Generation of shelf waves on the East Australian coast by wind stress

Abstract

A recent paper by Gill and Schumann (J. Phys. Oceanogr. 4, 1974, 83–90) gives expressions for the amplitude of continental shelf waves in terms of the longshore component of wind stress near the coast in the direction from which the waves are approaching. These expressions have been used to compute the continental shelf waves to be expected at a port on the east coast of Australia (Evans Head, 29°07'S.) as a function of the wind stress at six stations between Evans Head and Gabo Island (39°33'S., 149o57'E.). The results show high coherence and nearly zero phase shift in the frequency range 0.04–0.25 cycles per day, between shelf wave activity (as estimated by the adjusted sea level at Evans Head) and the appropriate function of the wind stresses. The coherence shows a sharp cut-off at 0.25 cpd.It is shown that a cut-off near this frequency would be expected, due to destructive interference, if the wind stresses are assumed independent of longshore distance. The observed shelf wave amplitude agrees to better than an order of magnitude with the amplitude computed from the wind stresses.The coherence, phase shift, amplitude and sharp cut-off results strongly support the hypothesis that, on the east coast of Australia, shelf waves are generated by the longshore component of wind stress, in the frequency range 0.04–0.25 cpd.