Tidal currents ebb and flow, but their net effect over a tidal cycle is not zero everywhere. The resulting weak, persistent flows—known as tidal residual currents—can significantly influence coastal water exchange, the long-term transport of water, nutrients, and pollutants, and the movement of red tides. A new numerical study, published in the Journal of Xiamen University (Natural Science) in May 2026, provides a comprehensive mapping of these currents from the Bohai Sea to the northern South China Sea and examines how they vary with water depth and seafloor topography.
Researchers from Xiamen University's College of Ocean and Earth Sciences and the 715th Research Institute of China State Shipbuilding Corporation Ltd. used the Regional Ocean Modeling System (ROMS) to simulate barotropic tidal motion over a domain spanning 99°E–150°E and 15°S–41°N at a horizontal resolution of 0.05°, with 50 vertical layers and 15 tidal constituents. Their analysis compares Eulerian residual currents, tidal Stokes drift, and Lagrangian residual currents to identify governing mechanisms. The study is available at http://dx.doi.org/10.6043/j.issn.0438-0479.202412018.
The simulations reveal distinct regional patterns. In the Bohai Sea, a large anticyclonic residual circulation dominates, with velocities of 0.5–3 cm/s, except in the northern Bohai Strait where speeds reach 4–10 cm/s. The Yellow Sea contains several small cyclonic and anticyclonic circulations near the coast, while a southward residual current emerges from the Bohai Strait and extends along the central Yellow Sea. In the Taiwan Strait, residual currents flow predominantly northeastward, with a strong anticyclonic circulation around the Taiwan Bank. The study also shows that tidal Stokes drift is comparable in magnitude to Eulerian residual currents in shallow waters but negligible in deep waters, causing Lagrangian residual currents to be directed more strongly toward the coast and slightly faster than Eulerian counterparts in shallow regions.
High-resolution results further indicate that bathymetric features such as coastlines, islands, shoals, and submarine ridges organize the residual-current field and generate numerous small-scale circulations. A residual-vorticity balance reveals that the interaction of bottom friction with velocity shear exerts dominant control on the overall distribution of Eulerian residual currents. The bottom-friction term associated with water-depth gradients acts mainly in localized regions, while the Coriolis term influences background residual vorticity and several regional structures.
These findings matter because tidal residual currents contribute to the long-term transport and dispersion of pollutants, sediment, nutrients, and other suspended material. Previous estimates cited in the study indicate that tidal residual currents account for about 50–80% of the local flow between the Changjiang Estuary and the Subei Shoal and may become the dominant component in some shallow coastal areas. By clarifying where these currents are strongest and which mechanisms shape them, the study provides a physical basis for coastal environmental assessment, marine engineering, channel maintenance, and sustainable use of coastal resources across China's continental shelves.
The research was supported by the National Natural Science Foundation of China (Grant No. 41776015) and the National Key Research and Development Program of China (Grant No. 2022YFF0801404). Additional information is available through Chuanlink Innovations.
