Introduction

Striking differences between the variability of current and temperature on each side of the ridge are found in the inertial to diurnal band. There is one order of magnitude more energy in the $0.7-1.1 cpd$ band at mooring DN compared to mooring DS. In this chapter, we consider whether incident NIWs, and/or the diurnal tide can account for the observations. As noted earlier, we discount the possibility that the reflection of internal waves at near-critical slopes alone can account for the increased Near-Inertial/Diurnal (NI/D) band at DN compared to DS. The critical frequencies are $1.79 cpd$ and $3.34 cpd$ at moorings DN and DS respectively. Velocity spectra at the 2 moorings (Figure 3.9) do not show any clear energy increase around these frequencies that is characteristic of critical reflections.

The intermittent nature of the near-inertial band energy leads us to believe that these are internal wave events forced by winter winds to the north of the ridge (described as the internal swell by Alford (2001)). This scenario is consistent with the higher energy observed on the north of the ridge compared to the south. These near-inertial waves propagate equatorward over the ridge and cannot reach the south mooring DS due to the steepness of the slope on the south side of the ridge, while there is no obstruction for these equator-ward propagating near-inertial waves to reach the north mooring DN. Chiswell (2003) hypothesized that such a shadowing effect occurs at his mooring site off the coast of New Zealand although data are not available to confirm this.

It is possible that the near-diurnal band energy is due to the diurnal tide. Diurnal internal tides can be generated at the ridge topography in the same manner as semidiurnal tides. However, we believe that the event like nature of the variability in this band is more consistent with the NIW band. Rainville and Pinkel (2005) and Carter and Gregg (2005) observed near-diurnal internal waves coupled with the semidiurnal tide at the nearby summit of Kaena Ridge. Here however, we note the absence of a peak at $M_2/2$ in our mooring data deep on the ridge north flank, suggesting that non-linear energy transfer from the dominant $M_2$ internal tide to $M_2/2$ internal waves is not the source of energy in the near-inertial to diurnal band (Figure 3.8).