Abstract
The sea surface velocity measurements obtained during the period of 2014–2016 using two high-frequency radars (HFR), which were installed on the northern coast of Vietnam, were reprocessed by using a variational approach (EOF/2dVar) to provide an extended dataset. The high temporal resolution of measurements and large spatial coverage of the radar data enabled the assessment of the surface circulation in the data-poor region of the southern Gulf of Tonkin (GoT) and the characterization of its temporal variations in a wide range of scales: from tidal frequency to annual cycle. It was found that tidal motions account for a large percentage (approximately 60%) of surface current variability in the GoT. The temporal variability of the kinetic energy of the surface currents is tightly correlated with wind variations in the monsoon-dominated atmospheric circulation system. Stratification, induced by the discharge of the Red River, also affects the tidal currents within the coastal region (~ 40 km wide) by modifying the tidal ellipses polarizations. Close to the shore, the prominent coastal flow experiences large temporal variations in response to wind and buoyancy forcing. The seasonal variability of the zonal wind component was found to play a significant role in coastal flow speed variations through the cross-shore mass exchange. The Red River freshwater discharge controls the seaward extension of the coastal flow and causes asymmetry in the seasonal variability of flow velocity. The data obtained and analysis results could be useful for the improvement of the regional circulation models, for further studies of the physical and biological processes in the GoT, and for marine resource management.
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Acknowledgments
This study was supported by the University of Littoral Côte d’Opale and the Région “Hauts-de-France” PhD fellowship. The study was also partly supported by the Vietnam state-level project KC09.14/16-20. The data set used in this study has been collected in the framework of the US-Vietnam collaboration project: “Gulf of Tonkin Circulation study” (NICOP N62909-15-1-2018), financed by the Office of Naval Research (ONR). We also thank the field team of the CFO for technical assistance. We appreciate the valuable comments from two anonymous reviewers for helping in improving the paper.
Funding
This study was funded by Université du Littoral-Côte d’Opale for PhD scholarship fellowship and partly supported by Vietnam state-level project KC09.14/16-20.
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M.C.T. performed the data analysis and wrote the original version of the manuscript. A.S. proposed the ideas and was deeply involved in the analysis, manuscript organization, and co-writing. K.C.N. gave valuable comments and revised the manuscript.
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Responsible Editor: Pierre De Mey-Frémaux
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Tran, M.C., Sentchev, A. & Nguyen, K.C. Multi-scale variability of circulation in the Gulf of Tonkin from remote sensing of surface currents by high-frequency radars. Ocean Dynamics 71, 175–194 (2021). https://doi.org/10.1007/s10236-020-01440-x
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DOI: https://doi.org/10.1007/s10236-020-01440-x