- On the predictability of sea surface height around Palau
Andres, M., Musgrave, R., Rudnick, D. L., Zeiden, K., Peacock, T. & Park, J. H.
Journal of Physical Oceanography, 50, (11), 3267-3294 (2020)
As part of the Flow Encountering Abrupt Topography (FLEAT) program, an array of pressure-sensor equipped inverted echo sounders (PIESs) was deployed north of Palau where the westward-flowing North Equatorial Current encounters the southern end of the Kyushu–Palau Ridge in the tropical North Pacific. Capitalizing on concurrent observations from satellite altimetry, FLEAT Spray gliders, and shipboard hydrography, the PIESs’ 10-month duration hourly bottom pressure p and round-trip acoustic travel time τ records are used to examine the magnitude and predictability of sea level and pycnocline depth changes and to track signal propagations through the array. Sea level and pycnocline depth are found to vary in response to a range of ocean processes, with their magnitude and predictability strongly process dependent. Signals characterized here comprise the barotropic tides, semidiurnal and diurnal internal tides, southeastward-propagating superinertial waves, westward-propagating mesoscale eddies, and a strong signature of sea level increase and pycnocline deepening associated with the region’s relaxation from El Niño to La Niña conditions. The presence of a broad band of superinertial waves just above the inertial frequency was unexpected and the FLEAT observations and output from a numerical model suggest that these waves detected near Palau are forced by remote winds east of the Philippines. The PIES-based estimates of pycnocline displacement are found to have large uncertainties relative to overall variability in pycnocline depth, as localized deep current variations arising from interactions of the large-scale currents with the abrupt topography around Palau have significant travel time variability.
- Uncovering Fine-Scale Wave-Driven Transport Features in a Fringing Coral Reef System via Lagrangian Coherent Structures
Leclair, M., Lowe, R., Zhang, Z., Ivey, G.R. & Peacock, T.
Fluids, 5, (4), 190 (2020)
Understanding the transport and exchange of water masses both within a reef and between a reef and the surrounding ocean is needed to describe a wide-range of ecosystem processes that are shaped by the movement of material and heat. We show how novel Lagrangian data processing methods, specifically developed to reveal key and often hidden transport structures, can help visualize flow transport patterns within and around morphologically complex reef systems. As an example case study, we consider the wave-driven flow transport within the Ningaloo Reef in Western Australia. We show that a network of attracting, repelling, and trapping flow transport structures organizes the flow transport into, around, and out of the reef. This approach is broadly applicable to coral reef systems, since the combination of well-defined bathymetry and persistent flow-forcing mechanisms (e.g., by wave breaking or tides) is conducive to the existence of persistent Lagrangian transport structures that organize material transport.
- Consideration of midwater ecosystems is required to fully evaluate the environmental risks of deep-sea mining
Drazen, J. C., Smith, C. R., Gjerde, K. M., Haddock, S. H. D., Carter, G., Choy, C. A., Clark, M., Dutrieux, P., Goetze, E., Hauton, C., Hatta, M., Koslow, J. A., Leitner, A., Pacini, A., Perelman, J. N., Peacock, T., Sutton, T., Watling, L. & Yamamoto, H
Proceedings of the National Academy of Sciences, 117, (30), 17455-17460 (2020)
Despite rapidly growing interest in deep seabed mineral exploitation, environmental research and management have focused on potential impacts to benthic environments and have paid little attention to pelagic ecosystems. Yet pelagic ecosystems have established ecological and societal importance. Seafloor mining activities will generate sediment plumes and noise in the benthic boundary layer and higher in the water column that may have extensive ecological effects in deep midwaters, depths from ~200m to the seafloor. These ecosystems represent more than 90% of the livable volume on our planet, contain a fish biomass 100 times greater than the global annual fish catch, connect shallow-living ecosystems to deeper ones including the benthos, and play key roles in carbon export, nutrient regeneration, and in the provisioning of harvestable fish stocks. These deep midwater ecosystem services as well as biodiversity could be negatively affected by mining. We will examine the potential effects of deep seabed mining on midwater ecosystems and provide specific recommendations on how ecosystem risks could be more comprehensively and effectively evaluated.
