James N. Moum

Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Abstract Detailed observations of the structure within internal solitary waves propagating shoreward over Oregons continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By as...

The Efficiency of Mixing in Turbulent Patches: Inferences from Direct Simulations and Microstructure Observations

The time evolution of mixing in turbulent overturns is investigated using a combination of direct numerical simulations (DNS) and microstructure profiles obtained during two field experiments. The focus is on the flux coefficient G, the ratio of the turbulent buoyancy flux to the turbulent kinetic energy dissipation rate e .I n observational oceanography, a constant value G5 0.2 is often used to infer the buoyancy flux and the turbulent diffusivity from measured e. In the simulations, the value of G changes by more than an order of magnitude over the life of a turbulent overturn, suggesting that the use of a constant value for G is an oversimplification. To account for the time dependence of G in the interpretation of ocean turbulence data, a way to assess the evolutionary stage at which a given turbulent event was sampled is required. The ratio of the Ozmidov scale LO to the Thorpe scale LT is found to increase monotonically with time in the simulated flows, and therefore may provide the needed time indicator. From the DNS results, a simple parameterization of G in terms of LO/ LT is found. Applied to observational data, this parameterization leads to a 50%‐60% increase in median estimates of turbulent diffusivity, suggesting a potential reassessment of turbulent diffusivity in weakly and intermittently turbulent regimes such as the ocean interior.

Comparison of Turbulence Kinetic Energy Dissipation Rate Estimates from Two Ocean Microstructure Profilers

Abstract Almost 1000 microstructure profiles from two separate groups on two separate ships using different instrumentation, signal processing, and calibration procedures were compared for a 3.5-day time period at 0°, 140°W and within 11 km of each other. Systematic bias in the estimates of ϵ is less than a factor of 2, which is within estimates of the cumulative uncertainties in the measurement of ϵ. Although there is no evidence for strong gradients in mean currents, water properties, or surface meteorology, occasional hourly averages of ϵ differ by several factors of 10. Both groups observed periods where ϵ estimates exceeded those of the other group by large factors. The authors believe that the primary reason for these large differences is natural variability, which appears to be greater in the meridional direction than in the zonal direction.

River plumes as a source of large-amplitude internal waves in the coastal ocean.

Satellite images have long revealed the surface expression of large amplitude internal waves that propagate along density interfaces beneath the sea surface. Internal waves are typically the most energetic high-frequency events in the coastal ocean, displacing water parcels by up to 100 m and generating strong currents and turbulence that mix nutrients into near-surface waters for biological utilization. While internal waves are known to be generated by tidal currents over ocean-bottom topography, they have also been observed frequently in the absence of any apparent tide–topography interactions. Here we present repeated measurements of velocity, density and acoustic backscatter across the Columbia River plume front. These show how internal waves can be generated from a river plume that flows as a gravity current into the coastal ocean. We find that the convergence of horizontal velocities at the plume front causes frontal growth and subsequent displacement downward of near-surface waters. Individual freely propagating waves are released from the river plume front when the fronts propagation speed decreases below the wave speed in the water ahead of it. This mechanism generates internal waves of similar amplitude and steepness as internal waves from tide–topography interactions observed elsewhere, and is therefore important to the understanding of coastal ocean mixing.

Length scales of turbulence in stably stratified mixing layers

Turbulence resulting from Kelvin–Helmholtz instability in layers of localized stratification and shear is studied by means of direct numerical simulation. Our objective is to present a comprehensive description of the turbulence evolution in terms of simple, conceptual pictures of shear–buoyancy interaction that have been developed previously based on assumptions of spatially uniform stratification and shear. To this end, we examine the evolution of various length scales that are commonly used to characterize the physical state of a turbulent flow. Evolving layer thicknesses and overturning scales are described, as are the Ozmidov, Corrsin, and Kolmogorov scales. These considerations enable us to provide an enhanced understanding of the relationships between uniform-gradient and localized-gradient models for sheared, stratified turbulence. We show that the ratio of the Ozmidov scale to the Thorpe scale provides a useful indicator of the age of a turbulent event resulting from Kelvin–Helmholtz instability.

