Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session X28: Geophysical Fluid Dynamics: Ocean II |
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Chair: Varvara Zemskova, University of Waterloo Room: 251 F |
Tuesday, November 26, 2024 8:00AM - 8:13AM |
X28.00001: Abstract Withdrawn
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Tuesday, November 26, 2024 8:13AM - 8:26AM |
X28.00002: ABSTRACT WITHDRAWN
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Tuesday, November 26, 2024 8:26AM - 8:39AM |
X28.00003: Improving Thorpe-length scale based estimates of turbulent kinetic energy dissipation rates in stably stratified turbulent flows kiarash Nayeb pashaei, Karan Venayagamoorthy A linear relationship between the Thorpe (vertical overturn) length scale LT and Ozmidov length scale LO is common assumption that is often invoked to estimate the dissipation rate of turbulent kinetic energy ε. This assumption is particularly attractive since vertical scales of overturns can be calculated using sorting algorithms from inversions in standard measurements of density profiles obtained from Conductivity-Temperature-Depth (CTD) measurements in the ocean. However, a number of studies highlight the lack of a linear relationship between LT and LO and indicate that inferred estimates of ε may be biased high by up to an order of magnitude. In this study, an attempt is made to provide a physically based correction to the Thorpe-inferred dissipation rates that is consistent with 'true' estimates of ε as obtained from direct numerical simulations (DNS) of stratified turbulence. The implications of these findings for improved estimates of ocean mixing rates will be discussed. |
Tuesday, November 26, 2024 8:39AM - 8:52AM |
X28.00004: Multiscale Interactions in the Oceanic Mixed Layer: Insights from Large-Eddy Simulations Shirui Peng, Simone Silvestri, Abigail Bodner The oceanic mixed layer is characterized by multiscale interactions ranging from the kilometer-scale turbulence typical of geostrophic turbulence down to micro-scale turbulence generated by shear and convection. However, due to the large scale separation of these phenomena, understanding energy transfers among oceanic mesoscale processes down to microscopic turbulence remains a significant challenge. We present large-eddy simulations that resolve multiscale interactions within the oceanic mixed layer, encompassing boundary layer turbulence, submesoscale, and mesoscale processes. We compare hydrostatic and non-hydrostatic models featuring strong convergence zones, distinct warm and cold temperature fronts, and surface forcing. We analyze the spatial and spectral properties of temperature, velocity, and vorticity fields, and decompose these fields to investigate multiscale momentum and buoyancy fluxes. These insights aim to enhance parameterizations for multiscale dynamics in large-scale ocean models, contributing to more accurate predictions of ocean behavior and energy distribution under climate change. |
Tuesday, November 26, 2024 8:52AM - 9:05AM |
X28.00005: Model-based study of coherent structures in Langmuir turbulence Anqing Xuan, Lian Shen The ocean boundary layer features Langmuir circulations across various scales, arising from the interactions of wave forcing and the turbulent boundary layer. The flow state, known as Langmuir turbulence, significantly impacts ocean mixing and transport. In this study, we examine the characteristics of coherent structures in Langmuir turbulence with a combined simulation- and model-based approach. Results obtained from large-eddy simulations indicate that the turbulent Langmuir number affects the scales of vortical structures. A linearized model with wave forcing is used to analyze this flow. The resolvent modes obtained from the model align well with the simulation and provide further insights into the dynamics of coherent vortices in Langmuir turbulence. |
Tuesday, November 26, 2024 9:05AM - 9:18AM |
X28.00006: A plume-based high-order closure scheme for upper ocean turbulence Amrapalli Garanaik, Luke van Roekel, Brodie Pearson Turbulent mixing in the ocean surface boundary layer (OSBL) plays a key role in the evolution of the Earth system on various scales. The current state-of-the-art mixing schemes show systematic biases in representing this mixing in general circulation models (GCMs). In this study, we present a new physically motivated mixing scheme that effectively captures the OSBL turbulent mixing produced by a wide range of realistic oceanic forcing, including surface waves, wind, and buoyancy. This new scheme is a hybrid mass-flux and high-order closure that maintains full energetic constraints by evolving a subset of second- and third-order turbulent moments utilizing a plume-based closure assumption. It inherently captures the non-diffusive mixing produced by both buoyancy-driven convective and wave-driven Langmuir turbulence. We have evaluated the scheme against a suite of large eddy simulations and other existing schemes and find that the new scheme compares well to the large eddy simulation results, has little sensitivity to vertical resolution and time step, making it feasible for GCMs. We extended the scheme to capture even more complex realistic oceanic forcing, such as time-varying diurnal forcing, sea ice melt effects, and testing in global simulation by optimizing the numerical implementation of the scheme. Finally, the new scheme is also well suited for implementation on GPU systems, allowing ocean models to be more amenable to emerging high-performance computing architectures. |
Tuesday, November 26, 2024 9:18AM - 9:31AM |
X28.00007: Effects of Offshore Wind Energy on Ocean Circulation and Mixing Miguel A Guzman Hernandez, John M Perez Medina, Umberto Ciri, Kianoosh Yousefi, Stefano Leonardi A one-way numerical coupling of the ocean and atmosphere was developed to study the effects of offshore wind turbines in the surrounding oceanic regions. Our in-house UTD-WF LES code was used to model the atmospheric boundary layer and wind turbines, whereas FVCOM (Finite Volume Community Ocean Model) was used to model the oceanic domain. |
Tuesday, November 26, 2024 9:31AM - 9:44AM |
