Observations of low-frequency circulation and amplified subinertial tidal currents at Cobb Seamount

Abstract

We present acoustic Doppler current profiler (ADCP) survey results resolving three-dimensional current structure in thermocline stratification atop Cobb Seamount (130.8°W, 46.8°N). Mean flow includes clockwise circulation near the bottom and an east-northeastward background current. Clockwise flow is not strong enough to isolate fluid from the background current as in a Taylor cap unless this occurs in the deepest 50 m, outside ADCP coverage. Diurnal currents (subinertial, 0.69f) are amplified within a few kilometers radially and about 100 m vertically of the seamount by up to 5.3 times ambient K1 tidal currents. Their diagnostics include clockwise propagation around the seamount with first azimuthal wavenumber, clockwise rotation in time, narrow current ellipses oriented nearly along bathymetric contours with positively correlated radial and azimuthal components, counterclockwise turning with depth, and downward propagation. They are compared to the stratified seamount-trapped wave of first azimuthal wavenumber and third-highest subinertial resonant frequency (0.70f) calculated numerically using measured bathymetric and stratification profiles. Inviscid free wave currents exhibit symmetric patterns not consistent with the measurements; in a forced damped wave maintained by ambient tidal currents against parameterized friction of 2 day timescale, as appropriate to dissipation timescales estimated from microstructure measurements, symmetries are broken such that each observed current diagnostic is matched. When the wave is linearized about a steady, baroclinic, clockwise current representing the measured mean flow, its frequency and structure are very weakly modified. Copyright 1997 by the American Geophysical Union.