Parametric Subharmonic Instability of the Semidiurnal Internal Tides at the East China Sea Shelf Slope

Abstract

On the basis of measurements from an observing mooring system, the observational evidence of parametric subharmonic instability (PSI) that transfers energy from semidiurnal internal tides (ITs) to the subharmonic waves at the East China Sea continental shelf slope is presented for the first time. Although the mooring station is very close to the energetic semidiurnal IT generation site, about 76% of the observed shear variance is contained in the near-inertial band, which is found to have comparable upward- and downward-propagating energy components. Bispectra and bicoherence estimates further confirm the occurrence of PSI transferring energy from the low-mode M2 ITs (vertical wavelength of ~1000 m) to high-mode subharmonic waves (vertical wavelength of ~200 m). The calculated energy transfer rate g reveals an averaged net value of ~5 × 10−9 W kg−1. Strong temporal variation of g is found that is not exactly in phase with the semidiurnal energy flux. After looking into the local vorticity fields, it is strongly suggested that the varying background relative vorticity associated with the evolving Kuroshio has modified the efficiency of PSI at the mooring location through changing the local effective inertial frequency.

Observation of Near-Inertial Waves Induced by Typhoon Mitag (2019) on the Southwestern East China Sea Continental Slope

“…This method can effectively estimate the phase-locking between waves with different frequencies. It has been widely used to distinguish between nonlinearly coupled waves and independent waves [12,13,[72][73][74][75][76]. The bispectrum is expressed in the frequency domain as:…”

“…The simultaneous enhancement of near-inertial currents and M2f currents as well as the significant bicoherence values of the [f, M2-f] frequency indicate that this was the result of nonlinear interaction between the NIWs and M2-f waves, which reflects a mechanism of energy dissipation of internal tides and NIWs. Moreover, Yang et al [76] showed that the M2 internal tide transmits energy on a smaller scale through the parametric subharmonic instability (PSI) mechanism, revealing a complex mechanism of energy dissipation of internal tides and NIWs on the southwestern continental slope of the ECS. It is also necessary to further study the contribution of ocean mixing caused by NIWs compared with internal tides.…”

Variation and Episodes of Near-Inertial Internal Waves on the Continental Slope of the Southeastern East China Sea

“…It has a wide continental shelf in the northwestern side which connects to a deep trough in the southeastern side through a steep continental slope. The ECS has variable types of internal waves, and previous studies have revealed characteristics of internal solitary waves [20][21][22][23] and internal tides [20,[24][25][26][27]. However, reports about near-inertial internal waves in ECS are few.…”

Intermittent Intense Thermocline Shear Associated With Wind‐Forced Near‐Inertial Internal Waves in a Summer Stratified Temperate Shelf Sea

“…We now calculate the depth‐integrated horizontal kinetic energy (HKE) within the near‐inertial band following $$$\text{HKE}={\int }_{{h}_{1}}^{{h}_{2}}({u}^{\prime 2}+{v}^{\prime 2}){\rho }_{0}/2dz,$$$ where, $${\rho }_{0}$$ = 1,024 kg m −3 is the reference density; h 1 and h 2 are the lower and upper integration limits, respectively (Yang, Wei, & Zhao, 2020). We first extract the near‐inertial signals through bandpass filtering in the frequency space (0.9–1.15 $${\omega }_{f}$$) and then calculate the HKE.…”