Observed water current and transport through Qiongzhou Strait during August 2010

The velocity structure of the residual current across an entire section of the Qiongzhou Strait (QS) in summer is presented for the first time. Shipboard Acoustic Doppler Current Profile measurements, from the mid-region of the QS (110.18°E), were collected on 1–4 August 2010. The diurnal tidal currents had their maximum amplitudes between 4.24 and 20.24 m. Their amplitude along the major axis ranged from approximately 0.55 m/s in the middle part of the strait (20.15°N) to 0.84 m/s in the north part of the strait (20.20°N). Both anticlockwise and clockwise tidal current rotations exist in the QS. During the observation period (neap tide), a significant westward residual current occupied almost the entire study section. Two velocity cores of westward current were observed at the northern part and near the deepest trough, although an eastward current appeared in the middle part of the transect. The deepest core was located near 62 m at 20.13°N, with a maximum velocity of −0.34 m/s. The shallower core was located at approximately 16 m at 20.20°N, with a maximum velocity of −0.33 m/s. The estimated total volume of water transported through the QS was −0.16 Sv. This value is an important boundary condition, applicable to numerical models studying coastal ocean circulation in the northwestern South China Sea.