- Whither the Chukchi Slope Current?
Boury, S., Pickart, R. S., Odier, P., Lin, P., Li, M., Fine, E. C., Simmons, H. L., MacKinnon, J. A. & Peacock, T.,
Journal of Physical Oceanography, 50, (6), 1717-1732 (2020)
Recent measurements and modeling indicate that roughly half of the Pacific-origin water exiting the Chukchi Sea shelf through Barrow Canyon forms a westward-flowing current known as the Chukchi Slope Current (CSC), yet the trajectory and fate of this current is presently unknown. In this study, through the combined use of shipboard velocity data and information from five profiling floats deployed as quasi-Lagrangian particles, we delve further into the trajectory and the fate of the CSC. During the period of observation, from early September to early October 2018, the CSC progressed far to the north into the Chukchi Borderland. The northward excursion is believed to result from the current negotiating Hanna Canyon on the Chukchi slope, consistent with potential vorticity dynamics. The volume transport of the CSC, calculated using a set of shipboard transects, decreased from approximately 2 Sv (1 Sv ≡ 106 m3 s−1) to near zero over a period of 4 days. This variation can be explained by a concomitant change in the wind stress curl over the Chukchi shelf from positive to negative. After turning northward, the CSC was disrupted and four of the five floats veered offshore, with one of the floats permanently leaving the current. It is hypothesized that the observed disruption was due to an anticyclonic eddy interacting with the CSC, which has been observed previously. These results demonstrate that, at times, the CSC can get entrained into the Beaufort Gyre.
- Search and rescue at sea aided by hidden flow structures
Serra, M., Sathe, P., Rypina, I., Kirincich, A., Lermusiaux, P., Allen, A., Peacock, T. & Haller, G.
Nature Communications, 11, 2525 (2020)
Every year, hundreds of people die at sea because of vessel and airplane accidents. A key challenge in reducing the number of these fatalities is to make Search and Rescue (SAR) algorithms more efficient. Here, we address this challenge by uncovering hidden TRansient Attracting Profiles (TRAPs) in ocean-surface velocity data. Computable from a single velocity-field snapshot, TRAPs act as short-term attractors for all floating objects. In three different ocean field experiments, we show that TRAPs computed from measured as well as modeled velocities attract deployed drifters and manikins emulating people fallen in the water. TRAPs, which remain hidden to prior flow diagnostics, thus provide critical information for hazard responses, such as SAR and oil spill containment, and hence have the potential to save lives and limit environmental disasters.
- Using braids to quantify interface growth and coherence in a rotor oscillator flow
Filippi, M., Budisic, M., Allshouse, M.R., Atis, S., Thiffeault, J.L. & Peacock,T.
Physical Review Fluids, 5, (5), 054504 (2020)
The growth rate of material interfaces is an important proxy for mixing and reaction rates in fluid dynamics, and can also be used to identify regions of coherence. Estimating such growth rates can be difficult, since they depend on detailed properties of the velocity field, such as its derivatives, that are hard to measure directly. When an experiment gives only sparse trajectory data, it is natural to encode planar trajectories as mathematical braids, which are topological objects that contain information on the mixing characteristics of the flow, in particular through their action on topological loops. We test such braid methods on an experimental system, the rotor-oscillator flow, which is well-described by a theoretical model. We conduct a series of laboratory experiments to collect particle tracking and particle image velocimetry data, and use the particle tracks to identify regions of coherence within the flow that match the results obtained from the model velocity field. We then use the data to estimate growth rates of material interface, using both the braid approach and numerical simulations. The interface growth rates follow similar qualitative trends in both the experiment and model, but have significant quantitative differences, suggesting that the two are not as similar as first seems. Our results shows that there are challenges in using the braid approach to analyze data, in particular the need for long trajectories, but that these are not insurmountable.
