Deep near-inertial waves

The majority of the transport of water below 4000 m depth into the North Pacific occurs in the Samoan Passage (168.5-170W, 7.5-10S). The Samoan Passage Abyssal Mixing Experiment (2012 - 2014) included seven long-term (approx. 1.5 year) and 17 short-term (6 - 30 days) moorings. Near-inertial waves were observed throughout the passage in moored profiler and point current meter velocity records. Although intermittent, there was no seasonal cycle. Maximum near-inertial energy was centered around the 1 degree C isotherm in the interface between the Antarctic origin bottom water and the overlying water. An array of four long term moorings, equipped with current meters, orientated east to west along the mouth of the passage, showed that the near-inertial wave energy entering the passage was higher in the water column to the west. A cluster of four moorings, north of the long term moorings at the month of the passage, located near a small sill (one 2km upstream, and three spaced 1km apart 3km downstream of the sill) show surprising differences in the inertial band energy. The upstream mooring spectra, 2 km south of the sill, has a double peak presumably from storms at different latitudes. The central downstream mooring retains the double peak, whereas the moorings 1 km on either side have a single near-inertial peak. This implies that interactions with topography, including generation of local vorticity, shadowing and flow steering, are important. An additional 12 short-term moored profiler records further north in the passage display near-inertial energy peaks. Together these moorings provide a unique insight in to the topographic interaction of deeply propogating near-inertial waves at a range of scales.