Alteration of wind flow by an oil film on the sea surface

Analyses of water waves propagating under oil films beginning with Osborne Reynolds in the nineteenth century and continuing through modern times have identified dissipative processes brought about by the inextensibility or elasticity of surface films. Capillary-gravity waves are rapidly attenuated by this means, but the reduction of large storm breakers by oil films, as reported since ancient times, cannot be explained solely by such small scale processes.

A published report in the late nineteenth century clearly indicates that storm breakers were annulled after an oil film extended over an area extending about one kilometer to windward of the observer, while breakers continued in distant clean water areas. This observation is most easily explained if energy transmitted to waves of up to 100 m wavelength ceases when the wind blows over an oil slick.

We present two hypotheses to explain the cessation of growth of such long waves. Either the critical height where wind speed equals wave phase velocity (Miles 1954) increases over an oil slick, or wind flow often separates down wind from large wave crests when capillary-gravity waves are absent. The view expressed here is that suppression of storm breakers is not a direct mechanical response of the sea surface to an oil film but rather that smoothing of the small scale roughness of the sea surface causes a major change in the structure of the wind profile over the sea such that energy input by Miles mechanism into gravity waves up to 100m wavelength range is greatly reduced.

Observation over an artificial pond show that the critical height tends to decrease over slicked water surfaces; this makes the first hypothesis unlikely. There are no measurements of wind structure over oil slicks on stormy seas to evaluate the second hypothesis.

In summary it appears that the influence of oil slicks has two aspects: the film attenuates short waves, and the lack of short waves alters the wind flow profoundly.