Long‐Term Statistics of Observed Bubble Depth Versus Modeled Wave Dissipation. Issue 2 (13th February 2020)

• Record Type: Journal Article
• Authors: Strand, Kjersti Opstad; Breivik, Øyvind; Pedersen, Geir; Vikebø, Frode Bendiksen; Sundby, Svein; Christensen, Kai Håkon
• Is Part Of: Journal of geophysical research. Volume 125:Issue 2(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-13
• Subjects: bubble depth -- wave breaking -- TKE flux -- upper ocean mixing -- air‐sea interaction -- echosounder bubble measurements; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015906
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Air 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. Plain Language Summary: Down‐mixed air bubbles measured with a bottom‐based echosounder provide an opportunity to observe upper ocean processes continuously through all types of weather. Here we investigate the correlation between air bubble depth and modeled energy flux from breaking waves. We investigate 1 year of data available from an offshore location in Northern Norway. Results show 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 and wave height. We find that the bubble depth is not limited by the depth of the mixed layer. A relationship between bubble depth and energy flux is found, similar to the relationship between wind and energy flux. This is important for prediction of both dispersal of particles (such as oil spills and marine plankton) and transfer of gases from the atmosphere to the ocean. Key Points: The longest (1 year) time series of echosounder measurements of bubble depth to date is presented Bubble depths in excess of 35 m are observed under storm conditions Modeled wave dissipation correlates strongly with observed bubble depth