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Two cycles of oceanographic observations were carried out in the shelf zone of South Vietnam in the summer of 2010 (summer monsoon period) and spring of 2011 (intermonsoon period) with CTD survey and ADCP mooring stations. Vertical structure, spatial distribution, and short-term variability of main oceanographic parameters and the dynamics of coastal currents are analyzed. In the summer of 2010, the surface water was well warmed up (28–31 °C) with a high salinity (33.1–33.6 psu) which may be explained by a significant area of anticyclonic gyre off the southeastern coast of Vietnam. Such a dynamic system caused the convergence of warm surface waters in the coastal zone of Vietnam instead of the upwelling usually observed here in the summer season. In the spring of 2011, the surface water of the open sea was slightly less heated and significantly less saline influenced by the flow of Sunda shelf water. Higher salinity of coastal water in Nha Trang Bay may be explained by the reduction of local rivers discharge in the dry season and tidal mixing. Vertical profiles of oceanographic parameters demonstrated baroclinic structure of the waters on the shelf with three water masses. A bottom layer of high turbidity water of 20–30 m thick at the outer edge of the shelf at depths of 50–75 m was found in Nha Trang Bay. Its formation is probably due to advection of coastal waters with a high content of suspended matter, or intensive tidal mixing, stirring up bottom sediments, and forming a zone of increased primary production and a nepheloid layer. The measurements of currents confirmed the strong impact of diurnal tide on dynamics of the shelf water. The maximum recorded current velocities exceeded 50 cm/s in the surface layer and 30 cm/s at the bottom. During the period of tide phase change, the flow had a baroclinic character with oppositely directed currents in the upper and lower layers. A soliton passage was recorded in Nha Trang Bay, probably caused by the interaction of internal waves on the shelf. At that time, the flow velocity has doubled, which determines a sharp increase in vertical mixing and can have significant environmental consequences.

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Structure and Dynamics of Shelf Water off Southern Vietnam During Summer Monsoon and Intermonsoon Period

  • 章节

  • 发表日期：

    2025年

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  • Vyacheslav B. Lobanov,
  • Nguyen Ba Xuan,
  • Aleksandr F. Sergeev,
  • Nguyen Kim Vinh,
  • Bui Hong Long,
  • Aleksandr A. Voronin,
  • Igor I. Gorin,
  • Pham Xuan Duong,
  • Nguyen Van Tuan,
  • To Duy Thai,
  • Pham Sy Hoan,
  • Nguyen Truong Тhanh Hoi,
  • Pavel E. Shcherbinin

摘要

Two cycles of oceanographic observations were carried out in the shelf zone of South Vietnam in the summer of 2010 (summer monsoon period) and spring of 2011 (intermonsoon period) with CTD survey and ADCP mooring stations. Vertical structure, spatial distribution, and short-term variability of main oceanographic parameters and the dynamics of coastal currents are analyzed. In the summer of 2010, the surface water was well warmed up (28–31 °C) with a high salinity (33.1–33.6 psu) which may be explained by a significant area of anticyclonic gyre off the southeastern coast of Vietnam. Such a dynamic system caused the convergence of warm surface waters in the coastal zone of Vietnam instead of the upwelling usually observed here in the summer season. In the spring of 2011, the surface water of the open sea was slightly less heated and significantly less saline influenced by the flow of Sunda shelf water. Higher salinity of coastal water in Nha Trang Bay may be explained by the reduction of local rivers discharge in the dry season and tidal mixing. Vertical profiles of oceanographic parameters demonstrated baroclinic structure of the waters on the shelf with three water masses. A bottom layer of high turbidity water of 20–30 m thick at the outer edge of the shelf at depths of 50–75 m was found in Nha Trang Bay. Its formation is probably due to advection of coastal waters with a high content of suspended matter, or intensive tidal mixing, stirring up bottom sediments, and forming a zone of increased primary production and a nepheloid layer. The measurements of currents confirmed the strong impact of diurnal tide on dynamics of the shelf water. The maximum recorded current velocities exceeded 50 cm/s in the surface layer and 30 cm/s at the bottom. During the period of tide phase change, the flow had a baroclinic character with oppositely directed currents in the upper and lower layers. A soliton passage was recorded in Nha Trang Bay, probably caused by the interaction of internal waves on the shelf. At that time, the flow velocity has doubled, which determines a sharp increase in vertical mixing and can have significant environmental consequences.