Wind-forced Motion on the Northern Great Barrier Reef

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  • 1 School of Mathematics, University of New South Wales, Kensington, Australia
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Abstract

Observations of current, bottom pressure, and wind from the shelf of the Northern Great Barrier Reefare interpreted using a two-layer, frictional, step-shelf model forced by propagating wind stress. Distinguishing features of the region are the many reefs on the shallow shelf, the well-mixed nature of the shelf waters, and the steep continental slope.

The instrument army was deployed from April to October 1982 and comprised 14 current meters, nine pressure gauges, and two weather stations. Low-pass [<0,6 cycles per day (cpd)] filtered velocity and pressure data are analyzed separately using a time domain empirical orthogonal function (EOF) analysis. The first EOFs of both velocity and pressure data are found to account for most of the observed variance on the shelf. Time series of these two EOFs are both highly coherent, with the local longshore wind stress at “weatherband” frequencies(<0.3cpd);however, the frequency transfer functions of the wind to the current and of the wind to the pressure are very different. The observed velocity transfer function (evaluated from data) is frequency independent and has almost zero phase whereas the observed pressure transfer function decreases with frequency and lags the wind by about 60°. Theoretical response functions of the step-shelf model (with a high coefficient of friction on the shelf) have these same characteristics. The difference in the response functions for velocity and pressure is due to the large contribution that the sea level response over the slope makes to the sea level over the shelf while having very little effect on shelf velocities. Thus, the observed pressure response over the shelf reflects the dynamic balance over the slope, which is that of an internal Kelvin wave, while the longshore velocity response on the shelf is determined simply by the balance between wind stress and bottom friction on the shelf.

Abstract

Observations of current, bottom pressure, and wind from the shelf of the Northern Great Barrier Reefare interpreted using a two-layer, frictional, step-shelf model forced by propagating wind stress. Distinguishing features of the region are the many reefs on the shallow shelf, the well-mixed nature of the shelf waters, and the steep continental slope.

The instrument army was deployed from April to October 1982 and comprised 14 current meters, nine pressure gauges, and two weather stations. Low-pass [<0,6 cycles per day (cpd)] filtered velocity and pressure data are analyzed separately using a time domain empirical orthogonal function (EOF) analysis. The first EOFs of both velocity and pressure data are found to account for most of the observed variance on the shelf. Time series of these two EOFs are both highly coherent, with the local longshore wind stress at “weatherband” frequencies(<0.3cpd);however, the frequency transfer functions of the wind to the current and of the wind to the pressure are very different. The observed velocity transfer function (evaluated from data) is frequency independent and has almost zero phase whereas the observed pressure transfer function decreases with frequency and lags the wind by about 60°. Theoretical response functions of the step-shelf model (with a high coefficient of friction on the shelf) have these same characteristics. The difference in the response functions for velocity and pressure is due to the large contribution that the sea level response over the slope makes to the sea level over the shelf while having very little effect on shelf velocities. Thus, the observed pressure response over the shelf reflects the dynamic balance over the slope, which is that of an internal Kelvin wave, while the longshore velocity response on the shelf is determined simply by the balance between wind stress and bottom friction on the shelf.

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