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Near surface ocean process : acoustical observations, ambient sound, and Langmuir circulation

University of British Columbia

This thesis describes a study of near surface ocean processes based on observations made with an instrument incorporating both active and passive acoustical systems. The topics addressed include the organization of bubble clouds by Langmuir circulation, the modulation of ambient sound levels by surface wave action, and the interactions of vortices associated with Langmuir circulation. Observations in a deep ocean environment reveal bubble clouds organized into plumes of 100 m length and widths of about 5 m aligned with the wind through the action of Langmuir circulation. The depth of these plumes varies somewhat with wind speed with the greatest depths of 12 m occurring at wind speeds of 13 msֿ ¹. Using acoustic Doppler techniques, downward vertical velocities of 0.06 msֿ ¹ are observed at 8 m depth within the bubble plumes. Ambient sound observations at 8 kHz are used to search for possible relationships between wave breaking and Langmuir circulation: no systematic relationship is identified. This investigation does reveal the occurrence of modulations to ambient sound levels in phase with the long (~ 150 m) surface waves passing over the instrumentation (positioned at 30 m depth). A model of sound generation at the ocean surface suggests that individual sources must have spacings of less than 6 m to reproduce the observations. Increased breaking activity (or greater source levels) are required at long wave crests to explain these modulations: it could be caused either by interactions between long and short waves, or variations in wind stress over the long waves. The observations of Langmuir circulation reveal many coexisting scales of spacing between the windrows. A mechanism capable of generating small scale vorticity of the appropriate orientation through wave breaking and vortex stretching is developed. The consequences of interactions between this small scale, two dimensional vorticity is then explored using a Lagrangian vorticity model. This model demonstrates that continuous injection of small scale vorticity close to the ocean surface can lead to circulation similar to that expected for Langmuir circulation: a distribution of circulations cell sizes results, down welling speeds exceed upwelling speeds, and the cell spacing scales vary in proportion to the depth at which a bottom boundary is placed in the model.

Text

2011-03-03

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http://hdl.handle.net/2429/32007

Oceanography

University of British Columbia

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