Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H39: Geophysical Fluid Dynamics Ocean II 
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Chair: François Blanchette, UC Merced Room: 6a 
Monday, November 25, 2019 8:00AM  8:13AM 
H39.00001: Turbulent mixing in oceanic flows: challenges and insights for improved prediction Karan Venayagamoorthy The understanding and quantitative prediction of diapycnal (irreversible) mixing of density and momentum in the oceans remains an important ongoing challenge. From a practical perspective, there is a critical need to obtain accurate prediction of turbulent heat, mass and momentum fluxes using indirect measurements in the field. Indirect methods for estimating mixing rates typically rely on the inference of three key quantities namely: (i) the rate of dissipation of turbulent kinetic energy; (ii) the mixing efficiency, which is a measure of the amount of turbulent kinetic energy that is irreversibly converted into background potential energy; and (iii) the background density stratification, respectively. In this talk, an overview on how these quantities are typically inferred and/or parameterized will be presented. Some important challenges, ambiguities and new insights will also be presented with an eye toward improved prediction of ocean mixing. [Preview Abstract] 
Monday, November 25, 2019 8:13AM  8:26AM 
H39.00002: Predicting regions of ocean vertical transport via surface coherent structures Michael Allshouse, H M Aravind Vertical transport in the upper ocean impacts the surface mixing, advection of nutrients, and the ocean energy budget. Observing regions of significant vertical transport is difficult because vertical velocities in the ocean are often orders of magnitude smaller than horizontal velocities. Some tools for predicting where large vertical velocities will occur include HF radar, satellite altimetry, and modeled horizontal velocity fields, which all provide ocean surface velocities. While Eulerian analysis of these fields can yield some information, Lagrangian coherent structures are more robust to noisy observational data and model parameter uncertainty. We correlate surface coherent structures to vertical transport below the surface to evaluate their capacity to predict regions of strong vertical transport. In particular, we compute the finitetime Lyapunov exponent field from the surface velocity and compare this with the corresponding local vertical subduction. This correlation is tested on a highfidelity simulation of a sheared submesoscale flow and an operational ocean forecast. The identification of coherent structures provides a target zone for anticipated vertical transport that could be observed via Lagrangian floats. [Preview Abstract] 
Monday, November 25, 2019 8:26AM  8:39AM 
H39.00003: Oceanic Submesoscale WaveVortical Interactions and Their Effect on Scalar Transport Gerardo HernandezDuenas, Pascale Lelong, Leslie Smith The mechanisms driving lateral dispersion in the ocean on scales of $100~m10km$ remain, by and large, not well identified. Dominant motions in this regime, known as the submesoscale, are internal waves and vortical motions. These two components have similar spatial scales but evolve on different temporal scales. Smallscale vortical mode are susceptible to instabilities and may not be as longlived as their largerscale geostrophic counterparts. While vortices are more efficient at dispersing a passive tracer than waves, the role of the latter remains less well understood. In this talk, we will present simulations using a set of intermediate models to identify the role of various nonlinear interactions between vortical and wave motions. These intermediate models range from the quasigeostrophic model which only includes PV/PV/PV nonlinearities and GGG model with only wave/wave/wave nonlinearities to the full Boussinesq model which retains all. Statistics such as energy transfer spectra and diffusivity will be shown to identify the effect of different nonlinear interactions on scalar transport. [Preview Abstract] 
Monday, November 25, 2019 8:39AM  8:52AM 
H39.00004: The Energetics of Seamount Wakes Brad Perfect, Nirnimesh Kumar, James Riley This work revisits the longstanding hypothesis that seamounts are the ``stirring rods'' of the ocean. Specifically, it has been proposed that the eddy motions generated by the interaction of underwater mountains with ocean currents might play a significant role in the broader field of ocean mixing. We report on the results of a series of numerical simulations for an idealized seamount in rotating, stratified flow at a range of Froude and Rossby numbers ($0.014 < $ Fr $ < 0.14$ and $0.053 < $ Ro $ < 0.21$). This parameter space is consistent with the strongly stratified, strongly rotating flow experienced by a large seamount. In each simulation, the seamount generates a distinctive wake and produces internal wave radiation. We explore the energetics of the wake and waves separately, and find that the wake is predominantly controlled by the Burger number, while the internal wave radiation has a more complex dependence that can be predicted by modifying preexisting linear wave models. Ultimately, our results suggest that for lowFroude number flows, the wake of the seamount extracts more energy from the mean flow than the internal wave flux, supporting the stirring rod view of seamount energetics. [Preview Abstract] 
