On the resonant triad interaction over mid-ocean ridges

Resonant triad interaction is an important mechanism via which the energy of internal tides (ITs) is dissipated. In this study, based on a two-dimensional high-resolution non-hydrostatic model, resonant triad interaction over mid-ocean ridges and corresponding energetics are investigated. Impacts of topographic criticality (the ratio of topographic slope to internal wave slope) on resonant triad interaction are examined by a series of simulations. Results indicate that the topographic criticality not only affects the occurrence of resonant triad interaction but also modulates the frequencies of generated subharmonic waves. Near-critical and supercritical topographies are beneficial to the occurrence of resonant triad interaction with enhanced energy production and dissipation. The impacts of topographic criticality are modulated by latitude. At the critical latitude where local Coriolis frequency is equal to half of the tidal frequency, subharmonic waves are intensified over critical and subcritical topographies, but their frequencies are fixed to local Coriolis frequency. In contrast, resonant triad interaction is suppressed poleward of the critical latitude despite the topographic criticality. Although the energy of subharmonic waves is less than a quarter of the total energy, they contribute to 50% of the total energy dissipation on average. Given that global ITs are dominated by the M2 and most of the M2 ITs are generated upon mid-ocean ridges equatorward of the critical latitude, a remarkable proportion of global IT energy may be dissipated through resonant triad interaction. Therefore, the contribution of resonant triad interaction should be taken into consideration when developing the parameterization of IT-driven mixing.