Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L29: Geophysical Fluid Dynamics: Internal Wave Dynamics |
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Sponsoring Units: DFD Chair: Raffaele Ferrari, MIT Room: 310 |
Monday, November 23, 2015 4:05PM - 4:18PM |
L29.00001: Turning Ocean Mixing Upside Down Raffaele Ferrari, Ali Mashayek, Jean-Michael Campin, Trevor McDougall, Maxim Nikurashin It is generally understood that small-scale mixing, such as is caused by breaking internal waves, drives upwelling of the densest ocean waters that sink to the ocean bottom at high latitudes. However the observational evidence that small-scale mixing is more vigorous close to the ocean bottom than above implies that small-scale mixing converts light waters into denser ones, thus driving a net sinking of abyssal water. It is shown that abyssal waters return to the surface along weakly stratified boundary layers, where the small-scale mixing of density decays to zero. The net ocean meridional overturning circulation is thus the small residual of a large sinking of waters, driven by small-scale mixing in the stratified interior, and an equally large upwelling, driven by the reduced small-scale mixing along the ocean boundaries. Thus whether abyssal waters upwell or sink in the net cannot be inferred simply from the vertical profile of mixing intensity, but depends also on the ocean hypsometry, i.e. the shape of the bottom topography. The implications of this result for our understanding of the abyssal ocean circulation will be presented with a combination of numerical models and observations. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L29.00002: Internal wave generation by tidal flow over random topography Jiajun Zhao, Likun Zhang, Harry Swinney The irregularity of oceanic topography plays a critical role in determining the power in internal waves generated by tidal flow over the seafloor. We conduct numerical simulations (for a fluid with a constant buoyancy frequency) for different synthetic random topographies. For topography with small rms height $H_{\mathrm{rms}}$ and small slopes the simulations yield a \textit{quadratic} dependence of the power on $H_{\mathrm{rms}}$, in accord with linear theory. However, for tall topography with steep slopes the internal wave power is found to vary \textit{linearly} with $H_{\mathrm{rms}}$. The transition from quadratic to linear scaling of the radiated internal wave power on $H_{\mathrm{rms}}$ occurs when the ``valley slope'' exceeds the internal wave slope. (The valley slope, to be defined in this talk, characterizes the maximum slope of topography between adjacent peaks.) The simulations also reveal that the radiated power saturates with increasing topographic resolution, as conjectured in previous studies. The present results should be helpful in improving estimates of the total internal wave power generated by the world's oceans. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L29.00003: Experiments with mixing in stratified flow over a topographic ridge Ross Griffiths, Yvan Dossmann, Madeleine Gamble Rosevear, Andy McC. Hogg, Graham Hughes, Michael Copeland The interaction of balanced abyssal ocean flow with submarine topography is expected to generate lee waves, which can carry energy into the ocean interior, as well as local turbulent mixing near the boundary. We report observations of lee waves and turbulence, and measurements of the mixing rate, in laboratory experiments with a topographic ridge towed through a density stratification. The experiments span three parameter regimes including linear lee waves, nonlinear wave radiation and an evanescent regime in which wave radiation is not possible. The stratification evolves from an initially uniform buoyancy frequency to a mixed boundary layer and pycnocline. Full field density measurements provide the depth-dependence of energy loss to turbulent mixing. The ratio of the local mixing in the turbulent wake and remote mixing by wave radiation takes a nearly constant value that is not sensitive to the stratification or dynamical regime; the average value $q_{mix}= 0.90 \pm 0.06$ in the linear lee wave regime, is three times larger than that assumed in parameterizations of internal wave-induced mixing in the ocean. The results suggest that mixing by local nonlinear mechanisms close to abyssal ocean topography may be much greater than remote mixing by lee waves. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L29.00004: Transient triadic instability of internal gravity wave a new track to turbulence in the lee of a topography Jean-Marc Chomaz, Gaétan Lerisson Internal gravity waves in a continuously stratified fluid propagate energy away from the source and are particularly important to understand the ocean mixing. We study the stability of different gravity wave inhomogeneous in space through fully non-linear direct numerical simulation and linear global stability analysis and transient growth computation. In particular the steady flow over an arbitrary topography is computed using the selective frequency algorithm and the stability properties of the flow are analysed using the Arnoldi-Krylov technique applied to the direct linear equation to retrieve the global spectrum and to the direct-adjoint technique