Poster Abstracts
Tuesday, 17 May 2016, 2:00 PM to 5:30 PM
Short Poster Introductions, 5 minutes, maximum 2 slides
Afterwards: Time to view the posters until 5:30 PM
Transport in the Gulf of Mexico
Rodrigo Duran (NETL-DOE / CEOAS-OSU)
Aided by Lagrangian coherent structure theory techniques we separate the surface-ocean flow in the Gulf
of Mexico into regions with distinct transport characteristics, using the output from a 12-year-long dataassimilative
simulation produced by the HYbrid-Coordinate Ocean Model (HYCOM).
Chaotic Advection and Mixing in an Idealized 3D Eddy Model
G. Jay Brett (Woods Hole Oceanographic Institution)
Typically, oceanic fluid flows are well-approximated by a restriction to two dimensions; however, this
approximation breaks down in the submesoscale. A laminar kinematic model for the flow within a rotating
Ekman-driven cylinder is used to approximate a fully three-dimensional submesoscale eddy. Patterns of
chaotic advection and stirring rates under a steady symmetry-breaking perturbation are described and
matched to those of a dynamic rotating cylinder simulation from prior work. The possibility of observing
such chaotic patterns in the ocean is then examined. Four methods are used to determine the importance
of chaotic advection's effect on mixing with respect to smaller-scale turbulent diffusion: scaling arguments
including a Lagrangian Batchelor scale, the spread of ensembles of trajectories, numerical simulations of
dye release, and the calculation of a Nakamura effective diffusivity. We find that chaotic advection will
dominate turbulent diffusion in the widest chaotic regions.
Spiral inertial waves emitted from geophysical vortices
Peng Wang (University of Miami)
By numerically simulating an initially unstable geophysical vortex, we discover for the first time a special
kind of inertial waves, which are emitted in a spiral manner from the vortices; we refer to these waves as
spiral inertial waves (SIWs). SIWs appear at small Rossby numbers (0.01 < Ro < 1) according to our
parameter sweep experiments; the amplitude, wavelength and frequency of SIWs are sensitive to Rossby
numbers. We extend the Lighthill-Ford radiation into inertial waves, and propose an indicator for the
emission of inertial waves; this indicator may be adopted into general circulation models to parameterize
inertial waves. Additionally, in our tracer releasing experiments, SIWs organize tracers into spirals, and
modify the tracer's local rate of change by advecting tracers vertically. Further, the spirals of SIWs
resembles some spiral features observed in the ocean and atmosphere, such as spiral ocean eddies and
spiral hurricane rainbands; thus, SIWs may offer another mechanism to form spiral eddies and rainbands.
Since no density anomaly is required to generate the spirals of SIWs, we infer that the density anomaly,
hence the baroclinic or frontal instability, is unlikely to be the key factor in the formation of these spiral
features.
The Objective Eulerian Skeleton of Fluid Flows
Mattia Serra (ETH Zürich)
We discuss a new objective (frame-invariant) theory for Eulerian Coherent Structure identification in two dimensional unsteady flows. Objective Eulerian Coherent Structures (OECS) reveal the time-varying
skeleton of fluid flows that instantaneously approximates the most influential material surfaces in
transport and mixing. We also describe an objective non-dimensional metric that quantifies the
persistence of vortex-type OECSs. In an application to persistent eddy detection in satellite-derived ocean
velocity data, we find that our OECS persistence metric significantly outperforms vortex predictions from
other customary Eulerian diagnostics, such as the potential vorticity gradient and the Okubo-Weiss
criterion.
