Topographic Internal Waves Research Articles

Near‐Surface Oceanic Kinetic Energy Distributions From Drifter Observations and Numerical Models

AbstractThe geographical variability, frequency content, and vertical structure of near‐surface oceanic kinetic energy (KE) are important for air‐sea interaction, marine ecosystems, operational oceanography, pollutant tracking, and interpreting remotely sensed velocity measurements. Here, KE in high‐resolution global simulations (HYbrid Coordinate Ocean Model; HYCOM, and Massachusetts Institute of Technology general circulation model; MITgcm), at the sea surface (0 m) and at 15 m, are compared with KE from undrogued and drogued surface drifters, respectively. Global maps and zonal averages are computed for low‐frequency (<0.5 cpd), near‐inertial, diurnal, and semidiurnal bands. Both models exhibit low‐frequency equatorial KE that is low relative to drifter values. HYCOM near‐inertial KE is higher than in MITgcm, and closer to drifter values, probably due to more frequently updated atmospheric forcing. HYCOM semidiurnal KE is lower than in MITgcm, and closer to drifter values, likely due to inclusion of a parameterized topographic internal wave drag. A concurrent tidal harmonic analysis in the diurnal band demonstrates that much of the diurnal flow is nontidal. We compute simple proxies of near‐surface vertical structure—the ratio 0 m KE/(0 m KE + 15 m KE) in model outputs, and the ratio undrogued KE/(undrogued KE + drogued KE) in drifter observations. Over most latitudes and frequency bands, model ratios track the drifter ratios to within error bars. Values of this ratio demonstrate significant vertical structure in all frequency bands except the semidiurnal band. Latitudinal dependence in the ratio is greatest in diurnal and low‐frequency bands.

Intermittent Frontogenesis in the Alboran Sea

AbstractWe present a phenomenological description and dynamical analysis of the Alboran fronts using a realistic simulation at submesoscale resolution. The study is focused on east Alboran fronts emerging within relatively strong flows that separate from the Spanish coast into the basin interior. Despite modest lateral shifting associated with the position of the Alboran anticyclonic gyres and variations in intensity, these fronts present a similar structure and dynamical configuration as the climatological Almeria–Oran front. The statistical analysis of our solution shows that strained-induced frontogenesis is a recurrent submesoscale mechanism associated with these fronts, and the process is assessed in terms of the advective Lagrangian frontogenetic tendencies associated with buoyancy and velocity horizontal gradients. Intermittency in their strength and patterns is indicative of high variability in the occurrence of active frontogenesis in association with the secondary (overturning) circulation across the frontal gradient. As a result, we find many episodes with strong surface fronts that do not have much associated downwelling. Frontogenesis and the associated secondary circulation are further explored during two particular frontal events, both showing strong downwelling of (1) cm s−1 extending down into the pycnocline. A frontogenetic contribution of turbulent vertical momentum mixing to the secondary circulation is identified in the easternmost region during the cold season, when the dynamics are strongly influenced by the intrusion of the salty Northern Current. The background vertical velocity fields observed during the analyzed events indicate other currents in the submesoscale range, including tidal and topographic internal waves.

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

AbstractThe generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi‐steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of IWs as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream IW beams due to a quasi‐Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a superbuoyancy (ω > N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream‐bias energy flux—and consequently net horizontal momentum—described in Shakespeare (2020, https://doi.org/10.1175/JPO‐D‐19‐0179.1) is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW‐induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.

Variability of the Turbulent Kinetic Energy Dissipation along the A25 Greenland–Portugal Transect Repeated from 2002 to 2012

AbstractThe variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland–Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. The 2002–12 averaged dissipation rate 〈ε2002–2012〉 in the upper ocean lays in the range 1–10 × 10−10 W kg−1. At depth, 〈ε2002–2012〉 is smaller than 1 × 10−10 W kg−1 except over rough topography found at the continental slopes, the Reykjanes Ridge, and in a region delimited by the Azores–Biscay Rise and Eriador Seamount. There, the vertical energy flux of internal waves is preferentially oriented toward the surface and 〈ε2002–2012〉 is in the range 1–20 × 10−10 W kg−1. The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the finescale parameterization occur at depth between the Azores–Biscay Rise and Eriador Seamount. This region is also marked by mesoscale eddying flows resulting in enhanced surface energy level and enhanced bottom velocities. Estimates of the vertical energy fluxes into the internal tide and into topographic internal waves suggest that the latter are responsible for the strong dissipation events. At Eriador Seamount, both topographic internal waves and the internal tide contribute with the same order of magnitude to the dissipation rate while around the Reykjanes Ridge the internal tide provides the bulk of the dissipation rate.

On improving the accuracy of the M2 barotropic tides embedded in a high-resolution global ocean circulation model

The ocean tidal velocity and elevation can be estimated concurrently with the ocean circulation by adding the astronomical tidal forcing, parameterized topographic internal wave drag, and self-attraction and loading to the general circulation physics. However, the accuracy of these tidal estimates does not yet match accuracies in the best data-assimilative barotropic tidal models. This paper investigates the application of an augmented state ensemble Kalman Filter (ASEnKF) to improve the accuracy of M2 barotropic tides embedded in a 1/12.5° three-dimensional ocean general circulation model. The ASEnKF is an alternative to the techniques typically used with linearized tide-only models; such techniques cannot be applied to the embedded tides in a nonlinear eddying circulation. An extra term, meant to correct for errors in the tide model due to imperfectly known topography and damping terms, is introduced into the tidal forcing. Ensembles of the model are created with stochastically generated forcing correction terms. The discrepancies for each ensemble member with TPXO, an existing data-assimilative tide model, are computed. The ASEnKF method yields an optimal estimate of the model forcing correction terms, that minimizes resultant root mean square (RMS) tidal sea surface elevation error with respect to TPXO, as well as an estimate of the tidal elevation. The deep-water, global area-averaged RMS sea surface elevation error of the principal lunar semidiurnal tide M2 is reduced from 4.4 cm in a best-case non-assimilative solution to 2.6 cm. The largest elevation errors in both the non-assimilative and ASEnKF solutions are in the North Atlantic, a highly resonant basin. Possible pathways for achieving further reductions in the RMS error are discussed.

