Thesis Outline

In this dissertation we examine temperature and current data from two deep ($2425 m$ ) moorings on the southern (DS) and northern (DN) flanks of Kaena Ridge (Figure 1.3). These moorings are located at depths well below the main thermocline.

We use estimates of dissipation based on a Thorpe scale analysis to show that mixing associated with the semidiurnal internal tide is vigorous at the DS mooring (dissipation events $\geqslant10^{-7}Wkg^{-1}$ ). The time series provide an unique view of deep mixing driven by a tidal current. At the DN mooring, we observe a less energetic tidal environment than at DS, and enhanced variability in the inertial to diurnal band. The overall inferred mixing at the north mooring is one order of magnitude smaller than at the south mooring.

The dissertation is organized as follows. The mooring locations and instrumentation are first described in chapter 2. The observed flow characteristics and the tidal variability are discussed in section 3. The character and possible generation of near-inertial flow events are considered in chapter 4. The method for estimating turbulent overturns and the associated dissipation, along with an estimation of the uncertainties of the method are described in section 5. In section 6 we identify two types of mixing events at the south mooring, related to different phases of the semidiurnal tide, and investigate their relationship with observed shear and strain. We attribute the mixing events occurring at the north mooring to enhanced strain in the context of the co-occurrence of near-inertial waves near the bottom and the semidiurnal internal tidal beam higher in the water column away from the boundary. A discussion and summary follow in chapter 7.

Figure 1.1: Intrusion experimental setup, for a continuously stratified fluid, showing the oscillatory grid that generated turbulence at the sloping boundary. Several evenly spaced intrusions, marked by fluorescine dye, are visible on the right of the picture.

Figure 1.2: Snapshots of the intrusion experiment for a two-layer fluid, taken a few seconds after the onset of grid oscillation and generation of turbulence at the sloping boundary. The blue line scale is $10 cm$ in length.

Figure 1.3: Bathymetric map of the Kaena Ridge showing the locations of the DS and DN moorings (this study), the Big Boy mooring (Levine and Boyd, 2005), and FLIP (Rainville and Pinkel, 2005). The scale is in meters.