A new numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 provides a 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 coastal water exchange and the long-term transport of water, nutrients, pollutants, and red tide organisms. The findings offer a physical basis for assessing material transport and can inform coastal environmental assessment, marine engineering, and sustainable resource use.
Tidal residual currents arise because tidal motion does not average to zero everywhere 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 studies in China focused mainly on Eulerian residual currents—time-averaged velocities at fixed locations—which do not directly represent the net transport of individual water parcels. Lagrangian residual currents, calculated from the net displacement of water parcels, provide a more direct measure. However, systematic high-resolution simulations spanning China's marginal seas were limited, and the relative effects of the Coriolis force, bottom friction, and velocity shear were not fully resolved.
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. The analysis compared Eulerian residual currents, tidal Stokes drift, and Lagrangian residual currents. The simulations reveal distinct regional patterns: in the Bohai Sea, a large anticyclonic 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 small cyclonic and anticyclonic circulations near the coast, with a southward residual current extending from the Bohai Strait 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 to Eulerian residual currents in shallow waters but negligible in deep waters, making Lagrangian residual currents in shallow regions more strongly directed toward the coast and slightly faster than their Eulerian counterparts. Bathymetric features such as coastlines, islands, shoals, and submarine 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 term influences background residual vorticity and regional structures.
These findings are relevant to coastal management because tidal residual currents contribute to the long-term transport and dispersion of pollutants, sediment, nutrients, and 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 dominant in some shallow coastal areas. By clarifying where these currents are strongest and which mechanisms shape them, the research provides a foundation for assessing long-term material transport across China's continental shelves. The study was funded 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.
