Preprints
https://doi.org/10.5194/os-2021-104
https://doi.org/10.5194/os-2021-104

  08 Nov 2021

08 Nov 2021

Review status: this preprint is currently under review for the journal OS.

Ocean bubbles under high wind conditions. Part 2: Bubble size distributions and implications for models of bubble dynamics

Helen Czerski1, Ian M. Brooks2, Steve Gunn3, Robin Pascal4, Adrian Matei1, and Byron Blomquist5,6 Helen Czerski et al.
  • 1Department of Mechanical Engineering, University College London, London, WC1E 7BT, UK
  • 2School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 3Department of Electronics and Computer Science, University of Southampton, SO17 1BJ, UK
  • 4National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 5Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 6NOAA Physical Sciences Laboratory, Boulder, Colorado, USA

Abstract. Bubbles formed by breaking waves in the open ocean influence many surface processes but are poorly understood. We report here on detailed bubble size distributions measured during the High Wind Speed Gas Exchange Study (HiWinGS) in the North Atlantic, during four separate storms with hourly averaged wind speeds from 10–27 m s−1. The measurements focus on the deeper plumes formed by advection downwards (at 2 m depth and below), rather than the initial surface distributions. Our results suggest that bubbles reaching a depth of 2 m have already evolved to form a heterogeneous but statistically stable population in the top 1–2 metres of the ocean. These shallow bubble populations are carried downwards by coherent near-surface circulations; bubble evolution at greater depths is consistent with control by local gas saturation, surfactant coatings and pressure. We find that at 2 m the maximum bubble radius observed has a very weak wind speed dependence and is too small to be explained by simple buoyancy arguments. For void fractions greater than 10−6, bubble size distributions at 2 m can be fitted by a two-slope power law (with slopes of −0.3 for bubbles of radius < 80 μm and −4.4 for larger sizes). If normalised by void fraction, these distributions collapse to a very narrow range, implying that the bubble population is relatively stable and the void fraction is determined by bubbles spreading out in space rather than changing their size over time. In regions with these relatively high void fractions we see no evidence for slow bubble dissolution. When void fractions are below 10−6, the peak volume of the bubble size distribution is more variable, and can change systematically across a plume at lower wind speeds, tracking the void fraction. Relatively large bubbles (80 μm in radius) are observed to persist for several hours in some cases, following periods of very high wind. Our results suggest that local gas supersaturation around the bubble plume may have a strong influence on bubble lifetime, but significantly, the deep plumes themselves cannot be responsible for this supersaturation. We propose that the supersaturation is predominately controlled by the dissolution of bubbles in the top metre of the ocean, and that this bulk water is then drawn downwards, surrounding the deep bubble plume and influencing its lifetime. In this scenario, oxygen uptake is associated with deep bubble plumes, but is not driven directly by them. We suggest that as bubbles move to depths greater than 2 m, sudden collapse may be more significant as a bubble destruction mechanism than slow dissolution, especially in regions of high void fraction. Finally, we present a proposal for the processes and timescales which form and control these deeper bubble plumes.

Helen Czerski et al.

Status: open (until 05 Jan 2022)

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Helen Czerski et al.

Helen Czerski et al.

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Short summary
The bubbles formed by breaking waves at the ocean surface are important because they are thought to speed up the movement of gases like carbon dioxide and oxygen between the atmosphere and ocean. We collected data on the bubbles in the top few metres of the ocean which were created by storms in the North Atlantic. The focus in this paper is the bubble sizes and their position in the water. We saw that there are very predictable patterns, and set out what happens to bubbles after a wave breaks.