Session H10: Geophysical Fluid Dynamics: Oceanographic (5:45pm - 6:30pm CST)

Turbulent exchanges between near-inertial waves and balanced flows

Observations collected over the past few decades reveal that the strength of wind generated near-inertial waves in the upper ocean can vary depending on the geographic region and season. Inspired by these observations, we investigate turbulent interactions and energy exchanges between near-inertial waves and balanced flows in different wave-energy regimes. We find accelerated vertical propagation and dissipation of the waves in regimes where balanced and wave fields have comparable strengths. In such regimes we also find that near-inertial waves directly extract energy from balanced flows, with O(10{\%})being the amount of balanced energy loss. In contrast, we find that near-inertial waves transfer energy to balanced flows in regimes where balance-to-wave energy is small, with the gain in balanced energy being dependent on the relative strength of waves. Furthermore, these regimes are characterized by relatively weaker vertically propagation and dissipation of the near-inertial wave field. One of the key outcomes of this study is the demonstration of the lack of a unique direction for near-inertial wave-balanced flow energy transfers. Depending on the balance-to-wave energy ratio, waves can act as an energy sink or energy source for balanced flows.   

Downscale transfer of quasi-geostrophic energy catalyzed by near-inertial waves

Wind forcing injects energy into mesoscale eddies and near-inertial waves (NIWs) in the ocean, and the NIWs are believed to solve the puzzle of mesoscale energy budget by absorbing energy from mesoscale eddies. We study the turbulent energy transfer in the NIW--quasigeostrophic mean mesoscale eddy coupled system based on a previously derived two-dimensional model which inherits conserved quantities in Boussinesq equations. The conservation of energy, potential enstrophy and wave action implies the existence of phase transition with changing the relative strength between NIW and mean-flow, quantified by a parameter R.Using forced-dissipative numerical simulations, we justify the existence of second-order phase transition around a critical value $R_c$. When $R< R_c$, energy transfers bidirectionally, wave action transfers downscale, and vorticity forms strong cyclones. When $R>R_c$, energy transfers downscale, wave action transfers bidirectionally, and vortex filaments are dominant. We find the catalytic wave induction mechanism where the NIW induces a downscale mean energy flux, which differs from the stimulated loss of balance mechanism observed in inertial value problems. The new mechanism is effective in the toy-model study, making it potentially important for ocean energetics.   

Shear-induced breaking of internal gravity waves

Motivated by observations of turbulence in the strongly stratified ocean thermocline, we use direct numerical simulations to investigate the interaction of a sinusoidal shear flow and a large-amplitude internal gravity wave. Despite strong nonlinearities in the flow, linear ray tracing theory proves qualitatively useful in describing the refraction of the wave by the shear. Consistent with the linear theory, the energy of the wave accumulates in regions of negative mean shear where we observe evidence of both convective instabilities and shear instabilities. Streamwise-aligned convective rolls emerge fastest, but their contribution to irreversible mixing is dwarfed by shear-driven billow structures that develop later. Although the wave strongly distorts the buoyancy field on which these billows develop, the mixing efficiency of the subsequent turbulence is similar to that arising from Kelvin–Helmholtz instability in a stratified shear layer. We discuss the complex interaction between the mean flow, internal gravity wave and turbulence, and its implications for internal wave-driven mixing in the ocean.   

Not Participating

Isolated fronts with large lateral density gradients in geostrophic and hydrostatic balance are common in the upper ocean. Such strong fronts may be the result of baroclinic frontogenesis or of sharp freshwater interfaces as are found in the northern Gulf of Mexico near the Mississippi-Atchafalaya river plume. These fronts may be unstable to symmetric or inertial instabilities which further enhance small-scale mixing and encourage vertical transport between the surface and the abyss. Here, we consider the problem of an initially balanced front of finite width and which is bounded by flat no-stress horizontal surfaces. We examine how the evolution and equilibration depends on the front strength and aspect ratio using nonlinear numerical simulations, and develop a model to predict the profile and effective width of the final equilibrated state in the absence of external forcing. While fronts with $Ro > 2.6$ collapse to a self-similar profile dependent only on the deformation radius, we find that for small enough $Ro < 1$, frontlets form as the front equilibrates. These frontlets increase both the kinetic and potential energy of the final balanced state, but are also found to interact with the boundaries if the front exhibits inertial oscillations.   

Interaction between a submesoscale front and convective turbulence

The coherent vortex filaments and eddies of the submesoscale (10 km - 0.1 km) play a crucial role in transporting heat, salt, dissolved gases, and organic matter across the mixed layer and in restratifying the upper ocean. In contrast, the finescale (smaller than O(100) m) is primarily responsible for mixing and dissipation. In the model problem, a warm filament with active submesoscales evolves in the presence of convective turbulence. The surface cooling flux, which drives the convective turbulence, is varied among cases. The flow, simulated using LES, is separated into the two scales with an explicit low-pass filter. We find that when the surface cooling flux increases, so do the downwelling velocity and the vertical buoyancy flux associated with the coherent submesoscale. The finescale dissipation also intensifies in the vortex filaments. Moreover, the finescale velocity that develops at the fronts is different from convective turbulence, e.g., in the level of anisotropy.   