to optimize transient growth. We show that, both exponential and transient growths are linked to the triadic instability of the lee wave but correspond respectively to the large scale branch and the small scale branch also known as the parametric subharmonic instability. Interpretation of this surprising selection principle is proposed in term of the absolute and convective instability of the 2D periodic planar wave (see the presentation by G. Lerisson et al.) [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L29.00005: Impact of a mean current on internal tide energy dissipation at the critical latitude Océane Richet, Jean-Marc Chomaz, Caroline Muller In many regions of the ocean, the abyssal flow is dominated by tidal flow. A large fraction of the tidal energy input in the ocean is dissipated via the generation of internal waves above rough topography. Idealised simulations suggest that internal tide energy is transferred and dissipated at smallerscales by the formation of a resonant triad between near-inertial waves, internal tides and subharmonics waves. Furthermore, the energy dissipation is enhanced at the critical latitude (28.8$^\circ$), corresponding to the \textit{Parametric Subharmonic Instability} (PSI). In the ocean, the presence of background flow, for instance due to the passage of a mesoscale eddy, can modify energy transfer mechanisms and the amount of energy dissipation. In this study, we investigate the generation and dissipation of internal tides in the presence of a background flow. We use a high-resolution two-dimensional nonhydrostatic numerical model (the MITgcm), with realistic multiscale topography representing the Brazil basin region. The purpose of this study is to understand the impact of the mean flow on the generation and dissipation of tidal waves. Our particular interest is how the maximum of energy dissipation at the critical latitude is impacted by the mean flow. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L29.00006: Determining Pressure and Velocity Fields from Experimental Schlieren Data Frank M. Lee, Michael R. Allshouse, P.J. Morrison, Harry L. Swinney Internal gravity waves generated by tidal flow over bottom topography in the ocean are important because they contribute significantly to the energy composition of the ocean. Determination of the instantaneous internal wave energy flux requires knowledge of the pressure and velocity fields, each of which is difficult to measure in the ocean or the laboratory. However, the density perturbation field can be measured using a laboratory technique known as ``synthetic schlieren.'' We present an analytical method for deducing both the pressure and velocity fields from the density perturbation field. This yields the instantaneous energy flux of linear internal waves. Our method is verified in tests with data from a Navier-Stokes direct numerical simulation. The method is then applied to laboratory schlieren data obtained for the conditions in the numerical simulations. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L29.00007: Internal Wave Generation by Tide-Topography Interactions in the Presence of a Vertically Sheared Background Current Kevin Lamb, Michael Dunphy Vertically sheared background currents alter the generation of internal waves by tide-topography interactions by introducing asymmetries and minimum phase speeds for horizontally propagating vertical modes. A linear theory for internal wave generation for arbitrary stratifications and background currents, restricted to lie above two-dimensional topography, has been developed. Rotational affects have not been considered. In this talk the results of fully nonlinear simulations of the internal wave generation process will be presented and compared with predictions of the linear theory. We have found that the theory gives good predictions for wide subcritical ridges. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L29.00008: Experiments on topographies lacking tidal conversion Leo Maas, Alexandre Paci, Bing Yuan In a stratified sea, internal tides are supposedly generated when the tide passes over irregular topography. It has been shown that for any given frequency in the internal wave band there are an infinite number of exceptions to this rule of thumb. This ``stealth-like'' property of the topography is due to a subtle annihilation of the internal waves generated during the surface tide's passage over the irregular bottom. We here demonstrate this in a lab-experiment. However, for any such topography, subsequently changing the surface tide's frequency does lead to tidal conversion. The upshot of this is that a tidal wave passing over an irregular bottom is for a substantial part trapped to this irregularity, and only partly converted into freely propagating internal tides. