Observations and physics of amplified subinertial tidal currents in stratification and mean shear flow at a seamount
Three-dimensional structure of amplified diurnal tidal currents (subinertial, 0.69f) at Cobb Seamount (130.8$\sp\circ W,$ 46.8$\sp\circ N),$ collected by ship-mounted Acoustic Doppler Current Profiler, agrees well with a stratified seamount-trapped wave resonant at 0.70f. The wave solution incorporates measured bathymetry, stratification, and baroclinic mean clockwise current, with parameterized damping of 2-day timescale based on observed dissipation. Physics and observable signatures of damping and mean flow on a wave are explained, including critical surface formation in mean shear.Observed diurnal currents propagate clockwise with first azimuthal wavenumber. Amplification extends a few km radially and about 100 m vertically from the seamount, reaching 5.3 times open-ocean K$\sb1$ currents. Characteristics are clockwise rotation in time, in narrow ellipses oriented nearly along isobaths with radial and azimuthal components positively correlated, and anticlockwise turning with depth such that phase propagates downward.Understanding of stratified seamount-trapped waves is extended in three ways. First, physics are explained using short topographic Rossby waves in stratification, or stratified slope-Kelvin waves. An inviscid stratified seamount-trapped wave superposes azimuthally resonant up- and down-going stratified slope-Kelvin waves, causing standing cross-slope (radial and vertical) structure. Second, observable signatures are described, including symmetries in inviscid-wave current patterns inconsistent with the measurements. A forced damped wave transports energy toward the summit where dissipation occurs, breaking cross-slope standing-wave symmetries, and exhibits flow characteristics similar to all those observed at Cobb. Third, because clockwise bottom-intensified mean flow is observed, waves linearized about a representative baroclinic azimuthal current are examined. Mean flow shifts the resonant frequency very weakly, because Doppler shifting is counteracted by the potential vorticity gradient of mean horizontal shear variations, and distorts wave structure weakly also.Stronger mean flow causes singularities at low (high) subinertial frequencies: stratified seamount-trapped wave (internal wave) critical surfaces. An internal wave critical surface bounds a superinertial cap, where subinertial motions are effectively superinertial. Though too weak at Cobb, mean flow at Fieberling Guyot (32.4$\sp\circ N,$ 127.8$\sp\circ W)$ forms an internal wave critical surface in diurnal currents (0.93f) that is coincident with high turbulence levels.
- Oceanography