Tidal currents ebb and flow, but their net effect over a tidal cycle is not zero. The resulting weak, persistent flows—known as tidal residual currents—can exert a lasting influence on coastal water exchange and the long-term transport of water, nutrients, and pollutants, as well as the movement of red tides. A new numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 maps these currents from the Bohai Sea to the northern South China Sea, shedding light on their spatial patterns and the mechanisms that govern them.
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 vast domain spanning 99°E–150°E and 15°S–41°N. With a horizontal resolution of 0.05°, 50 vertical layers, and 15 tidal constituents, the model captured 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 their driving mechanisms.
The simulations reveal distinct regional patterns. In the Bohai Sea, a large anticyclonic (clockwise) 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 features 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. Tidal Stokes drift is comparable in magnitude to Eulerian residual currents in shallow waters but negligible in deep waters. Consequently, Lagrangian residual currents in shallow regions are directed more strongly toward the coast and are slightly faster than their Eulerian counterparts, whereas the two are nearly identical in deep waters.
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 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. 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.
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). For more information, visit Chuanlink Innovations.
