Tidal Mixing Events on the Deep Flanks of Kaena Ridge, Hawaii

Abstract

A 3-month mooring deployment (August–November 2002) was made in 2425-m depth, on the south flank of Kaena Ridge, Hawaii, to examine tidal variations within 200 m of the steeply sloping bottom. Horizontal currents and vertical displacements, inferred from temperature fluctuations, are dominated by the semidiurnal internal tide with amplitudes of ≥ 0.1 m s⁻¹ and ∼100 m, respectively. A series of temperature sensors detected tidally driven overturns with vertical scales of ∼100 m. A Thorpe scale analysis of the overturns yields a time-averaged dissipation near the bottom of 1.2 × 10⁻⁸ W kg⁻¹, 10–100 times that at similar depths in the ocean interior 50 km from the ridge. Dissipation events much larger than the overall mean (up to 10⁻⁶ W kg⁻¹) occur predominantly during two phases of the semidiurnal tide: 1) at peak downslope flows when the tidal stratification is minimum (N = 5 × 10⁻⁴ s⁻¹) and 2) at the flow reversal from downslope to upslope flow when the tidal stratification is ordinarily increasing (N = 10⁻³ s⁻¹). Dissipation associated with flow reversal mixing is 2 times that of downslope flow mixing. Although the overturn events occur at these tidal phases and they exhibit a general spring–neap modulation, they are not as regular as the tidal currents. Shear instabilities, particularly due to tidal strain enhancements, appear to trigger downslope flow mixing. Convective instabilities are proposed as the cause for flow reversal mixing, owing to the oblique propagation of the internal tide down the slope. The generation of similar tidally driven mixing features on continental slopes has been attributed to oblique wave propagation in previous studies. Because the mechanical energy source for mixing is primarily due to the internal tide rather than the surface tide, the observed intermittency of overturn events is attributed to the broadbanded nature of the internal tide.