Indirect evidence for substantial damping of low‐mode internal tides in the open ocean

AbstractA global high‐resolution ocean circulation model forced by atmospheric fields and the M2 tidal constituent is used to explore plausible scenarios for the damping of low‐mode internal tides. The plausibility of different damping scenarios is tested by comparing the modeled barotropic tides with TPXO8, a highly accurate satellite‐altimetry‐constrained tide model, and by comparing the modeled coherent baroclinic tide amplitudes against along‐track altimetry. Five scenarios are tested: (1) a topographic internal wave drag, argued here to represent the breaking of unresolved high vertical modes, applied to the bottom flow (default configuration), (2) a wave drag applied to the barotropic flow, (3) absence of wave drag, (4) a substantial increase in quadratic bottom friction along the continental shelves (with wave drag turned off), and (5) application of wave drag to the barotropic flow at the same time that quadratic bottom friction is substantially increased along the shelves. Of the scenarios tested here, the default configuration (1) yields the most accurate tides. In all other scenarios (2–5), the lack of damping on open ocean baroclinic motions yields baroclinic tides that are too energetic and travel too far from their sources, despite the presence of a vigorous mesoscale eddy field which can scatter and decohere internal tides in the model. The barotropic tides are also less accurate in the absence of an open ocean damping on barotropic motions, that is, in scenarios (3) and (4). The results presented here suggest that low‐mode internal tides experience substantial damping in the open ocean.

Topographically induced internal solitary waves in a pycnocline: Secondary generation and selection criteria

Geophysical flows support the propagation of stable nonlinear internal waves (internal solitary waves or ISWs) with complex generation mechanisms. At least two regimes of ISWs generation in the pycnocline, both involving the interaction between a tidal flow and the bottom topography, are known in the ocean. They can either be directly induced above topographies (primary generation) or by a topographic internal wave beam impinging on the pycnocline. This “secondary generation” process is the subject-matter of the present study. The present work relies on direct numerical simulations of an academic configuration inspired by oceanic observations. It aims at describing the different steps involved in the secondary generation process. To mimic the oceanic case, the internal wave beam is emitted from the topography at the bottom of the flow. First, the linear scattering of the internal wave beam at the pycnocline is studied in a linear configuration. Increasing the forcing amplitude leads to the generation of steep isopycnal troughs in the pycnocline, at the locations of the internal wave beam impacts. The dynamics of these troughs is studied in details, which permits to associate them with propagating ISW2s that emerge from the second normal mode. Finally, the evolution of the structure of normal modes 2 and 3 with respect to the pycnocline strength, as well as the role played by the topography, is analyzed. This study is a step to complete and unify previous independent analytical studies of the secondary generation process.

Mathematical modeling of the generation of topographic internal waves by nonstationary currents

In the linear statement, within the framework of long-wave approximation, we develop a mathematical model for the numerical analysis of the fields of internal waves generated by a local unevenness of the bottom in a nonstationary spatially homogeneous flow. For the corresponding problem, we obtain the solution suitable for numerical calculations. We study the effects connected with periodic changes in the direction of currents and analyze the horizontal distribution of amplitudes for a single internal mode and the dependence of this distribution on the Coriolis parameter.

The boiling-water phenomena at Camarinal Sill, the strait of Gibraltar

Abstract A detailed description of high-amplitude steady topographic internal waves recorded at Camarinal Sill during a survey on the R.V. “Investigador” is presented. These internal waves are generated during the maximum outflow (westward) stage of the tidal current and remain over the sill for more than 4 h until the outflow slackens, then being released towards the Mediterranean. Their amplitudes are comparable to the well-known internal bore of Camarinal Sill, occurring during maximum outflow during spring tides. However, they respond to a different physical origin and their spatial features are also quite different. In fact, the favourable hydraulic conditions for the generation of steady waves over the sill inhibit the internal bore generation, and vice versa. Analysis of the observations suggests that steady internal waves are the result of a resonant response of the stratified fluid over the sill to the forcing of the flow on the across-sill topography. An important consequence of the steady internal waves with clear biological implications is the significant mixing phenomena that are induced. Mixing is enabled by an enhancement of the shear at the trough together with a significant induced vertical advection.

A Growth Mechanism for Topographic Internal Waves Generated by an Oscillatory Flow

On the basis of ray tracing of individual waves generated at various phases of the tidal flow, an amplification mechanism is presented for a new class of topographically generated internal waves identified by Nakamura et al., which develop across a broad latitude range and can exist even above the critical latitude where the tidal frequency equals the inertial frequency. The results show that unsteady lee waves are always amplified when the maximum frequency is much smaller than the buoyancy frequency because their phase speeds (amplitudes) are equal (proportional) to the tidal flow speed at their time of generation. Fast mixed tidal–lee waves are also effectively amplified, when the rotation effect is significant. Accordingly, amplification of unsteady lee and fast mixed tidal–lee waves can occur even if the requirements of previous theories (e.g., the critical slope and critical Froude number conditions) are not satisfied. Since the result here covers the generation and amplification processes of topographic internal waves across a broader parameter range than earlier theories, it should contribute to a better understanding of boundary mixing processes, especially in high-latitude regions.