Observations of bottom intensification of temperature and velocity fluctuations induced by oblique tidal interactions with a slope

Abstract

Long-term measurements of temperature and velocity collected at six depths in 302 m of water off the North West Cape of Western Australia (21°37′35″S, 113°56′11″E) revealed several periods of extreme near-bed currents. The dominant forcing at the site is the M₂ tide, with energy levels generally decreasing as the bottom is approached. There is, however, a dramatic change in the energy distribution for all frequencies in excess of the M₄ tidal frequency between 80 and 10 m above the seabed. Waves in this frequency bandwidth are critical to the local bottom slope and show a strong linear internal wave signature; however, they do not appear to conform with linear internal wave reflection theory. Dissipation estimates reveal energetic motions with dissipation rates of ~10⁻⁵ m² s⁻³. Superimposed on this ambient state, three energetic events with duration varying between 8 and 24 h can also be detected. These are characterised by large increases in energy levels in the high-frequency range, and peak speeds varying from 0.59 m s⁻¹ to 1.87 m s⁻¹. These events appear to be driven by direct local energy inputs at high frequencies.