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

• Main Author: Matei, Adrian Victor
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UCL (University College London) 2021
• Subjects: North Atlantic
• Online Access: https://discovery.ucl.ac.uk/id/eprint/10121140/1/Matei_000_thesis.pdf; https://discovery.ucl.ac.uk/id/eprint/10121140/

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 ...