Innovative experiments have provided new insights into how bubbles are created by breaking waves. These findings might ultimately lead to more accurate models of global climate.
When ocean waves break, air and sea water mix to form whitecaps. Beneath the surface of the whitecap, a mixture of air and sea water form a violent turbulent flow known as a bubble plume. The plumes generated by typical breaking waves evolve rapidly for approximately 10 seconds as large bubbles rise quickly back to the surface1. After this time, a relatively diffuse plume of small bubbles persists in the ocean for up to several minutes. Study of the rapidly evolving flows inside these plumes is a challenging task: until now, such studies have not shed any light on the physical mechanisms responsible for bubble creation, nor have they identified the factors that govern bubble size2. On page 839 of this issue3, Deane and Stokes report the results of a pioneering study of bubble-creation mechanisms. Their analysis and interpretation of laboratory and field measurements represent a significant step towards unravelling the mystery of bubble creation by breaking waves.
Why should we care about the bubbles generated by breaking waves? Bubbles have a surprisingly important role in many physical, chemical and biological processes occurring at the air–sea interface. Bubble formation increases gas transfer between the air and sea4, and rising bubbles scavenge organic material and bacteria from the water column and transport them to the ocean surface5. Bubbles are both sources and scatterers of underwater sound6, and when they rise back to the surface they burst and eject tiny droplets into the atmosphere. The resulting marine aerosols influence cloud and hurricane dynamics, as well as Earth's radiative balance and biogeochemical cycles7.
The foundation for Deane and Stokes's work is the idea that bubbles break up or fragment when the differential pressure forces associated with turbulence exceed the restoring force of surface tension8. Thus, in a given turbulent flow, bubbles smaller than a certain size — defined as the Hinze scale — are stable, whereas larger bubbles tend to break up. Deane and Stokes discovered that two primary mechanisms are involved. Larger bubbles are formed by turbulent fragmentation when the air cavity trapped by the overturning wave crest collapses. Smaller bubbles are created by the impact and subsequent splashing that occurs when the toe of the wave crest plunges onto the ocean surface. This mechanism is referred to as 'jet and drop impact'.
Deane and Stokes used video recordings to measure the size and number of bubbles generated by breaking waves. From these data the authors calculated size distributions showing the number of bubbles per unit volume plotted against bubble radius, and observed a distinct change in the slope of the size distributions at a radius of approximately 1 mm in both laboratory and field data. The video recordings (Fig. 1) revealed that the smallest bubbles that fragmented were about 1 mm in radius. On the basis of these two results and additional analysis, Deane and Stokes conclude that the Hinze scale is equal to approximately 1 mm.
Deane and Stokes's argument that it is turbulent fragmentation that determines the size and number of large bubbles (radius 1 mm and over) is particularly convincing: the observed slope of the size distribution of large bubbles is consistent with an earlier analysis9 and with previous measurements of bubble size made beneath the crests of more gently breaking waves10. The authors acknowledge that their arguments regarding the creation of smaller bubbles by jet and drop impact are more speculative. However, the dimensional analysis and underwater sound data that they present to support their arguments are consistent and persuasive.
One of the most encouraging aspects of the new work is that it reconciles the authors' laboratory and field measurements of bubble size distributions with their mechanistic model of bubble creation. This not only gives us confidence in the findings but also provides hope that an accurate model of the size and number of bubbles entrained by breaking waves is within reach. Development of such a model is an essential step in accurately assessing the significance of gas transfer by bubbles in the global cycling of gases. As well as using Deane and Stokes's results to provide the shape of the bubble size distribution, a future model will require a reliable method for predicting the occurrence of breaking waves and the volume of air entrained.
The randomness of wave breaking makes it unlikely that theoretical approaches will solve this problem in the near future. But developments in visible7 and infrared11,12 remote sensing of breaking ocean waves might lead to accurate statistical descriptions of their properties that can be used in lieu of a theoretical model.
Global climate models currently used to predict climate change do not have sufficient resolution to allow direct modelling of small-scale processes such as bubble production by breaking waves. So the effect of these small-scale physical processes on air–sea gas transfer must be incorporated into the models with relatively simple equations. If forecasts of global warming are to be accurate, these equations will have to be based on sound physical principles. The best approach will be to develop equations that combine the statistical information obtained from remote sensing of breaking waves with the fundamental knowledge that stems from experimental studies such as those of Deane and Stokes.
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Loewen, M. Inside whitecaps. Nature 418, 830 (2002). https://doi.org/10.1038/418830a