Tidal and tidal current characteristics in the Guangxi Gulf of Tonkin, South China Sea

The Guangxi Gulf of Tonkin (GXGT) is rich in mineral, oceanic energy, and biological resources; however, it has not attracted much attention. Based on ECOMSED, a high-resolution three-dimensional numerical model is constructed in this study, and the accuracy is validated using long-term observational data. The results showed that the GXGT is dominated by regular diurnal tide and mixed mainly diurnal tidal currents and some regions to the northwest and south of Weizhou Island are dominant by mixed mainly semi-diurnal tidal currents. Furthermore, the rotation direction of O-1 is found to be counter-clockwise and clockwise in the western and eastern domains, respectively, whereas the rotation directions of the other three tidal constituents are clockwise and counter-clockwise in the open sea and coastal bays, respectively. Similarly, the propagation direction of the tidal energy flux of O-1 (northwestward) is different from them of the other three tidal constituents (northeastward) in the western GXGT. On the other hand, the propagation directions of the four tidal constituents are all northeastward in the eastern GXGT. A comparison of the reference run with the summed-up effect of six runs, where each of the tidal constituents has been considered separately, revealed that the nonlinear effect due to the interaction of the six tidal constituents cannot be neglected in the GXGT, especially in the western GXGT. Interestingly, energy input from wind to O-1 is larger than that to M-2 whereas the tidal dissipation of O-1 is less than that of M-2 at BBL. Additionally, tidally induced residual currents mostly flow westward in the GXGT, with clockwise eddies present outside the Qinzhou Bay and Tieshan Port. Driven by the monsoon wind, the current structure and strength in the GXGT have obvious seasonal variation. And sensitive experiments show that the residual currents in the GXGT are deeply influenced by the monsoon wind and bottom topography.