Origins of features in wave number‐frequency spectra of space‐time images of the ocean

Wave number‐frequency spectra measured with remote sensing systems consist of energy along the ocean wave dispersion relation and additional features that lie above and below this relation. At low frequencies a feature passing through the origin as a straight line is observed while one or more high‐frequency features exhibit substantial curvature. Here we utilize images obtained on the open ocean from microwave Doppler shifts, which are directly related to scatterer velocity and allow us to calculate expected wave‐wave interaction effects. We show that the strongest features lying off the first‐order dispersion relation are not primarily due to second‐order interactions, breaking caused by wind turbulence, advection by turbulence, or shadowing. The low‐frequency feature can be seen traveling in the opposite direction to swell when looking nearly crosswind. We show that the most probable cause of these features is the interference of long ocean waves, which causes breaking near local maxima of surface slope. Doppler spectra observed by the radars indicate that the maximum speed reached by water particles on the open ocean is less than 6 m/s and usually close to the speed of the low‐frequency feature in the wave number‐frequency spectrum. Since this is much less than the phase speeds of dominant wind waves and swell, neither of these waves can be the breaking wave. Rather, we hypothesize that the superposition of these waves steepens short gravity waves on the surface, which then break to produce water parcels traveling near their phase speed, the speed observed by the radar.