Subtidal current structure and variability of the continental shelf and slope of the northern South China Sea

Abstract

The spatial structures and temporal variations in subtidal currents in the northern South China Sea (SCS) are quantified in this study 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 cm-2. The potential transient along-shelf flow formation mechanisms during winter are attributed to the variation in cross-shelf and along-shelf sea levels. The variation in cross-shelf sea level is due to Ekman transports varying over a period of 100 h. The variation in cross-shelf sea level gradient induced by the wind generates an along-shelf geostrophic current. When the sea level gradient anomaly is negative (i.e., sea level anomaly increased seaward), a transient shelf current appears, whereas the opposite phenomenon occurs as the sea level anomaly decreases seaward. The variation in the along-shelf sea level is due to the along-shelf sea level setup or set down 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.