Tidal currents ebb and flow, but their net effect over a tidal cycle is not zero everywhere. The resulting weak, persistent flows—tidal residual currents—play a crucial role in coastal water exchange and the long-term transport of materials, including pollutants, nutrients, and the organisms responsible for red tides. A new study published in the Journal of Xiamen University (Natural Science) provides a comprehensive mapping of these currents from the Bohai Sea to the northern South China Sea, offering insights that could improve coastal management and environmental protection.
Conducted by researchers from Xiamen University's College of Ocean and Earth Sciences and the 715th Research Institute of China State Shipbuilding Corporation Ltd., the study used the Regional Ocean Modeling System (ROMS) to simulate barotropic tidal motion at a high horizontal resolution of 0.05 degrees, with 50 vertical layers and 15 tidal constituents. The 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 researchers compared Eulerian residual currents, tidal Stokes drift, and Lagrangian residual currents to identify the mechanisms governing these flows.
The simulations revealed 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 features several small cyclonic and anticyclonic circulations near the coast, with a southward residual current emerging from the Bohai Strait. In the Taiwan Strait, residual currents flow predominantly northeastward, with a strong anticyclonic circulation around the Taiwan Bank. The study also found that tidal Stokes drift is comparable to Eulerian residual currents in shallow waters but negligible in deep waters, causing Lagrangian residual currents in shallow regions to be directed more 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 numerous 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. The bottom-friction term associated with water-depth gradients acts mainly in localized regions, while the Coriolis term influences background residual vorticity and several regional structures.
These findings are significant for coastal management 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). The full study is available at DOI: 10.6043/j.issn.0438-0479.202412018. For more information about the journal, visit Chuanlink Innovations.