Monday, November 25, 2019 8:52AM  9:05AM 
H39.00005: Observations of nonlinear internal wave evolution and mixing from the shelf to the surf zone. Kristen Davis, Gregory Sinnett, Emma Reid Internal waves strongly influence the physical and chemical environment of coastal ecosystems worldwide. We report novel observations from a distributed temperature sensing (DTS) system that tracked the transformation of internal waves from the shelf break to the surf zone over a shelfslope region of a coral atoll in the South China Sea. The spatiallycontinuous view of the nearbottom temperature field provided by the DTS offers a perspective of physical processes previously available only in laboratory settings or numerical models. Additionally, we report observations of turbulent dissipation during the passage of a shoaling internal wave train and examine the implications for irreversible mixing of subthermocline water into the nearshore region and onto a shallow coral reef. We find that during summer, internal waves shoaling on the shallow atoll regularly transport cold, nutrientrich water shoreward, altering nearsurface water properties on the fore reef. This water is transported shoreward of the reef crest by tides, breaking surface waves and winddriven \textunderscore ow, where it signi\textunderscore cantly alters the water temperature and nutrient concentrations on the reef \textunderscore at. [Preview Abstract] 
(Author Not Attending)

H39.00006: Towards an accurate parameterization of mixed layer deepening during ocean convection Taimoor Sohail, Bishakhdatta Gayen, Andrew Hogg We investigate the growth and equilibrium of the oceanic mixed layer during a deep convective event using a Direct Numerical Simulation (DNS) and a largescale ocean model. The DNS resolves all scales of flow, providing the opportunity to quantify vertical buoyancy flux and mixing in a deep convective event. The inferred vertical diffusivity and mixed layer depth (MLD) are simulated over a range of background stratifications and surface buoyancy forcings in the DNS model. A scaling theory for convective mixing is proposed which robustly predicts the rate of MLD growth and vertical diffusivity during deep convection. We directly compare the MLD predicted from the scaling theory with existing vertical parameterizations in a largescale ocean model. Specifically, we run analogous deep convection experiments with the Modular Ocean Model (MOM) 6 running three different vertical parameterizations for the ocean boundary layer: CVMix, KPP and ePBL. We quantify the difference between the MLD derived from these parameterization schemes and the predictions from the theoretical scaling, thereby quantifying the accuracy of existing parameterization schemes. [Preview Abstract] 
Monday, November 25, 2019 9:18AM  9:31AM 
H39.00007: Impacts of inertial/symmetric instabilities on ocean fronts Nicolas Grisouard Oceanic submesoscale density fronts are structures in geostrophic and hydrostatic balance. They tend to be small (100 m to 10 km wide at midlatitudes) and ubiquitous features of the nearsurface of the oceans. They tend to be unstable, which may be key to understanding the kinetic energy budget of the ocean, as well as their effects on gas and nutrient exchanges between the surface and the abyss. In this presentation, we focus on the inertial or symmetric instability. We present a series of numerical experiments to investigate energetic impacts of these instabilities on fronts. Our set of experiments covers the submesoscale portion of a threedimensional parameter space consisting of the Richardson and Rossby numbers, and a measure of stratification or latitude. We first argue that contrary to parameterization prototypes that are currently being developed, drainage of available potential energy from the geostrophic flow can be a leadingorder source of their growth. We also argue that a front is relatively robust when experiencing this instability, and provide hints as to its contribution to the shape of fronts. We also caution modellers about a possibly large impact of the choices of the dissipation operator on the dynamics of the instability. [Preview Abstract] 
Monday, November 25, 2019 9:31AM  9:44AM 