X28.00008: Quantifying irreversible transport and mixing of biogeochemical scalars in a stratified shear flow Vincent Laroche, Alexis K Kaminski Biogeochemical scalars, representing many different species of phytoplankton, zooplankton, nutrients, etc., play an important role in marine ecosystems. Here we examine a simplified two-species system of generic phytoplankton and nutrient scalars. We construct a model that governs the biological reaction (nutrient uptake, phytoplankton death/remineralization) and examine, via direct numerical simulation, how these two scalars interact when mixed by stratified shear instabilities (Kelvin-Helmholtz, Holmboe). Turbulent advective fluxes show large oscillations during the evolution of the shear instabilities, associated with reversible fluid motions. To isolate the irreversible turbulent scalar fluxes, we instead calculate diascalar fluxes, that is, diffusive fluxes across isoscalar surfaces. This concept has been previously used to quantify irreversible mixing of buoyancy in stratified mixing events; here, we extend this approach to also account for the effects of biogeochemical reactions. Our results show that this analysis proves useful for measuring the net effect of mixing, as well as for capturing the downward propagation of a phytoplankton front as it grows and consumes nutrients beneath the interface of our simulated domain. |
Tuesday, November 26, 2024 9:44AM - 9:57AM |
X28.00009: The evolution and fate of a submesoscale frontal instability -- simulation and spectral analysis Arjun Jagannathan, Eshwar Ramanathan, Cesar B Rocha We analyze submesoscale-resolving simulations of frontal instabilities inspired by recent observations in the Northeastern Pacific. The initial state is a finite-width front in thermal-wind balance, i.e., a horizontal buoyancy gradient $\bar{b}_x$ balanced by the vertical shear $\overline{V}_{\!z}$ of the geostrophic flow. The choice of the base flow profiles and frontal width are motivated by recent saildrone measurements off the northern California coast as part of NASA's Submesoscale Ocean Dynamics Experiment (S-MODE). To probe this configuration for instabilities, we superimpose upon the background flow field a white noise density perturbation. The simulations have a horizontal resolution of 100 m and a vertical resolution of 2 m and are performed using flow\_solve, a spectral code that solves the non-hydrostatic, primitive equations under the Bousinnesq approximation. Turbulence closure is achieved through eigth order hyperviscous and hyperdiffusive operators that act to dissipate energy and buoyancy variance only at the smallest resolved scales, leaving the larger scales untouched. Over $\mathcal{O}(1)$ days a mixed layer baroclinic instability manifests, with $\mathcal{O}(1)$ Rossby number and horizontal scale comparable to a mixed layer deformation radius scale $\mathcal{O}(Nh_{ml}/f)$ ($\mathcal{O}(1)$ km). This instability acts to partially restratify the front by slumping isopycnals. By day 10, secondary barotropic instabilities are observed along the flanks of the baroclinically unstable mode. By day 15, there is a forward energy flux towards the smallest resolved scales, as confirmed by a $k^{-2}$ slope of horizontal kinetic energy spectra over the entire submesoscale range. A spectral proper orthogonal decomposition (SPOD) of the vorticity field on different 2D planes provides insight into the modal structure of the instabilities. |
Tuesday, November 26, 2024 9:57AM - 10:10AM |
X28.00010: Effects of Topographic Characteristics on Internal Wave Generation in Uniform & Non-Uniform Stratifications Sai Saandeep Sampatirao, Manikandan Mathur Internal tide generation by barotropic flow over topography is well-recognized to be a significant source of mechanical energy that can contribute to deep ocean vertical mixing. Advances in observations and numerical modelling have led to improved global estimates of barotropic-to-baroclinic conversion via internal tide generation. However, features such as the wavenumber distribution of the generated internal tides, the role of nonuniform stratification, and the effects of topographic shapes, remain poorly understood. In this study, we use the Green function approach to model internal tide generation by ocean floor topography to address some of these questions. A specific question concerns how the internal tide characteristics vary with the criticality and height ratio of the topography. Does the answer depend on the stratification profile and the shape of the topography? |
Tuesday, November 26, 2024 10:10AM - 10:23AM |
X28.00011: Accumulation of particles with different characteristics in the global ocean Zih-En Tseng, Guangyao Wang, Yue Cynthia WU, Christopher Ruf, Yulin Pan Assessing the distribution of plastic debris in the ocean has proven to be important. Previous research, utilizing Lagrangian models, has confirmed the influence of convergent surface flows on tracer accumulation and identified five major garbage patches within subtropical ocean gyres. However, to understand the distribution of particles with different characteristics, an Eulerian model is preferred for its enhanced ability to incorporate parametrizations and fragmentation. For this purpose, we present an Eulerian microplastic transport model using MITgcm. This model incorporates ECCOv4r4 climate data and includes a 25-year simulation of seasonal cycles. |
Tuesday, November 26, 2024 10:23AM - 10:36AM |
X28.00012: Internal stresses in low-Reynolds-number fractal aggregates Matteo Polimeno, Francois Blanchette, Changho Kim We present a numerical model of the stresses around and within fractal-structured aggregates in low-Reynolds-number flows. Assuming that aggregates are made of cubic particles, we first use a boundary integral method to compute the stresses acting on the boundary of the aggregates. From these external stresses, we compute the internal stresses to gain insight on their breakup, or disaggregation. We focus on systems in which aggregates are either settling under gravity or subjected to a background shear flow and study two types of aggregates with fractal dimension slightly less or slightly more than two. We partition the aggregates into multiple shells based on the distance between the individual cubes in the aggregates and their center of mass and observe the distribution of internal stresses in each shell. Our findings indicate that the magnitude of large internal stresses is distributed in a manner consistent with a power-law and that large stresses are least likely to occur near the far edges of the aggregates. In addition, after breaking aggregates at the face with the maximum internal stress, we compute the mass distribution of sub-aggregates and observe significant differences between the settling and shear setups for the two types of aggregates, with the low-fractal-dimension aggregates being more likely to split approximately evenly. |
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