Despite being far weaker than the ebb and flow of daily tides, tidal residual currents—the net displacement of water over a tidal cycle—can have a lasting influence on China's coastal seas. A new numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 provides the most detailed mapping yet of these currents from the Bohai Sea to the northern South China Sea, shedding light on their role in moving water, nutrients, pollutants, and even red tides.
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 at a horizontal resolution of 0.05° with 50 vertical layers and 15 tidal constituents. Their domain spanned 99°E–150°E and 15°S–41°N, covering the Bohai Sea, Yellow Sea, East China Sea, northern South China Sea, and adjacent western North Pacific. The study, available via DOI: 10.6043/j.issn.0438-0479.202412018, compares Eulerian residual currents, tidal Stokes drift, and Lagrangian residual currents to identify the mechanisms that govern them.
The simulations reveal distinct regional patterns. In the Bohai Sea, a large anticyclonic (clockwise) circulation dominates, with velocities of 0.5–3 cm/s, except in the northern Bohai Strait where speeds can reach 4–10 cm/s. The Yellow Sea features several small cyclonic and anticyclonic eddies 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. Notably, tidal Stokes drift is comparable in magnitude 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.
The high-resolution results also show 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 indicates that the interaction of bottom friction with velocity shear exerts the 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 the background residual vorticity and several regional structures.
These findings matter for coastal management and environmental protection. 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. The results can inform coastal environmental assessment, marine engineering, channel maintenance, and the sustainable use of coastal resources. By clarifying 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.
