Tidally Driven Vorticity, Diurnal Shear, and Turbulence atop Fieberling Seamount

Abstract

Fine- and microstructure profiles collected over Fieberling Seamount at 32°26?N in the eastern North Pacific reveal a variety of intensified baroclinic motions driven by astronomical diurnal tides. The forced response consists of three phenomena coexisting in a layer 200 m thick above the summit plain: (i) an anticyclonic vortex cap of core relative vorticity ? 0.5f, (ii) diurnal fluctuations of ±15 cm s?1 amplitude and 200-m vertical wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity ?e ? 10 ? 10?4 m2 s?1. The vortex cannot be explained by Taylor?Proudman dynamics because of its ? 0.3fN2 negative potential vorticity anomaly. The ±0.3f fortnightly cycle in the vortex?s strength suggests that it is at least partially maintained against dissipative erosion by tidal rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit plain. They also exhibit a fortnightly cycle in their amplitude, pointing to seamount amplification of impinging barotropic tides. Their horizontal structure resembles that of a seamount-trapped topographic wave. However, the counterclockwise turning with depth of the horizontal velocity vector and the 180° phase difference between radial velocity u?r and vertical displacement ?? = ?T?/Tz (producing a net positive radial heat flux ?u?rT??) are more consistent with a vortex-trapped near-inertial internal wave of upward energy propagation. The strong negative vorticity of the vortex cap allows the diurnal frequency to be effectively superinertial; that is, diurnal fluctuations satisfy a hyperbolic equation within the vortex. A vortex- trapped wave would encounter a vertical critical layer at the top of the cap where its energy would be lost to turbulence. Observed turbulent kinetic energy dissipation rates of ε = 3 ? 10?8 W kg?1 are sufficiently high to deplete the wave and vortex in less than 3 days, emphasizing the strongly forced/damped nature of the system. Inferred eddy diffusivities two orders of magnitude larger than those found in the ocean interior suggest that, locally, seamounts are important sites for diapycnal transport. On basin scales, however, there are too few seamounts or ridges penetrating the main pycnocline to support a basin-averaged diffusivity of O(10?4 m2 s?1) above 3000-m depth.