Computing Soliton Currents from Synthetic Aperture Radar Images of the South China Sea

Abstract

Abstract Solitons (solitary internal waves) are generated by the interaction between tides and bottom topography. Energetic solitons with vertical density displacements of over 100 m, accompanied by 3-plus knot surface currents, are often observed in the South China Sea. The strong episodic currents associated with these internal waves have significantly affected offshore oil operations. A space-borne synthetic aperture radar (SAR) has the ability to provide all-weather imagery of ocean surface roughness. These images cover swaths of ~50 km at a spatial resolution of ~20 m. These lines are manifestations of surface roughness variations due to the modulation of short surface waves as they propagate through the spatially varying soliton surface currents. An inversion model is developed to retrieve surface currents of solitons from SAR images. This data inversion is based on iterative fitting of SAR data to models of wave-current interaction, radar scattering and SAR imaging. The inversion model is applied to Space Shuttle SAR images of solitons in the South China Sea, and the surface currents thus retrieved are compared with in-situ current measurements. Due to the large area coverage and fine resolution, the ability to obtain accurate surface currents from SAR images makes them a cost-effective means for assessing soliton currents for offshore platform design. Introduction Solitons are generated by the interaction between tides and bottom topography. A packet of several solitons is typically generated at a time. The width of an individual soliton can range from 100 m in the New York Bight1 to several km in the Sulu Sea2. The length of a soliton can extend to more than 50 km1–6. Extensive oceanographic surveys in the South China Sea near Pratas Reef southwest of Taiwan (Figure 1) revealed energetic solitons with vertical density displacements of over 100 m, accompanied by 3-plus knot near-surface currents5,6. Studies using a ray tracing model indicate that solitons in this area are generated between Batan and Sabtang Islands in the Luzon Strait, more than 500 km to the east of Pratas Reef5,6. After being generated at the source region, the solitons propagate westward towards the continental shelf. The presence of strong currents associated with these solitons has significantly affected offshore oil operations.