Experimental evidence of transverse modulation and frequency downshift of uniform waves in a long tank

We report experiments in a long tank showing that transverse Benjamin–Feir instability of Stokes waves can lead to a significant energy transfer into oscillations across the tank. We observe frequency downshift in the long-term evolution of Stokes waves essentially when significant energy is transferred to narrow-banded transverse modes. Experiments with Stokes waves are often carried out with wavelengths that are not long compared with the width of the tank, permitting transverse instabilities to be excited. With insufficient resolution of measurements across the tank, transfer of energy to transverse modes can be misinterpreted as dissipation. Our experiments suggest that the frequency downshift depends as much on energy-preserving transverse modulations of type I as it does on damping or wave breaking. Broad-banded unstable modulations of type II do not imply downshift.