Session 8
Friday, 20 May 2016, 9:00 AM to 10:00 AM
20 minutes per talk, 10 minutes for questions and speaker change
From |
To |
Name |
Affiliation |
Title |
|---|---|---|---|---|
9:00am |
9:30am |
Ryan Abernathey | Columbia University |
Lagrangian Coherent Structures and Eulerian Eddy Fluxes in the East Pacific |
9:30am |
10:00am |
A. D. Kirwan, Jr. |
SMSP, University of Delaware |
Dynamics of time-dependent transport boundaries in rotating stratified Euler flow |
Session 8 Abstracts
Lagrangian Coherent Structures and Eulerian Eddy Fluxes in the East Pacific
Ryan Abernathey (Columbia University)
Two very different perspectives on the nature of mesoscale (10-300 km) eddies exist in the oceanographic
literature. One one hand, ocean modelers emphasize the Eulerian concept of the "eddy flux" (i.e. Reynolds
flux) arising from time-varying small-scale correlations between between the velocity field and an
advected scalar (e.g. the eddy heat flux). This quantity is important because it contributes significantly to
the climatological budgets of heat, salt, nutrients, etc. On the other hand, observationalists, and,
increasingly, applied mathematicians, conceptualize "eddies" as discrete coherent structures which may
trap fluid and propagate spatially. This study attempts to unify these two perspectives by asking, what is
the role of Lagrangian coherent structures in the Eulerian eddy flux?
We examine the East Pacific sector, from 180 - 130 E longitude, for which previous studies have examined
the Eulerian eddy flux in great detail. We use the AVISO surface geostrophic velocity dataset to drive the
advection of a dense array of Lagrangian particles (over 8 million); from these trajectories, Rotationally
Coherent Vortices (RCVs) are identified using the objective, frame-invariant definition of Haller et al.
(2016). The meridional eddy diffusivity is computed from the full trajectory data using Taylor's formula,
and from this, the relative contribution of the RCVs to the net meridional flux is quantified. The coherent
vortices are found to make a relatively minor contribution to the net meridional flux, indicating the ocean
mesoscale eddy transport is achieved primarily through more disorganized motions. These results contrast
with the findings of some previous studies which, using non-objective eddy tracking techniques, may have
overestimated the contribution of coherent mesoscale eddies to the Eulerian eddy flux.
Dynamics of time-dependent transport boundaries in rotating stratified Euler flow
A. D. Kirwan, Jr. (SMSP, University of Delaware)
The problem considered here is quantification of time-dependent flow boundaries in stratified flow. The
velocity field is specified as an analytic solution to the time dependent stratified Euler equations on an fplane.
Results are presented for model parameters scaled for ocean submesoscale dynamics. Particle
trajectories live on both cyclonic and anti-cyclonically winding tori. The simplicity of the velocity field is in
contrast to the intricate structure of the transport boundary surfaces. They exhibit remarkable depth
dependency and vibrate at super-inertial frequencies. Implications for analyses of ocean circulation data
are offered.
(Authors: Henry Chang, SMSP University of Delaware; H. S. Huntley, SMSP University of Delaware; A. D.
Kirwan, Jr., SMSP University of Delaware; M. H. M. Sulman, Department of Mathematics, Wright State
University; B. L. Lipphardt, Jr; bruce.lipphardt@gmail.com)
