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Modeling upwelling circulation over continental shelf in the northern South China sea

Authors Cheung, Yee Ying
Issue Date 2006
Summary To-date the upwelling process and the associated forcing mechanism over the continental shelf of the Northern South China Sea (NSCS) remain unclear. Therefore a three-dimensional Regional Ocean Model System (ROMS), was used to conduct a process-orientated investigation in order to better understand the intrinsic upwelling dynamics in the NSCS. The model uses an orthogonal curvilinear grid with a horizontal resolution of about 3 km and it has 30 terrain-following vertical levels. A physical-sensible forced open boundary condition is implemented, which divides total velocity into global and local solutions. The local solution is provided by a two-dimensional submodel and the global solution is obtained from a radiation condition. Forced with idealized upwelling favorable winds, the model simulates the observed upwelling characteristics in the NSCS well. In particular, it was found that the upwelling over the shelf of NSCS has a strong alongshore variation with intensified upwelling occurring off Shantou where the shelf extends offshore abruptly and in the lee of coastal promontories. Strong upwelling was also found in the waters south of Hainan Island where wind-induced Ekman transport is strong. Dynamic analysis indicates that the strength of the onshore transport in the upwelling circulation is highly controlled by the interaction between the coastal upwelling jet and the local shelf topography. The strong onshore current represented by a positive Coriolis force is balanced by a negative pressure gradient force induced at the lee of coastal promontories or sharply widened shelf. Relatively strong upwelling also occurs where the shelf slope is steep such as Daya Bay - Red Sea Gulf. The steep continental slope with offshore tongue-shaped bottom isobaths tends to enhance the onshore current at depth. Upwelled water at the sea surface is advected offshore by the intensified upwelling jet at the same location. Cold and nutrient-rich water is more likely to be found east of Hong Kong due to stronger upwelling over the steep slope. The relatively flat bottom at the mid-shelf around Shantou, where the shelf extends offshore abruptly, acts as a platform to allow upwelled water to be retained at the mid-shelf. The upward current in the south of the Taiwan Shoals, forced by the upwelling favorable wind, is not strong enough to push the bottom water to the sea surface, suggesting that the observed dense water is likely formed by a force other than local wind stress. The effects of fresh water discharge from the Pearl River Estuary (PRE) on the upwelling circulation were investigated. It was found that the fresh water plume is formed in the central part of the shelf and advected towards the eastern part of the shelf by the upwelling jet. The river discharge tends to strengthen onshore currents in the interior of the water column over the inner shelf. A simulation forced with observed upwelling favorable winds reveal that the upwelling intensity at the near shore location was sensitive to the orientation and strength of the local wind stress.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006
Language English
Format Thesis
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