A new numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 provides the first high-resolution mapping of tidal residual currents across China's marginal seas, from the Bohai Sea to the northern South China Sea. The research, conducted by scientists from Xiamen University's College of Ocean and Earth Sciences and the 715th Research Institute of China State Shipbuilding Corporation Ltd., highlights the importance of these weak but persistent currents in shaping coastal environments and transporting materials. The findings are crucial for coastal management, pollution control, and sustainable use of marine resources.
Tidal residual currents arise because tidal motion does not average to zero over a tidal cycle, resulting in a net displacement of water parcels. While much weaker than instantaneous tidal currents, they can dominate long-term transport. 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. This means they play a key role in the movement and dispersion of pollutants, sediments, nutrients, and even red tides.
Using the Regional Ocean Modeling System (ROMS), the team simulated 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. They compared Eulerian residual currents (time-averaged velocity at fixed locations) with Lagrangian residual currents (net displacement of water parcels) and tidal Stokes drift. The results reveal distinct regional patterns: a large anticyclonic circulation in the Bohai Sea with velocities of 0.5–3 cm/s (up to 4–10 cm/s in the northern Bohai Strait); small cyclonic and anticyclonic eddies in the Yellow Sea; a southward residual current from the Bohai Strait along the central Yellow Sea; and a strong northeastward flow with an anticyclonic circulation around the Taiwan Bank in the Taiwan Strait.
The study also shows that in shallow waters, tidal Stokes drift is comparable in magnitude to Eulerian residual currents, making Lagrangian residual currents more coastward and slightly faster. In deep waters, the two are nearly identical. Bathymetric features such as coastlines, islands, shoals, and ridges organize the residual-current field and generate small-scale circulations. A residual-vorticity balance indicates that bottom friction interacting with velocity shear exerts dominant control on the overall distribution of Eulerian residual currents, while the Coriolis force influences background residual vorticity and regional structures.
These findings have direct implications for coastal environmental assessment, marine engineering, channel maintenance, and resource management. By identifying where tidal residual currents are strongest and which mechanisms shape them, the study provides a physical basis for assessing long-term material transport 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). The full study is available at http://dx.doi.org/10.6043/j.issn.0438-0479.202412018.