H39.00008: Equilibration of symmetric instability and inertial oscillations at an idealised submesoscale front Aaron Wienkers, Leif Thomas, John Taylor Submesoscale fronts with large lateral buoyancy gradients and O(1) Rossby numbers are common in the upper ocean. These fronts are associated with large vertical transport and are hotspots for biological activity. Submesoscale fronts are susceptible to symmetric instability (SI)  a form of stratified inertial instability which can occur when the potential vorticity is of the opposite sign to the Coriolis parameter. Growing SI modes eventually break down through a secondary shear instability, leading to 3D turbulence and vertically mixing the geostrophic momentum. Once out of thermal wind balance, the front undergoes inertial oscillations which can drive further smallscale turbulence. \\Here, we consider the idealised problem of a balanced front with uniform horizontal buoyancy gradient and bounded by flat nostress horizontal surfaces. We study the equilibration of this unstable front using a linear stability analysis and 3D numerical simulations. We find drastically different behavior emerging at late times. While weak fronts develop frontlets and excite subinertial oscillations, stronger fronts produce borelike gravity currents. We describe the details of these energy pathways as the front evolves toward the final adjusted state in terms of the dimensionless front strength. [Preview Abstract] 
Monday, November 25, 2019 9:44AM  9:57AM 
H39.00009: Global Distribution of the Oceanic Bottom Mixed Layer Thickness PengQi Huang, XianRong Cen, YuanZheng Lu, ShuangXi Guo, ShengQi Zhou With the increasing observations in the abyssal ocean, the bottom ocean has received unprecedented attention. The bottom mixed layer (BML) is the only pathway for communication between the ocean interior and underlying, where interrelated physical, geochemical, and biological processes actively take place. In this study, over $30,000$ fulldepth conductivitytemperaturedepth (CTD) profiles archived during the past 30 years by the World Ocean Circulation Experiment (WOCE) Program were used to obtain the first approximation of the global distribution of the oceanic BML thickness $H_{BML}$ by applying an integrated method. We found that $H_{BML}$ had inhomogeneous distributions in different ocean basins and appeared thicker around midocean ridges. In particular, the median $H_{BML}$ values were $40$, $42$, and $64$ m in the Atlantic, Indian, and Pacific Oceans respectively, and $47$ m globally. In the abyssal ocean, $H_{BML}$ became thicker in the deeper ocean ($D$), according to a statistical fitting result of an exponential function of $H_{BML} = 26.34+0.85e^{(D/1271.8)}$. [Preview Abstract] 
Monday, November 25, 2019 9:57AM  10:10AM 
H39.00010: Settling of randomly formed marine aggregates Eunji Yoo, Shilpa Khatri, François Blanchette Settling marine aggregates plays an important role in transporting carbon from the surface ocean to the deep ocean. Investigation of settling rates is critical to understand the ecological importance of these particles. We study the settling of DiffusionLimitedAggregates as a model of marine aggregates in the ocean. The aggregates are assembled as a collection of cubic particles formed by clustertocluster aggregation, resulting in fractal objects. The stresses on the surface and flow around the aggregates are computed in the limit of zero Reynolds number using a singlelayer potential boundary integral method and we handle the challenges of singularities analytically. We first validate and analyze the performance of our numerical method. We then present the statistical distribution of drag and torque on aggregates of various sizes. We determine that the gyration radius is the most relevant length scale to describe the dynamics of those aggregates and obtain expressions for average drag and torque acting on settling aggregates. [Preview Abstract] 
Monday, November 25, 2019 10:10AM  10:23AM 
H39.00011: A Brownian dynamics model for the formation of marine aggregates Francois Blanchette, Changho Kim Marine aggregates, the largest of which are called marine snow, play an important role in the oceanic carbon cycle. As microorganisms form, grow, and die near the ocean surface, they tend to cluster and form aggregates. We study numerically the formation of these aggregates using a Brownian dynamics model. The mobility tensor of each aggregate particle, which significantly depends on its shape and size, is computed by a boundary integral method to solve the corresponding Stokes equations. Thus, our model provides a more accurate description of the formation mechanism of aggregates. We investigate the fractal dimension and size distribution of aggregates and also characterize the settling speed as a function of a properly defined size of an aggregate. [Preview Abstract] 
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