Stokes Waves at the Critical Depth are Modulationally Unstable

The paper fully answers a long standing open question concerning the stability/instability of pure gravity periodic traveling water waves—called Stokes waves—at the critical Whitham–Benjamin depth hWB = 1.363... and nearby values. We prove that Stokes waves of small amplitude O() are, at the critical depth hWB, linearly unstable under long wave perturbations. The same holds true for slightly smaller values of the depth h > hWB − c2, c > 0, depending on the amplitude of the wave. This problem was not rigorously solved in previous literature because the expansions degenerate at the critical depth. To solve this degenerate case, and describe in a mathematically exhaustive way how the eigenvalues change their stable-to-unstable nature along this shallow-todeep water transient, we Taylor-expand the computations of Berti et al. (Arch Ration Mech Anal 247:91, 2023) at a higher degree of accuracy, starting from the fourth order expansion of the Stokes waves. We prove that also in this transient regime a pair of unstable eigenvalues depict a closed figure “8”, of smaller size than for h > hWB, as the Floquet exponent varies