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On the imprint of surfactant‐driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps
Author(s) -
Callaghan A. H.,
Deane G. B.,
Stokes M. D.
Publication year - 2017
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2017jc012809
Subject(s) - breaking wave , pulmonary surfactant , seawater , environmental science , oceanography , geology , meteorology , chemical engineering , physics , engineering , wave propagation , optics
Surfactants are ubiquitous in the global oceans: they help form the materially‐distinct sea surface microlayer (SML) across which global ocean‐atmosphere exchanges take place, and they reside on the surfaces of bubbles and whitecap foam cells prolonging their lifetime thus altering ocean albedo. Despite their importance, the occurrence, spatial distribution, and composition of surfactants within the upper ocean and the SML remains under‐characterized during conditions of vigorous wave breaking when in‐situ sampling methods are difficult to implement. Additionally, no quantitative framework exists to evaluate the importance of surfactant activity on ocean whitecap foam coverage estimates. Here we use individual laboratory breaking waves generated in filtered seawater and seawater with added soluble surfactant to identify the imprint of surfactant activity in whitecap foam evolution. The data show a distinct surfactant imprint in the decay phase of foam evolution. The area‐time‐integral of foam evolution is used to develop a time‐varying stabilization function, ϕ ( t ) and a stabilization factor, Θ , which can be used to identify and quantify the extent of this surfactant imprint for individual breaking waves. The approach is then applied to wind‐driven oceanic whitecaps, and the laboratory and ocean Θ distributions overlap. It is proposed that whitecap foam evolution may be used to determine the occurrence and extent of oceanic surfactant activity to complement traditional in‐situ techniques and extend measurement capabilities to more severe sea states occurring at wind speeds in excess of about 10 m/s. The analysis procedure also provides a framework to assess surfactant‐driven variability within and between whitecap coverage data sets.