Machine learning model of quasi-geostrophic dynamics

We extend the machine learning technique of reservoir computing (RC) to two elementary fluid models of ocean circulation: the well-known double gyre stream function model with time-variable forcing and a one layer quasi-geostrophic (QG) basin model. In both cases, the models are used to generate flow data that sample a range of possible dynamical behavior in such flows for particular flow parameters. In the case of QG, a PDE system with 3 physically relevant dimensionless parameters is solved, including Munk and Stommel type solutions. We present results on the effectiveness of the RC approach in capturing the characteristics of these systems and assess the accuracy and usefulness of the RC models by comparisons to descriptive models, including FTLE and POD, and the role of both physical and numerical parameters on these results.   

Towards Simulation of Stratified Turbulent Wakes at Very High Reynolds number

We present the components of an upgraded Fourier/spectral-element (SEM) flow solver designed for the simulation of stratified wakes with non-zero net momentum. Use of a modal SEM in the vertical direction retains the flexibility of localized flow resolution of the wake core and offers improved numerical stability properties, as compared to its predecessor spectral multidomain penalty method scheme. The selection of the polynomial basis functions in combination with static condensation results in a large number of small tridiagonal systems, which enables code performance speed-up. Following a brief discussion of code performance, we will discuss results from implicit Large Eddy Simulations of stratified sphere wakes at internal Froude number, $Fr=4$, which extend to body-based Reynolds number $Re \sim O(10^6)$. Our preliminary analysis will focus on characterizing the persistence of turbulent fine-structure deep into the intermediate stage of wake evolution. The numerical dissipation caused by spectral filtering will be quantified as a fraction of physical dissipation resolved by the grid. The potential of a distinct and sufficiently long window in time at $Re \sim O(10^6)$ which supports strongly layered turbulence in the strongly stratified regime will be discussed.   

Coherent pathways for vertical transport from the surface mixed layer to ocean interior

The dynamical pathways of subduction, by which water from the oceanic surface mixed layer makes its way into the pycnocline, are influenced by both geostrophic frontogenesis and submesoscale instabilities in the mixed layer. We explore the pathways and mechanisms for subduction using a submesoscale-resolving numerical model of a mesoscale front. We use particle tracking to identify Lagrangian trajectories that exit the mixed layer and study the evolution of the dynamical properties during subduction from a statistical standpoint. Water parcels subduct within coherent regions along the front. These coherent subduction regions set the \textasciitilde 10 km length scales of the subducted features. As a result, the vertical transport rate of a tracer has a spectrum that is flatter than the spectrum of vertical velocity. Contrary to the forced submesoscale processes that sequester low potential vorticity anomalies in the interior, we find that PV can be elevated in subducting water masses. The rate of subduction that we estimate is of similar magnitude to previous studies (\textasciitilde 100 m/year), but the pathways that are unraveled in this study along with the Lagrangian evolution of properties on water parcels, emphasize the role of submesoscale dynamics coupled with mesoscale frontogenesis.   

Characterization of shoaling internal waves from optical fibre cable data using space-time statistics

A distinctive pattern of shoaling internal waves (IWs) offshore of La Jolla, CA, has been revealed by a temperature sensing array of fibre optics on the sea floor and autonomous wave-powered profiling moorings. The measurements reveal that the end-state of the on-shore propagating IWs exhibits coherent wake structures that radiate in the offshore direction. Previously undocumented, this feature constitutes a non-turbulent mechanism that extracts energy from the shoaling waves. Despite the distinctive signature of the radiating wake, its intermittent and stochastic nature hampers the analysis. We hence use conditional statistics to extract an ensemble of IW structures. Important wave characteristics, like the mean cross/along-shore velocity (ranging from $0.1-0.4$ m/s) and wave approach angle (ranging from $70^{\circ}-77^{\circ}$) are obtained from the ensemble mean and using wavelet transforms of the prewhitened data. The entire ensemble of the realization is then used to obtain the dominant coherent wave structure using a space-time proper orthogonal decomposition approach.   

