Topography and Stratification

The Kaena Ridge topography has been surveyed with multibeam sonars from different agencies and compiled into a bathymetric map with a horizontal resolution of $150 m$ (Figure 1.3). DS was located in a steep region on the south flank of the ridge, in a slight depression between two ridges running down the slope and separated by $\sim10 km$ (Figure 1.3). Averaged over a $1.5 km$ square, the topographic slope is $1/4.5$ at DS, and oriented at a $35^\circ$ angle relative to true north (NNE). Mooring DN was deployed on the north flank of the ridge, on a relatively flatter plateau, where the topographic slope is $1/11.6$ , and oriented $200^\circ$ relative to true north (SSW).

At mooring DS, the overall vertical- and time-averaged buoyancy frequency is $\bar N=1.02\times10^{-3} s^{-1}$ or $14.5 cpd$ , and the Coriolis frequency is $f=5.36 \times 10^{-5} s^{-1}$ or $0.74 cpd$ . This equates to a critical frequency $\omega _c=\sqrt{f^2sin^2\theta+N^2cos^2\theta}$ of $3.34 cpd$ averaged over a $1.5 km$ square around the mooring. $\omega _c$ varies depending on the area used to average the topographic slope ($4.04 cpd$ for a $0.6 km$ square, $3.68 cpd$ for a $0.9 km$ square). In comparison, at mooring DN, $\bar N = 1.19\times10^{-3} s^{-1}$ or $16.43 cpd$ , and the Coriolis frequency is $f=5.43 \times 10^{-5} s^{-1}$ or $0.75 cpd$ , leading to $\omega _c = 1.79 cpd$ .

The difference in $N$ between the 2 moorings ( $\sim 1.7 \times10^{-4} s^{-1}$ ) is significantly larger than the $95\%$ confidence interval of $1\times10^{-6}$ and $0.8\times10^{-6} s^{-1}$ for DS and DN, respectively. Changes in stratification related to mesoscale eddy activity along the Hawaiian Ridge likely would not affect the 3 and 8 month averages at each moorings.

Stratification averages were calculated for each month over the 16 years of CTD data at station ALOHA. No regular annual variations of stratification were observed at the depths of interest ( $2000-3000 m$ ). We therefore suggest that the difference in $N$ between the 2 moorings is unlikely to be related to the different deployment times (August to November 2002 for DS and November 2002 to June 2003 for DN).

Time-average vertical temperature profiles along $160 ^\circ W$ , obtained as part of the World Ocean Circulation Experiment (WOCE), do not show a meridional change large enough on either side of the ridge, although the data are too sparse for a definitive comparison.

The implications of the different stratification at the two moorings will be discussed in section 7 in regards to turbulent dissipation and mixing. Also, at all depths, absolute temperatures at the moorings are within $0.03 ^\circ C$ of the climatological absolute temperature values (16 years of repeat CTD profiles) at nearby station ALOHA (Figure 2.3). The Temperature-Salinity (TS) diagrams for the two moorings indicate the similarity of water mass properties on either side of the Ridge (Figure 2.4). The more elongated cloud of TS points at mooring DS compared to DN is related to the higher vertical excursions measured on the south flank. The narrow distribution of T and S allowed us to infer the salinity at depths where only temperature was measured. We used the measured temperature and the inferred salinity to calculate potential density. In chapter 6, we used these potential density profiles to detect statically unstable patches and to estimate turbulent dissipation.

Figure 2.1: Cross section of the ridge and $M_2$ modeled baroclinic energy fluxes from Merrifield (2005)

Figure 2.2: Position of the sensors on the DS (left) and DN (right) moorings. T indicates temperature measurement, C salinity measurement, and U current measurement.

Figure 2.3: Time-averaged vertical profiles of absolute temperature at DS (red) and DN (blue), corrected (dashed lines), and uncorrected (thick lines), (see chapter 6 for the details of the correction). Instrument locations are depicted by a '*'. The black line shows the climatological temperature profile at Station Aloha, located $\sim100 km$ to the NNE of mooring DN.

Figure 2.4: TS diagrams at mooring DS (left) and mooring DN (right). The data are organized in bins of $0.01 ^\circ C$ and $0.001 PSU$ and the contours indicate the percentage of the data in each bins.