Non linear interactions between waves and tides on a coral reef atoll

Mermaid Reef is one of Rowley Shoal’s three atolls (15 x 7 km), located on the edge of the North West Shelf of Australia. This atoll is regularly exposed to a combination of wind waves (remote swell waves or locally generated storm waves) and large tides, making it an ideal site to investigate how waves and tides linearly interact to drive the overall circulation and flushing on coral reef atolls. Here, we describe the hydrodynamic processes that drive the circulation using observations from a unique one-year experiment (November 2017 to October 2018) based on monitoring of surface waves, currents and water levels spanning the reef flat, lagoon and channel. Complementary numerical simulations were conducted using high-resolution LiDAR bathymetry, tidal forcing and offshore forcing from incident wave conditions, resolving the detailed hydrodynamics of the entire reef at very fine resolution (~30 m). Mermaid reef experiences a meso-tidal range (2.4 m) and a mean significant wave height of 1.5 m and up to 2.8m during extreme events such as tropical cyclones. The observed circulation shows alternating periods when either waves or tides dominantly drive the atoll circulation. The relative importance of waves and tide forcing on the circulation and flushing revealed the dominant role of tidal oscillations, both during weak wave energy conditions (significant wave height <1m) with the ebb/flood cycles of the tide but also during strong wave energy conditions (significant wave height >1m) through a tidal modulation of the wave-driven flows into the lagoon.The combination of in situ observations and numerical simulations provide a key insight into the atoll flushing processes, which are essential to ensure water renewal and the overall water quality within atoll reefs. It also provides insights on the dominant flow pathways, connectivity patterns and potential thermal stress refuges. The latter is of particular interest as Mermaid reef has avoided significant coral bleaching unlike many of the other atolls off Western Australia, which has been hypothesised to be linked to the surrounding open ocean processes. Yet, this is the first hydrodynamic study conducted on this critically important coral reef atoll.