Observations of Water Mass Flux at the North Channel of Dongsha Atoll

The water flow in Dongsha Atoll is important for biogeochemical and biological status in the atoll lagoon. Here, we present a four-week in-situ measurement of waves and currents to study the water mass flux at the north channel of the atoll. The near-bottom current profiles were measured by one bottom mounted pulse-coherent acoustic Doppler profiler and the turbulent flow motions were measured by two acoustic Doppler velocimeters. The dataset covers two spring-neap cycles where the major solar tide modulated the major lunar tide. The Fourier spectrum analysis is used to obtain the dominant tidal period. The observed current speed is up to 0.5 m/s during the observational period and is obviously at opposite direction between flood and ebb tides. During the flood tide, the current flow is toward the direction of northwest to southeast while it changes to northwestward direction during the ebb tides. The current speed is much larger at the ebb tides than that at flood tides, which indicates that the net water mass flux was not balanced at the north cannel. The observed mean net mass flux is negative (outward the lagoon), which suggests that the water mass was transported throughout the channel from the atoll lagoon. From the mass conservation, the result implies that the south channel or the wave-induced driven current along the barrier reef flat of the atoll may provide a positive net flux to bring the water into the lagoon. However, the value of the mean net mass flux is small, which indicates that the exchange of the water mass between the atoll lagoon system and the adjacent open ocean was largely determined by the tidal currents. The Hilbert-Huang transform is applied to extract the time-varying modes to study the variation of the daily net mass flux. In addition, the local wind is found to affect the local wave height and period though the fetch is limited. The observed wave height and wave period can be predicted well by the SWAN model.