Tides in Vietnam
Tidal dynamics in the Gulf of Tonkin
Researchers: P. Marchesiello, Minh Nguyen, Sylvain Ouillon, Din Van Uu, Marine Hermann, Gildas Cambon
The Gulf of Tonkin, situated in the South China Sea, is a zone of strong ecological, touristic and economic interest. The coastal zone off Haiphong and Halong in the Gulf is one of the largest port systems in Vietnam, constituting a center for trade and industries. It is is also a major touristic attraction with beautiful, scenic beaches, bays and islands (Halong bay is a UNESCO world heritage center). The changing rates of supply/replenishment from the fluvial environment, the concentration and high growth rate of populations in large urban areas on the coast, marine traffic, tourist activities, river and coastal management (barriers, reservoirs, dams.) are dominant factors in controlling deposition of beaches and waterways. Studying sediment transports in this region is thus of major importance.
The scientific objective of this study is to revisit the dominant physical processes that characterize tidal dynamics in the Gulf of Tonkin using a high-resolution model and the combination of all available data. Particular attention is thus given to model-data cross-examination using tidal gauges and coastal satellite altimetry data and to model calibration derived from a set of sensitivity experiments to model parameters. The tidal energy budget of the gulf (energy flux and dissipation) is then analyzed and its resonance properties are evaluated and compared with idealized models and observations. Then, the tidal residual flow in both Eulerian and Lagrangian frameworks is evaluated. Finally, the problem of tidal frontogenesis is addressed showing that tidal mixing alone does not explain the observed summer frontal structures in chlorophyll concentrations.
ROMS_AGRIF is found to reproduce the tides of the Gulf of Tonkin with improved accuracy over the existing state of the art. The model errors are estimated by a compilation of all available tide gauge measurements along the coast and from ten satellite-altimeter ground-tracks data specifically reprocessed for coastal oceanography. Another specificity of our satellite data set is that it contains 6 ground tracks of 5-year TOPEX–Jason-1 interleaved data that increases the number of measurement locations and brings significant sample improvement in key areas like the Hainan Strait. On the other hand, it reduces the reliability of semidiurnal tides (particularly M2 amplitude and S2 phase) due to its limited time series. Nevertheless, the combination of 16-year primary data and 5-year interleaved data provide the best data set available to date for the Gulf of Tonkin, which is dominated by diurnal tides.
The model-data comparison shows good results near the open boundaries of the computational domain implying that the TPXO tidal product provides adequate forcing for our model. Model-data differences increase significantly near shallow coastal regions. There are two possible reasons for this. First, model errors may increase near the coast due to bathymetric and bottom roughness uncertainties that have a larger impact in shallow water. The lowest RMS errors were obtained with a choice of low values for drag coefficients and bottom roughness, which is consistent with the fact that the bottom of the gulf is made of fine sediments (Ma et al. 2010). In addition, the choice of a logarithmic drag profile appears crucial in 3D simulations to reproduce bed shear stress distribution and magnitude as it accounts for the increased vertical resolution in shallow water. The second source of discrepancy between model and data is the loss of quality of satellite altimetry measurements near the coastline due to land contamination and inaccurate geophysical corrections. A comparison between satellite and tide gauge data provides an observational error estimate, which appear to fall within model-data differences. Measurement errors for M2 and S2 are particularly large without possibility for now to point to any of the data set for explanations.
Evaluation of resonant modes
The validation of our model allows us to review the analysis of tidal characteristics in the Gulf of Tonkin. We used the model to explore for the first time the resonance spectrum of the gulf. Uncertainties in the damping process may alter our resonance analysis, but it provides consistent results with theoretical models and observations. It shows the three modes of resonance consistent with those expected from the idealized rectangular bay model with constant slope, with resonance peak at periods: 29 h, 12.5 h and 8 h. The rectangular bay model with constant depth used previously by several authors for the Gulf of Tonkin is thus disqualified. The latter model wrongly places O1 at the peak period of mode 0 and predicts no semi-diurnal resonance. Our results suggest that semi-diurnal tides are resonant at mode 1 but they are small in the Gulf because that they are already small in the South China Sea. On the contrary, diurnal tides are large in the South China Sea because of Helmholtz resonance (Zu et al., 2008) involving Luzon Strait as the main opening for tidal energy flux in this basin. Then, it appears that O1 amplitude is larger in the gulf than K1 because of a larger resonant effect (O1 is closer to the resonant peak period of 29 hours), even though its amplitude in the South China Sea is smaller than K1. This analysis may bring new insights and for example be useful to predict the future evolution of local tidal regime associated with changes in mean sea level and coastal morphology caused by human activity. For example, the non-uniform rectangular bay model predicts that a 50-cm rise in sea level (expected rise for a century in the Gulf from measurements) would shift down the resonant peak by a quarter hour bringing it closer to the diurnal modes. The amplification factor could then increase slightly for O1 and K1. Chnages in coastal morphology would probably be of greater importance.
Residual flow, tidal transport and mixing
Next, we explored the residual tidal flow. The strongest residual currents are found in the Hainan Strait (~20 cm/s flowing inside the gulf) and western Hainan Island (~8 cm/s). We show that the Hainan strait is a convergence zone for tidal energy flux that leaves little energy entering or escaping the Gulf. Nevertheless, it is there that residual currents are largest with consequences for the transport of water properties inside the gulf. The Hainan Strait is also a region of strongest energy dissipation with consequences for tidal mixing. Along the western coast of the gulf, residual currents are much weaker and only reach a maximum of 2 cm/s around headlands. These may be underestimated by the low resolution of local coastal morphology. A residual streamfunction was computed that reveals a coherent anticlockwise transport pattern around Hainan Island with maximum in the middle of the gulf. The Lagrangian flow appears qualitatively close to the Eulerian flow but weaker, as a result of the clockwise rotation of tidal ellipses in the gulf.
The tidal residuals can be locally strong but weaker in average than wind-driven currents (not shown), especially during the winter monsoon season. This has potential implications for transport properties too. In addition, the wind stress has a larger impact than expected from previous studies on vertical mixing. We showed in the last part of this study that it is the combination of winds and tides that can explain the location of seasonal fronts in the Gulf of Tonkin. It suggests that the study of primary production in this region should include a realistic set of forcing. The present model appears adequat for such applications in the fields of biogeochemistry and sediment transport. However, further investigation should be made at smaller scales where erosion and transport properties can be properly addressed. Our understanding is that an effort should then be made to achieve high resolution bathymetry and better acknowledge the diversity of bottom sediment type and its impact on the bottom boundary layer dynamics.
Nguyen, N.M., P. Marchesiello, F. Lyard, S. Ouillon, G. Cambon, D. Allain, U.V. Dinh, 2014: Tidal characteristics of the Gulf of Tonkin. Continetal Shelf Research, in press.