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Dynamical coupling of wind and ocean waves through wave-induced air flow


Understanding the physical mechanisms behind the generation of ocean waves by wind has been a longstanding challenge1,2. Previous studies3,4,5,6 have assumed that ocean waves induce fluctuations in velocity and pressure of the overlying air that are synchronized with the waves, and numerical models have supported this assumption7. In a complex feedback, these fluctuations provide the energy for wave generation. The spatial and temporal structure of the wave-induced airflow therefore holds the key to the physics of wind–wave coupling, but detailed observations have proved difficult. Here we present an analysis of wind velocities and ocean surface elevations observed over the open ocean. We use a linear filter8 to identify the wave-induced air flow from the measurements and find that its structure is in agreement with ‘critical-layer’ theory3. Considering that the wave-induced momentum flux is then controlled by the wave spectrum and that it varies considerably in vertical direction, a simple parameterization of the total air–sea momentum flux is unlikely to exist.

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Figure 1: Numerical solutions for the Rayleigh equation (1).
Figure 2: Floating Instrument Platform FLIP moored during the experiment.
Figure 3: Theoretical and experimental transfer functions for the wave-coherent velocities T u ˜ ( c ) = A u ˜ e i Φ u ˜ and T w ˜ ( c ) = A w ˜ e i Φ w ˜ and a surface elevation power spectrum.
Figure 4: The measured phase of the vertical wave-induced velocity fluctuations Φ in degrees throughout a five-day period.


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J. Edson's help and the expert assistance of FLIP's captain Tom Golfinos and crew made it possible to carry out the experiment. In discussions, comments or criticisms came from O. Phillips, M. Donelan, J. McWilliams, P. Sullivan, J.C.R. Hunt, P. Taylor, M. McIntyre, S. Belcher, V. Makin and V. Kudryavtsev. ONR (Marine Meteorology) has provided the funding for this work.

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Correspondence to T. S. Hristov.

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Hristov, T., Miller, S. & Friehe, C. Dynamical coupling of wind and ocean waves through wave-induced air flow. Nature 422, 55–58 (2003).

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