Sutanu Sarkar

Abstract Oceanic turbulence influences the transport and mixing of freshwater, heat, nutrients, and other biogeochemical tracers. It also has broader implications for oceanic and atmospheric circulations. Tides contribute substantially to the mechanically driven turbulent ocean mixing through the internal waves resulting from tide–topography interactions. Tidal currents also drive turbulent wakes and shear layers when the topography is three-dimensional (3D). The hypothesis that seamounts are the “stirring rods” of the ocean has motivated considerable recent interest in turbulent flow features near 3D topography. It also motivates the present large-eddy simulations (LES) of tidally dominated flows (tidal oscillations superposed on a weaker mean) past an idealized steep seamount. Complex interactions occur between the topography, the near wake, and previously shed vortices, especially during the tidal phases when the flow direction is reversed. The topographic wake is shown to be a hotspot for mixing, featuring large dissipation rates in the attached shear layers, hydraulic jet, recirculation region in the near wake, and peripheries of shed vortices. The majority of the observed dissipation is due to the vertical shear. Over a tidal cycle, the volume-integrated local dissipation within the wake is at least 4 times greater than the internal wave flux that may be dissipated elsewhere. Furthermore, normalized dissipation rates are maximized for the purely tidal setting. Within the tidal cycle, bulk mixing efficiency η varies substantially and is maximized at η ≈ 0.25 around flow reversals. Significance Statement Tidal forcing, primarily due to the gravitational attraction from the moon and the sun, constitutes a major source of mechanical energy input into the World Ocean. Delineating the energy pathways that ultimately lead to dissipation of the input is crucial to understanding the ocean circulation and mixing, which in turn have broader implications. A significant portion of energy dissipation is attributed to breaking of internal waves generated due to flow–topography interactions. We show that steep seamounts, hypothesized to be stirring rods of the ocean, not only generate internal waves but also act as local hotspots of energy dissipation. The local dissipation in tidally dominated flows is at least 4 times greater than the energy transferred to the internal waves, to potentially be dissipated elsewhere.

Oceanic turbulence influences the transport and mixing of freshwater, heat, nutrients, and other biogeochemical tracers. It also has broader implications for oceanic and atmospheric circulations. Tides contribute substantially to the mechanically driven turbulent ocean mixing through the internal waves resulting from tide-topography interactions. Tidal currents also drive turbulent wakes and shear layers when the topography is 3D. The hypothesis that seamounts are the ``stirring rods'' of the ocean has motivated considerable recent interest in turbulent flow features near 3D topography. It also motivates the present LES of tidally dominated flows (tidal oscillations superposed on a weaker mean) past an idealized steep seamount. Complex interactions occur between the topography, the near wake, and previously shed vortices, especially during the tidal phases when the flow direction is reversed. The topographic wake is shown to be a hotspot for mixing, featuring large dissipation rates in the attached shear layers, hydraulic jet, recirculation region in the near wake, and peripheries of shed vortices. The majority of the observed dissipation is due to the vertical shear. Over a tidal cycle, the volume-integrated local dissipation within the wake is at least four times greater than the internal wave flux that may be dissipated elsewhere. Furthermore, normalized dissipation rates are maximized for the purely tidal setting. Within the tidal cycle, bulk mixing efficiency ($η$) varies substantially and is maximized at $η\approx 0.25$ around flow reversals.

Stratified wakes past an isolated conical seamount are simulated at a Froude number of $Fr = 0.15$ and Rossby numbers of $Ro = 0.15$, 0.75, and $\infty$. The wakes exhibit a K{\' a}rm{\' a}n vortex street, unlike their unstratified, non-rotating counterpart. Vortex structures are studied in terms of large-scale global modes, as well as spatially localised vortex evolution, with a focus on rotation effects. The global modes are extracted by spectral proper orthogonal decomposition (SPOD). For all three studied $Ro$ ranging from mesoscale, submesoscale, and non-rotating cases, the frequency of the SPOD modes at different heights remains coupled as a global constant. However, the shape of the SPOD modes changes from slanted `tongues' at zero rotation ($Ro=\infty$) to tall hill-height columns at strong rotation ($Ro=0.15$). A novel method for vortex centre tracking shows that, in all three cases, the vortices at different heights advect uniformly at about $ 0.9U_{\infty}$ beyond the near wake, consistent with the lack of variability of the global modes. Under system rotation, cyclonic vortices (CVs) and anticyclonic vortices (AVs) present considerable asymmetry, especially at $Ro = 0.75$. The vorticity distribution as well as the stability of AVs are tracked downstream using statistics conditioned to the identified vortex centres. At $Ro=0.75$, intense AVs with relative vorticity up to $ω_z/f_{\rm c}=-2.4$ are seen with small regions of instability but all AVs evolve towards a more stable state. Recent stability analysis that accounts for stratification and viscosity is found to improve on earlier criteria.

