Bubble-Turbulence Dynamics and Dissipation Beneath Laboratory Breaking Waves

Bubbles directly link sea surface structure to the dissipation rate of turbulence in the ocean surface layer through wave breaking, and they are an important vehicle for air-sea transfer of heat and gases and important for understanding both hurricanes and global climate. Adequate parameterization of bubble dynamics, especially in high winds, requires simultaneous measurements of surface waves and breaking-induced turbulence; collection of such data would be hazardous in the field, and they are largely absent from laboratory studies to date. We therefore present data from a series of laboratory wind-wave tank experiments designed to observe bubble size distributions in natural seawater beneath hurricane conditions and connect them to surface wave statistics and subsurface turbulence. A shadowgraph imager was used to observe bubbles in three different water temperature conditions. We used these controlled conditions to examine the role of stability, surface tension, and water temperature on bubble distributions. Turbulent kinetic energy dissipation rates were determined from subsurface ADCP data using a robust inertial-subrange identification algorithm and related to wind input via wave-dependent scaling. Bubble distributions shift from narrow to broadbanded and toward smaller radius with increased wind input and wave steepness. TKE dissipation rate and shear were shown to increase with wave steepness; this behavior is associated with a larger number of small bubbles in the distributions, suggesting shear is dominant in forcing bubbles in hurricane wind-wave conditions. These results have important implications for bubble-facilitated air-sea exchanges, near-surface ocean mixing, and the distribution of turbulence beneath the air-sea interface in hurricanes. SIGNIFICANCE STATEMENT: Bubbles are a vehicle for the flux of heat, momentum, and gases between the atmosphere and ocean. These fluxes contribute to the energy budgets of hurricanes, climate, and upper-ocean biology. Few to no simultaneous measurements of surface waves, bubbles, and turbulence have been made in hurricane conditions. To improve numerical model representation of bubbles, we performed laboratory experiments to parameterize bubble size distributions using physical variables including wind and waves. Bubble distributions were found to become broadbanded and shift toward smaller radius with increased wind stress and wave steepness. Turbulence dissipation rate and shear were shown to increase with wave steepness. Our results give the first physically based bubble distribution parameterization from naturally breaking waves in hurricane-force conditions.

Automated summary

Bubbles directly connect sea surface structure to turbulence dissipation rate through wave breaking, playing a crucial role in air-sea heat and gas transfer and understanding hurricanes and global climate. This study presents laboratory experiments on bubble size distribution in natural seawater under hurricane conditions, and reveals the influence of wind input and wave steepness on bubble distributions. The results demonstrate a shift towards smaller radius and broader distribution of bubbles with increasing wind and wave conditions, indicating the dominance of shear in forcing bubbles in hurricane wind-wave conditions. These findings are of great importance for air-sea exchanges, ocean mixing, and turbulence distribution in hurricanes.