The formation and fate of internal waves in the South China Sea

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

Efficiency of mixing in the main thermocline

Estimates of heat flux from direct measurements of vertical velocity-temperature fluctuation correlations have been obtained from vertical profiles through turbulent patches in the main thermocline. These have been compared to more indirect flux estimates derived from dissipation rates of turbulent kinetic energy and temperature variance. Because record lengths are limited by the thickness of observed turbulent patches, uncertainties are larger than would be expected from relatively longer horizontal records. The best estimate of dissipation flux coefficient from these data is about 0.15–0.2, but it is characterized by a large range of sample values. This implies mixing efficiencies (flux Richardson numbers) are about 0.13–0.17. This is within the range of laboratory estimates but is different from measurements in turbulent tidal fronts.

Energy-containing scales of turbulence in the ocean thermocline

From measurements of the energy-containing scales of turbulence in the ocean thermocline, two new formulations are examined: (1) an inviscid estimate for the viscous dissipation rate of turbulent kinetic energy and (2) a mixing length estimate for the turbulent heat flux. These formulations are tested using coincident measurements of the relevant properties of both energy-containing and dissipation scales of stratified turbulence in the oceans main thermocline obtained from a vertical microstructure profiler. It is found that energy-containing scale estimates of both dissipation rate and heat flux compare favorably with dissipation scale estimates. Since the energy-containing scales are many times greater than the dissipation scales, the measurement constraints on these new estimates are considerably less strict than for dissipation scale estimates of the same quantities. These observations also suggest that the timescale for viscous decay of turbulent motions is greater than that for diffusive smoothing of scalar fluctuations. It is argued that this is consistent with current estimates of mixing efficiencies.

Observations of Boundary Mixing over the Continental Slope

Observations of mixing over the continental slope using a towed body reveal a great lateral extent (several kilometers) of continuously turbulent fluid within a few hundred meters of the boundary at depth 1600 m. The largest turbulent dissipation rates were observed ove ra5k mhorizontal region near a slope critical to the M2 internal tide. Over a submarine landslide perpendicular to the continental slope, enhanced mixing extended at least 600 m above the boundary, increasing toward the bottom. The resulting vertical divergence of the heat flux near the bottom implies that fluid there must be replenished. Intermediate nepheloid layers detected optically contained fluid with u‐S properties distinct from their surroundings. It is suggested that intermediate nepheloid layers are interior signitures of the boundary layer detachment required by the near-bottom flux divergance.

Energy Transport by Nonlinear Internal Waves

Wintertime stratification on Oregon’s continental shelf often produces a near-bottom layer of densefluid that acts as an internal waveguide on which nonlinear internal waves propagate. Shipboard profiling and bottom lander observations capture disturbances that exhibit properties of internal solitary waves, bores and gravity currents. Wave-like pulses are highly turbulent (instantaneous bed stresses are 1 N m 2 ), resuspending bottom sediments into the water column and raising them 30 + m above the seafloor. The waves’ cross-shelf transport of fluid counters the time-averaged Ekman transport in the bottom boundary layer. In the nonlinear internal waves we have observed, the kinetic energy is roughly equal to the available potential energy and is O(0.1) MJ per m of coastline. The energy transported by these waves includes a nonlinear advection term huEi that is negligible in linear internal waves. Unlike linear internal waves, the pressure-velocity energy flux hupi includes important contributions from nonhydrostatic effects and surface displacement. It is found that, statistically, huEi ’ 2hupi. Vertical profiles indicate that up(z) is more important in transporting energy near the seafloor while uE(z) dominates farther from the bottom. With the wave speed, c, estimated from weakly nonlinear wave theory it is verified experimentally that the total energy transported by the waves, hupi + huEi ’ chEi. The high but intermittent energyflux by the waves is, in an averaged sense, O(100) W per m of coastline. This is similar to independent estimates of the shoreward energy flux in the semidiurnal internal tide at the shelfbreak.