Spectral up- and downshifting of Akhmediev breathers under wind forcing

Abstract

We experimentally and numerically investigate the effect of wind forcing on the spectral dynamics of Akhmediev breathers, a wave-type known to model the modulation instability. We develop the wind model to the same order in wave steepness as the higher order modification of the nonlinear Schrödinger equation, also referred to as the Dysthe equation. This results in an asymmetric wind term in the higher order, in addition to the leading order wind forcing term. The derived model is in good agreement with laboratory experiments within the range of the facility’s length. We show that the leading order forcing term amplifies all frequencies equally and therefore induces only a broadening of the spectrum, while the asymmetric higher order term in the model enhances the higher frequencies more than the lower ones. Thus, the latter term induces a permanent upshift in the spectral mean. On the other hand, in contrast to the direct effect of wind forcing, wind can indirectly lead to frequency downshifts, due to dissipative effects such as wave breaking, or through the amplification of the intrinsic spectral asymmetry of the Dysthe equation. Furthermore, the definitions of the up- and downshift in terms of peak frequency and mean frequency, which are critical to relate our work to previous results, are highlighted and discussed.