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Even though the physics of wave breaking for ocean surface waves may not be well understood, what wave breaking is and what it looks like is no mystery to the average beach-goer. However, I am confused to as what "whitecapping" is, in the context of "whitecapping dissipation" of waves that appears in literature.

  1. What is whitecapping? Is it the white "stuff" that is created when a wave break?
  2. If so, what causes the white "stuff"? Is it the same as "foam", a term that is found in the literature as well?
  3. Are the terms "whitecapping" and "wave breaking" synonymously used? Are they in fact one and the same?

Relevant references would also be appreciated.

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Whitecapping refers to the steepness-induced wave dissipation in deep water during which some air is entrained into the near-surface water, forming an emulsion of water and air bubbles (foam) that appears white. It occurs when the velocity of individual water particles near the wave crest exceed the phase speed of the wave, causing the front face of the wave to become too steep and "break". Steeper and more vigorous breakers are more efficient at entraining air bubbles into the water column, and they generate more foam. Whitecapping is an essential process for air-sea gas exchange.

In the wave modeling community, because current spectral wave prediction models do not resolve air entrainment by the breaking waves, we use the term "whitecapping" to describe all steepness-induced deep-water wave dissipation. For example, see Hasselmann (1974).

Waves also break due to shoaling as they enter shallow water, and these are usually referred to as plunging breakers. While plunging breakers without doubt generate a lot of foam, we don't use the term whitecapping to describe them. Thus, whitecapping is a specific kind of wave breaking, as described in the first paragraph.

See a comprehensive review by Cavaleri et al. (2007) for a description of all the processes represented by current wave models.

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  • $\begingroup$ @milancurcic this is a good answer, +1, but the kinematic condition you mention for breaking is not very informative and very hard to quantify in practice. See the review by Perlin et al. (2013) and in particular the study by Stansell and MacFarlane (2002). Understanding the kinematics of these focusing waves is a current area of research. $\endgroup$
    – Nick P
    Jun 16, 2015 at 7:49
  • $\begingroup$ @NickP Thanks, good point on the kinematic condition, I agree with you. Can you write the full reference for Perlin et al. 2013? I can't seem to find that one. Also, feel free to either expand on my answer by submitting an edit, or by writing an answer of your own that would contribute to the topic. $\endgroup$
    – milancurcic
    Jun 16, 2015 at 15:06
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    $\begingroup$ @milancurcic The full reference is Perlin, Marc, Wooyoung Choi, and Zhigang Tian. "Breaking waves in deep and intermediate waters." Annual Review of Fluid Mechanics 45 (2013): 115-145. $\endgroup$
    – Nick P
    Jun 16, 2015 at 21:59

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