Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session H10: Geophysical Fluid Dynamics: Oceanographic (5:45pm  6:30pm CST)Interactive On Demand

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H10.00001: Turbulent exchanges between nearinertial waves and balanced flows Jim Thomas Observations collected over the past few decades reveal that the strength of wind generated nearinertial 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 nearinertial waves and balanced flows in different waveenergy 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 nearinertial waves directly extract energy from balanced flows, with O(10{\%})being the amount of balanced energy loss. In contrast, we find that nearinertial waves transfer energy to balanced flows in regimes where balancetowave 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 nearinertial wave field. One of the key outcomes of this study is the demonstration of the lack of a unique direction for nearinertial wavebalanced flow energy transfers. Depending on the balancetowave energy ratio, waves can act as an energy sink or energy source for balanced flows. [Preview Abstract] 

H10.00002: Downscale transfer of quasigeostrophic energy catalyzed by nearinertial waves JinHan Xie Wind forcing injects energy into mesoscale eddies and nearinertial 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 NIWquasigeostrophic mean mesoscale eddy coupled system based on a previously derived twodimensional 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 meanflow, quantified by a parameter R.Using forceddissipative numerical simulations, we justify the existence of secondorder 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 toymodel study, making it potentially important for ocean energetics. [Preview Abstract] 

H10.00003: Shearinduced breaking of internal gravity waves Colmcille Caulfield, Christopher Howland, John Taylor 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 largeamplitude 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. Streamwisealigned convective rolls emerge fastest, but their contribution to irreversible mixing is dwarfed by sheardriven 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 Kelvinâ€“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 wavedriven mixing in the ocean. [Preview Abstract] 
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H10.00004: The influence of symmetric and inertial instabilities on the evolution of oceanic fronts Aaron Wienkers, John Taylor 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 MississippiAtchafalaya river plume. These fronts may be unstable to symmetric or inertial instabilities which further enhance smallscale 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 nostress 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 selfsimilar 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. [Preview Abstract] 

H10.00005: Interaction between a submesoscale front and convective turbulence Vicky Verma, Hieu T. Pham, Sutanu Sarkar 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 lowpass 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. [Preview Abstract] 

H10.00006: Machine learning model of quasigeostrophic dynamics Kevin Yao, Eric Forgoston, Philip Yecko We extend the machine learning technique of reservoir computing (RC) to two elementary fluid models of ocean circulation: the wellknown double gyre stream function model with timevariable forcing and a one layer quasigeostrophic (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. [Preview Abstract] 

H10.00007: Towards Simulation of Stratified Turbulent Wakes at Very High Reynolds number Nidia Cristina Reyes Gil, Kristopher Rowe, Greg Thomsen, Peter Diamessis We present the components of an upgraded Fourier/spectralelement (SEM) flow solver designed for the simulation of stratified wakes with nonzero 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 speedup. 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 bodybased Reynolds number $Re \sim O(10^6)$. Our preliminary analysis will focus on characterizing the persistence of turbulent finestructure 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. [Preview Abstract] 

H10.00008: Coherent pathways for vertical transport from the surface mixed layer to ocean interior Mara Freilich, Amala Mahadevan 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 submesoscaleresolving 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. [Preview Abstract] 

H10.00009: Characterization of shoaling internal waves from optical fibre cable data using spacetime statistics Isha Shukla, Andrew Lucas, Oliver Schmidt 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 wavepowered profiling moorings. The measurements reveal that the endstate of the onshore propagating IWs exhibits coherent wake structures that radiate in the offshore direction. Previously undocumented, this feature constitutes a nonturbulent 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/alongshore velocity (ranging from $0.10.4$ 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 spacetime proper orthogonal decomposition approach. [Preview Abstract] 

H10.00010: Langmuir circulation without wind or surface waves: shear flow interacting with wavy topography Andreas Holm Akselsen, Simen {\AA}. Ellingsen Langmuir circulations in their traditional form are large rolling fluid flow pattern created by the interplay of surface waves and a nearsurface 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 Langmuirlike 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 steadystate viscous flow, simple equations can be derived and easily solved numerically. For the special case of a powerlaw velocity profile, explicit leadingorder 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 noslip boundary conditions. [Preview Abstract] 

H10.00011: Dynamic Mode Decomposition Uncovers Hidden Oceanographic Features Around the Strait of Gibraltar Sudam Surasinghe, Sathsara Dias, Kanaththa Priyankara, Erik Bollt, Marko Budisic, Larry Pratt, Jose SanchezGarrido Oceanic flow around the Strait of Gibraltar comprises dynamic submesoscale 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 welldefined 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 nonparametrically 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. [Preview Abstract] 

H10.00012: How observed drifter convergence corresponds to ocean surface convergence zones H. M. Aravind, Michael Allshouse 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 MaxeyRiley 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. [Preview Abstract] 

H10.00013: A Reynoldsaveraged simulation of coastal Langmuir cells and comparison to large eddy simulation Andres TejadaMartinez, Juan Penaloza Gutierrez, Anthony Perez, Michel Boufadel 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 fulldepth LCs. Here these cells are treated as a secondary component to the wind and/or pressure gradientdriven primary flow. The strategy is used to investigate LCs engulfing unstratified shallow water regions representative of a shallow shelf zone and a surfshelf transition zone. The resolved LCs and associated statistics will be compared with their counterparts in largeeddy simulation (LES). The comparison shows that the Reynoldsaveraged approach can successfully reproduce cell meandering and merging (i.e. the socalled Yjunctions), 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 Reynoldsaveraged approach will be presented investigating the impact of variable depth and wave direction on the width of the LCs and their intensity. [Preview Abstract] 

H10.00014: Predicting subduction regions in upper ocean using surface signatures Michael Allshouse, H.M. Aravind, Vicky Verma, Sutanu Sarkar, Mara Freilich, Amala Mahadevan, Patrick Haley, Pierre Lermusiaux 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 submesoscaleresolving ocean models spanning different lengthscales 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. [Preview Abstract] 
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