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The relationship of subsurface bubble plumes to wind speed and sea state in the open ocean

Matei, Adrian Victor; (2021) The relationship of subsurface bubble plumes to wind speed and sea state in the open ocean. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

When winds blow over the ocean, surface waves grow as energy and momentum are transferred from the air to the sea surface. The waves steepen as energy is added and may eventually break, generating turbulence and plumes of bubbles in the near-surface ocean. This thesis focuses on measurements made during a six-week research trip in the North Atlantic Ocean, in a wide range of wind speeds (8–30 m/s). The complex effects of wind speed, wave age, wind sea wave height and subsurface turbulence on the structure, penetration depth, duration and production rate of bubble plumes are investigated in this thesis. The major data set used here was collected using an upward-pointing sonar during three separate four-day deployments. Individual bubble plumes were detected using an algorithm based on simultaneous sonar and resonator acoustic signals from subsurface bubbles. The acoustical backscattering caused by bubble plumes decays approximately exponentially with depth. The decay constant is related to the bubble entrainment in the water column. The entrainment of a larger void fraction may lead to a lower backscattering decay constant due to the possibility of multiple scattering. Higher wind speed induces younger, rougher waves and higher turbulence, which generate deeper bubble entrainment that promotes longer-lasting plumes (> 100 seconds). The plume production rate is amplified by low-medium wind speeds (< 20 m/s) in a non-linear way, reaching up to 32 plumes per hour. However, at very high winds (> 20 m/s), the plume production rate is significantly reduced to only 8 plumes per hour in the presence of younger less developed seas. The duration variation of these plumes is associated with large scatter. The wave height was found to influence the bubble-plume activity only in a sea state where swell does not contaminate the wind-driven waves. Given the complex and often chaotic sea state in the open ocean, wind speed remains the main factor that controls the bubble plume structure. During the persistent storms, it is difficult to directly assess the effect of individual breaking waves on subsurface bubble plumes because the upper ocean may become supersaturated with gas, and plumes are often advected from a side into the sonar sample volume. In supersaturated seas (at wind speeds > 20 m/s), the plumes become continuous fields of bubbles persisting for very long times (minutes or tens of minutes), without a clear separation between individual events. The total backscatter distribution along the water column (0–8 metres, which was the distance from the sonar to the sea surface) varies with plume penetration depth. Most of the backscatter signal (over 90%) is contained in the upper one metre when the plume is shallow and dense, spreading down the water column more evenly as the plume deepens and becomes more acoustically transparent

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The relationship of subsurface bubble plumes to wind speed and sea state in the open ocean
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
URI: https://discovery.ucl.ac.uk/id/eprint/10121140
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