Preprints
https://doi.org/10.5194/npg-2021-29
https://doi.org/10.5194/npg-2021-29

  01 Sep 2021

01 Sep 2021

Review status: this preprint is currently under review for the journal NPG.

Observations of Shoaling Internal Wave Transformation Over a Gentle Slope in the South China Sea

Steven R. Ramp1, Yiing Jang Yang2, Ching-Sang Chiu3, D. Benjamin Reeder3, and Frederick L. Bahr4 Steven R. Ramp et al.
  • 1Soliton Ocean Services LLC, Falmouth, MA 02540
  • 2Institute of Oceanography, National Taiwan University, Taipei, Taiwan
  • 3Dept. of Oceanography, Naval Postgraduate School, Monterey, CA 93943
  • 4Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039

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.

Steven R. Ramp et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on npg-2021-29', Anonymous Referee #1, 25 Oct 2021
  • RC2: 'Comment on npg-2021-29', Peter Diamessis, 06 Nov 2021

Steven R. Ramp et al.

Steven R. Ramp et al.

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Short summary
Earlier work in the vicinity of the shelf and slope in the northeastern South China Sea serendipitously revealed the presence of large, stunning bed forms ("sand dunes") whose height (> 15 m) and length (> 350 m) are quite unique and unusual. We hypothesize that the dunes formed due to shoaling very large amplitude nonlinear internal waves that scour the bottom and re-suspend and redistribute the sediments. As a first step, the wave characteristics are observed and described in detail.