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Title: Lagrangian perspectives on ocean ventilation
Author: MacGilchrist, Graeme
ISNI:       0000 0004 7232 6536
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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Within the global climate system, the ocean operates as a vast store of important components such as heat and carbon dioxide that it exchanges with the atmosphere on timescales from days to millennia. Its facility to do so is in large part due to the process of ocean ventilation by which water is moved from the surface mixed layer, where it is in contact with the atmosphere, and transported through the subsurface. An understanding of ocean ventilation, therefore, is crucial in establishing the magnitudes and timescales of ocean-atmosphere exchange. In this thesis, a predominantly Lagrangian approach is adopted, evaluating trajectories in an eddy-permitting numerical ocean circulation model to explore different aspects of the ventilation process. Following critical assessment of the fidelity of the trajectory analysis, a dynamical systems approach is applied to assess the role of ocean turbulence in the transport of ventilated water in the subsurface, with application to the subtropical gyre of the North Atlantic. The pathways of ventilation, represented by Lagrangian maps, are found to be highly chaotic and characterised by a non-dimensional filamentation number that compares the ventilation and filamentation timescales of the flow. Subsequently, the mechanisms and variability of ventilation of dense water masses in the high-latitude North Atlantic are considered, a crucial component in present-day oceanic uptake of heat and carbon dioxide. A Lagrangian approach allows us to link surface processes to their subsurface signature, and reveals that variations in annual re-entrainment establish substantial inter-annual variability in the water that enters the deep ocean. Furthermore, the results reveal that mechanisms of ventilation in the numerical simulation differ substantially from observations, with possible implications for the fidelity of future climate projections. Finally, the translation into the ocean interior of a biologically and chemically active tracer, such as carbon dioxide, is considered, and how this is dependent on the interaction between ventilation, circulation and biogeochemical processes. From observations in the Weddell Gyre, it is shown that biological export and the horizontal circulation are critical in sustaining regional uptake of carbon dioxide from the atmosphere. Broadly, these various results serve to emphasise the complexity of the ventilation process and its associated role in the oceanic storage of climate-relevant components, from the chaotic nature of pathways, short timescales of variability, its difficulty of representation in numerical models, and its interaction with the biogeochemical system.
Supervisor: Marshall, David ; Johnson, Helen Sponsor: Natural Environment Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available