Langmuir circulation without wind or surface waves: shear flow interacting with wavy topography

Langmuir circulations in their traditional form are large rolling fluid flow pattern created by the interplay of surface waves and a near-surface shear current, typically both created by wind. Craik and Leibovich (1976) describe two kinematic mechanisms which cause instabilities which grow into Langmuir rolls, both involving only the interaction of mean current shear and wave motion. The same ingredients are present also in boundary layer flow over a wavy bottom topography.\\ \\ We present a theory of Langmuir-like circulations (LLC) created by boundary layer flow over a topography pattern of two monochromatic waves crossing at an angle. Thus, the mechanissm often called CL1 is triggered, we describe it with the theory of Craik (1970), slightly modified.\\ \\ A flow of arbitrary shear profile is assumed over the bottom topography. In the opposite limits of transient inviscid flow and steady-state viscous flow, simple equations can be derived and easily solved numerically. For the special case of a power-law velocity profile, explicit leading-order solutions are found. We map out the LCC response to varying wavelength, crossing angle and wave amplitude. The study is supplemented with DNS which verify the manifestation of LLC over wavy geometries with a no-slip boundary conditions.   

Dynamic Mode Decomposition Uncovers Hidden Oceanographic Features Around the Strait of Gibraltar

Oceanic flow around the Strait of Gibraltar comprises dynamic sub-mesoscale features arising due to topographic and tidal forcing, instabilities, and strongly nonlinear hydraulic processes, all governed by nonlinear equations of fluid motion. The purpose of this study is to isolate dominant features from 3D MIT general circulation model simulations and to investigate their physics. To this end, we use the Dynamic Mode Decomposition (DMD) that decomposes the sequence of simulation snapshots into a sum of Koopman modes: spatial profiles with well-defined exponential growth/decay rates and oscillation frequencies. To identify known features, we correlate identified DMD modes with the tidal forcing and demonstrate that DMD is able to non-parametrically detect the prominent waves known to occur in the western Mediterranean. Additionally, the analysis reveals previously undocumented Kelvin waves and demonstrates that meandering motions in the Atlantic Jet entering the Mediterranean Sea are associated with the diurnal tidal forcing. The DMD thus recovers the results obtained by classical harmonic analysis of tidal constituents, and also highlights features that have eluded attention so far, suggesting that DMD could be a useful part of an oceanographer's toolbox.   

How observed drifter convergence corresponds to ocean surface convergence zones

The identification of surface convergence zones is an established approach for identifying regions in the upper ocean where significant subduction occurs. While Eulerian fields such as the horizontal divergence of velocity can be used to estimate where instantaneous convergence zones occur, Lagrangian metrics such as dilation rate better identify subduction zones where vertical transport occurs over a time interval. Both of these analyses rely on velocity field information that may not be available for observational investigations, so drifters have been used to locate these convergence zones. However, inertia, buoyancy, and windage effects result in drifter trajectories that differ from the fluid trajectories in the upper ocean. First, we will evaluate the efficacy of using sparse drifter data to approximate the Lagrangian fields. Then, we use a Maxey-Riley framework that more accurately accounts for the phenomena impacting the drifter trajectories to calculate the Lagrangian fields and discuss how the observable fields differ from the fluid fields. This analysis will evaluate the effectiveness of using drifters and deployment procedures to identify the convergence zones.   

A Reynolds-averaged simulation of coastal Langmuir cells and comparison to large eddy simulation

Langmuir turbulence in the coastal ocean is driven by winds and waves and is characterized by Langmuir cells (LCs) that can span the full depth of the water column. A~solution strategy based on Reynolds averaging is introduced, relying on the coherency and persistence of full-depth LCs. Here these cells are treated as a secondary component to the wind and/or pressure gradient-driven primary flow. The strategy is used to investigate LCs engulfing unstratified shallow water regions representative of a shallow shelf zone and a surf-shelf transition zone. The resolved LCs and associated statistics will be compared with their counterparts in large-eddy simulation (LES). The comparison shows that the Reynolds-averaged approach can successfully reproduce cell meandering and merging (i.e. the so-called Y-junctions), a requisite for capturing the proper crosswind width of the LCs. The merging occurs less frequently over time as the cells grow after being spun from rest. Additional studies via the Reynolds-averaged approach will be presented investigating the impact of variable depth and wave direction on the width of the LCs and their intensity.   

Predicting subduction regions in upper ocean using surface signatures

Subduction in the upper ocean impacts surface mixing, advection of nutrients, and the ocean energy budget. Direct observations of subduction are difficult because vertical velocities in the ocean are often orders of magnitude smaller than horizontal velocities. New Lagrangian drifters can move vertically with the fluid but must be released in areas of large subduction like density fronts. To identify locations for targeted releases, target zones are identified from surface signatures computed using velocity and density fields. Eulerian analysis of these potentially noisy fields may highlight instantaneous convergence zones, but we propose a Lagrangian analysis to identify regions where subduction occurs over a time interval. Comparison with standard Eulerian based targets demonstrates the significant benefit of using Lagrangian analysis to target subduction. Analysis of three different submesoscale-resolving ocean models spanning different length-scales demonstrates the Lagrangian target zones identify larger subduction on average and are more likely to predict regions of high vertical subduction. An ensemble analysis of an ocean model demonstrates that the proposed Lagrangian predictors locate persistent subduction regions even without knowledge of the true surface velocity.