Transport and mixing by persistent and materially coherent ocean structures

• Main Author: Denes, Michael
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UNSW, Sydney 2022
• Subjects: Dynamical Systems; Oceanography; Turbulence; Lagrangian Coherent Structures; Geophysical Fluid Dynamics; anzsrc-for: 370803 Physical oceanography; anzsrc-for: 490105 Dynamical systems in applications; Laplace; Southern Ocean; South Atlantic Ocean
• Online Access: http://hdl.handle.net/1959.4/100951; https://unsworks.unsw.edu.au/bitstreams/f0656fb1-2d32-479f-8a97-c84b214a2a6a/download; https://doi.org/10.26190/unsworks/24658

The ocean is dominated by kinematic features, such as gyres, fronts, and mesoscale eddies, that persist for much longer than typical dynamical timescales. Due to their capacity to transport heat, salt, carbon, and other biogeochemical tracers over long distances, these coherent structures play an important role in climate, biology, and small-scale mixing. However, because of their Lagrangian (or flow-following) nature, identifying and tracking these features, and ultimately quantifying their contribution to transport processes, is challenging. In this thesis, we study transport and mixing in the ocean by coherent structures through the framework of finite-time coherent sets. This approach is motivated by a dynamic isoperimetric problem whose approximate solution is derived from the associated dynamic Laplace operator. Coherent sets describe regions of phase space that minimise mixing along their boundaries over a finite time window. They identify barriers to transport and provide the skeleton around which more complex or turbulent dynamics occurs. Chapter 2 introduces the formalism of the dynamic isoperimetric problem and associated dynamic Laplace operator and develops the necessary extensions for oceanographic applications. Chapter 3 examines the persistence and material coherence of a mesoscale ocean eddy in the South Atlantic Ocean and applies the framework to quantify, for the first time, transport by the outer ring of the eddy. Chapter 4 extends the framework to mixed boundary conditions, which we use to investigate material transport across circumpolar Southern Ocean fronts. We reveal a previously unobserved global pattern of alternating poleward and equatorward transport across Southern Ocean fronts, resulting from frontal meandering influenced by prominent sea-floor obstacles. Chapter 5 adapts the framework to global and regional domains containing multiple coherent features and studies two applications: basin-scale gyres and a train of ocean eddies. The results of this thesis demonstrate that the ...