Tuesday 23 August 2005

PB3
1550-1710 hours

211
Tidal mixing events on the deep flanks of Kaena Ridge, Hawaii
Aucan, Jerome¹, Merrifield, Mark¹, Luther, Doug¹, Flament, Pierre¹
1 University Of Hawaii, USA
Author email: jaucan@soest.hawaii.edu
A 3-month mooring deployment (Aug.-Nov. 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 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 1.5x10-8 W/kg, 10 to 100 times higher than at similar depths in the ocean interior . Dissipation events much larger than the overall mean (up to 10-6 W/kg) occur predominantly during two phases of the semidiurnal tide: 1) at peak downslope flows when the tidal stratification is minimum (N=5x10-4 1/s), and 2) at the flow reversal from down to upslope flow when the tidal stratification is ordinarily increasing (N=10-3 1/s). Dissipation associated with flow reversal mixing is twice 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 tidal 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 broad-banded nature of the internal tide.

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