Stability and transition of wind-driven water surfaces

Scripps Institution of Oceanography
University of California, San Diego

Abstract-
The air-sea interface plays a key role in the transfers of momentum, gas, heat and mass between the atmosphere and the ocean. These exchanges, while important for their effect on nutrient distribution and life in the ocean, are also crucial to weather, climate and the general circulation of both the atmosphere and the ocean. Wind generates waves, wind generates currents, but despite the interest in both processes since antiquity, there is no comprehensive understanding of the full complexity of the evolution of the wind-generated wavy surface current field and with the globally averaged wind speed in the range of 6-8 m/s, much of the time the air-sea interface is in a low-wind regime. It is clear that the stability, transition and subsequent mixing at the surface of the ocean and other natural water bodies are of great importance for many processes, from local to global, and play key roles in the fluxes between the atmosphere and the ocean.

We present the results of laboratory and field experiments on the stability of a wind-driven water surface to surface waves and Langmuir circulations. Using modern remote flow visualization techniques, we show that wave generation is accompanied by other phenomena, including the generation of small coherent Langmuir circulations which grow, dislocate, and constitute the transition to fully-developed turbulence of the surface flow. The length and velocity scales associated with the transition are comparable to those of the surface wave field, providing clear evidence of the influence of Langmuir circulations on the structure of the growing wave field. The field observations show a similar transition of the surface flow occurring over comparable length and time scales to those in the laboratory.

We conclude that the observed transition is an important phenomenon embedded within the complex dynamics of surface flows. The coherent structures rapidly disrupt the surface skin layer and efficiently mix mass, momentum, and heat from the surface to depth. The results are discussed in the context of ocean-atmosphere interactions and compared with available theoretical results.

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