Evolution of the semidiurnal (M₂) internal tide on the continental slope of the northern South China Sea

Abstract

[1] Kinetic energy spectra from a site on the continental slope in the South China Sea reveal that significant peaks appear at some nonlinear interaction frequencies, namely M 3 (M 1 + M 2 ) and fM 1 (M 1 + f), where f is the inertial frequency, M 1 is the diurnal internal wave, and M 2 is the lunar semidiurnal internal tide. A possible generation mechanism of M 3 is explored. Analysis of bicoherence and shear spectra suggests that strong M 3 is indirectly associated with parametric subharmonic instability (PSI) of M 2 . In another word, under the effect of PSI the energy of M 2 is first transferred to M 1 ; then via other nonlinear coupling, some nonlinear waves (e.g. fM 1 , M 3 ) are generated. Moreover, M 1 is also present at another site near the bottom of the continental slope. The shear spectra from these two sites show, for the first time, that M 1 can be significantly distinguished from lunar diurnal O 1 and lunisolar diurnal K 1 .

Current Observations of Internal Tides and Parametric Subharmonic Instability in Luzon Strait

“…The presence of spectral peaks at higher harmonics of the semi-diurnal tides and at linear combinations of these frequencies with the Coriolis frequency is not unique in the NSCS. Xie et al (2008) and Xie, Shang, & Chen (2010) already reported the presence of such peaks in a spectrum derived from observations in the slope region of the NSCS. These are assumed to be caused by non-linear interactions, especially the advection of slower moving high-frequency waves by the dominant M 2 internal tides.…”

“…The ridge lies equatorward of 28.9°latitude; thus, non-linear interactions, such as parametric subharmonic instability (PSI) (McComas & Bretherton, 1977;Müller, Holloway, Henyey, & Pomphrey, 1986), can potentially transfer energy from M 2 to M 2 /2 (defined as M 1 ) without violating the internal wave requirement that f ≤ ω ≤ N, where ω is the wave frequency, N the buoyancy frequency, and the inertial frequency is f = 2Ωsinφ, where Ω is the earth's rotation vector, and φ is the latitude. Some non-linear interactions have been revealed from a site on the continental slope in the SCS, and the distinct subharmonic of M 2 (M 1 ) can be clearly distinguished from the lunar diurnal O 1 and lunisolar diurnal K 1 (Xie et al, 2008, Xie, Shang, van Haren, Chen, & Zhang, 2011. Non-linear waves and internal tides in the northern SCS mainly originate from LS (Buijsman, Kanarska, & McWilliams, 2010;Cai & Xie, 2010).…”

“…Lien, Tang, Chang, and D'Asaro (2005) suggested that the strong internal tides generated in LS propagate into the SCS as a narrow tidal beam and produce nonlinear internal waves through interactions with islands and the continental slope. In recent years, many studies have focused on internal tides in the NSCS Guo, Fang, Gan, Chen, & Long, 2006;Ramp et al, 2004;Xie, Chen, Shang, & Fang, 2008). However, internal tides have also been observed in LS.…”

Observed upper ocean response to typhoon Megi (2010) in the Northern South China Sea

“…At Luzon Strait, strong diurnal and semidiurnal internal tides are generated when the barotropic tide flows over seamounts, and then propagate away into the SCS [ Niwa and Hibiya , ; Jan et al ., ]. Via the active nonlinear wave‐wave interactions among the near‐inertial internal waves, diurnal and semidiurnal tides [ Alford , ; Xie et al ., ], these internal tidal energy finally dissipates in the SCS and contributes to the enhanced diapycnal mixing in the deep SCS [ Tian et al ., ].…”

Tidal variability in the Hong Kong region

“…The presence of strong M3 and MO3 tides at most gauges in Hong Kong (Fig 9) 567 indicates a connection to the dynamics at the shelf where significant D3 energy has been 568 observed (Xie et al, 2008 (Fig 10). In the Beibu Gulf and the southern SCS, the N2 variation is almost 585 nonexistent, and the M3 signal is much smaller.…”