From Topographic Internal Gravity Waves to Turbulence (original) (raw)
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Deep Sea Research Part II: Topical Studies in Oceanography, 2006
Much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean. Here the process of internal-tide generation and dissipation by tidal flow over an isolated Gaussian topography is examined, using two-dimensional numerical simulations employing the MITgcm. Four different topographies are considered, for five different amplitudes of barotropic forcing, thereby allowing a variety of combinations of key nondimensional parameters. While much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes, here attention is focused on parameters dependent on the flow amplitude. For narrow topography, large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency. Tall narrow topographies are found to be the most conducive to mixing. Dissipation rates in these calculations are most efficient for the narrowest topography. r
Turbulent mixing driven by mean-flow shear and internal gravity waves in oceans and atmospheres
2012
This study starts with balances deduced by Baumert and Peters (2004, 2005) from results of stratified-shear experiments made in channels and wind tunnels by Itsweire (1984) and Rohr and Van Atta (1987), and of free-decay experiments in a resting stratified tank by Dickey and Mellor (1980). Using a modification of Canuto's (2002) ideas on turbulence and waves, these balances are merged with an (internal) gravity-wave energy balance presented for the open ocean by Gregg (1989), without mean-flow shear. The latter was augmented by a linear (viscous) friction term. Gregg's wave-energy source is interpreted on its long-wave spectral end as internal tides, topography, large-scale wind, and atmospheric low-pressure actions. In addition, internal eigen waves, generated by mean-flow shear, and the aging of the wave field from a virginal (linear) into a saturated state are taken into account. Wave packets and turbulence are treated as particles (vortices, packets) by ensemble kinetics so that the loss terms in all three balances have quadratic form. Following a proposal by Peters (2008), the mixing efficiency of purely wave-generated turbulence is treated as a universal constant, as well as the turbulent Prandtl number under neutral conditions. It is shown that: (i) in the wind tunnel, eigen waves are switched off, (ii) due to remotely generated long waves or other non-local energy sources, coexistence equilibria of turbulence and waves are stable even at Richardson numbers as high as 10310^3103; (iii) the three-equation system is compatible with geophysically shielded settings like certain stratified laboratory flows. The agreement with a huge body of observations surprises. Gregg's (1989) wave-model component and the a.m. universal constants taken apart, the equations contain only one additional dimensionless parameter for the eigen-wave closure, estimated as Yapprox1.35.Y\approx 1.35.Yapprox1.35.
Eddy-Modulated Internal Waves and Mixing on a Midocean Ridge
Journal of Physical Oceanography, 2012
Mesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales ,50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number (Ri) , 1 /4. In contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a ''cascade'') from subinertial flows, including mesoscale eddies, to nearinertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.
Observations of turbulence in a tidal beam and across a coastal ridge
Journal of Geophysical Research: Oceans, 2001
During a microstructure survey off California in Monterey Bay, we found a midwater beam of strong turbulence emanating from the shelf break along the ray path of the semidiurnal M 2 internal tide. Within the 50-m-thick beam the turbulence kinetic energy dissipation rate ε exceeded 10 −6 W kg −1 , and the diapycnal eddy diffusivity K ρ was > 0.01 m 2 s −1. The beam extended 4 km off the shelf break. Several factors suggest that this beam of strong turbulence resulted from the breaking of semidiurnal internal tides: the beam appeared to originate from the shelf break, which is a potential generation site for semidiurnal internal tides; the beam closely followed the ray path of the semidiurnal internal tide; the average ε off the shelf break varied by a factor of 100 with a semidiurnal tidal periodicity; the isopycnal displacement confirmed the presence of semidiurnal internal tides. Processes associated with the breaking of internal tides are intermittent and sporadic. At the same location we also observed equally intense turbulence in a ∼100-m-thick layer of stratified water across the ridge of a sea fan. This layer of strong turbulence was separated from the bottom and was clearly not generated by bottom friction. Although less well resolved in time, the strong turbulence above the bottom seemed to vary with the semidiurnal tide and existed at the lee of the ridge, where the isopycnal surface dipped and rebounded in a pattern resembling that of internal hydraulic jumps. On the basis of the behavior of the density field, we believe that the deep mixing was most likely produced by the across-ridge current of internal tides. The breaking of internal tides at middepth, where the Richardson number is close to the critical value, is likely due to shear instability. The presence of the coastal ridge provides an alternative pathway for converting energy from internal tides to turbulence via internal hydraulics. Multiplying the average ε in the midwater beam by the length of the global coastline gives 31 GW, only a small fraction of the estimated 360 GW dissipated globally by M 2 internal tides. Our observations suggest that either most internal tides are generated away from shelf breaks or most internal tides generated at shelf breaks propagate away from their generation sites, rather than dissipate locally, and eventually contribute to pelagic mixing.
Tidal mixing processes amid small-scale, deep-ocean topography
Geophysical Research Letters, 2015
The nature of tide-topography interaction reflects the topographic scales experienced by water parcels during their tidal excursions. In the deep ocean these scales are typically subkilometer, yet direct observations of tidal processes on such scales are lacking. At one site, a saddle amid steep and complex Mid-Atlantic Ridge topography, observations reveal tidally pulsed, bottom-trapped fronts, overflows, and lee waves in response to a tide combined with a mean flow of similar amplitude. The tidal pulsing of the fronts and overflows was only evident locally, and their phase became unpredictable over scales of hundreds of meters. Enhanced turbulence in a 100-200 m thick bottom boundary layer had an estimated dissipation rate of 2.6 × 10 À2 W m À2 , exceeding the large-scale average of tidal dissipation in mid-ocean ridge environments but by less than an order of magnitude. This site was not a dissipation "hotspot," and the processes observed could provide widely distributed mixing to the meridional overturning circulation.
