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Observations of the run-up of internal bores in a shallow bay were made with a tow-yo instrument and mooring arrays with high spatial and temporal resolution. Shoreward propagating internal bores have been studied with laboratory experiments and numerical models, but few observational studies have shown the detailed structure of the run-up of internal bores induced by internal tides. Our observations showed that internal bores propagate along the slope, accompanied by strong turbulent mixing and strong sediment resuspension in a shallow bay. The isothermal displacement due to the bores reached 20 m vertically in a water depth of 40 m. Turbidity measurements showed suspended particles transported from the sloping bottom and offshore above the thermocline, forming an intermediate nepheloid layer (INL). At the head of the bore (dense water), a vortex accompanied by strong vertical motion induced strong vertical sediment resuspension and a steep isothermal displacement. The rate of turbulent kinetic energy dissipation reached 10 −6 W kg −1 at the head of the wave. A nonhydrostatic numerical simulation in a two-dimensional domain reproduced fine features associated with the run-up of an internal bore and the vortex motion at its head.

In this study, turbulence and mixing are investigated in a constricted channel at the mouth of Tokyo Bay, using two instruments: a microstructure profiler, TurboMAP-L, and an ADCP. We observed three different stratified conditions, i.e., SSC (Strongly Stratified Condition), MSC (Moderately Stratified Condition) and WSC (Weakly Stratified Condition) over a complete tidal period. The water exchange at the mouth of Tokyo Bay was in the transitional regime between the hydraulic and the diffusive limit with SSC and MSC close to the hydraulic limit and WSC close to the diffusive limit. For SSC and MSC, the enhancement of the stratification and vertical mixing showed asymmetry over a semidiurnal tide cycle. Mixing was stronger during ebb than during flood for SSC. On the other hand, mixing was elevated during flood for MSC. For WSC, weak stratification and strong mixing occurred during both flood and ebb. The SSC asymmetry resembled the strongly stratified condition of Columbia River ( Kay and Jay, 2003a , Kay and Jay, 2003b ), and the MSC asymmetry resembled the Strain-Induced Periodic Stratification condition ( Simpson et al., 1990 ). A principle component analysis applied to the observed velocity profiles showed that both barotropic and baroclinic flow played a role during SSC and MSC, and in turn, barotropic flow dominated during WSC. The probability density function of the turbulent kinetic energy dissipation rate is practically expressed as a single lognormal population for all three conditions.

Subduction, upwelling, and phytoplankton blooms are commonly observed features at oceanic fronts. This study isolates the role of vertical mixing for enhanced production and water mass subduction near fronts, considering the time-developing problem with a Semi-Geostrophic circulation model coupled to a planktonic ecosystem model. Our model results show that vertical mixing in the surface boundary layer strongly modifies the time evolution of the front and of its associated biology. Ageostrophic flows caused by the combined effects of confluence and vertical mixing enhance primary production on the less dense side and increase water mass subduction on the dense side of the front. Confluence alone results in the intensification of the front by the same advective response, while the phytoplankton bloom on the less dense side does not arise without vertical mixing. Vertical mixing alone slumps the front near the surface and provides weak subduction on the dense side and uplift of the isopycnals at the center of the front. We find that it is possible to sustain an isolated phytoplankton patch above the domed isopycnals at the center of the front with the nutrients supplied by the secondary circulations arising due to vertical mixing. These results suggest that the phytoplankton bloom and patches found on the less dense side of fronts in many field observations are likely caused by fine-scale along-isopycnal upwelling of nutrients forced by adiabatic confluence in the meander trough of fronts and further pumping and entrainment of nutrients by the secondary circulation due to vertical mixing. Isolated patches observed at the center of the front in many frontal surveys could be caused by secondary flows due to vertical mixing.

A steady deepening of the diurnal mixed layer was observed in a large Japanese freshwater lake Lake Biwa during nocturnal cooling. The observed dissipation rates normalized with surface buoyancy flux and wind stress are in good agreement with previously reported values. The observed time series of the water column was simulated using the Mellor–Yamada second-order turbulence closure scheme. The time series of the observed diurnal mixing layer depth agrees well with the simulated time series. The details of entrainment were investigated using both data sets. Because the deepening was due to both convection and wind stress, the entrainment was separated into two parts. Each part followed the empirical relationships reported from laboratory studies. In order to use both forcings, namely convection and wind stress, we propose a new entrainment diagram taking both into consideration.