Session 5

Wednesday, 18 May 2016, 2:00 PM to 3:30 PM

20 minutes per talk, 10 minutes for questions and speaker change

From	To	Name	Affiliation	Title
2:00pm	2:30pm	Andrew Poje	City University of New York, CSI	Cluster-based, finite-time partitions for identifying Lagrangian transport properties
2:30pm	3:00pm	Joe LaCasce	University of Oslo	Deducing energy spectra from drifters
3:00pm	3:30pm	Francisco Beron-Vera	RSMAS, University of Miami	Nonlocal pair dispersion in the Gulf of Mexico

 

Session 5 Abstracts

Cluster-based, finite-time partitions for identifying Lagrangian transport properties

Andrew Poje (City University of New York, CSI)
We apply a cluster-based approach to identify the temporal evolution of Lagrangian structures in a classic
oceanographic transport problem, namely the cross-stream flux induced by the interaction of a meso-scale
Gulf Stream Ring eddy with the main jet. The model for the highly time-dependent, motion of the Gulf
Stream is a nominally 2 km, submesoscale permitting HYCOM simulation of the North Atlantic. The focus
is on a single mixing event driven by the interaction between an energetic cold core ring (a cyclone), the
strong jet, and a number of smaller scale cyclones and anticyclones. The timescales of interest are 1-3
weeks corresponding to model lifetimes of the larger eddies. The goal is to characterize and organize the
time-dependent Lagrangian kinematics in both the horizontal and vertical directions. The approach is
based on computing time-averages of fixed spatial basis functions over the trajectories and then applying
standard clustering algorithms to identify the finite-time structures within which trajectories possess
similar statistics. The resulting partitions are found to be robust to changes in the specifics of both the
basis sets and the clustering parameters. In addition to the synthetic model trajectories, applications of
the clustering method to a set of 300 in-situ surface trajectories from the Lagrangian Submesoscale
ExpeRiment, recently conducted in the northern Gulf of Mexico, will be discussed.

Deducing energy spectra from drifters

Joe LaCasce (University of Oslo)
Relative dispersion is often quantified using the second order velocity structure function, the mean square
velocity difference on pairs of particles. However, the results are frequently ambiguous in terms of
turbulent inertial ranges. We explore why this is so, and demonstrate how pair velocities can instead be
used to estimate energy spectra directly.

Nonlocal pair dispersion in the Gulf of Mexico

Francisco Beron-Vera (RSMAS, University of Miami)
Pair-separation statistics of in-situ and synthetic surface drifters deployed near the Deepwater Horizon
site in the Gulf of Mexico are investigated. The synthetic trajectories derive from a submesoscalepermitting
Navy Coastal Ocean Model (NCOM) simulation. The in-situ drifters were launched in the Grand
LAgrangian Deployment (GLAD). Various measures of the dispersion are calculated and compared to
theoretical predictions. For the NCOM pairs, the measures indicate nonlocal pair dispersion at the smallest
sampled scales. At separations exceeding 100 km, pair motion is uncorrelated, indicating absolute rather
than relative dispersion. With the GLAD drifters however the statistics suggest local dispersion (in which
pair separations exhibit power law growth), in line with previous findings. The disagreement stems in part
from inertial oscillations, which affect the energy levels at small scales without greatly altering the net
particle displacements. They were significant in GLAD but much weaker in the NCOM simulation. In
addition the GLAD drifters were launched close together, producing few independent realizations and
hence weaker statistical significance. Restricting the NCOM set to those launched at the same locations
yields very similar statistics.