Interactions Between Nonlinear Internal Ocean Waves and the Atmosphere
Geophysical Research Letters, 2019
The heterogeneity in surface roughness caused by transient, nonlinear internal ocean waves is readily observed in coastal waters. However, the quantifiable impact this heterogeneity has on the marine atmospheric surface layer has not been documented. A comprehensive data set collected from a unique ocean platform provided a novel opportunity to investigate the interaction between this internal ocean process and the atmosphere. Relative to the background atmospheric flow, the presence of internal waves drove wind velocity and stress variance. Furthermore, it is shown that the wind gradient adjusts across individual wave fronts, setting up localized shear that enhanced the air-sea momentum flux over the internal wave packet. This process was largely mechanical, though secondary impacts on the bulk humidity variance and gradient were observed. This study provides the first quantitative analysis of this phenomenon and provides insights into submesoscale air-sea interactions over a transient, internal ocean feature. Plain Language Summary The ocean surface appears rough because the wind applies a tangential force to the water, which deforms the surface, generating short and steep waves. These small waves, in turn, increase the friction felt by the wind as it blows across the ocean surface, thereby setting up a feedback mechanism that physically links, or couples, the lower atmosphere to the upper ocean. However, our understanding of this interaction in the case of a heterogeneously rough ocean surface is limited. Using a unique ocean platform, we have collected a novel and complete data set demonstrating the impact internal ocean waves have on the near-surface atmospheric variability, through their modulation of the ocean surface roughness. The surface currents associated with internal waves generate bands of smooth and rough water that travel coherently with the internal wave packet. Our analysis shows that these transient surface features have a distinct and profound impact on the physical characteristics and structure of the near-surface atmosphere. In particular, internal waves enhance the wind forcing over the ocean and individual wave fronts significantly alter the vertical wind gradient. Our results provide the first documentation of the impact internal waves have on the atmosphere and suggest that these dynamics should be accounted for when studying fine-scale atmosphere-ocean interactions and the impact internal waves have on the marine environment. U∕u * = 1 log (u * z) + C,
Topographic generation of internal waves by nonlinear superposition of tidal harmonics
Deep Sea Research Part I: Oceanographic Research Papers, 2005
A numerical study of topographic generation of internal waves by superposition of the semidiurnal and the diurnal tidal harmonics was motivated by the experimental data collected in the area of the Malin Shelf. In situ measurements and synthetic aperture radar (SAR) images revealed the evidence of the packets of internal waves propagating on-shelf. The tidal periodicity of these waves and their location (between the isobaths 160 m and 1000 m, i.e. far from the shelf break) as well as the spatial orientation of wave fronts suggest that they were not locally generated but came from some distant source. A detailed analysis performed with the help of a two-dimensional nonlinear nonhydrostatic numerical model shows that the most probable place of wave generation is the eastern flank of the Rockall Bank located 150 km off the Malin Shelf. The result demonstrates that a nonlinear superposition of diurnal and semidiurnal barotropic tidal harmonics was capable of generating the waves even though each of harmonics separately was too weak to produce a substantial baroclinic response. Such a case of efficient superposition of two tidal forcings takes place only over relatively steep bottom topography when fundamentally unsteady lee waves are excited. The same mechanism of generation can take place in some other parts of the World Ocean where the barotropic tides comprise several harmonics. r
Scattering of internal tides by barotropic quasigeostrophic flows
Oceanic internal tides and other inertia-gravity waves propagate in an energetic turbulent flow whose length scales are similar to the wavelengths. Advection and refraction by this flow cause the scattering of the waves, redistributing their energy in wavevector space. As a result, initially plane waves radiated from a source such as a topographic ridge become spatially incoherent away from the source. To examine this process, we derive a kinetic equation which describes the statistics of the scattering under the assumptions that the flow is quasigeostrophic, barotropic and well represented by a stationary homogeneous random field. Energy transfers are quantified by computing a scattering cross-section and shown to be restricted to waves with the same frequency and identical vertical structure, hence the same horizontal wavelength. For isotropic flows, scattering leads to an isotropic wave field. We estimate the characteristic time and length scales of this isotropisation, and study their dependence on parameters including the energy spectrum of the flow. Simulations of internal tides generated by a planar wavemaker carried out for the linearised shallow-water model confirm the pertinence of these scales. A comparison with the numerical solution of the kinetic equation demonstrates the validity of the latter and illustrates how the interplay between wave scattering and transport shapes the wave statistics.
Turbulence During the Generation of Internal Tide on a Critical Slope
Physical Review Letters, 2010
Three-dimensional direct numerical simulations are performed to examine nonlinear processes during the generation of internal tides on a model continental slope. An intense boundary flow is generated in the critical case where the slope angle is equal to the natural internal wave propagation angle. Wave steepening, that drives spanwise wave breaking via convective instability, occurs. Turbulence is present along the entire extent of the near-critical region of the slope. The turbulence is found to have a strong effect on the internal wave beam by distorting its near-slope structure. A complicated wave field with a broadband frequency spectrum is found. This work explains the formation of boundary turbulence during the generation of internal tides in the regime of low excursion numbers.