An Autonomous Dynamical System Captures all Lagrangian Coherent Structures in Three Dimensions
David Oettinger (ETH Zürich)
Lagrangian coherent structures (LCSs) are material surfaces that shape tracer patterns in flows with
arbitrary time dependence. Depending on their deformation properties, different types of LCSs can be
distinguished. Here we observe that, in three dimensions, initial positions of all types of LCSs can be viewed
as invariant manifolds of an autonomous dynamical system. This dual dynamical system, therefore, allows
for locating LCSs in finite-time, time-aperiodic flows by methods developed for autonomous dynamical
systems, such as Poincaré maps. Since the dual dynamical system is generally dissipative, its invariant
manifolds are normally hyperbolic and hence mark the LCS locations robustly. We consider both steady
and time-aperiodic flow models, and use the dual dynamical system to obtain both hyperbolic and elliptic
LCSs from a single computation.
Observations of Vorticity and Strain from Drifters in the Ocean
Sebastian Essink (WHOI/MIT)
Lagrangian velocity estimates from a large, but closely-spaced, drifter array in the Bay of Bengal are used
to calculate vorticity and strain. The along-trajectory vorticity signal reveals complex patterns, supposedly
generated by tides, inertial oscillations, and mesoscale and submesoscale flow. In particular, banded
structures of alternating positive and negative vorticity are observed, where trajectories are parallel to the
coastline. This work aims at a mechanistic understanding of the contributing factors that generate the
observed vorticity patterns. By understanding the role of tides and the mesoscale circulation, estimates
for the vorticity generating effect of submesoscale flows will be retrieved. The footprint of submesoscale
dynamics is evidenced from a) positively-skewed distributions of vorticity, b) regions, in which high strain
and high vorticity co-occur and c) a correlation between vorticity and lateral density gradients or fronts.
Title TBA
Margaux Filippi (MIT/WHOI)
(Abstract not available.)
Revisiting velocity structure functions in the Gulf of Mexico
Dhruv Balwada (WHOI)
Lagrangian velocity measurements from surface drifters in the Gulf of Mexico (GLAD experiment) provided
one of the few high resolution (spatial and temporal) datasets that could be used to evaluate dynamics of
small scale transport in the ocean. This was meant to be the decisive study after the BP oil spill to show if
the transport in the surface ocean was local (cloudy) or non-local (filamented). However, instead of solving
the age old problem, it has left the community in more disarray than before. Poje et al 2014 showed that
the spreading is local, while Beron-Vera & LaCasce 2016 have shown the behavior to be non-local.
In this study we revisit the data, for the third time in the literature, and take a careful view at the structure
functions of the velocity by decomposing into the divergent and rotational components. This Helmholtz
decomposition shows that the total energy in the second order structure function is dominated by
divergent motions at scales smaller than 5km and by rotational motions above that, indicating the
presence of strong vertical velocities at small scales. The rotational component of the second order
structure function has a slope of 2/3 between 20 – 100km, possibly indicating the presence of an inverse
energy cascade. Thus the slope of 2/3 over the entire range of length scales is due to an opportune overlap
of these two regimes.
Kinematic simulations using recreated velocity fields with appropriately prescribed structure functions (KE
spectra) are performed to look at the signature of the observed dynamics on particle dispersion.
Multiple quasi-zonal jets in the South Indian Ocean
Vivian V. Menezes (GFDI, Florida State University)
The large-scale current systems of the world ocean is now considered as a well-known subject, mostly
based on ocean circulation of the Northern Hemisphere. One distinctly different large-scale circulation
system and whose dynamics is still not understood is observed in the subtropical South Indian Ocean (SIO).
The SIO is dominated by a system of multiple eastward quasi-zonal jets extending from Madagascar to
Western Australia, which are collectively known as the South Indian Countercurrent (SICC). These jets flow
in a direction opposite to that predicted by classical theories of wind-driven circulation. Further, the
subtropical countercurrent theory (also known as mode-water-induced subsurface frontogenesis) was
found to be unsatisfactory to explain the observed multiple jet structure. Here, we conjecture that this
persistent multiple zonal jet structure may be explained as a result of frontal perturbations associated to
meridional discontinuities in the PV distribution (PV staircase-like), formed by interactions between
turbulence, waves and eddies.
This is joint work with Marcio Vianna (VM Oceanica).
