Variability of deepwater currents at the South-China-Sea-side of the Luzon Strait

A bottom-anchored sub-surface mooring system was deployed at the South China Sea side of the Luzon Strait, close to the south end of the Hengchun Ridge. Current velocity in the depth range of 1700-2500 m was obtained by two downward-looking acoustic doppler current profilers (Teledyne RDI 75-kHz Long Ranger) for 2 calendar years (May 2013 to May 2015). The sub-inertial currents were obtained by low-pass filtering of the raw velocity at a cut-off frequency of 0.125 d^-1. The average velocity of sub-inertial current was 1-3 cm/s in the depth range of 1700-2500 m, with high velocity up to 5-7 cm/s occurred periodically. The major direction of sub-inertial currents alternated between northeasternward and northwestward. The northwestward currents were usually corresponding to periods of high velocities, while northeastward currents were corresponding to periods of relatively lower velocities. As the station was located close to the turning point of the bathymetric contours, from NE-SW at the south of the station and NW-SE at the north of the station. Therefore, the results can be explained by the variations of the contour currents: when the velocity of the contour currents is high, the direction turn from northeasternward to northwestward at a more north position because of the inertia of the flows.