Resonant interactions of surface and internal gravity waves

A stably stratified, two-layer fluid system with a free surface admits resonant triads, including ones involving one surface and two internal waves. Resonant triads that couple surface and internal gravity waves are thought to play an important role in transferring energy from wind-generated surface waves to internal waves. Ball used a geometric construction to show that the linearized dispersion relation for a two-layer liquid system with a free upper surface and no stream velocity admits resonant triads.