Estimates of Mode-1 Internal Tide Harmonic Generation in the Global Ocean

Recent theoretical and experimental work has demonstrated the generation of nonlinear harmonics when internal waves propagate in non-uniform stratification, if the harmonic frequency is near resonance. This effect may impact the evolution of low-mode internal tides in the ocean, possibly leading to the formation of strongly nonlinear waves. Here, measured ocean stratification profiles are used to quantify the potential for nonlinear harmonic generation for mode 1 semi-diurnal internal tides. The results show that the first harmonic, with double the wavenumber of the parent mode, is often expected to attain a significant amplitude in the Equatorial Pacific due to near-resonance, but not in the Atlantic. This is because weaker and more shallow Atlantic pycnoclines do not provide a sufficient harmonic forcing. Hence near-resonance is found to be a necessary, but not sufficient, condition for harmonic generation in measured ocean stratifications. A second criterion, based on pycnocline depth and density change, is proposed as an additional condition for harmonic generation.