A Numerical Study of Water Exchange Processes in the Dongsha Atoll

Wind and ambient currents affect water exchange in coral reef atolls. Their degree of influence and the characteristics of the consequent water exchange depend on the atoll morphology and the local atmospheric and hydrographic conditions. The process of how the ambient currents drive water exchange, however, has drawn little attention in the literature. To investigate the effects of wind and ambient currents on water exchange in coral reef atolls, we built up hydrodynamic models of circulations around the Dongsha Atoll for an eighteen-day summer event. The Dongsha Atoll is located near the shelf break in the northern South China Sea. A shallow reef flat covering about 60% of the perimeter encloses the central lagoon in the east, south, and north. An island is in the west with two wide channels to its north and south. Water is exchanged through the reef flat and the two channels. The analysis results suggest that wind dominates the residual flow rates on the reef flat, and ambient currents dominate the residual flow rates in the channels. A key finding is that the ambient currents could drive water exchange with a magnitude comparable to the wind forcing. The predominantly westward ambient currents induce northward sea-level tilt across the entrances of north and south channels, driving inflow through the north channel. But this inflow pattern reverses periodically when cyclonic eddies, which are shed from the northeastern atoll corner due to flow separation, propagates to the north channel entrance. This means that the variability of channel volume flux depends critically on the strength and the angle of ambient currents impinging on Dongsha. Lagrangian float tracking indicates that wind is more efficient in replacing lagoon water with the open ocean than ambient currents, even though the residual flow rates induced by wind and by ambient currents are comparable. The results suggest that both Eulerian and Lagrangian approaches should be considered when assessing water exchange in an atoll system.