AbstractIn an idealized two-layer fluid, surface waves can generate waves at the internal interface through class 3 resonant triads in which all waves are propagating in the same direction. The triads are restricted to wavenumbers above a critical value kcrit that depends on the density ratio R between the two layers, and their depths. We perform numerical simulations to analyze the evolution of a surface wave field, initially specified by a Pierson-Moskowitz type spectrum, for R = 0:97 (representing a realistic lower a bound for oceanic stratification). At high initial steepness and peak wavenumber kp ≪ kcrit, the energy increases in the spectral tail; as a parameterization of resulting wave breaking, at each time step individual waves with a steepness greater than the limiting Stokes steepness are removed. The energy change of the surface wave field is a combination of energy transfer to the interfacial waves, spectral downshift, and wave breaking dissipation. At wavenumbers ≳ 0:6kp there is a net loss of energy, with the greatest dissipation at ≈ 1:3kp. The maximum gain occurs at ≈ 0:5kp. The onset of the spectral change shows a strong threshold behaviour with respect to the the initial wave steepness. For steep initial waves the integrated energy dissipation can reach > 30% of the initial energy, and only ≈ 1% of the initial surface wave energy is transferred to the interfacial wave field. The spectral change could be expressed as an additional dissipation source term, and coupled ocean/wave models should include additional mixing associated with the interfacial waves and enhanced wave breaking turbulence.