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On the imprint of surfactant-driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps

TitleOn the imprint of surfactant-driven stabilization of laboratory breaking wave foam with comparison to oceanic whitecaps
Publication TypeJournal Article
Year of Publication2017
AuthorsCallaghan AH, Deane GB, Stokes MD
JournalJournal of Geophysical Research-Oceans
Date Published2017/08
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000410790600004
Keywordsactive material; air-sea interface; bubbles; dependence; dissipation; fraction; gas transfer velocity; microlayers; Remote sensing; stability; surfactant; whitecap; wind-speed

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, phi(t) and a stabilization factor, circle dot, 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 H 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 10m/s. The analysis procedure also provides a framework to assess surfactant-driven variability within and between whitecap coverage data sets. Plain Language Summary The foam patches made by breaking waves, also known as "whitecaps'', are an important source of marine sea spray, which impacts weather and climate through the formation of cloud drops and ice. Sea spray chemistry depends on the chemistry of the whitecap that makes it. This chemistry is poorly understood, especially during storms when whitecaps are most prevalent but chemistry measurements are also the most difficult. In this article, we show that foam chemistry affects the persistence of laboratory whitecaps: the more surfactant a whitecap contains, the longer it persists. This effect has enabled us to develop a remote sensing tool to detect the presence of chemistry in whitecaps by analyzing a time-series of photographs of the foam. We have applied the technique to an existing set of whitecap images, and get reasonable values for implied surfactant concentrations in the ocean but validation of the technique in the field will have to await simultaneous measurement of whitecaps and sea surface chemistry. If validated, the new remote sensing tool will provide the first large-scale observations of ocean surface chemistry and its variation in space and time on wind-driven seas.

Short TitleJ Geophys Res-Oceans
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