A turbulent circular disk wake \cite{chongsiripinyo_decay_2020} is investigated through visualizations, conditional averaging, and spectral analysis to analyze the presence of large-scale streaks. The near wake and intermediate wake are dominated by the vortex shedding mode residing at $m=1, \St=0.135$ in the unstratified wake ($\Fr = \infty$). Once the influence of the $m=1$ mode is filtered, large-scale streaks become apparent in the flow. These streaks predominantly reside at $m=2$, $St \rightarrow 0$. The effect of stratification on streaks is investigated through the analysis of two body-based Froude numbers $\Fr = 2$ and $10$. Streaks are present and the lift-up mechanism is active in the near wake of the $\Fr = 10$ wake. However, as the $\Fr = 10$ wake evolves downstream, the signature of these streaks get weaker. The $\Fr = 2$ wake is significantly different from the other two cases from the very beginning. Large-scale streaks are significantly attenuated and the energy at $\St \rightarrow 0$ is notably lower than the other two cases throughout the course of its evolution. Conditionally averaged streamwise vorticity fields reveal that streaks are generated through the lift-up mechanism. Higher stratification diminishes the strength of the streamwise vortices thereby diminishing the lift-up effect.

Diurnal Warm Layers (DWLs) form near the surface of the ocean on days with strong solar radiation, weak to moderate winds, and small surface-wave effects. Here, we use idealized second-moment turbulence modelling, validated with Large Eddy Simulations (LES), to study the properties, dynamics and energetics of DWLs across the entire physically relevant parameter space. Both types of models include representations of Langmuir turbulence (LT). We find that LT only slightly modifies DWL thicknesses and other bulk parameters under equilibrium wave conditions, but leads to a strong reduction in surface temperature and velocity with possible implications for air-sea coupling. Comparing tropical and the less frequently studied high-latitude DWLs, we find that LT has a strong impact on the energy budget and that rotation at high latitudes strongly modifies the DWL energetics, suppressing net energy turnover and entrainment. We identify the key non-dimensional parameters for DWL evolution and find that the scaling relations of Price et Al. (19869 provide a reliable representation of the DWL bulk properties across a wide parameter space, including high-latitude DWLs. We present different sets of revised model coefficients that include the deepening of the DWL due to LT and other aspects of our more advanced turbulence model to describe DWL properties at midday and during the DWL temperature peak in the afternoon, which we find to occur around 15:00-16:30 for a broad range of parameters.

Stably stratified Taylor–Green vortex simulations are performed by lattice Boltzmann methods (LBM) and compared to other recent works using Navier–Stokes solvers. The density variation is modeled with a separate distribution function in addition to the particle distribution function modeling the flow physics. Different stencils, forcing schemes, and collision models are tested and assessed. The overall agreement of the lattice Boltzmann solutions with reference solutions from other works is very good, even when no explicit subgrid model is used, but the quality depends on the LBM setup. Although the LBM forcing scheme is not decisive for the quality of the solution, the choice of the collision model and of the stencil are crucial for adequate solutions in underresolved conditions. The LBM simulations confirm the suppression of vertical flow motion for decreasing initial Froude numbers. To gain further insight into buoyancy effects, energy decay, dissipation rates, and flux coefficients are evaluated using the LBM model for various Froude numbers.

Thin Diurnal Warm Layers (DWLs) form near the surface of the ocean on days of large solar radiation,weak to moderate winds, and small surface waves. DWLs are characterized by complex dynamics,and are relevant to the ocean especially by modifying surface-layer mixing and atmosphere-oceanfluxes. Here, we use idealized Large Eddy Simulations (LES) and second-moment turbulencemodelling, both including the effects of Langmuir turbulence, to identify the key non-dimensionalparameters of the problem, and explore DWL properties and dynamics across a wide parameterspace. Comparison of LES and the second-moment turbulence models shows that the latter providean accurate representation of the DWL structure and dynamics. We find that, for equilibrium waveconditions, Langmuir effects are significant only in the Stokes layer very close to the surface. Whilewe see pulses in the turbulent stresses and shear in the LES, there are no relevant effects ofLangmuir turbulence on DWL bulk properties and total entrainment. Results of the parameter spaceanalysis agree with the midday scaling by Pollard et al. (1986), however, with modified modelcoefficients and deviations of up to 30% especially at high-latitudes. We develop non-dimensionalexpressions for the strength and timing of the DWL temperature peak in the afternoon, and discussthe mixing efficiency and energetics of DWLs in the presence of Langmuir turbulence.