Abstract:

Moorings deployed on the south (Aug-Nov 2002) and north (Nov 2002 - June 2003) flanks of the Kaena Ridge, Hawaii are used to examine current and temperature variability within $200 m$ of the steeply sloping bottom near the $2400 m$ isobath. On the south flank, horizontal currents and vertical displacements are dominated by the semidiurnal internal tide over the depths sampled. On the north flank, the semidiurnal tide is less energetic than on the south, with a different vertical structure as tidal amplitudes decrease toward the boundary. Near the boundary, near-inertial to diurnal oscillations associated with winter wind forcing north of the islands are present which were comparatively weak on the south flank. At both sites, strong temperature inversions are detected with vertical scales of $\sim 100 m$ .

A Thorpe scale analysis of the overturns yields a time-averaged dissipation near the bottom of $1.2\times10^{-8} Wkg^{-1}$ , 10 to 100 times higher than at similar depths in the ocean interior $50 km$ from the ridge at the south mooring. The dissipation at the north mooring is estimated to be an order of magnitude smaller at $1.8\times10^{-9} Wkg^{-1}$ . At the south mooring, dissipation events much larger than the overall mean (up to $10^{-6} Wkg^{-1}$ ) occur predominantly during two phases of the semidiurnal tide: 1) during peak downslope flows when the tidally modulated stratification is minimum ( $N=5\times10^{-4} s^{-1}$ ), and 2) when the flow reverses from down to upslope flow as the tidally modulated stratification is ordinarily increasing ( $N=10^{-3} s^{-1}$ ). Shear instabilities, particularly due to tidal and near-inertial strain enhancements, appear to trigger downslope flow mixing at the south mooring. Convective instabilities are proposed as the cause for flow reversal mixing at the south mooring, owing to the oblique propagation of an internal tidal beam down the slope.

At the north mooring, dissipation occurs predominantly during periods of high strain at the maximum upward displacement of the semidiurnal tide. Overturns are particularly prevalent when the tidal strain is enhanced by near-inertial wave driven downwelling near the slope. The intermittency of near-inertial waves at the north mooring limits the predictability of mixing events.