Abstract
AbstractAir bubble penetration depths are investigated with a bottom‐mounted echosounder at a seabed observatory in northern Norway. We compare a 1‐year time series of observed bubble depth against modeled and estimated turbulent kinetic energy flux from breaking waves as well as wind speed and sea state. We find that the hourly mean and maximum bubble depths are highly variable, reaching 18 and 38 m, respectively, and strongly correlated with wind and sea state. The bubble depth is shallowest during summer following the seasonal variations in wind speed and wave height. Summertime shallowing of the mixed layer depth is not limiting the penetration depth. A strong relationship between bubble depth and modeled turbulent kinetic energy flux from breaking waves is found, similar in strength to the relationship between bubble depth and wind speed. The wind sea is more strongly correlated with bubble depth than the total significant wave height, and the swell is only weakly correlated, suggesting that the wave model does a reasonable separation of swell and wind sea.
Highlights
Breaking surface waves generate turbulent mixing and entrain air bubbles in the water column (Gemmrich & Farmer, 1999; Thomson et al, 2016; Thorpe, 1982)
Following the procedure of Thorpe (1992), Trevorrow (2003), and Wang et al (2016), we investigate how the bubble depth varies with surface wind speed and sea state
Before we look at the relationship between the turbulent kinetic energy (TKE) flux from the wave model and the bubble depth, we briefly investigate the sensitivity of the parameterized TKE flux given by equation (3)
Summary
Breaking surface waves generate turbulent mixing and entrain air bubbles in the water column (Gemmrich & Farmer, 1999; Thomson et al, 2016; Thorpe, 1982). This controls the air-sea gas exchange crucial for the ventilation of the ocean and the uptake of carbon dioxide and oxygen (Rhein et al, 2013). Vertical size-specific bubble distributions for radii between 8 and 130 μm were measured by Vagle and Farmer (1992) using a multifrequency acoustic backscatter technique Such detailed bubble distributions allow estimates of the total amount of air entrained by breaking waves and the associated gas transfer. An assessment of the echosounder's ability to measure significant wave height is presented in Appendix A
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