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

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 similar to 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.