Bimodal Directional Distribution of the Second Kind: Resonant Propagation of Wind-Generated Ocean Waves

Over the last several decades, it has been accepted that under steady forcing, wind-generated waves travel in the direction of wind. Last year, we presented two-dimensional (2D) spectral analysis of 3D ocean wave topography at equilibrium stage. The results demonstrate unequivocally a robust bimodal directionality in wave components shorter than the dominant wavelength. The generation mechanism of the bimodality is clarified to be nonlinear wave-wave interaction. Continued investigation reveals a second kind of bimodal directional distribution produced by resonant propagation of waves with the forcing wind field. In this situation, the dominant waves in a young sea align in oblique angles with the wind direction to maintain propagation resonance for a more efficient air-sea momentum transfer. As a result, two symmetric wave systems straddle the wind vector. The results from these analyses will revise our fundamental understanding of the physics of wind-wave generation and the forcing functions governing the dynamics of ocean waves. The implications of these directional observations on remote sensing (directional characteristics of ocean surface roughness) and air-sea interaction (directional properties of mass, momentum, and energy transfers) are significant. Published in the NRL Review, p160-162, 2001.