- Axisymmetric internal wave transmission and resonant interference in nonlinear stratifications
Boury, S., Odier, P. & Peacock. T.
Journal of Fluid Mechanics, 886, 8 (2020)
To date, the influence of nonlinear stratifications and two layer stratifications on internal wave propagation has been studied for two-dimensional wave fields in a Cartesian geometry. Here, we use a novel wave generator configuration to investigate transmission in nonlinear stratifications of an axisymmetric internal wave. We demonstrate that, despite the additional geometric complexity, with associated features such as an inhomogeneous spatial distribution of the energy flux, results for plane waves can be generalised to axisymmetric wave fields. Two configurations are studied, both theoretically and experimentally. In the case of a free incident wave, a transmission maximum is found in the vicinity of evanescent frequencies. In the case of a confined incident wave, resonant effects, in the sense of constructive interference, lead to enhanced transmission rates from an upper layer to a layer below. We consider the oceanographic relevance of these results by applying them to an example oceanic stratification, finding that there can be real-world implications.
- Interference and transmission of spatiotemporally locally forced internal waves in non-uniform stratifications
Supekar, R. & Peacock, T.
Journal of Fluid Mechanics, 866, 350-368 (2019)
Studies of the effects of constructive or destructive interference on the transmission of internal waves through non-uniform stratifications have typically been performed for internal wave fields that are spatiotemporally harmonic. To understand the impacts of spatiotemporal localization, we present a theoretical and experimental study of the transmission of two-dimensional internal waves that are generated by a boundary forcing that is localized in both space and time. The model analysis reveals that sufficient localization leads to the disappearance of transmission peaks and troughs that would otherwise be present for a harmonic forcing. The corresponding laboratory experiments that we perform provide clear demonstration of this effect. Based on the group velocity and angle of propagation of the internal waves, a practical criterion that assesses when the transmission peaks or troughs are evident is obtained.
- Excitation and resonant enhancement of internal wave modes
Boury, S., Peacock, T. & Odier, P.
Physical Review Fluids, 4, 034802 (2019)
To date, axisymmetric internal wave fields, which have relevance to atmospheric internal wave fields generated by storm cells and oceanic near-inertial wave fields produced by surface perturbations, have been experimentally realized using an oscillating sphere or torus as the source. Here we use a wave generator configuration capable of exciting axisymmetric internal wave fields of arbitrary radial form to generate axisymmetric internal wave modes. After establishing the theoretical background for axisymmetric mode propagation, taking into account lateral and vertical confinement, and also accounting for the effects of weak viscosity, we study modes of different order. We characterize the efficiency of the wave generator through careful measurement of the wave amplitude based upon group velocity arguments, and then consider the effect of vertical confinement to induce resonance, identifying a series of experimental resonant peaks that agree well with theoretical predictions. In the vicinity of resonance, the wave fields undergo a transition to nonlinear behavior that is initiated on the central axis of the domain and proceeds to erode the wave field throughout the domain.
- Model investigations of dewatering plumes generated by deep-sea mining operations
Rzeznik, A. J., Flierl, G. & Peacock, T.
Ocean Engineering, 172, (15), 684-696 (2019)
Deep-sea nodule mining operations potentially involve plans for discharge plumes to be released into the water column by surface operation vessels. As such, we consider the effects of non-uniform, realistic stratifications with vertical shear on forced compressible plumes for application to nodule mining applications. The plume model is developed to account for the influence of thermal conduction through the discharge pipe and an initial adjustment phase. We investigate and elucidate the substantial role of compressibility, for which a dimensionless number is introduced to determine its importance compared to that of the background stratification. Our results show that (i) small-scale stratification features can have a significant impact, (ii) in a static ambient there is a discharge flow rate that minimizes the plume vertical extent, (iii) the ambient velocity profile plays an important role in determining final plume scale and dilution factor, and (iv) for a typical plume the dilution factor is expected to be several hundred to a thousand.
