Near‐inertial motions over a mid‐ocean ridge: Effects of topography and hydrothermal plumes

Abstract

We investigate the spatial structure of near‐inertial motions in the vicinity of the Endeavour segment of Juan de Fuca Ridge (approximately 48°N, 129°W) in the northeast Pacific Ocean. On the basis of time series current and water property data collected from September 1984 to September 1987, near‐inertial motions are ubiquitous features of the 2200‐m water column, with root‐mean‐square (rms) current speeds comparable to those of the dominant M2 tidal currents. Within the lower 1000 m of the water column where most of the observations were obtained, near‐inertial oscillations have rms current speeds of O(1 cm/s) and vertical isotherm displacements of O(10 m). The fluctuations are confined to the frequency band 0.966–1.079 f(f is the local Coriolis parameter) and have characteristic event durations of 1 week. Although the spectra of subsurface motions are dominated by the “blue‐shifted” superinertial band, significant spectral peaks are found also in the subinertial and inertial frequency bands. Marked alteration of the near‐inertial current amplitudes occurs over two well‐defined depth zones within the study region. Within the 200‐m zone immediately above the 2100‐m ridge crest, current amplitudes are amplified by a factor of 1.2–1.7 because of bottom reflection and/or scattering of the downward propagating energy. Evidence that the amplification may be linked to bottom reflection rather than to scattering is provided by flattening and cross‐slope rotation of the near‐inertial current ellipses with increased proximity to the top of the ridge. Reflection would occur at grazing angles of less than 1° and would be associated with surface‐generated waves originating at distances of over 100 km from the observational site. In contrast to the enhanced amplitudes immediately above the top of the ridge, near‐inertial currents within the 1600‐ to 1800‐m depth range undergo pronounced attenuation and frequency alteration. Amplitude attenuation is especially pronounced for motions in the superinertial band and may arise through critical‐layer absorption of downward propagating waves as they encounter increased vertical shear in the background flow. The increased shear is most likely associated with buoyancy‐induced flow formed by the extensive hydrothermal plume emanating from vent sites in the axial valley along the ridge crest, but it could also be related to bottom‐trapped oscillations over the steep ridge topography. Near‐inertial motions are estimated to have vertical coherence scales of the order of 10–100 m, while horizontal coherence scales exceed the 50‐km separation between the mooring locations. Minimum vertical and horizontal coherences are found for the depth zone 1600–1800 m, while maximum correlation occurs for near‐bottom motions immediately above the crest of the ridge. Weak near‐inertial motions are observed within the 100‐m‐deep axial valley.