Observations of internal waves travelling across the deep basin of the South China Sea provide an opportunity for exploring the effects of rotation and non‐linearity on their evolution. Time series measurements using inverted echo‐sounders at three locations illustrate the progressive steepening of the internal tide generated in Luzon Strait and the subsequent development of short non‐linear internal wave trains. Potential mechanisms for internal tide generation are discussed in terms of tidal beam interaction with near‐surface stratification and mode 1 response to flow over a ridge. For transformation of an internal tide under the influence of non‐linearity and rotation, we apply Boyd's (2005) criterion for wave stability in a rotating flow to separate waves dominated by non‐linearity, which can be expected to steepen and break, from waves that are inhibited from breaking due to rotational dispersion of energy into internal inertial gravity waves. Wave breaking in this context refers to the poin... Résumé [Traduit par la rédaction] L'observation des ondes internes parcourant le bassin profond de la mer de Chine méridionale fournit une possibilité d'étudier les effets de la rotation et de la non‐linéarité sur leur évolution. Des séries chronologiques de mesures faites à l'aide d'échosondeurs inversés à trois endroits montrent l'accentuation progressive de la marée interne générée dans le détroit de Luzon et la formation subséquente de courts trains d'ondes internes non linéaire. Nous discutons des mécanismes possibles de génération de marée interne en fonction de l'interaction du faisceau de marée avec la stratification superficielle et la réponse du premier mode à l'écoulement au‐dessus d'une crête. Pour la transformation d'une marée interne sous l'effet de la non‐linéarité et de la rotation, nous appliquons le critère de Boyd (2005) sur la stabilité des vagues dans un écoulement en rotation pour séparer les vagues dominées par la non‐linéarité, qui devraient éventuellement devenir plus abruptes et ...

The three-dimensional (3D) double-ridge internal tide interference in the Luzon Strait in the South China Sea is examined by comparing 3D and two-dimensional (2D) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6-km-deep trench in the strait. As in an earlier 2D study, barotropic-to-baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than in the 2D simulations for the central strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance and not of the along-ridge length; that is, the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking of the barotropic flow by the 3D ridges, affecting wave generation, and a more constructive phasing between the remotely generated internal waves, arriving under oblique angles, and the barotropic tide. Most of the resonance occurs for the first mode. The contribution of the higher modes is reduced because of 3D radiation, multiple generation sites, scattering, and a rapid decay in amplitude away from the ridge.

This paper investigates the internal tidal energy distribution in the southwestern Japan/East Sea using vertical round-trip travel time (τ) data from 23 pressure-sensor-equipped inverted echo sounders (PIES). The τ records are analyzed by bandpass filtering to separate time-dependent variability of the semidiurnal and diurnal bands. The semidiurnal internal tides exhibit a horizontal beam pattern of high energy, propagating into the open basin. They originate from a restricted portion of the shelf break where the Korea Strait enters the Ulleung Basin. The generation appears to occur at ∼200-m water depth near 35.5°–35.7°N and 130°–131°E, where the slope of bottom topography matches that of the wave characteristics, coinciding with the location where the semidiurnal barotropic cross-slope tidal currents are strongest. Maximum vertical displacement of the thermocline interpreted as a long-wave first baroclinic mode from the measured τ is about 25 m near the generation region. Annual and monthly variations of the propagation patterns and generation energy levels are observed, and these are closely associated with changes in the mesoscale circulation and stratification. Eastward (westward) refraction is observed when a warm (cold) eddy crosses the path of internal tide propagation. Moreover, when the generation region is invaded by cold eddies that spoil the match between shelf break and thermocline depth, the internal tidal energy level decreases by a factor of about 2. A simple geometric optics model is proposed to explain the observed horizontal refraction of the beam of semidiurnal internal tides in which stratification and current shear play essential roles. In contrast, diurnal internal tides are observed to be trapped along the continental slope region around 36°N.