The tides of the central Great Barrier Reef

Abstract

The central third of the Great Barrier Reef is a diffuse matrix of platform reefs on the outer half of a 300 km long segment of continental shelf. The subset of constituents O 1, P 1, K 1, N 2, M 2, S 2 and K 2 describes adequately both the vertical tides and tidal currents in the reef zone and lagoon. Sea level amplitudes and phases display quasi-linear changes in space throughout the region away from the coastal boundary, while sea level amplitude and phase gradients are very small along the shelf break. Cross-shelf sea level and phase gradients are relatively large, but decay equatorward, particularly where the shelf narrows from 120 to 60 km. This variation is reflected in current amplitudes, which range from spring maxima near 30 cm s −1 in the south to within the background noise in the north. The tidal data obtained in the field part of this study are used as boundary conditions to force a nonlinear numerical tidal model of the region, which is driven by the five principal constituents over a period of a month. This provides an overall picture of the dynamics, with results comparing well with measured data on a relatively coarse 5 nmi grid. Nowhere in this region are the reefs concentrated as an identifiable barrier, and it is found by comparative numerical experiments that, due to their scattered nature, they need not be modelled explicitly at this resolution. The M 2 tides dominate and energy propagates across the shelf break in the south of this central section to turn poleward on the shelf. Elsewhere, the M 2 energy flow is basically longshore and equatorward.