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Abstract Shoaling internal solitary waves (ISWs) were observed at three mooring sites on the upper continental slope in the northern South China Sea over a period of 5–11 months at water depths of 600, 430, and 350 m. Their properties exhibit a fortnightly variation because of their origination from internal tides. ISW amplitudes, current speeds, and propagation speeds are greater and wave widths narrower in summer than in winter, consistent with the effect of increased stratification in summer, as confirmed by Dubreil‐Jacotin‐Long (DJL) solutions. As ISWs propagate up the slope, the differential response of current and propagation speeds to bottom topography provides an opportunity for convective breaking of ISWs. Convective breaking occurs mostly between 430 and 600‐m depths and exhibits a marginal convective instability status such that (a) the maximum current speed remains nearly equal to the propagation speed and (b) for large‐amplitude waves the current speed and propagation speed decrease at nearly the same rate between 600 and 430‐m depths. The marginal convective instability occurs because ISWs adjust gradually to the gently sloping bottom and preserve their structural integrity after the onset of breaking. Vertical velocity variances behind the leading ISWs, which serve as a surrogate for the number of trailing waves, increase when ISWs reach the convective breaking limit, suggesting that convective breaking may accelerate the fission process in leading ISWs or that convective breaking is accompanied by an enhanced nonlinear dispersion of waves trailing ISWs generated by internal tides.
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Large-Amplitude Internal Wave Transformation into Shallow Water
Abstract Large-amplitude internal solitary wave (ISW) shoaling, breaking, and run-up was tracked continuously by a dense and rapidly sampling array spanning depths from 500 m to shore near Dongsha Atoll in the South China Sea. Incident ISW amplitudes ranged between 78 and 146 m with propagation speeds between 1.40 and 2.38 m s−1. The ratio between wave amplitude and a critical amplitudeA0controlled breaking type and was related to wave speedcpand depth. Fissioning ISWs generated larger trailing elevation waves when the thermocline was deep and evolved into onshore propagating bores in depths near 100 m. Collapsing ISWs contained significant mixing and little upslope bore propagation. Bores contained significant onshore near-bottom kinetic and potential energy flux and significant offshore rundown and relaxation phases before and after the bore front passage, respectively. Bores on the shallow forereef drove bottom temperature variation in excess of 10°C and near-bottom cross-shore currents in excess of 0.4 m s−1. Bores decelerated upslope, consistent with upslope two-layer gravity current theory, though run-up extentXrwas offshore of the predicted gravity current location. Background stratification affected the bore run-up, withXrfarther offshore when the Korteweg–de Vries nonlinearity coefficientαwas negative. Fronts associated with the shoaling local internal tide, but equal in magnitude to the soliton-generated bores, were observed onshore of 20-m depth.
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- Award ID(s):
- 1753317
- PAR ID:
- 10375880
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 52
- Issue:
- 10
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- p. 2539-2554
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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