- Is deep-sea mining worth it?
Peacock T. & Alford, M. R.
Scientific American, 318, (5), 72-77 (2018)
- The generation of Rossby waves and wake eddies by small islands
Musgrave, R., Flierl, G. & Peacock, T.
Journal of Marine Research, 76, (2), 63-91 (2018)
The influence of small islands on zonal geostrophic currents is examined in a two-layer configuration. An analytic solution for steady quasigeostrophic flow is derived under the assumption of no upstream influence and is validated numerically in a time-dependent quasigeostrophic model. Under these conditions solutions are the sum of two eigenmodes, which are either arrested Rossby waves or evanescent depending on background flow conditions (layer speeds, stratification, and latitude). In contrast to homogeneous flows, arrested Rossby waves in two layers can occur even when the depth mean flow is westward and can be generated both to the east and west of the island. A third blocking mode may play a role in general, altering the meridional structure of the zonal flow upstream and downstream of the island. The influence of the quasigeostrophic modes on submesoscale island wake eddies is considered in a two-layer primitive equation model with no-slip boundary conditions at the island. Wake eddy formation is inhibited in the presence of an arrested Rossby wave, though the overall drag is similar.
https://elischolar.library.yale.edu/journal_of_marine_research/457
- Tilting at wave beams: a new perspective on the St. Andrew’s cross
Kataoka T., Ghaemsaidi, S.J., Holzenberger, N., Peacock,T. & Akylas, T.R.
Journal of Fluid Mechanics, 830, 660-680 (2017)
The generation of internal gravity waves by a vertically oscillating cylinder that is tilted to the horizontal in a stratified fluid of constant buoyancy frequency, is investigated theoretically and experimentally. This forcing arrangement leads to a variant of the classical St Andrew’s Cross that has certain unique features: (i) radiation of wave beams is limited due to a lower cut-off frequency set by the cylinder tilt angle to the horizontal; (ii) the response is essentially three-dimensional, as end effects eventually come into play when the cut-off frequency is approached, however long a cylinder might be. These results follow from kinematic considerations and are also confirmed by laboratory experiments. The kinematic analysis, moreover, suggests a resonance phenomenon near the cut-off frequency, where viscous and nonlinear effects are likely to play an important part. This scenario is examined by an asymptotic model as well as experimentally.
- An axisymmetric inertia-gravity wave generator
Maurer P., Ghaemsaidi, S. J., Joubaud, S., Peacock, T. & Odier, P.
Experiments in Fluids, 58, 143 (2017)
There has been a rich interplay between laboratory experimental studies of internal waves and advancing understanding of their role in the ocean and atmosphere. In this study, we present and demonstrate the concept for a new form of laboratory internal wave generator that can excite axisymmetric wave fields of arbitrary radial structure. The construction and operation of the generator are detailed, and its capabilities are demonstrated through a pair of experiments using a Bessel function and a bourrelet (i.e., ring-shaped) configuration. The results of the experiments are compared with the predictions of an accompanying analytical model.
- Impact of windage on ocean surface Lagrangian coherent structures
Allshouse M.R., Ivey G.N., Lowe R.J., Jones N.L., Beegle-Krause C.J., Xu J. & Peacock T.
Environmental Fluid Mechanics, 17, (3), 473-483 (2017)
Windage, the additional direct, wind-induced drift of material floating at the free surface of the ocean, plays a crucial role in the surface transport of biological and contaminant material. Lagrangian coherent structures (LCS) uncover the hidden organizing structures that underlie material transport by fluid flows. Despite numerous studies in which LCS ideas have been applied to ocean surface transport scenarios, such as oil spills, debris fields and biological material, there has been no consideration of the influence of windage on LCS. Here we investigate and demonstrate the impact of windage on ocean surface LCS via a case study of the ocean surrounding the UNESCO World Heritage Ningaloo coral reef coast in Western Australia. We demonstrate that the inclusion of windage is necessary when applying LCS to the study of surface transport of any floating material in the ocean.
