Preprints
https://doi.org/10.5194/os-2021-103
https://doi.org/10.5194/os-2021-103

  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 1: Bubble distribution and development

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. The bubbles generated by breaking waves are of considerable scientific interest due to their influence on air-sea gas transfer, aerosol production, and upper ocean optics and acoustics. However, a detailed understanding of the processes creating deeper bubble plumes (extending 2–10 metres below the ocean surface) and their significance for air-sea gas exchange is still lacking. Here, we present bubble measurements from the HiWinGS expedition in the North Atlantic in 2013, collected during several storms with wind speeds of 10–27 m s−1. A suite of instruments was used to measure bubbles from a self-orienting free-floating spar buoy: a specialised bubble camera, acoustical resonators, and an upward-pointing sonar. The focus in this paper is on bubble void fractions and plume structure. The results are consistent with the presence of a heterogeneous shallow bubble layer occupying the top 1–2 m of the ocean which is regularly replenished by breaking waves, and deeper plumes which are only formed from the shallow layer at the convergence zones of Langmuir circulation. These advection events are not directly connected to surface breaking. The void fraction distributions at 2 m depth show a sharp cut-off at a void fraction of 10−4.5 even in the highest winds, implying the existence of mechanisms limiting the void fractions close to the surface. Below wind speeds of 16 m s−1 or RHw = 2 × 106, the probability distribution of void fraction at 2 m depth is very similar in all conditions, but increases significantly above either threshold. Void fractions are significantly different during periods of rising and falling winds, but there is no distinction with wave age. There is a complex near-surface flow structure due to Langmuir circulation, Stokes drift, and wind-induced current shear which influences the spatial distribution of bubbles within the top few metres. We do not see evidence for slow bubble dissolution as bubbles are carried downwards, implying that collapse is the more likely termination process. We conclude that the shallow and deeper bubble layers need to be studied simultaneously to link them to the 3D flow patterns in the top few metres of the ocean. Many open questions remain about the extent to which deep bubble plumes contribute to air-sea gas transfer. A companion paper (Czerski, 2021) addresses the observed bubble size distributions and the processes responsible for them.

Helen Czerski et al.

Status: open (until 03 Jan 2022)

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  • CC1: 'Comment on os-2021-103; "Flux and Presence"', David Woolf, 21 Nov 2021 reply

Helen Czerski et al.

Helen Czerski et al.

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Short summary
The bubbles formed by breaking waves speed up the movement of gases like carbon dioxide and oxygen between the atmosphere and the ocean. Understanding where these gases go is an important part of understanding Earth's climate. In this paper we describe measurements of the bubbles close to the ocean surface during big storms in the North Atlantic. We observed small bubbles collecting in distinctive patterns which help us to understand the contribution they make to the ocean breathing.