Island wakes in shallow coastal waters

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Abstract
Rattray Island, northeast Australia, is 1.5 km long, 300 m wide, and lies in well‐mixed water approximately 25 m deep. Its long axis is inclined at about 60° into the direction of the dominant semidiurnal tidal current. The length of the wake in the lee of the island, as documented by aerial photographs and satellite imagery, appears to equal that of the wake behind a flat plate in a two‐dimensional flow at a Reynolds number of about 10. However, current metering, drogues measurements, and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low Reynolds number model. Further, estimates of the turbulent eddy coefficient suggest an effective Reynolds number more in the vicinity of 103. To reconcile these observational differences and to explain the observed upwelling in the core of the wake, an Ekman pumping model is proposed. It is postulated that the Ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation, to be negated by the vorticity of opposite sign introduced at the bottom. Further, it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies, leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands, coral reefs, and rock outcrops.