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L29.00009: Transient Growth in Internal Solitary Waves Karl Helfrich, Pierre-Yves Passaggia, Brian White Internal solitary waves of large amplitude are common in the atmosphere and ocean and play an important role in mixing and transport. While these waves can propagate over long distances, observations suggest they are susceptible to a range of instabilities, which promote breakdown, overturning, and mixing. To gain insight into these instabilities, we consider the optimal transient growth of a family of solitary waves, which are solutions to the Dubreil-Jacotin-Long (DJL) equation for increasing phase speed and varying background stratification. Optimal initial disturbances are computed by means of direct-adjoint iterations of the Navier-Stokes system in the Boussinesq approximation. The most amplified disturbances resemble Kelvin-Helmholtz instabilities and are localized near the bottom of the wave, where the Richardson number is minimum, and are maximized for short time horizons. The optimal transient growth of these perturbations is shown to increase with the phase speed. Implications for breakdown and mixing will be discussed. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L29.00010: Impulse response of an internal gravity vawe and the absolute or convective nature of the triadic instability Ga\'etan Lerisson, Jean-Marc Chomaz, Sabine Ortiz Internal gravity waves propagate energy from the source and are important to understand the ocean mixing. We compute the 2D impulse response of a infinite internal wave by direct numerical simulation using an extremely extended computational domain with a resolution up to 16384 by 8192 and integration time (up to 300 Brunt V\"ais\"al\"a periode). Such extended domain and long time are necessary since the base flow is periodic in space and the impulse response has to converge on each ray onto the spatio-temporal Floquet mode. We observe the splitting of the impulse response into 3 different wave packets and show that each of them corresponds to a different branch of the triadic instability. Reanalysis of the triadic instability taking into account the detuning from the exact resonance allows us to show that the group velocity of each leading triads is the average of the group velocity of the two resonant waves. The small-scale wave packet then moves with the fluid where as the large scale mode has a group velocity comparable or larger than the base wave itself. We deduce from this impulse response the absolute and convective nature of each branch of the triadic instability and predict a selection of the instability mode strongly sensitive to the mean advection speed. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L29.00011: Instability and mixing of stratified shear layers forced by internal wave strain Alexis Kaminski, John Taylor Mixing of the stably-stratified ocean interior plays an important role in determining the vertical stratification and the transport of key biological and geochemical tracers. Shear instabilities are thought to be a key mechanism in triggering small-scale mixing in the ocean, and a large literature is devoted to examining the stability properties of steady, parallel stratified shear flows. However, geophysical flows are frequently complicated by additional processes, such as internal waves, leading to variation in space and in time. Not only is the breaking of internal waves an important source of mixing, but the vertical strain caused by these waves may also impact the stability of the flows through which they propagate. Here, we idealize this process by imposing a standing wave which is spatially and temporally periodic onto a stably-stratified shear flow. We use a direct-adjoint looping method to examine the linear stability of this complicated base flow over a range of parameters in order to identify and quantify the effect of the wave strain on the overall flow stability. Direct numerical simulations are then used to examine the nonlinear evolution and subsequent mixing. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L29.00012: Internal Wave Apparatus for Copepod Behavior Assays S. Jung, K.A. Haas, D.R. Webster Internal waves are ubiquitous features in coastal marine environments and have been observed to mediate vertical distributions of zooplankton \textit{in situ}. Internal waves are generated through oscillations of the pycnocline in stratified waters and thereby create fine-scale hydrodynamic cues that copepods and other zooplankton are known to sense, such as fluid density gradients and velocity gradients (quantified as shear deformation rate). The role of copepod behavior in response to cues associated with internal waves is largely unknown. Thus, a coupled quantification of copepod behavior and hydrodynamic cues will provide insight to the bio-physical interaction and the role of biological versus physical forcing in mediating organism distributions. We constructed a laboratory-scale internal wave apparatus to facilitate fine-scale observations of copepod behavior in flows that replicate \textit{in situ} conditions of internal waves in a two-layer stratification. Three cases are chosen with density jump ranging between 0.75 -- 1.5 kg/m$^{3}$. Analytical analysis of the two-layer system provides guidance of the target forcing frequency to generate a standing internal wave with a single dominate frequency of oscillation. Flow visualization and signal processing of the interface location are used to quantify the wave characteristics. A copepod behavior assay is conducted, and sample trajectories are analyzed to identify copepod response to internal wave structure. [Preview Abstract] |
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