Structure and Variability of Abyssal Current in Northern South China Sea Based on CPIES Observations

• Published in: Journal of geophysical research. Oceans 2021-04, Vol.126 (4), p.n/a
• Main Authors: Zheng, Hua, Zhang, Chuanzheng, Zhao, Ruixiang, Zhu, Xiao‐Hua, Zhu, Ze‐Nan, Liu, Zhao‐Jun, Wang, Min
• Format: Article
• Language: English
• Subjects: abyssal current; Abyssal currents; Abyssal zone; Autumn; current and pressure‐recording inverted echo sounder; Current observations; Cyclonic circulation; Diurnal; Diurnal tides; Diurnal variations; Echo sounding; Echoes; Energy conversion; Frequencies; Geophysics; Hurricanes; Inertial waves; Oceans; Planetary waves; Renewal; Rossby waves; Semidiurnal tides; South China Sea; Spring; Spring (season); Summer; temporal and spatial variability; Temporal variability; Temporal variations; Tidal circulation; Tides; Typhoons; Velocity; Water transport; Wave propagation; Winter
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2020JC016780

The abyssal current is important for water renewal and energy conversion in the deep South China Sea (SCS), and it potentially contributes to the upper and middle circulations. A mooring array including 11 current and pressure‐recording inverted echo sounders was deployed in the northern SCS between July 2016 and April 2019 to observe large‐scale and long‐term abyssal currents. The yearly averaged current flows southwestward following the boundary with a maximum velocity of 2.25–2.52 cm/s. The spatial structure exhibits a weaker and wider current in the north, where topography is subdued, whereas the current enhances and narrows on the steep slopes. The southwestward current is strong in the summer and autumn but is insignificant in the winter and spring. In the autumn, the current is narrower and closer to the boundary than that in the summer, and a northeastward countercurrent is found ∼60 km away from the western boundary. The southwestward and northeastward currents constitute the cyclonic circulation. Drastic temporal variability is observed in the deep ocean. The abyssal current is dominated by semidiurnal tides, diurnal tides, and near‐inertial waves (NIWs) in the high‐frequency band. Bursts of NIWs are observed in the deep ocean after two typhoons, and the NIWs propagate southwestward at a velocity of 2.4 m/s. The low‐frequency band is dominated by a westward‐propagating 70‐day fluctuation following the characteristics of topographic Rossby waves. Plain Language Summary The abyssal cyclonic current in the South China Sea plays an important role in water transport and energy conversion in the deep ocean, and it potentially contributes to the circulations in the upper and middle ocean. Although model simulations and field observations have previously confirmed the existence of this abyssal current, the knowledge about abyssal current is lacking due to the sparseness of in situ observations in time and space. We conducted large‐scale abyssal current observations in the northern South China Sea from July 2016 to April 2019, and found that the yearly averaged current flows follow the western boundary and exhibit a maximum velocity of 2.25–2.52 cm/s. The southwestward current is strong in the summer and autumn but weak in the winter and spring. The current is narrower in the autumn than in the summer, and a northeastward current occurs far away from the boundary in the autumn. The southwestward and northeastward current constitute the cyclonic circulatio