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Estimates of mode-1 internal tide harmonic generation in the global ocean
Phys. Rev. Fluids 8, 124801 – Published 6 December, 2023
DOI: https://doi.org/10.1103/PhysRevFluids.8.124801
Abstract
Recent theoretical and experimental work has demonstrated the generation of nonlinear harmonics when internal waves propagate in nonuniform stratification. This effect is most pronounced if the harmonic frequency is near resonance with a freely propagating mode. It may play a significant role in the evolution of low-mode internal tides in the ocean, possibly leading to the formation of strongly nonlinear waves. Here, global ocean stratification profiles measured by ARGO floats are used to quantify the potential for nonlinear harmonic generation for semidiurnal mode-1 internal tides. The results show that the first harmonic, with double the wave number of the parent mode, is often expected to attain an amplitude exceeding a fifth of the parent mode in equatorial regions. Potentially strong harmonic responses were calculated most frequently for the Pacific and Indian Oceans. A strong harmonic is not expected in the Atlantic, despite near-resonance conditions, because weaker and more shallow 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.
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