Superinertial tidal enhancement and cross-reef flushing at islands

It is well-known that, when stratification is strong relative to topographic effects, coastal-trapped waves along a straight coast can extend from subinertial to superinertial frequencies, although topographic coupling allows energy leakage to the internal wave field. This study expands on this phenomenology, using a linear model of the response of waters around an idealized circular island (with realistic topography and stratification) to ambient spatially uniform oscillating currents at a range of superinertial frequencies, i.e., to idealized tides. In addition to the expected critical tidal internal wave reflections, there are well-defined “leaky mode” resonances at distinct frequencies. These amplified responses, if excited, can affect cross-reef tidal flushing dramatically. The sensitivities of these near-resonances to dissipation, topography, and stratification (among other variables) are explored. The implication of these findings is that tidal flushing across a bounding reef can vary dramatically depending on whether a major tidal component happens to be at or near a leaky-mode resonance. This, in turn, might affect nutrient availability and so help explain how otherwise similar islands can vary in terms of biological productivity.