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 have a lasting influence on coastal water exchange and the long-term transport of water, nutrients, pollutants, and even red tides. A new numerical study published in the Journal of Xiamen University (Natural Science) in May 2026 provides the first systematic, high-resolution mapping of these currents from the Bohai Sea to the northern South China Sea, revealing their regional 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 (99°E–150°E, 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 (time-averaged velocity at fixed points), tidal Stokes drift, and Lagrangian residual currents (net displacement of water parcels), which more directly represent material transport. The study, published with DOI 10.6043/j.issn.0438-0479.202412018, fills a gap in understanding how the Coriolis force, bottom friction, and velocity shear shape these flows.
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 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. 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.
High-resolution results further 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.
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 original source URL is available at http://dx.doi.org/10.6043/j.issn.0438-0479.202412018.
