Energy Flux and Dissipation in Luzon Strait: Two Tales of Two Ridges

Abstract

Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP-CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s 21 and 300 m, respectively. Energy fluxes exceeding 60 kW m 21 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500-1000 m but was strongest by far atop the western ridge on the northern line, where .500-m overturns resulted in dissipation exceeding 2 3 10 26 W kg 21 (implying diapycnal diffusivity K r . 0.2 m 2 s 21 ). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

The formation and fate of internal waves in the South China Sea

“…This feature exists because the double-ridge structure creates a 100km-scale resonant cavity for the ~100km wavelength semi-diurnal internal tide 17 , and is further confirmed by observations of very high energy density but little energy flux between the two ridges that is characteristic of a standing wave pattern 15 There was no evidence of significant nonlinear internal waves east of 120.5 o E, finally laying to rest a widely stated conjecture that the large-amplitude internal solitary waves arise in the immediate vicinity of the Luzon Strait via a hydraulic lee wave mechanism 11 . Rather, what emerges immediately to the west of the Luzon Strait is a broad, energetic and spatially-coherent, nearly sinusoidal internal tide ( Figure 1a The combination of IWISE numerical models and field measurements enables us to provide the first assessment of an energy budget for internal waves throughout the entire South China Sea, as summarized in Figure 3c.…”

“…Our moorings, shipboard stations, and autonomous gliders within and to the west of the Luzon Strait reveal time-averaged westward energy fluxes of 40±8 kWm -1 ( Figure 6 2a; see Methods). Such values are about 100 times typical open-ocean values 16 and exceed any other known generation site around the world 15 . Measured fluxes agree to within a factor of two in magnitude and ±30 o in direction with predictions of our farfield numerical model (see Methods), as well as previous numerical predictions 9 .…”

“…All were forced with predictions using 8 tidal constituents (K1, O1, P1, Q1, K2, M2, N2 and S2) from a barotropic tidal model, TPXO7.2 31 which was validated against measured currents in the region in this and a previous experiment 15,25 . Bathymetry was from multibeam depth soundings where available, and from the 30 arc-second database 32 elsewhere.…”

“…A pilot study was performed in the summer of 2010 to determine the feasibility of operating at desired locations 15 , and the full field program was executed throughout the summer of 2011. This Letter is a synthesis of the results from both programs.…”

Generation sites of internal solitary waves in the southern Taiwan Strait revealed by MODIS true-colour image observations

“…They can propagate over several hundreds of kilometres, produce strong turbulence and mixing with a considerable velocity shear, and modify thermohaline structures of the water column (Gerkema and Zimmerman 2008;Alford et al 2011;Guo and Chen 2014).…”