- Internal tide generation using Green function analysis: to WKB or not to WKB?
Mathur M., Carter G. & Peacock T.
Journal of Physical Oceanography, 46, (7), 2157-2168 (2016)
An established analytical technique for modeling internal tide generation by barotropic flow over bottom topography in the ocean is the Green function–based approach. To date, however, for realistic ocean studies this method has relied on the WKB approximation. In this paper, the complete Green function method, without the WKB approximation, is developed and tested, and in the process, the accuracy of the WKB approximation for realistic ridge geometries and ocean stratifications is considered. For isolated Gaussian topography, the complete Green function approach is shown to be accurate via close agreement with the results of numerical simulations for a wide range of height ratios and criticality; in contrast, the WKB approach is found to be inaccurate for small height ratios in the subcritical regime and all tall topography that impinges on the pycnocline. Two ocean systems are studied, the Kaena and Wyville Thomson Ridges, for which there is again excellent agreement between the complete Green function approach and numerical simulations, and the WKB approximate solutions have substantial errors. This study concludes that the complete Green function approach, which is typically only modestly more computationally expensive than the WKB approach, should be the go-to analytical method to model internal tide generation for realistic ocean ridge scenarios.
- The impact of multiple layering on internal wave transmission
Ghaemsaidi S.J., Dosser H.V., Rainville L. & Peacock T.
Journal of Fluid Mechanics, 789, 617-629 (2016)
Given the ubiquity of layering in environmental stratifications, an interesting example being double-diffusive staircase structures in the Arctic Ocean, we present the results of a joint theoretical and laboratory experimental study investigating the impact of multiple layering on internal wave propagation. We first present results for a simplified model that demonstrates the non-trivial impact of multiple layering. Thereafter, utilizing a weakly viscous linear model that can handle arbitrary vertical stratifications, we perform a comparison of theory with experiments. We conclude by applying this model to a case study of a staircase stratification profile obtained from the Arctic Ocean, finding a rich landscape of transmission behaviour.
- Nonlinear internal wave penetration via parameteric subharmonic instability
Ghaemsaidi S.J., Joubaud S., Dauxois T., Odier P. & Peacock T.
Physics of Fluids, 28, (1), 011703 (2016)
We present the results of a laboratory experimental study of an internal wave field generated by harmonic, spatially periodic boundary forcing from above of a density stratification comprising a strongly stratified, thin upper layer sitting atop a weakly stratified, deep lower layer. In linear regimes, the energy flux associated with relatively high frequency internal waves excited in the upper layer is prevented from entering the lower layer by virtue of evanescent decay of the wave field. In the experiments, however, we find that the development of parametric subharmonic instability in the upper layer transfers energy from the forced primary wave into a pair of subharmonic daughter waves, each capable of penetrating the weakly stratified lower layer. We find that around 10% of the primary wave energy flux penetrates into the lower layer via this nonlinear wave-wave interaction for the regime we study.
- Introduction to focus issue: objective detection of coherent structures
- Refining finite-time Lyapunov exponent ridges and the challenges of classifying them
Allshouse M.R. & Peacock T.
CHAOS, 25, 087410 (2015)
While more rigorous and sophisticated methods for identifying Lagrangian based coherent structures exist, the finite-time Lyapunov exponent (FTLE) field remains a straightforward and popular method for gaining some insight into transport by complex, time-dependent two-dimensional flows. In light of its enduring appeal, and in support of good practice, we begin by investigating the effects of discretization and noise on two numerical approaches for calculating the FTLE field. A practical method to extract and refine FTLE ridges in two-dimensional flows, which builds on previous methods, is then presented. Seeking to better ascertain the role of a FTLE ridge in flow transport, we adapt an existing classification scheme and provide a thorough treatment of the challenges of classifying the types of deformation represented by a FTLE ridge. As a practical demonstration, the methods are applied to an ocean surface velocity field data set generated by a numerical model.
