Chaotic advection, mixing, and property exchange in three-dimensional ocean eddies and gyres
Citable URI
https://hdl.handle.net/1912/10413DOI
10.1575/1912/10413Abstract
This work investigates how a Lagrangian perspective applies to models of two oceanographic flows:
an overturning submesoscale eddy and the Western Alboran Gyre. In the first case, I focus on
the importance of diffusion as compared to chaotic advection for tracers in this system. Three
methods are used to quantify the relative contributions: scaling arguments including a Lagrangian
Batchelor scale, statistical analysis of ensembles of trajectories, and Nakamura effective diffusivity
from numerical simulations of dye release. Through these complementary methods, I find that
chaotic advection dominates over turbulent diffusion in the widest chaotic regions, which always
occur near the center and outer rim of the cylinder and sometimes occur in interior regions for
Ekman numbers near 0.01. In thin chaotic regions, diffusion is at least as important as chaotic
advection. From this analysis, it is clear that identified Lagrangian coherent structures will be
barriers to transport for long times if they are much larger than the Batchelor scale. The second
case is a model of the Western Alboran Gyre with realistic forcing and bathymetry. I examine
its transport properties from both an Eulerian and Lagrangian perspective. I find that advection
is most often the dominant term in Eulerian budgets for volume, salt, and heat in the gyre, with
diffusion and surface fluxes playing a smaller role. In the vorticity budget, advection is as large as
the effects of wind and viscous diffusion, but not dominant. For the Lagrangian analysis, I construct
a moving gyre boundary from segments of the stable and unstable manifolds emanating from two
persistent hyperbolic trajectories on the coast at the eastern and western extent of the gyre. These
manifolds are computed on several isopycnals and stacked vertically to construct a three-dimensional
Lagrangian gyre boundary. The regions these manifolds cover is the stirring region, where there is
a path for water to reach the gyre. On timescales of days to weeks, water from the Atlantic Jet and
the northern coast can enter the outer parts of the gyre, but there is a core region in the interior
that is separate. Using a gate, I calculate the continuous advective transport across the Lagrangian
boundary in three dimensions for the first time. A Lagrangian volume budget is calculated, and
challenges in its closure are described. Lagrangian and Eulerian advective transports are found to
be of similar magnitudes.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy
at the
Massachusetts Institute of Technology
and the
Woods Hole Oceanographic Institution June 2018
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Suggested Citation
Thesis: Brett, Genevieve, "Chaotic advection, mixing, and property exchange in three-dimensional ocean eddies and gyres", 2018-06, DOI:10.1575/1912/10413, https://hdl.handle.net/1912/10413Related items
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