In this study, the spatial structures and temporal variations of subtidal currents inthe northern South China Sea (SCS) were quantified using 20-day to 9-month measurements from eight acoustic Doppler current profiler moorings deployed on the shelf and slope. The moored observations demonstrated that subtidal currents varied considerably with no persistent current feature on the shelf or slope. In winter, transient northeastward subtidal flows appeared sporadically but only when the wind stress was lower than 1 dyne/cm2.
The potential mechanisms of transient along-shelf flow formation during winter were attributed to the variation in cross-shelf and along-shelf sea levels. The variation in the cross-shelf sea level was due to Ekman transports varying over a period of 100 h. The variation in the cross-shelf sea level gradient induced by the wind wouldgenerate an along-shelf geostrophic current. When the sea level gradient anomaly was negative (i.e., sea level anomaly increased seaward), a transient shelf current appeared, whereas the opposite phenomenon occurred as the sea level anomaly decreased seaward. The variation in the along-shelf sea level was due to the along-shelf sea level setup or setdown as a result of shelf water accumulation or reduction near Hainan Island over a period of approximately 400 h. Other possible factors affecting subtidal currents include fresh water inputs from the Taiwan Strait, typhoons, and eddies in the northern SCS.