- Lagrangian based methods for coherent structure detection
Allshouse M.R. & Peacock T.
CHAOS, 25, 097617 (2015)
There has been a proliferation in the development of Lagrangian analytical methods for detecting coherent structures in fluid flow transport, yielding a variety of qualitatively different approaches. We present a review of four approaches and demonstrate the utility of these methods via their application to the same sample analytic model, the canonical double-gyre flow, highlighting the pros and cons of each approach. Two of the methods, the geometric and probabilistic approaches, are well established and require velocity field data over the time interval of interest to identify particularly important material lines and surfaces, and influential regions, respectively. The other two approaches, implementing tools from cluster and braid theory, seek coherent structures based on limited trajectory data, attempting to partition the flow transport into distinct regions. All four of these approaches share the common trait that they are objective methods, meaning that their results do not depend on the frame of reference used. For each method, we also present a number of example applications ranging from blood flow and chemical reactions to ocean and atmospheric flows.
- The formation and fate of internal waves in the South China Sea
Alford M., Peacock T. et al.
Nature, 521, 65-69 (2015)
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.
- The Morning Glory Cloud: flow visualization by Mother Nature
Ouazzani, Z., Hacker, J., Thompson, R. & Peacock T.
Physics of Fluids, 26, 091110 (2014)
- Self-propulsion of immersed objects via natural convection
Mercier M., Ardekani A.M., Allshouse M.R., Doyle B. & Peacock T.
Physical Review Letters, 112, 204501 (2014)
Natural convection of a fluid due to a heated or cooled boundary has been studied within a myriad of different contexts due to the prevalence of the phenomenon in environmental and engineered systems. It has, however, hitherto gone unrecognized that boundary-induced natural convection can propel immersed objects. We experimentally investigate the motion of a wedge-shaped object, immersed within a two-layer fluid system, due to a heated surface. The wedge resides at the interface between the two fluid layers of different density, and its concomitant motion provides the first demonstration of the phenomenon of propulsion via boundary-induced natural convection. Established theoretical and numerical models are used to rationalize the propulsion speed by virtue of balancing the propulsion force against the appropriate drag force.
- Topographic scattering of the low-mode internal tide in the deep ocean
Mathur M., Carter G. & Peacock T.
Journal of Geophysical Research: Oceans, 119, 2165 (2014)
We investigate the role of deep-ocean topography in scattering energy from the large spatial scales of the low-mode internal tide to the smaller spatial scales of higher modes. The complete Green function method, which is not subject to the restrictions of the WKB approximation, is used for the first time to study the two-dimensional scattering of a mode-1 internal tide incident on subcritical and supercritical topography of any form in arbitrary stratifications. For an isolated Gaussian ridge in a uniform stratification, large amplitude critical topography is the most efficient at mode-1 scattering and small amplitude topography scatters with an efficiency on the order of 5–10%. In a nonuniform stratification with a pycnocline, the results are qualitatively the same as for a constant stratification, albeit with the key features shifted to larger height ratios. Having validated these results by direct comparison with the results of nonlinear numerical simulations, and in the process demonstrated that WKB results are not appropriate for reasonable ocean predictions, we proceed to use the Green function approach to quantify the role of topographic scattering for the region of the Pacific Ocean surrounding the Hawaiian Islands chain. To the south, the Line Islands ridge is found to scatter ∼40% of a mode-1 internal tide coming from the Hawaiian Ridge. To the north, realistic, small-amplitude, rough topography scatters ∼5–10% of the energy out of mode 1 for transects of length 1000–3000 km. A significant finding is that compared to large extents of small-amplitude, rough topography a single large topographic feature along the path of a mode-1 internal tide plays the dominant role in scattering the internal tide.
- Large-scale, realistic laboratory modeling of M2 internal tide generation at the Luzon Strai
Mercier M., Gostiaux L.,Helfrich K.,Sommeria J., Viboud S., Didelle H., Ghaemsaidi S.J., Dauxois T. & Peacock T.
