Unpicking the details of deep bubble plumes in stormy conditions: bubble populations and plume spatial distributions measured in the North Atlantic during the HiWINGS expedition.

Helen Czerski, University College London, London, United Kingdom, Adrian Matei, University College London, United Kingdom, Steve Gunn, University of Southampton, Southampton, United Kingdom, Ian M Brooks, University of Leeds, Leeds, United Kingdom and Raied Al-Lashi, Nottingham Trent University, Nottingham, United Kingdom
The subsurface bubble plumes generated by breaking waves in high wind conditions have been studied for several decades. However, progress in understanding the details of bubble plumes and their dependence on the environmental conditions has been limited by opportunities for field studies, the availability of the specialised equipment required and the necessary auxiliary data to interpret the measurements. We report here on the detailed bubble size distributions and plume spatial distributions measured during an extensive field campaign in the North Atlantic. Hourly averaged wind speeds varied from 10 m/s to 28 m/s, and measurements were made from a free-floating 11m spar buoy which drifted for several days during each of four separate storms. Bubble size distributions were measured at depths of 2 m and 4 m using a specialised bubble camera and an acoustical resonator respectively, and plume distribution was simultaneously monitored using an upward-looking sonar. The measured bubble size range was 10-4000 microns in radius.

Detailed analysis of this data provides several insights into bubble distribution and lifetime in stormy seas. We identify size limits on the bubbles found at the two depths, and consider the implications for bubble dissolution. Normalising instantaneous bubble size distributions by their void fraction is observed to collapses the data to a relatively narrow range. We consider the possibility that the steepening of bubble size distributions with time previously seen may be an artefact of averaging processes and the bubble size limitations of the instruments used. Further analysis allows the comparison of the void fraction and peak bubble volume at 2m and 4m depth in various wind conditions. These insights provide context for measurements in the existing literature and are relevant for subsurface acoustical measurements and air-sea gas transfer studies.