are involved in mixing. While intense turbulence is observed near the front between the Oyashio and the Tsugaru Current, double diffusion occupies a noticeable fraction in both the Tsugaru Water and the Mixed Water between the Oyashio and the Kuroshio. After determining a criterion to distinguish double diffusion from turbulence, vertical diffusivities and buoyancy fluxes are estimated using microstructure data. When turbulence is weak, double diffusion is observed around temperature and salinity anomalies, partly due to interleaving, and dominates the buoyancy flux. Vertical diffusivities due to double diffusion are parameterized as a function of the 10-m-scale density ratio. The 10-m-scale diffusivity estimates are consistent with the microstructure data when an appropriate criterion to reproduce a probability density function for the Turner angle is applied. A weighted-average diffusivity model is proposed to account properly for turbulence and double diffusion simultaneously.

[1] Strong modulation of turbulent mixing by a westward-propagating tropical instability wave (TIW) was observed in the stratified shear layer between the equatorial undercurrent (EUC) and the surface mixed layer during October and November 2008 at 0 N 140 W. The unique deep diurnal-cycle mixing in the stratified layer beneath the equatorial cold tongue was observed where nighttime turbulent mixing was a factor of 10 greater than during daytime. The turbulent kinetic energy dissipation rate, ɛ, was Ο(10, and the turbulent heat flux was $À500 W m À2, at least 5-10 times greater than observed previously in the central equatorial Pacific. Turbulence mixing varied significantly during the four distinct phases of the meridional flow associated with the TIW. Observations during the northward-to-southward transition recorded the largest values of reduced shear squared, the thickest nighttime surface mixed layer, the deepest penetration of the deep-cycle turbulence, and the largest turbulent heat flux and largest integrated ɛ in the deep-cycle layer (DCL). During steady southward flow, the depth of the bases of the nighttime surface mixed layer and of the DCL were the shallowest. A 50-m-thick layer of strong turbulence was observed immediately above the EUC core during the northward-to-southward and steady southward phases. Here, the average ɛ exceeded 10 À6 W kg À1, the eddy diffusivity exceeded 10 À3 m 2 s À1 , and the turbulent heat flux was $À500 W m À2. To parameterize mixing in the central equatorial Pacific accurately, numerical models must simulate the enhancement of mixing associated with TIWs and also the variability of mixing in different TIW phases.Citation: Inoue, R., R.-C. Lien, and J. N. Moum (2012), Modulation of equatorial turbulence by a tropical instability wave,

Equatorial Internal Wave Experiment observations at 0°, 140°W from October 2008 to February 2009 captured modulations of shear, stratification, and turbulence above the Equatorial Undercurrent by a series of tropical instability waves (TIWs). Analyzing these observations in terms of a four‐phase TIW cycle, we found that shear and stratification within the deep‐cycle layer being weakest in the middle of the N‐S phase (transition from northward to southward flow) and strongest in the late S phase (southward flow) and the early S‐N phase (transition from southward to northward flow). Turbulence was modulated but showed less dependence on the TIW cycle. The vertical diffusivity (KT) was largest during the N (northward flow) and N‐S phases, when stratification was weak, despite weak shear, and was smallest from the late S phase to the S‐N phase, when stratification was strong, despite strong shear. This tendency was less clear in turbulent heat flux because vertical temperature gradients were small at times when KT was large, and large when KT was small. We investigated the dynamics of shear and stratification variations within the TIW cycle by using an ocean general circulation model forced with observed winds. The model successfully reproduced the observed strong shear and stratification in the S phase, except for a small phase difference. The strong shear is explained by vortex stretching by TIWs. The strong stratification is explained by meridional and vertical advection.

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In this study, a Navis‐MicroRider microstructure float and an EM‐APEX float were deployed along the Kuroshio Extension Front. The observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front, presumably generated through mesoscale stirring and near‐inertial oscillations. In these interleaving structures, microscale thermal dissipation rates χ are very high scriptO( >10−7 K2s−1), while turbulent kinetic energy dissipation rates ϵ are relatively low scriptO( 10−10−10−9 Wkg−1), with effective thermal diffusivity Kθ of scriptO( 10−3 m2s−1) consistent with the previous parameterizations for double‐diffusion, and, Kθ is two orders of magnitude larger than the turbulent eddy diffusivity for density Kρ. The average observed dissipation ratio Γ in salt finger and diffusive convection favorable conditions are 1.2 and 4.0, respectively, and are larger than that for turbulence. Our results suggest that mesoscale subduction/obduction and near‐inertial motions could catalyze double‐diffusive favorable conditions, and thereby enhancing the diapycnal tracer fluxes below the Kuroshio Extension Front.

The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1–2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are >O(10−7 W kg−1), and estimated vertical eddy diffusivity shows >O(10−4 m2 s−1) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.

Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO2 uptake.

If a current is composed of a number of constituents with different frequencies, then quadratic friction may be analyzed at the same frequencies. The ratios of the constituents of the friction differ from the ratios for the current itself, with a classic result being that for unidirectional flow a very weak current constituent experiences proportionately 50% more friction than a strong constituent. Here, exact results for the magnitude of the friction constituents are derived and confirmed numerically. The results are applied to the tidal currents in Juan de Fuca Strait and the Strait of Georgia, showing that minor constituents experience proportionately more friction than the main constituent by an amount that varies spatially but is typically less than the classic result of 50%. For two-dimensional currents it is shown that, if there are two current constituents with the same ellipticity and major axis direction, the friction coefficients are separable functions of the current constituent ratio and the ellipticity. Some results are derived for two constituents with different ellipticity and major axis direction. For the case of two constituents with rectilinear but misaligned currents, each constituent experiences friction inclined at an angle to its current. Last, the effect of a tidal current on the bottom friction experienced by a steady flow is investigated for arbitrary relative magnitudes and directions of the tide and steady flow. In particular, the inclination of the mean friction to the mean flow is quantified.