Session Q32: Geophysical Fluid Dynamics: Oceanographic

Development of ice floe tracker algorithm to measure Lagrangian statistics in the eastern Greenland coast

Sea ice export through Fram Strait is a key component of the Arctic climate system. The East Greenland Current (EGC) carries most of the sea ice southwards until it melts. Lagrangian methods using sea ice buoys have been used to map ice features in polar regions. However, their spatial and temporal coverage is limited. Satellite data can provide a better tool to map sea ice flow and its variability. Here, an automated sea ice floe detection algorithm uses ice floes as tracers for surface ocean currents. We process Moderate Resolution Imaging Spectroradiometer satellite images to track ice floes (length scale 5-10 km) in the north-eastern Greenland Sea region. Our matlab-based routines effectively filter out clouds and adaptively modify the images to segment and identify ice floes. Ice floes were tracked based on persistent surface features common in successive images throughout 2016. Their daily centroid locations were extracted and its resulting trajectories are used to describe surface circulation and its variability using differential kinematic parameters. We will discuss the application of this method to a longer time series and larger spatial coverage. This enables us to derive the inter-annual variability of mesoscale features along the eastern coast of Greenland.   

A nonlinear self-similar solution to barotropic flow over rapidly varying topography

Beginning from the Shallow Water Equations (SWE), a nonlinear self-similar analytic solution is derived for barotropic flow over rapidly varying topography. We study conditions relevant to the ocean slope where the flow is dominated by Earth's rotation and topography. Examples of the solution's relevance are presented. The solution is found to extend the topographic $\beta$-plume solution of Kuehl (2014) in two ways: 1) The solution is valid for intensifying jets. 2) The influence of nonlinear advection is included. The SWE are scaled to the case of a topographically controlled jet, then solved by introducing a similarity variable, $\eta = cx^{n_x}y^{n_y}$. The nonlinear solution, valid for topographies $h = h_0-xy^3$, takes the form of the Lambert W Function for sudo velocity. The linear solution, valid for topographies $h = h_0-\alpha xy^{-\gamma}$, takes the form of the Error Function for transport. Kuehl's results considered the case $ \gamma=1< 1$ which admits expanding jets, while the new result consider the case $\gamma<-1$ which admits intensifying jets and a non-linear case with $\gamma = - 3$.   

Subduction at upper ocean fronts by baroclinic instability

Large eddy simulations of upper ocean fronts that are initially in geostrophic balance show that the linear and subsequent nonlinear evolution of baroclinic intability are effective in restratifying the front. During the growth of baroclinic instability, the front develops thin regions with enhanced vertical vorticity, i.e., vorticity filaments. Moreover, the vorticity filaments organize into submesoscale eddies. The subsequent frontal dynamics is dominated by the vorticity filaments and the submesoscale eddies. Diagnosis of the horizontal force balance reveals that the regions occupied by these coherent structures have significantly large imbalance, and are characterized by large vertical velocity. High density fluid from the heavier side of the front is subducted by the vertical velocity to the bottom of the mixed layer. The process of subduction is illustrated by Lagrangian tracking of fluid particles released at a fixed depth.   

Evolution of turbulence at a frontal zone

Large-eddy simulations are used to investigate processes leading to turbulence at an ocean front. The geostrophically-balanced front follows Eady's model with a uniform lateral density gradient. In the vertical direction, the front consists of two layers, each with a linear stratification. The upper 50-m surface mixed layer is weakly stratified with the gradient Richardson number, Ri, equal to 0.25. The lower 50-m pycnocline has a stronger stratification with Ri = 3. The evolution of turbulence includes both symmetric instability and baroclinic instability. Consistent with linear theory, symmetric instability only develops in the surfaced mixed layer. Secondary shear instabilities subsequently grow along the slanted isopycnals and induce turbulence in the front. At a later time, baroclinic instability develops into full-depth submesoscale eddies which are intertwined with thin filaments of enhanced lateral shear. Strong turbulence is localized around the periphery of the eddies and along the filaments. Spectral analysis indicates the energy transfer at the geostrophic scales differs significantly from that at the smaller turbulent scales.   

