Nonlinear damped spatially periodic breathers and the emergence of soliton-like rogue waves

Abstract: The spatially periodic breather solutions (SPBs) of the nonlinear Schr\"odinger equation, prominent in modeling rogue waves, are unstable. In this paper we numerically investigate the effects of nonlinear dissipation and higher order nonlinearities on the routes to stability of the SPBs in the framework of the nonlinear damped higher order nonlinear Schr\"odinger (NLD-HONLS) equation. The initial data used in the experiments are generated by evaluating exact SPB solutions at time $T_0$. The number of instabilities of the background Stokes wave and the damping strength are varied. The Floquet spectral theory of the NLS equation is used to interpret and provide a characterization of the perturbed dynamics in terms of nearby solutions of the NLS equation. Significantly, as $T_0$ is varied, tiny bands of complex spectrum are observed to pinch off in the Floquet decomposition of the NLD-HONLS data, reflecting the breakup of the SPB into a waveform that is close to either a one or two "soliton-like" structure. For wide ranges of $T_0$, i.e. for solutions initialized in the early to middle stage of the development of the MI, all rogue waves are observed to occur when the spectrum is close to a one or two soliton-like state. When the solutions are initialized as the MI is saturating, rogue waves also can occur after the spectrum has left a soliton-like state. Other novel features arise due to nonlinear damping: enhanced asymmetry, two timescales in the evolution of the spectrum and a delay in the growth of instabilities due to frequency downshifting.