Research Article: Tides and Their Dynamics over the Sunda Shelf of the Southern South China Sea

Date Published: September 13, 2016

Publisher: Public Library of Science

Author(s): Farshid Daryabor, See Hai Ooi, Azizan Abu Samah, Abolghasem Akbari, Vanesa Magar.

http://doi.org/10.1371/journal.pone.0162170

Abstract

A three-dimensional Regional Ocean Modelling System is used to study the tidal characteristics and their dynamics in the Sunda Shelf of the southern South China Sea. In this model, the outer domain is set with a 25 km resolution and the inner one, with a 9 km resolution. Calculations are performed on the inner domain. The model is forced at the sea surface by climatological monthly mean wind stress, freshwater (evaporation minus precipitation), and heat fluxes. Momentum and tracers (such as temperature and salinity) are prescribed in addition to the tidal heights and currents extracted from the Oregon State University TOPEX/Poseidon Global Inverse Solution (TPXO7.2) at the open boundaries. The results are validated against observed tidal amplitudes and phases at 19 locations. Results show that the mean average power energy spectrum (in unit m2/s/cph) for diurnal tides at the southern end of the East Coast of Peninsular Malaysia is approximately 43% greater than that in the East Malaysia region located in northern Borneo. In contrast, for the region of northern Borneo the semidiurnal power energy spectrum is approximately 25% greater than that in the East Coast of Peninsular Malaysia. This implies that diurnal tides are dominant along the East Coast of Peninsular Malaysia while both diurnal and semidiurnal tides dominate almost equally in coastal East Malaysia. Furthermore, the diurnal tidal energy flux is found to be 60% greater than that of the semidiurnal tides in the southern South China Sea. Based on these model analyses, the significant tidal mixing frontal areas are located primarily off Sarawak coast as indicated by high chlorophyll-a concentrations in the area.

Partial Text

The South China Sea (SCS) is a semi-enclosed tropical sea, located between several land-masses that include Peninsular Malaysia, Borneo, the Philippines and East Asia. The SCS has a complex bathymetry with a depth ranging from over 1000 m in the middle and northern parts to less than 100 m in the continental shelf (Fig 1). The southern South China Sea (SSCS) is bounded by Peninsular Malaysia’s eastern continental shelf, the Gulf of Thailand and the sea off Borneo and the southern coast of Vietnam. It is connected to the Java Sea through the Sunda Shelf in the south. Monsoonal winds have great influence on the sea circulation in the SSCS [1–11]. Furthermore, sea surface and seabed also have different significant impacts on wind-induced circulations as well as on the distribution, propagation and dissipation of tidal energy flux [12] with complicated tidal dynamics, especially at the bottom [13–15]. In this region, several successful 2D numerical studies have been performed by Ye and Robinson [16] on M2 and K1 tidal constituents with approximately 34 km resolution, Fang et al. [17] on M2, S2, K1, and O1 with 28 km resolution. Zu et al. [18] used simulation model of 10 km resolution and ETOPO5 with a model integration of 240 days to investigate characteristics and dynamics of M2, S2, K1, O1, N2, K2, P1, and Q1. Most recently, Green et al. [19] used the Oregon State University Tidal Inversion Software (OTIS) without data assimilation but with a realistic tidal conversion scheme to demonstrate that the modelled dissipation levels are overestimated over the entire SCS, noting that the discrepancies are far larger in regions with steep bathymetry.

From the last two months run for the inner domain of ROMS, u- and v-components as well as sea surface height are used to estimate the current and elevation for each tidal constituent based on the T-Tide harmonic analysis [33]. The estimates of simulated tidal phase and amplitude are compared with the tide gauges (TGs) data. Data from 19 TGs provided by the Department of Survey and Mapping Malaysia are used to validate the modelled sea surface elevation. Seventeen of these TGs are in the Malaysian waters, mostly distributed along the east coast and the southern end of Peninsular Malaysia, as well as Borneo. The remaining two gauges (Laut Island and Natuna Islands) are located within SSCS (Fig 1B). Tidal harmonic constants for these tidal gauges are computed and used for model validation.

The mixing parameter (given by log10(hU3)) from Simpson and Hunter [46] is used to assess the contribution of tidal waves in shallow water on key biological processes that lead to the transport of nutrients and phytoplankton blooms across a tidal mixing fronts, where h (m), is the water depth and U, the tidal current amplitude (m/s).Yanagi et al. [47] shows the existence of the tidal mixing fronts in the SSCS for the northern Gulf of Thailand and the offshore area of the ECPM. Previous findings have pointed out that the regions with smaller parameter values of ~<3 are locations of tidal mixing fronts [47–49]. Hence, coastal areas of Sarawak, namely Tanjung Datu (Station 6), Kuching (Station 9), and Kuala Paloh (Station 10), are found to be the locations of tidal mixing fronts due to the existence of strong M2 tide because of its maximum tidal current and strong tidal energy flux. For the diurnal tides, the maximum tidal current is clearly correlated with the strong energy flux in the region between southern tip of the ECPM and western East Malaysia (Fig 10C and 10D). Results of the simulated tides in the Continental Shelf Area of the SSCS from a 3D, one-way nested regional ocean modelling system indicate that the modelled tides compare well with the observations at 19 tidal stations. Existence of high tidal elevation at the southern tip of the ECPM and East Malaysia reflect the significant role of tides in these regions. Moreover, the M2 tidal current tends to flow towards ECPM due to the convergence of the counter clockwise tidal current from the coast of Vietnam and the clockwise tidal current from East Malaysia. Depth of the bathymetry can alter significantly the phase change. In general, the region between the southern end of the ECPM and coastal Sarawak is dominated by the diurnal tide with a counter clockwise phase rotation. However, along the coastal areas of East Malaysia, especially the coastal areas of Sarawak in places like Tanjung Datu, Kuching and Kuala Paloh are affected by the M2 tide with the high energy flux and strong tidal current rotating in a clockwise direction. The patterns of the co-tidal lines (lines of constant tidal phase) for the two main tidal constituents, namely M2 and K1 are the result of their differing amplitudes, leading to different contributions of tidal energies for each tidal constituent in the SSCS. The largest tidal energy flux in the study area is related mostly to the diurnal tides. There is a good correlation between the areas of high tidal energy flux and strong tidal current, causing the areas to be well mixed by the tidal mixing fronts. The areas affected significantly by the tidal mixing fronts are the coastal areas of Sarawak as exemplified by high chlorophyll concentrations. Nonetheless, more work needs to be done by considering the use of more refined model and bathymetry resolutions as well as more robust harmonic analysis with favourable hydrographic conditions.   Source: http://doi.org/10.1371/journal.pone.0162170