Effects of Small-Scale Turbulent Mixing on Upper Ocean Carbonate Chemistry

The effects of both shear- and wave-driven turbulence on the evolution of carbonate chemical species in the upper ocean are examined at scales from one to several hundred meters using large eddy simulations. The simulations model reactive carbonate species in the presence of realistic upper-ocean turbulence by solving the wave-averaged Boussinesq equations with and without an imposed Stokes drift velocity, leading to wave- (i.e., Langmuir) and shear-driven turbulence, respectively. Carbonate chemistry is represented in the simulations using a reduced seven-species mechanism solved using a Runge-Kutta-Chebyshev method, and comparisons are made between simulations with time-dependent chemistry, equilibrium chemistry, and no chemistry. By examining different reaction chemistries and surface forcing scenarios, coupled turbulence-reactive tracer dynamics are connected to spatial and statistical properties of the resulting species fields. In particular, Langmuir turbulence has a pronounced effect by increasing the uptake of carbon dioxide in the upper ocean. Implications of these results for coarse-resolution models of the global carbon cycle are discussed.   

Assessing uncertainty in the turbulent upper-ocean mixed layer using an unstructured finite-element solver

Vertical transport flux in the ocean upper mixed layer has recently been attributed to submesoscale currents, which occur at scales on the order of kilometers in the horizontal direction. These phenomena, which include fronts and mixed-layer instabilities, have been of particular interest due to the effect of turbulent mixing on nutrient transport, facilitating phytoplankton blooms. We study these phenomena using a non-hydrostatic, large eddy simulation for submesoscale currents in the ocean, developed using the extensible, open-source finite element platform FEniCs. Our model solves the standard Boussinesq Euler equations in variational form using the finite element method. FEniCs enables the use of parallel computing on modern systems for efficient computing time, and is suitable for unstructured grids where irregular topography can be considered in the future. The solver will be verified against the well-established NCAR-LES model and validated against observational data. For the verification with NCAR-LES, the velocity, pressure, and buoyancy fields are compared through a surface-wind-driven, open-ocean case. We use this model to study the impacts of uncertainties in the model parameters, such as near-surface buoyancy flux and secondary circulation, and discuss implications.   

Driving Turbulence by Generic Mean Flow - A New Model Formulation for Large-Eddy Simulation

Shelf sea models are currently at the stage where computing capability permits the resolving of internal tides (\textless 2 km). However, the underpinning turbulence closure schemes which govern the vertical flux of momentum and scalars do not have the skill to provide meaningful estimates at regions of appreciable stratification. The implications of this are large, as modelling of important phenomena, such as plankton bloom formations and the carbon cycle will not yield meaningful results. This deficiency in turbulence parametrisations promotes motivation for the model development of a turbulence resolving Large-eddy simulation. A new model formulation will be presented, whereby mean flow and stratification can be prescribed by observational, analytical or model data. The resultant velocity and density fluctuations, which govern the mediation of turbulent flux quantities, are explicitly solved for. We will present compelling comparisons between model and observations to demonstrate the skill of the new model approach.   

Turbulent convection in geostrophic circulation with wind and buoyancy forcing

We conduct a direct numerical simulation of geostrophic circulation forced by surface wind and buoyancy to model a circumpolar ocean. The imposed buoyancy forcing (represented by Rayleigh number) drives a zonal current and supports small-scale convection in the buoyancy destabilizing region. In addition, we observe eddy activity which transports heat southward, supporting a large amount of heat uptake. Increasing wind stress enhances the meridional buoyancy gradient, triggering more eddy activity inside the boundary layer. Therefore, heat uptake increases with higher wind stress. The majority of dissipation is confined within the surface boundary layer, while mixing is dominant inside the convective plume and the buoyancy destabilizing region of the domain. The relative strength of the mixing and dissipation in the system can be expressed by mixing efficiency. This study finds that mixing is much greater than viscous dissipation, resulting in higher values of mixing efficiency than previously used.   

