Measurements of sub-surface bubble populations and the modelling of air-sea gas flux

Abstract

Bubbles, formed by breaking waves, play an important role in the transfer of gases between the Earth’s oceans and atmosphere and have been shown to increase the flux of gases during periods of heightened sea state. Having been
formed, these bubble clouds evolve through the effects of buoyancy, gas exsolution and dissolution, and the fragmentation and coalescence of bubbles.
A number of experimenters have successfully measured sub-surface bubble clouds using a variety of acoustic and optical techniques, although data over a wider range of bubble radii are required for fuller comparison with models
of how these clouds evolve and contribute to air-sea transfers of mass, momentum and energy. This study details the design of an acoustic system deployed on an 11 metre spar buoy during two sea trials in the Atlantic
Ocean. Through the measurement of the additional attenuation due to bubbles, bubble size distributions were inferred over the broadest range of bubble
radii ever measured using active acoustics in the open ocean. The volumetric backscatter strength from the bubble clouds were also measured to gain a profile of these bubble populations. A gas transfer model was then developed, with the measured data used as an input to calculate the associated fluxes. With this method, bubble-mediated transfer velocities and equilibrium supersaturations were found for the first time based on experimental work. These parameters aid the characterisation of air-sea gas transfer and
therefore help improve the accuracy of existing climate models.

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Identifiers

ORCID for T.G. Leighton: orcid.org/0000-0002-1649-8750

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