TIDALLY DRIVEN VORTICITY, DIURNAL SHEAR, AND TURBULENCE ATOP FIEBERLING-SEAMOUNT

Fine-and microstructure profiles collected over Fieberling Seamount at 32 degrees 26'N in the eastern North Pacific reveal a variety of inte nsified baroclinic motions driven by astronomical diurnal tides. The f orced 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 c orresponding to an eddy diffusivity kappa(epsilon) congruent to 10 x 1 0(-4) m(2) s(-1): The vortex cannot be explained by Taylor-Proudman dy namics because of its -0.3fN(2) negative potential vorticity anomaly. The +/-0.3f fortnightly cycle in the vortex's strength suggests that i t is at least partially maintained against dissipative erosion by tida l rectification. The diurnal motions are slightly subinertial, turning clockwise in time and counterclockwise with depth over the summit pla in. They also exhibit a fortnightly cycle in their amplitude, pointing to seamount amplification of impinging barotropic tides. Their horizo ntal structure resembles that of a seamount-trapped topographic wave. However, the counterclockwise turning with depth of the horizontal vel ocity vector and the 180 degrees phase difference between radial veloc ity u'(r) and vertical displacement xi' = -T'/(T) over bar(z) (produci ng a net positive radial heat flux [u'T-r']) are more consistent with a vortex-trapped near-inertial internal wave of upward energy propagat ion. The strong negative vorticity of the vortex cap allows the diurna l frequency to be effectively superinertial; that is, diurnal fluctuat ions 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 kinet ic energy dissipation rates of epsilon = 3 x 10(-8) W kg(-1) are suffi ciently high to deplete the wave and vortex in less than 3 days, empha sizing the strongly forced/damped nature of the system. Inferred eddy diffusivities two orders of magnitude larger than those found in the o cean interior suggest that, locally, seamounts are important sites for diapycnal transport. On basin scales, however, there are too few seam ounts or ridges penetrating the main pycnocline to support a basin-ave raged diffusivity of O(10(-4) m(2) s(-1)) above 3000-m depth.