New Layer Thickness Parameterization of Diffusive Convection

Double-diffusion convection is one of the most important non-mechanically driven mixing processes. Its importance has been particular recognized in oceanography, material science, geology, and planetary physics. Double-diffusion occurs in a fluid in which there are gradients of two (or more) properties with different molecular diffusivities and of opposing effects on the vertical density distribution. It has two primary modes: salt finger and diffusive convection. Recently, the importance of diffusive convection has aroused more interest due to its impact to the diapycnal mixing in the interior ocean and the ice and the ice-melting in the Arctic and Antarctic Oceans. In our recent work, we constructed a length scale of energy-containing eddy and proposed a new layer thickness parameterization of diffusive convection by using the laboratory experiment and in situ observations in the lakes and oceans. The new parameterization can well describe the laboratory convecting layer thicknesses (0.01\textasciitilde 0.1 m) and those observed in oceans and lakes (0.1\textasciitilde 1000 m).   

Interaction between Langmuir circulation and the bottom boundary layer in shallow water

Results are reported from large eddy simulations (LES) of a shear flow in an unstratified finite-depth water column driven by a surface wind stress and a constant crosswind pressure gradient. The Craik-Leibovich vortex force in the LES equations serves to generate Langmuir circulation (LC) consisting of parallel counter rotating vortices aligned downwind. The vortex force parameterizes the interaction between surface gravity waves and the wind-driven shear current resulting in LC at the surface of the ocean. In the LES without crosswind pressure gradient, LC is generated at the surface and over time penetrates deeper into the water column while also growing in the crosswind direction. The growth of the LC is restricted by the bottom of the water column. In LES with constant pressure gradient such that the induced crosswind current is 1.5 times the wind-driven current at mid-depth, the growth of LC is interrupted by the crosswind current's bottom boundary layer. Turbulent ejections from this boundary layer interact with the LC leading to a full-depth coherent structure characterized by a blend between Langmuir turbulence (associated with the LC) and bottom-generated shear turbulence. Diagnostics of this hybrid turbulence and budgets of the Reynolds shear stress will be analyzed.   

Shoaling internal solitary waves of depression over gentle slopes

The shoaling of an internal solitary wave (ISW) of depression over gentle slopes is explored through fully nonlinear and non-hydrostatic simulations using a high resolution/accuracy deformed spectral multidomain penalty method. During shoaling, the wave does not disintegrate as in the case of steeper slope but, instead, maintains its symmetric shape. At the core of the wave, an unstable region forms, characterized by the entrapment of heavier-over-light fluid. The formation of this convective instability is attributed to the vertical stretching by the ISW of the near-surface vorticity layer associated with the baroclinic background current. According to recent field observations in the South China Sea, the unstable region drives localized turbulent mixing within the wave, estimated to be up to four times larger than that in the open ocean, in the form of a recirculating trapped core. In this talk, emphasis is placed on the structure of the unstable region and the persistence of a possible recirculating core using simulations which capture 2D wave propagation combined with 3D representation of the transition to turbulence. As such, a preliminary understanding of the underlying fluid mechanics and the potential broader oceanic significance of ISWs with trapped cores is offered.   

Numerical simulation of ocean surface oil plume dispersion and its impact on upper-ocean light field.

Crude oil plumes from offshore spills can be dispersed over a wide area by the upper-ocean turbulence and Langmuir circulations, inducing significant impact on the ocean ecosystem. Clouds of suspended crude oil droplets can cause significant light absorption and scattering, strongly affecting the sunlight penetration in the ocean euphotic zone where photosynthesis occurs. In this study, the turbulent dispersion of surface oil plumes and the resultant variations of upper-ocean light field are studied using high-fidelity numerical simulations. In particular, the ocean flow field and the oil plume dispersion are simulated using large-eddy simulation, and the sunlight transport is simulated using Monte Carlo method. The simulation results show that oil plumes of different droplet sizes are dispersed very differently by the ocean turbulence and Langmuir circulations, with large oil droplets concentrating in confined regions near the ocean surface, while small oil droplets being diluted smoothly in the ocean mixed layer. The differences in oil droplet size and dilution rate yield different inherent optical properties for the mixture of oil droplets and seawater, resulting in very different light field variations for different ocean flow and oil plume conditions.