Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session G13: Geophysical Fluid Dynamics: Oceans I, Stratification, Waves & Tides |
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Sponsoring Units: DFD GPC Chair: Hussein Aluie, University of Rochester Room: C124 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G13.00001: Restratification at oceanic fronts by baroclinic instabilities Vicky Verma, Hieu Pham, Vamsi Chalamalla, Sutanu Sarkar Large eddy simulation with adaptive mesh refinement is used to investigate how stratification in the upper ocean surface layer evolves at frontal zones. The model includes a front with both lateral and vertical density gradients that is initially in geostrophic balance. The vertical density gradient consists of a mixed layer and a thermocline with constant stratification. Cases with different mixed layer depth are explored to contrast how the front equilibrates in different seasons. The evolution of the flow consists of the growth of baroclinic instability followed by nonlinear evolution into three-dimensional eddies that stir fluid across the front. These eddies create thin regions that have elevated shear, density gradient and turbulent mixing. The difference in the flow dynamics between the mixed layer and the thermocline is described using the organization of vortical structures and quantified through momentum and energy budgets. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G13.00002: Energetics of baroclinic response to tidal forcing at steep topography Masoud Jalali, Sutanu Sarkar Topographic features with steep, supercritical slope are sites of large energy conversion from the oscillating barotropic tide to internal waves according to linear theory. However, large local energy loss is also reported in regions with steep supercritical (topographic slope larger than the slope of the wave propagation angle) topography, e.g at Luzon strait. High-resolution, three-dimensional LES have been performed for a triangular obstacle and a more realistic obstacle taken as a scaled-down model of a Luzon Strait cross-section. These simulations resolve turbulence, compute a closed baroclinic energy budget and quantify the local baroclinic energy loss, $q$. The results are used to investigate the dependence of terms in the baroclinic energy budget on the tidal forcing amplitude, $U_0$. Stronger barotropic forcing in the regime of low-to-moderate excursion number with $Ex \leq O(1)$ corresponding to broad, tall topography leads to stronger wave response and higher value of $q$. The rise in the energy loss to turbulence, $P$, is faster than $U_0^2$, varying approximately as $U_0^3$. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G13.00003: Direct Measurements of the Baroclinic Instability in the Ocean Mahmoud Sadek, Hussein Aluie, Matthew Hecht, Geoffrey Vallis The ocean is mechanically driven by wind and buoyancy at the surface which produce sloping isopycnals with a reservoir of available potential energy (APE). Large scale APE can be converted to kinetic energy via the baroclinic instability, which produces mesoscale eddies. Mesoscale eddies are ubiquitous in mid- and high-latitudes, and play a primary role in determining the strength and trajectories of currents and in generating intrinsic climate variability. The widespread belief that mesoscale eddies are generated through baroclinic instability is based on general accord between observations and linear stability analysis and the predicted behavior of nonlinear models. However, these models are unable to give us quantitative evidence of the extent to which the instability is responsible for eddy generation at various locations in the ocean. To this end, we implement a new coarse-graining framework, recently developed to study flow on a sphere, to directly analyze the baroclinic instability as a function of scale and geographic location, and implement it using strongly eddying high-resolution simulations in the North Atlantic and in the Southern Ocean. The results give us new information about location and intensity of the instability in both physical and spectral space. [Preview Abstract] |
Monday, November 21, 2016 8:39AM - 8:52AM |
G13.00004: Optimal Transient Growth of Submesoscale Baroclinic Instabilities Brian White, Varvara Zemskova, Pierre-Yves Passaggia Submesoscale instabilities are analyzed using a transient growth approach to determine the optimal perturbation for a rotating Boussinesq fluid subject to baroclinic instabilities. We consider a base flow with uniform shear and stratification and consider the non-normal evolution over finite-time horizons of linear perturbations in an ageostrophic, non-hydrostatic regime. Stone (1966, 1971) showed that the stability of the base flow to normal modes depends on the Rossby and Richardson numbers, with instabilities ranging from geostrophic ($Ro\rightarrow 0$) and ageostrophic (finite $Ro$) baroclinic modes to symmetric ($Ri<1$, $Ro>1$) and Kelvin-Helmholtz ($Ri<1/4$) modes. Non-normal transient growth, initiated by localized optimal wave packets, represents a faster mechanism for the growth of perturbations and may provide an energetic link between large-scale flows in geostrophic balance and dissipation scales via submesoscale instabilities. Here we consider two- and three-dimensional optimal perturbations by means of direct-adjoint iterations of the linearized Boussinesq Navier-Stokes equations to determine the form of the optimal perturbation, the optimal energy gain, and the characteristics of the most unstable perturbation. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G13.00005: Numerical modeling of convective instabilities in internal solitary waves of depression shoaling over gentle slopes Gustavo Rivera, Peter Diamessis The shoaling of an internal solitary wave (ISW) of depression over gentle slopes is explored through fully nonlinear and non-hydrostatic simulations based on a high-accuracy deformed spectral multidomain penalty method. As recently observed in the South China Sea, in high-amplitude shoaling ISWs, the along-wave current can exceed the wave celerity resulting in convective instabilities. If the slope is less than 3{\%}, the wave does not disintegrate as in the case of steeper slope shoaling but, instead, maintains its symmetric shape; the above convective instability may drive the formation of a turbulent recirculating core. The sensitivity of convective instabilities in an ISW is examined as a function of the bathymetric slope and wave steepness. ISWs are simulated propagating over both idealized and realistic bathymetry. Emphasis is placed on the structure of the above instabilities, the persistence of trapped cores and their potential for particle entrainment and transport. Additionally, the role of the baroclinic background current on the development of convective instabilities is explored. A preliminary understanding is obtained of