Turbulent mixing at the surface of natural water bodies: Breaking waves and Langmuir circulations

A laboratory and field investigation of mixing and turbulence in the surface-wave zone, and specifically, the role played by breaking waves and Langmuir circulations is presented.; Breaking waves are believed to be a significant source of mixing of surface boundary layer of the oceans. We show in both the laboratory and the field that, with acoustical instrumentation, it is possible to directly resolve part of the inertial subrange in the turbulence generated by breaking events. This technique permits the measurement of the turbulent kinetic energy dissipation without invoking Taylor's hypothesis. The results show enhanced levels of dissipation when compared to the classical wall-layer scaling. In addition the turbulence under breaking waves is highly localized and decays very rapidly. We also present an investigation of the generation and evolution of surface waves and Langmuir circulations. In the laboratory, experiments using a variety of modern flow visualization techniques show that the classical wave generation problem is accompanied by phenomena that occur over comparable time and length scales. Of interest is the generation of Langmuir circulations which provide the transition to turbulence of the accelerating surface flow. During this transition, the surface skin layer is disrupted thus enhancing heat and gas transfer. In addition, a large fraction of momentum is mixed from the surface to depth by the circulations, at rates greater than those of viscous transport. We subsequently show that the wave field is strongly modulated by the Langmuir circulations. Thus, the stability of the wind-driven surface flow involves multiple interacting phenomena, rendering the problem both rich and intricate. Field observations confirm that small-scale Langmuir circulations occur in the field as well, at least under low wind-speed conditions. The data acquired in both the laboratory and the field show that these phenomena play an important role in the mixing of the upper layers of natural water bodies and in small-scale air-sea interaction in general.