W. D. Smyth

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.

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.

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.

Anisotropy of turbulence in stably stratified mixing layers

Direct numerical simulations of turbulence resulting from Kelvin–Helmholtz instability in stably stratified shear flow are used to study sources of anisotropy in various spectral ranges. The set of simulations includes various values of the initial Richardson and Reynolds numbers, as well as Prandtl numbers ranging from 1 to 7. We demonstrate that small-scale anisotropy is determined almost entirely by the spectral separation between the small scales and the larger scales on which background shear and stratification act, as quantified by the buoyancy Reynolds number. Extrapolation of our results suggests that the dissipation range becomes isotropic at buoyancy Reynolds numbers of order 105, although we cannot rule out the possibility that small-scale anisotropy persists at arbitrarily high Reynolds numbers, as some investigators have suggested. Correlation-coefficient spectra reveal the existence of anisotropic flux reversals in the dissipation subrange whose magnitude decreases with increasing Reynolds n...

Resonant Wind-Driven Mixing in the Ocean Boundary Layer

Abstract The role of resonant wind forcing in the ocean boundary layer was examined using an ocean large-eddy simulation (LES) model. The model simulates turbulent flow in a box, measuring ∼100–300 m on a side, whose top coincides with the ocean surface. Horizontal boundary conditions are periodic, and time-dependent wind forcing is applied at the surface. Two wind forcing scenarios were studied: one with resonant winds, that is, winds that rotated at exactly the inertial frequency (at 45°N), and a second with off-resonance winds from a constant direction. The evolution of momentum and temperature for both cases showed that resonant wind forcing produces much stronger surface currents and vertical mixing in comparison to the off-resonance case. Surface wave effects were also examined and found to be of secondary importance relative to the wind forcing. The main goal was to quantify the main processes via which kinetic energy input by the wind is converted to potential energy in the form of changes in the ...

Upper-Ocean Turbulence during a Westerly Wind Burst: A Comparison of Large-Eddy Simulation Results and Microstructure Measurements

The response of the upper ocean to westerly wind forcing in the western equatorial Pacific was modeled by means of large-eddy simulation for the purpose of comparison with concurrent microstructure observations. The model was initialized using currents and hydrography measured during the Coupled Ocean‐Atmosphere Response Experiment (COARE) and forced using measurements of surface fluxes over a 24-h period. Comparison of turbulence statistics from the model with those estimated from concurrent measurements reveals good agreement within the mixed layer. The shortcomings of the model appear in the stratified fluid below the mixed layer, where the vertical length scales of turbulent eddies are limited by stratification and are not adequately resolved by the model. Model predictions of vertical heat and salt fluxes in the entrainment zone at the base of the mixed layer are very similar to estimates based on microstructure data.

Local ocean response to a multiphase westerly wind burst. 2. Thermal and freshwater responses

A westerly wind burst observed in the warm pool of the western equatorial Pacific Ocean cooled the oceans surface layer by about 0.8oC. Turbulent entrainment at the base of this layer caused cooling but also heating due to the reversal of the vertical temperature gradient during rain events. Consequently, the cumulative effect of turbulent entrainment was minimized. Following the wind burst, a sustained eastward surface current contributed to high current shear and turbulent dissipation rates at the top of the thermocline. As a result, most of the heat transfer into the thermocline occurred after the wind burst had ended. The cruise-averaged turbulent flux into the thermocline was 17 + 10 W m -2, which suggests that the annual mean is only a few watts per square meter. The restratification of the upper ocean in the aftermath of the wind burst is accounted for partly (but not wholly) by local turbulent entrainment. Despite heavy precipitation, upper ocean salinity generally increased during the cruise. Advection appears to have been the dominant factor governing local salinity changes.

Air–Sea Interactions from Westerly Wind Bursts During the November 2011 MJO in the Indian Ocean

The life cycles of three Madden–Julian oscillation (MJO) events were observed over the Indian Ocean as part of the Dynamics of the MJO (DYNAMO) experiment. During November 2011 near 0°, 80°E, the site of the research vessel Roger Revelle, the authors observed intense multiscale interactions within an MJO convective envelope, including exchanges between synoptic, meso, convective, and turbulence scales in both atmosphere and ocean and complicated by a developing tropical cyclone. Embedded within the MJO event, two bursts of sustained westerly wind (>10 m s−1; 0–8-km height) and enhanced precipitation passed over the ship, each propagating eastward as convectively coupled Kelvin waves at an average speed of 8.6 m s−1. The ocean response was rapid, energetic, and complex. The Yoshida–Wyrtki jet at the equator accelerated from less than 0.5 m s−1 to more than 1.5 m s−1 in 2 days. This doubled the eastward transport along the oceans equatorial waveguide. Oceanic (subsurface) turbulent heat fluxes were compara...

Narrowband Oscillations in the Upper Equatorial Ocean. Part II: Properties of Shear Instabilities

Abstract Narrowband oscillations observed in the upper equatorial Pacific are interpreted in terms of a random ensemble of shear instability events. Linear perturbation analysis is applied to hourly averaged profiles of velocity and density over a 54-day interval, yielding a total of 337 unstable modes. Composite profiles of mean states and eigenfunctions surrounding the critical levels suggest that the standard hyperbolic tangent model of Kelvin–Helmholtz (KH) instability is a reasonable approximation, but the symmetry of the composite perturbation is broken by the stratification and vorticity gradient of the underlying equatorial undercurrent. Unstable modes are found to occupy a range of frequencies with a peak near 1.4 mHz, consistent with the frequency content of the observed oscillations. A probabilistic theory of random instabilities predicts this peak frequency closely. An order of magnitude estimate suggests that the peak frequency is of order N, in accord with the observations. This results not ...