the transition to turbulence within a high-amplitude ISW shoaling over progressively varying bathymetry. [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G13.00006: Effects of stratification on the equilibration of shallow fronts Hieu Pham, Sutanu Sarkar Turbulence at frontal zones in the upper ocean is influenced by shear and density stratification that can vary in both lateral and vertical directions. The present study uses large eddy simulation to investigate the evolution of turbulence at a shallow front that consists of a geostrophic jet and stratification in both directions. The density gradient is represented by a hyperbolic-tangent profile in the lateral direction and a linear profile in the vertical direction. The vertical density gradient is varied among cases to explore the dynamics that evolve in different regimes of Richardson number, $Ri_g$. In the cases with $Ri_g < 0.25$, the turbulence rapidly develops throughout the surface layer and spreads laterally outward from the middle of the front. The front is quickly equilibrated within a fraction of an inertial period. In the cases with $0.25 \, < \, Ri_g\, <\, 1$, turbulence is initiated in a thin layer of elevated shear that forms near the surface. The turbulence that occurs in patches in the lateral direction spreads downward across the surface layer. The budgets of momentum, potential vorticity and energy are discussed to illustrate the different processes leading to the equilibration of the front. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G13.00007: The response of an ocean front to small-scale turbulence Matthew Crowe, John Taylor Fronts, or regions with large horizontal density gradients, are common features of the upper ocean. Ocean fronts are hotspots for air/sea exchange and marine life. Observations indicate elevated levels of small scale turbulence at fronts, which nevertheless often have a stable density stratification. The dynamical processes that govern this stratification are not well understood. We consider the evolution of an initially balanced front to an imposed turbulent viscosity and diffusivity. Over long times the dominant balance is found to be the quasi-steady Turbulent Thermal Wind (TTW) balance with time-evoluton due to an advection-diffusion balance in the buoyancy equation. We use the leading order balance to analytically determine similarity solutions for the spreading of a front and compare our results with numerical simulations. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G13.00008: Instability properties under a model mode-1 internal tide. John Segreto, Peter Diamessis The instability properties of the bottom boundary layer (BBL) under a model mode-1 internal tide in linearly stratified finite-depth water are studied, using 2-D direct numerical simulations (DNS) based on a spectral multidomain penalty method model. This model internal tide is a proxy for its lower-mode oceanic counterpart which is generated when stratified water is forced over topography by barotropic tidal currents. Such low-mode internal tidal waves tend to propagate long distances from the point of generation, carrying with them large amounts of energy. One mechanism through which this energy is dissipated is through wave-BBL interactions, where strong shear layers develop along the bed, leading to focused instabilities which are precursors for localized turbulent events. Such events in the BBL can cause sediment resuspension and drive benthic nutrient fluxes, playing a crucial role in ecosystem balances. In the model problem, the stability response of the time-dependent BBL is examined by introducing low-amplitude perturbations near the bed. The corresponding time-evolving BBL-integrated perturbation energy growth rates are then computed, by comparing both the perturbed and unperturbed cases. When an instability actually occurs, its vorticity structure and preferred location is identified. Ultimately, a stability boundary is constructed as a function of perturbation amplitude and internal wave steepness, aspect ration and Reynolds number. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G13.00009: Capturing remote mixing due to internal tides using multi-scale modeling tool: SOMAR-LES Edward Santilli, Vamsi Chalamalla, Alberto Scotti, Sutanu Sarkar Internal tides that are generated during the interaction of an oscillating barotropic tide with the bottom bathymetry dissipate only a fraction of their energy near the generation region. The rest is radiated away in the form of low- high-mode internal tides. These internal tides dissipate energy at remote locations when they interact with the upper ocean pycnocline, continental slope, and large scale eddies. Capturing the wide range of length and time scales involved during the life-cycle of internal tides is computationally very expensive. A recently developed multi-scale modeling tool called SOMAR-LES combines the adaptive grid refinement features of SOMAR with the turbulence modeling features of a Large Eddy Simulation (LES) to capture multi-scale processes at a reduced computational cost. Numerical simulations of internal tide generation at idealized bottom bathymetries are performed to demonstrate this multi-scale modeling technique. Although each of the remote mixing phenomena have been considered independently in previous studies, this work aims to capture remote mixing processes during the life cycle of an internal tide in more realistic settings, by allowing multi-level (coarse and fine) grids to co-exist and exchange information during the time stepping process. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G13.00010: An experimental investigation of the Rossby two-slit problem Alexis Kaminski, Joseph Pedlosky, Karl Helfrich Rossby waves, which arise in response to buoyancy or winds at the sea surface, are a common feature of the oceans, and the problem of Rossby wave propagation in closed basins is a classical problem in geophysical fluid dynamics. Theoretical models of ocean circulation in basins with incomplete barriers such as ocean ridges or island chains (e.g.~Pedlosky \& Spall, JPO(29), 1999; Pedlosky, JPO(31), 2001) suggest that barriers extending through most of a basin are surprisingly inefficient at blocking the transmission of Rossby wave energy from one subbasin to the next. However, the existing theory neglects nonlinear effects and friction in the main basin interiors. To examine these effects, here we present the results of a series of experiments performed over a range of forcing frequencies and amplitudes, in which particle image velocimetry is used to measure the flow field. We find that while the linear theory appears to capture the large-scale structures of the flow, viscosity and nonlinearity significantly affect the flow along the boundaries and near the gaps in the barrier. [Preview Abstract] |
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