Geophysical Research Letters, 40, 5704 (2013)
The complex double-ridge system in the Luzon Strait in the South China Sea (SCS) is one of the strongest sources of internal tides in the oceans, associated with which are some of the largest amplitude internal solitary waves on record. An issue of debate, however, has been the specific nature of their generation mechanism. To provide insight, we present the results of a large-scale laboratory experiment performed at the Coriolis platform. The experiment was carefully designed so that the relevant dimensionless parameters, which include the excursion parameter, criticality, Rossby, and Froude numbers, closely matched the ocean scenario. The results advocate that a broad and coherent weakly nonlinear, three-dimensional, M2 internal tide that is shaped by the overall geometry of the double-ridge system is radiated into the South China Sea and subsequently steepens, as opposed to being generated by a particular feature or localized region within the ridge system.
- Visualization of the 3D conical internal wave field generated by an oscillating sphere using Stereo PIV
Ghaemsaidi S.J. & Peacock T.
Experiments in Fluids, 54, 1454 (2013)
To date, experimental studies of internal wave velocity fields have been limited to two-dimensional investigations of planar or axisymmetric systems. Here, we present results of the first three-dimensional stereoscopic Particle Image Velocimetry (PIV) visualizations of an internal wave field. The experiments utilize the canonical arrangement of a vertically oscillating sphere, which enables rigorous comparison with recently published theoretical results. The excellent level of agreement between experiment and theory demonstrates the utility of using stereoscopic PIV to study three-dimensional internal waves. Furthermore, the ability to measure all three components of the velocity field gives an alternative perspective on the significance of harmonics generated via nonlinear processes in the vicinity of the oscillating sphere.
- Lagrangian Coherent Structures: the hidden skeleton of fluid flows
Peacock T. & Haller G.
Physics Today, 66, 41 (2013)
New techniques promise better forecasting of where damaging contaminants in the ocean or atmosphere will end up.
- Tidally generated internal-wave attractors between double ridges
Echeverri P., Yokossi T., Balmforth N.J. & Peacock T.
Journal of Fluid Mechanics, 669, 354 (2011)
A study is presented of the generation of internal tides by barotropic tidal flow over topography in the shape of a double ridge. An iterative map is constructed to expedite the search for the closed ray paths that form wave attractors in this geometry. The map connects the positions along a ray path of consecutive reflections from the surface, which is double-valued owing to the presence of both left- and right-going waves, but which can be made into a genuine one-dimensional map using a checkerboarding algorithm. Calculations are then presented for the steady-state scattering of internal tides from the barotropic tide above the double ridges. The calculations exploit a Green function technique that distributes sources along the topography to generate the scattering, and discretizes in space to calculate the source density via a standard matrix inversion. When attractors are present, the numerical procedure appears to fail, displaying no convergence with the number of grid points used in the spatial discretizations, indicating a failure of the Green function solution. With the addition of dissipation into the problem, these difficulties are avoided, leading to convergent numerical solutions. The paper concludes with a comparison between theory and a laboratory experiment.
- Internal tide generation by arbitrary two-dimensional topography
Echeverri P. & Peacock T.
Journal of Fluid Mechanics, 659, 247 (2010)
To date, analytical models of internal tide generation by two-dimensional ridges have considered only idealized shapes. Here, we advance the Green function approach to address the generation of internal tides by two-dimensional topography of arbitrary shape, employing the Wentzel-Kramers-Brillouin (WKB) approximation to consider the impact of non-uniform stratifications. This allows for a more accurate analytical estimation of tidal conversion rates. Studies of single and double ridges reveal that the conversion rate and the nature of the radiated internal tide can be sensitive to the topographic shape, particularly around criticality and when there is interference between wave fields generated by neighbouring ridges. The method is then applied to the study of two important internal tide generation sites, the Hawaiian and Luzon Ridges, where it captures key features of the generation process.
