11:00 AM - 11:15 AM
[AOS16-08] Two steps in the energy transfer from M2 internal tides to high-frequency internal waves
Internal tides (ITs) represent one of the major energy sources available for small-scale mixing in the ocean. Where and how the ITs lose their energy is crucial for understanding the global distribution of diapycnal mixing. Here, the energy pathway associated with ITs is examined based on measurements obtained at the East China Sea shelf slope.
We first show the occurrence of parametric subharmonic instability (PSI) which is one kind of resonant triad wave-wave interaction mechanism. This is as anticipated at the measurement location (25.4 °N) since the PSI is a latitude-dependent mechanism can theoretically work below the M2 critical latitude (28.8 °N). Bispectral estimates show that the PSI can transfer energy from the low-mode M2 ITs (vertical wavelength of 1000 m) to high-mode M1 subharmonic waves (vertical wavelength of 200 m). After that, we reveal an interesting energy pathway from the PSI-generated M1 subharmonic waves to high-frequency internal waves (HFIWs) through nonlinear interactions between them. This interprets the observed similar varying trends of magnitude between internal waves at near-inertial and high-frequency bands. Bispectral estimates confirm that the M1subharmonic wave interacts with a pair of HFIW which have larger vertical scales and a broad range of frequencies. The average net value of the energy transfer rate reaches 2 × 10−7 W kg−1. As a shift in both wave frequency and vertical mode, the HFIWs have many different properties with the M1 subharmonic waves, such as the propagating speeds and distances. These interactions after the occurrence of PSI can then be responsible for the redistribution of PSI-transferred energy in the ocean, modulating the spatial distribution of diapycnal mixing. We suggest that these two steps may represent an important energy pathway for ITs in the ocean, providing important implications for understanding the global distribution of diapycnal mixing.
We first show the occurrence of parametric subharmonic instability (PSI) which is one kind of resonant triad wave-wave interaction mechanism. This is as anticipated at the measurement location (25.4 °N) since the PSI is a latitude-dependent mechanism can theoretically work below the M2 critical latitude (28.8 °N). Bispectral estimates show that the PSI can transfer energy from the low-mode M2 ITs (vertical wavelength of 1000 m) to high-mode M1 subharmonic waves (vertical wavelength of 200 m). After that, we reveal an interesting energy pathway from the PSI-generated M1 subharmonic waves to high-frequency internal waves (HFIWs) through nonlinear interactions between them. This interprets the observed similar varying trends of magnitude between internal waves at near-inertial and high-frequency bands. Bispectral estimates confirm that the M1subharmonic wave interacts with a pair of HFIW which have larger vertical scales and a broad range of frequencies. The average net value of the energy transfer rate reaches 2 × 10−7 W kg−1. As a shift in both wave frequency and vertical mode, the HFIWs have many different properties with the M1 subharmonic waves, such as the propagating speeds and distances. These interactions after the occurrence of PSI can then be responsible for the redistribution of PSI-transferred energy in the ocean, modulating the spatial distribution of diapycnal mixing. We suggest that these two steps may represent an important energy pathway for ITs in the ocean, providing important implications for understanding the global distribution of diapycnal mixing.