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

Abstract

Acoustic Doppler Current Profiler (ADCP) measurements of the velocity structure in the thermocline in Luzon Strait are presented. The statistics for current, vertical variation of the inertia-gravity waveband, parametric subharmonic instability (PSI), and current shear are analyzed. It was found that 1) barotropic flow primarily consists of a nearly circular mixed tide. Diurnal tides are strongest and show smooth variance with a fortnightly spring-neap cycle, indicative of the astronomical tide-generating force. However, the semi-diurnal band power exhibits a high-frequency oscillation as a result of non-linear interactions. The high-frequency band power with high values during the spring tide oscillates with the tidal cycle. Near-inertial wave motions showing random variance may be caused by changes in the wind forcing at the sea surface or by random forcing. 2) Baroclinic velocities exhibit strong shear structure. The observed large changes in the amplitude of the baroclinic velocity and the limited vertical extent of the high-velocity cores may be interpreted as internal wave beams that pass through the observed water column. Semi-diurnal tides are dominant in the baroclinic velocity. Kinetic energy spectra also revealed that additional peaks were centred at sum-tidal-inertial interaction frequencies (such as M 2 + f) and difference-interaction frequencies (such as M 2 − f). The spectral exponent of the baroclinic velocity is ω −α (1 < α < 3). 3) Strong non-linear interactions among internal waves exist, and the semi-diurnal (M 2 ) component plays a key role in these interactions. Bicoherence analysis showed that M 2 /2 waves were non-linearly coupled with the dominant M 2 internal tide. 4) The polarization relations were used to diagnose observational internal tidal motions. Diurnal waves propagate to the east-northeast, and the semi-diurnal-diurnal waves propagate westward. In the case of diurnal tides, the minor to major axis ratio is different from the expected value of f/ω K1 because of the deviation of inclinations, whereas, for semi-diurnal tides, it is close to the expected value of f/ω M2 at depths from 30 to 150 m.RÉSUMÉ [Traduit par la rédaction] Nous présentons les mesures de la structure des vitesses dans la thermocline faites par profileur de courant à effet Doppler (ADCP) dans le détroit de Luçon. Nous analysons les statistiques sur le courant, la variation verticale de la gamme d'ondes d'inertie-gravité, l'instabilité subharmonique paramétrique (PSI) et le cisaillement du courant. Il ressort que 1) l'écoulement barotrope consiste principalement en une marée mixte presque circulaire. Les marées diurnes sont les plus fortes et présentent une variance régulière dans un cycle vives-eaux mortes-eaux de deux semaines qui révèle la nature astronomique de la force qui produit les marées. Cependant, la puissance de bande semi-diurne affiche une oscillation de haute fréquence causée par des interactions non linéaires. La puissance de bande de haute fréquence avec des valeurs élevées durant les ...

Temporal variations in internal tide multimodal structure on the continental shelf, South China Sea

“…The overall quasi-periodic temporal variation of the current values for IT D 1 is shown, which is a signal for the spring-neap tide cycle. A similar phenomenon has been observed by other researchers (Liao et al, 2012;Xie et al, 2013). During most of the temporal series, the fi rst-mode IT D 1 currents dominated, and as a result, the overall D 1 component ( K 1 ) showed a fi rst-mode structure (Fig.3a).…”

Moored observation of abyssal flow and temperature near a hydrothermal vent on the Southwest Indian Ridge

“…Following the recent observations [ van Haren et al ., ; van Haren, ; Xie et al . ; Liao et al, ], the internal wave band is not always a smooth broadband spectrum. The conventional view is that the bulk of internal wave energy is input at inertial and tidal frequencies, which cause nonlinear interactions (coupling) between near‐inertial and/or semidiurnal tidal motions to transfer energy to smaller scales, eventually ending in dissipation.…”

“…Some observational studies [ Mihaly et al, ; van Haren et al ., ; van Haren, ; Alford et al, ; Alford, ; Xie et al . ; Liao et al, ; Chou et al ., ] provided solid evidence of nonlinear interactions between internal waves. Many numerical experiments [ Hibiya et al, 1998, ; Xing and Davies , ; MacKinnon and Winters, ; Furuichi et al, ; Gerkema et al, ] were carried out to investigate these phenomena.…”

An empirical investigation of nonlinear energy transfer from the M2 internal tide to diurnal wave motions in the Kauai Channel, Hawaii

“…The bispectral analyses presented earlier (Figures a and 4b) showed that the [−O 1 , –K 1 , –M 2 ] triad has both elevated bispectral energy and significant bicoherence throughout a depth range > 200 m where the M 2 tide is energetic and beam‐like, evidence that has been interpreted as indicating nonlinear energy transfer in recent studies of PSI [e.g., Xie et al ., ; Liao et al ., ]. However, an experiment using the same bispectral techniques applied to a synthetic data set containing only deterministic tidal inputs and white Gaussian noise also resulted in elevated bispectral values and high bicoherence for the [−O 1 , −K 1 , −M 2 ] triad (Figures c and 4d), showing that these results do not always carry physical meaning [ Yao , ].…”

“…Numerical and observational studies have suggested that direct and rapid energy transfer from the large-scale M 2 tide to small-scale oscillations at the M 2 subharmonic (M 2 /2, or M 1 ) frequency can occur for a range of latitudes equatorward of~29˚[e.g., Hibiya and Nagasawa, 2004;Gerkema et al, 2006]. Specifically, studies around the Kauai Channel [e.g., Carter and Gregg, 2006;Rainville and Pinkel, 2006;Sun and Pinkel, 2013] and the Luzon Strait [e.g., Xie et al, 2011;Liao et al, 2012] have inferred from observations that PSI is an identifiable mechanism of nonlinear energy transfer, even at tropical latitudes (~21˚N). The geographical extent of PSI acting on energetic internal tides is an important aspect of its relevance to diapycnal mixing in the global ocean.…”