Characteristics of winter tidal and residual surface circulation in the Northwestern Gulf of Tonkin from HF radar observations

Kim Cuong Nguyen; Ngoc Anh Giap; Thanh Huyen Tran; Ngoc Anh Tran; Duy Thai To; Cong Viet Nguyen

doi:10.15625/1859-3097/24320

Author affiliations

Authors

Nguyen Kim Cuong University of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam https://orcid.org/0000-0002-5154-3377
Giap Ngoc Anh University of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam
Tran Thanh Huyen Helzel Messtechnik GmbH, Carl-Benz-Straße 9, 24568 Kaltenkirchen, Germany
Tran Ngoc Anh University of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam
To Duy Thai Institute of Oceanography, VAST, Vietnam
Nguyen Cong Viet The Center of Marine Environmental Monitoring and Analysis, Navy High Command, Hai Phong, Vietnam

DOI:

https://doi.org/10.15625/1859-3097/24320

Keywords:

Tidal currents, Gulf of Tonkin, HF radar, residual circulation, cluster analysis

Abstract

Surface current observations collected from December 2024 to February 2025 were analyzed to investigate tidal dynamics and residual circulation in the coastal waters off the Red River Delta, northern Vietnam. Harmonic analysis was applied to identify the dominant tidal constituents and their spatial characteristics, while residual currents were obtained by removing the principal tidal components. The results show that the tidal regime is primarily dominated by the diurnal constituents K1 and O1, which exhibit larger current magnitudes than the semi-diurnal constituents M2 and S2. The strongest tidal currents occur within the bathymetric zone between 10 and 25 m depth, indicating a strong influence of seabed topography on the spatial distribution of tidal motion. Tidal ellipse analysis reveals predominantly meridional orientations in deeper areas, while flatter ellipses in intermediate depths indicate a mainly oscillatory tidal flow. Near the Van Uc estuary, nearly circular tidal ellipses suggest locally complex hydrodynamic conditions influenced by coastal geometry and bathymetry. Residual currents derived from the tidal analysis were further examined using cluster analysis (SOM and K-means), which identified four representative circulation patterns. The most common regime (38.28%) is characterized by weak offshore-directed flow with relatively uniform velocities of approximately 5 cm/s across the domain, representing the background circulation state. Another frequent regime (31.93%) shows weak alongshore currents aligned with the coastline, reflecting the influence of coastal boundaries and bathymetric steering. A third pattern (21.93%) indicates rotational circulation in the northern sector associated with the influence of winter monsoon conditions. The least frequent but strongest regime (7.86%) occurs near the Ba Lat estuary, where intensified northeastward residual currents may enhance the transport of riverine water and suspended materials toward the open shelf. In addition, the residual flow is consistently weak near the 20 m depth contour, suggesting a frontal zone that limits cross-shelf exchange. Temporal analysis indicates alternating periods dominated by weak and stronger circulation states, highlighting the combined influence of tides, winds, and river discharge on residual transport. These results demonstrate that, despite strong instantaneous tidal currents, net surface transport in this coastal region is controlled by a limited number of recurring residual circulation patterns.

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