18 Oct 2023
 | 18 Oct 2023
Status: this preprint is open for discussion.

On the short-term response of entrained air bubbles in the upper ocean: a case study in the North Adriatic Sea

Alvise Benetazzo, Trygve Halsne, Oyvind Breivik, Kjersti Opstad Strand, Adrian Callaghan, Francesco Barbariol, Silvio Davison, Filippo Bergamasco, Cristobal Molina, and Mauro Bastianini

Abstract. Air bubbles in the upper ocean are generated mainly by wave breaking at the air-sea interface. As such, after the waves break, entrained air bubbles evolve in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the relationships between bubble penetration depth, mechanical and thermal forcings, and air-sea transfer velocity of CO2, a field experiment was conducted from an oceanographic research platform in the North Adriatic Sea. Air bubble plumes were measured using high-resolution echosounder data from an up-looking 1000-kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organised into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17-m depth under strong forcing. We verify that bubble depths adapt rapidly to wind and wave conditions and scale approximately linearly with wind speed. A scaling with the wind/wave Reynolds number is proposed to account for the sea-state severity in the depth prediction. Results also show a strong connection between measured bubble depths and theoretical air-to-sea CO2 transfer velocity parametrised with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed, cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air-sea interaction studies and pave the way for progress in CO2 gas exchange formulations.

Alvise Benetazzo et al.

Status: open (until 23 Dec 2023)

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Alvise Benetazzo et al.

Alvise Benetazzo et al.


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Short summary
We investigated the behaviour of air bubble plumes in the upper ocean, focusing on their shape in various stormy conditions. We conducted a field experiment in the North Adriatic Sea using high-resolution sonar. We found that bubble penetration depths respond to wind/wave parameters and are triggered by the cooling of the water masses. We also find a strong connection between bubble depths and theoretical CO2 gas transfer. Our findings have implications for air-sea interaction modelling.