Abstract
Four oceanographic moorings were deployed across the South China Sea continental slope near 21.85° N, 117.71° E, from May 30 to July 18, 2014 for the purpose of observing high-frequency nonlinear internal waves (NLIWs) as they shoaled across a rough, gently sloping bottom. Individual waves required just two hours to traverse the array and could thus easily be tracked from mooring-to-mooring. In general, the amplitude of the incoming NLIWs was a good match with the fortnightly tidal envelope in the Luzon Strait, lagged by 48.5 hours, and were smaller than the waves observed 50 km to the southwest near the Dongsha Plateau. The now-familiar type a-waves and b-waves were observed, with the b-waves always leading the a-waves by 6–8 hours. Most of the waves were remotely generated, but a few of the b-waves formed locally via convergence and breaking at the leading edge of the upslope internal tide. Waves incident upon the array with amplitude less than 50 m and energy less than 100 MJ m−1 propagated adiabatically upslope with little change of form. Larger waves formed packets via wave dispersion. For the larger waves, the kinetic energy flux decreased sharply upslope between 342 m to 266 m while the potential energy flux increased slightly, causing an increasing ratio of potential-to-kinetic energy as the waves shoaled. The results are in rough agreement with recent theory and numerical simulations of shoaling waves.
Highlights
Considerable field work has been dedicated to observing and understanding the very large amplitude, high-frequency nonlinear internal waves (NLIW) in the northeastern South China Sea (SCS)
It has been well established that the waves emerge from an impressive internal tide which is generated by the flux of the barotropic tide across the two ridges in the Luzon Strait [Buijsman et al, 2010a, 2010b; Zhang et al, 2011]
This paper addresses how the high-frequency nonlinear internal waves were transformed under shoaling, while the tidal dissipation and dune-building processes will be addressed in separate works
Summary
Considerable field work has been dedicated to observing and understanding the very large amplitude, high-frequency nonlinear internal waves (NLIW) in the northeastern South China Sea (SCS). It has been well established that the waves emerge from an impressive internal tide which is generated by the flux of the barotropic tide across the two ridges in the Luzon Strait [Buijsman et al, 2010a, 2010b; Zhang et al, 2011] Both tidal conversion and dissipation are high around the ridges [Alford et al, 2011], but adequate energy survives to escape the ridges and propagate WNW across the sea. The internal tides steepen nonlinearly until eventually the NLIW are formed [Farmer et al, 2009; Li and Farmer, 2011; Alford et al, 2015] The longitude where this takes place depends on the details of the forcing and stratification but based on satellite imagery it is not until at least 120° 30’E, roughly 50 km west of the western (Heng-Chun) ridge [Jackson, 2009]. Once the waves start to shoal on the continental slope roughly between 1000m to
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