Moorings Description

Two moorings were deployed during the nearfield phase of the HOME experiment, at the same depth ( $\sim 2425 m$ ), on the north and south sides of the Kaena Ridge, $40 km$ apart (Figure 1.3). Mooring Deep South (DS) was deployed on the south side of the ridge ( $158.65^\circ W, 21.58 ^\circ N$ ), from August to November 2002. Mooring Deep North (DN) was deployed on the north side of the ridge ( $158.4^\circ W, 21.87^\circ N$ ) between November 2002 and June 2003. The locations of the moorings were chosen in an attempt to sample downgoing semidiurnal tidal beams identified in the model runs of Merrifield and Holloway (2002) and Merrifield (2005) (Figure 2.1). It was hypothesized that the internal tide would be the primary energy source for near-boundary mixing at this depth.

The mooring designs were similar for DS and DN (Figure 2.2). Temperature was recorded every 3 and 5 minutes at mooring DS and DN, respectively, by Seabird Electronics temperature sensors (SBE 39). Sensor spacing varied between $16$ and $24 m$ on mooring DS, while sensors were spaced every $8 m$ between $28$ and $68 m$ above the bottom ($mab$ ) and every $24$ to $32 m$ between $68$ and $220 mab$ on mooring DN. According to the manufacturer, the SBE 39 sensors have a very small measurement error of 2 millidegrees, and a slow drift, typically less than 0.2 millidegree per month. One Seabird Electronics temperature and conductivity sensor (SBE 37) was located at $100 mab$ and $44 mab$ at mooring DS and DN, respectively.

Each mooring included an upward-looking RDI 300kHz ADCP, at $22 mab$ with a nominal range of approximately $100 m$ . To reduce the shock on the instruments during deployment, a nylon line was used between the anchor and the acoustic releases, and hence we were not able to sample closer to the bottom.

The ADCP data were 8 minute averages with a vertical resolution of $4 m$ . Ancillary data from the ADCP (i.e. amplitude and correlation of the acoustic return signal) indicate that the acoustic return from some depth bins was contaminated by the side lobe reflection off the mooring elements, which biases the current speed towards zero. These bins were not used in the analysis, although the data are included in some figures for reference (Figure 3.6 for example). Reliable current estimates (standard deviation of $\sim 0.02 ms^{-2}$ for 40 pings averaged every 8 minutes) were obtained out to a range of only $40 mab$ to $60 mab$ , presumably due to a low concentration of acoustic scatterers at this depth.

One pressure sensor was included at the top of each mooring to estimate the effect of the mooring layover. We found that these lateral and vertical motions were negligible at DS, confirming the mooring line was near-vertical at all times. We found that the measured pressure does not differ from the predicted tidal pressure by more than $0.2 dbar \sim 0.2 m$ . This was consistent with the measurement of the ADCP tilt and roll, which never exceeded $5^\circ$ . At DN, the pressure sensor failed, but considering the similarity of mooring designs and horizontal current speeds, we believe the mooring line at DN also remained near vertical at all times.