On certain properties of the compact Zakharov equation

Abstract

Long-time evolution of a weakly perturbed wavetrain near the modulational instability threshold is investigated within the framework of the compact Zakharov equation for unidirectional deep-water waves, recently derived by Zakharov & Dyachenko (2011). Multiple-scale solutions reveal that a perturbation to a slightly unstable uniform wavetrain of steepness μslowly evolves according to a Nonlinear Schrodinger equation. In particular, for small carrier wave steepness μ<μ_1~0.27 the perturbation dynamics is of focusing type and the long-time behavior is characterized by Fermi-Pasta-Ulam recurrence, the signature of breather interactions. However, the amplitude of breathers and their likelihood of occurrence tend to diminish as μincreases while the Benjamin-Feir index decreases and becomes nil at \mu1. Thus, homoclinic orbits persist only for small values of wave steepness μ<<μ_1, in agreement with recent experimental and numerical observations of breathers. When the Zakharov equation is already beyond its range of nominal validity for μ>μ_1, predictions seem to foreshadow a dynamical trend to wave breaking. In particular, the perturbation dynamics becomes of defocussing type, and nonlinearities tend to stabilize a linearly unstable wavetrain as Fermi-Pasta-Ulam recurrence is suppressed. At μ=μ_c~0.577, subharmonic perturbations restabilize and superharmonic instability appears, possibly indicating that wave dynamic behavior changes at large steepness, in qualitative agreement with the numerical simulations of Longuet-Higgins and Cokelet (1978) for steep waves. Indeed, for μ>μ_c a multiple-scale perturbation analysis reveals that a weak narrowband perturbation to a uniform wavetrain evolves in accord with a modified Korteweg-de Vries/Camassa-Holm type equation, again implying a possible mechanism conducive to initiating wave breaking.