Study of Large-Amplitude Nonlinear Internal Waves in the South China Sea
Date Issued
2008
Date
2008
Author(s)
Chang, Ming-Huei
Abstract
The study is focused on the large-amplitude nonlinear internal waves (NLIW) in the South China Sea (SCS): the propagation characteristics, the energy, energy flux and dissipation, and the relationship between the interior properties of NLIW and its surface scattering strength.hree sets of long-term ADCP measurements taken on the Dongsha plateau, integrating with both the shipboard measurement and the remote sensing data, are used to study the propagation characteristics of the NLIWs. The moorings were aligned along 21o05’N near the eastern edge of the Dongsha plateau. From east to west, the distances between the two successive moorings are ~8.5’ and ~17’, respectively. The NLIW propagating directions and speeds were computed by NLIW-induced current velocity and NLIW arrival time between two successive mooring stations, respectively. The averaged propagating direction of NLIW is 165o, which is northwestward. The averaged propagating speeds between two successive mooring stations are 1.83±0.38 m/s and 1.61±0.20 m/s from east to west. The above estimations are further verified by the observations of both shipboard marine radar and satellite images. The propagating directions reveal irregular variation. Nonetheless, the propagating speeds, which are higher in Aug.-Oct. and are slower in Jan.-Mar, reveal apparently seasonal variation. Such seasonal variation could relate with the typically seasonal stratification in the SCS, strong stratification in Aug.-Oct and weak stratification in Jan.-Mar. The linear phase speed, which is calculated using the climatological density profiles of Generalized Digital Environmental Model (GDEM) output, has good correlation with the measured NLIW propagating speed. Both the propagation direction and speed reveal daily inequality. Two types of NLIW appear reciprocally around the spring tide. One of them propagates faster and mainly northwestward and the other propagates slower and more northward than the previous one. It could be associated with the tidal current in the Luzon Strait.hree sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea are analyzed. The data show strong divergences of energy and energy flux of nonlinear internal waves along and across waves’ prevailing westward propagation path. The NLIW energy flux is 8.5 kW m-1 on the plateau, only 0.25 kW m-1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m-1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is ~0.04 W m-2, which corresponds to a dissipation rate of O(10-7-10-6)Wkg-1. Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of ~100 km width that is centered at ~210N, and dissipate nearly all their energy before reaching the continental shelf.urface signatures and interior properties of NLIWs were measured during a period of weak northeast wind (~2 m s-1) using shipboard marine radar, ADCP, CTD, and echo sounder. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ~6 m s-1 surface wind speed, i.e., three times greater than the actual surface wind speed. The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW. The observed average half-amplitude full width of NLIWs is 1.09±0.2 km; the average half-amplitude full width of the enhanced scattering strength is ~0.57 . The average half-amplitude full width of the enhanced horizontal velocity convergence of NLIWs is approximately equal to . The peak of the enhanced surface scattering leads the center of NLIWs by ~0.46 . NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. The analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of our empirical formulas in different conditions of wind speed, surface waves, and NLIWs, or with other remote sensing methods are encouraged.
Subjects
internal waves
nonlinear internal waves
Internal Solitary waves
South China Sea
Dongsha
Radar
SDGs
Type
thesis
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