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

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Tidally Driven Vorticity, Diurnal Shear, and Turbulence Atop Fieberling Seamount DECEMBER 1997 KUNZE AND TOOLE 2663 Tidally Driven Vorticity, Diurnal Shear, and Turbulence atop Fieberling Seamount ERIC KUNZE School of Oceanography, University of Washington, Seattle, Washington JOHN M. TOOLE Woods Hole Oceanographic Institution, Woods Hole, Massachusetts (Manuscript received 6 March 1996, in ®nal form 13 January 1997) ABSTRACT Fine- and microstructure pro®les collected over Fieberling Seamount at 328269N in the eastern North Paci®c reveal a variety of intensi®ed 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 2 0.5 f, (ii) diurnal ¯uctuations of 615 cm s21 amplitude and 200-m vertical 24 2 21 wavelength, and (iii) turbulence levels corresponding to an eddy diffusivity ke ù 10 3 10 m s . The vortex cannot be explained by Taylor±Proudman dynamics because of its 2 0.3 fN 2 negative potential vorticity anomaly. The 60.3 f fortnightly cycle in the vortex's strength suggests that it is at least partially maintained against dissipative erosion by tidal recti®cation. 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 ampli®cation 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 1808 phase difference between radial velocityur9 and vertical displacement j952T9/Tz (producing a net positive radial heat ¯ux ^ur9T9&) 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 ¯uctuations 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 « 5 3 3 1028 Wkg21 are suf®ciently 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(1024 m2 s21) above 3000-m depth. 1. Introduction Seamount (32.58N) at slightly subinertial diurnal tidal The peaks of seamounts are sites of intense near- frequencies. Since subinertial internal waves are not al- bottom ¯uctuations (Noble et al. 1988; Noble and Mul- lowed in a quiescent ocean, these diurnal ¯uctuations lineaux 1989; Genin et al. 1989; Padman et al. 1992; might be seamount-trapped topographic waves (Brink Eriksen 1991; Kunze and Sanford 1986; Kunze et al. 1989, 1990). Codiga and Eriksen (1997) demonstrated 1992) that support abundant populations of benthic ®l- that the ampli®ed diurnal motions around Cobb Sea- terfeeders (Genin et al. 1986; 1992; Levin et al. 1994). mount (478N) had properties consistent with forced But how seamounts intensify oscillating currents has not damped seamount-trapped topographic waves. been clearly established. Kunze and Sanford (1986) in- Sloping topography allows bottom-trapped topo- terpreted the velocity signal turning counterclockwise graphic waves with frequencies v # Nsina (Rhines with depth above Caryn Seamount (36.68N) as a criti- 1970; Huthnance 1978). Sloping bottom topography cally re¯ected near-inertial internal wave. Genin et al. also causes critical re¯ection of impinging internal grav- 2 2 2 2 (1989), Eriksen (1991, 1995), Noble et al. (1994), and ity waves with characteristics ÏÏv 2 f / N 2 v Brink (1995) report dominant motions above Fieberling 5 kH/kz 5 Cgz / CgH identical to the bottom slope a (Wunsch 1969; Phillips 1977; Eriksen 1982, 1985). Rough topography scatters tidal and internal wave en- ergy to high wavenumbers (Baines 1971; Bell 1975; Corresponding author address: Dr. Eric Kunze, School of Ocean- ography, University of Washington, Box 357940, Seattle, WA 98195- Gilbert and Garrett 1989; MuÈller and Xu 1992). Both 7940. processes amplify ®nescale internal wave shear and E-mail: [email protected] strain. Elevated shear and strain will support stronger q 1997 American Meteorological Society Unauthenticated | Downloaded 09/23/21 01:37 PM UTC 2664 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 27 and more frequent shear/advective instabilities to induce ocean. The global signi®cance of the near-seamount in- intensi®ed turbulent dissipation and mixing (Gregg tensi®cation of mixing is addressed in section 7c. 1989; Kunze et al. 1990; Polzin et al. 1995). Relative to the ocean interior, enhanced turbulence has been doc- umented over seamounts (Nabatov and Ozmidov 1988; 2. Measurement and analysis techniques Lueck and Mudge 1997; Toole et al. 1997). This has a. Data led to speculation that seamounts act as stirring rods for the World Ocean, with topographically ampli®ed mixing Pro®les were collected over the summit and ¯anks of near boundaries dominating over the weak diapycnal Fieberling Seamount (328269N, 1278459W) during mixing of the ocean interior (Munk 1966; Lueck and March 1991 as part of the ONR-sponsored Topographic Mudge 1997). Interactions Program to investigate the impact of sea- Strong quasi-steady circulations are also expected mounts on physical and biological oceanography. Time over seamounts. Both Taylor±Proudman dynamics series and spatial surveys were made with the High- (Hogg 1973; Owens and Hogg 1980; Swaters and My- Resolution Pro®ler (HRP; Schmitt et al. 1988) and Sip- sak 1985; Roden 1987) and topographic recti®cation pican expendable current pro®lers (XCP; Sanford et al. (Loder 1980; Maas and Zimmerman 1989a,b; Haidvogel 1982, 1993). Other relevant measurements are (i) year- et al. 1993; Codiga 1993) could produce a vortex cap long mooring deployments from September 1990 to over a seamount's summit. Strati®cation limits the ver- September 1991 at ®ve sites on Fieberling Seamount tical extent of such vortices (Huppert 1975; Zyryanov and two sites on the abyssal plain (Wichman et al. 1993; 1981; Chapman and Haidvogel 1992) to O( fL/N). For Noble et al. 1994; Brink 1995; Eriksen 1995, 1997), a Coriolis frequency f 5 7.8 3 1025 s21, an ensemble- and (ii) mesoscale CTD surveys conducted in August average buoyancy frequency N 5 4.3 3 1023 s21 and 1989 (Roden 1991) and April±May 1991 (Roden 1994). seamount radius L 5 7 km, the expected vortex cap Fieberling Seamount is the northwestmost member of thickness is about 100 m at Fieberling. A Taylor cap the Fieberling seamount chain, rising from an abyssal would be anticyclonic and requires an impinging geo- plain at 4000±4500-m depth to a summit plain at 500± strophic ¯ow. In the absence of dissipative processes, 700-m depth (Fig. 1). It is a guyot with a ¯at crown this vortex would have the same potential vorticity as (apart from a narrow pinnacle southwest of the summit's the surrounding ocean. The properties of a recti®ed vor- geometric center that attains 440-m depth) due to sur- tex depend partly on the nature of the forcing and damp- face wave erosion before the peak sunk below the sur- ing as well as the topography. Anticyclonic circulations face. Bottom slopes on the summit plain are about 0.05 appear most likely, at least in barotropic laboratory and compared to ¯ank slopes as high as 0.5. The abrupt numerical experiments (Boyer et al. 1991; Verron et al. change in slope near the 700-m isobath will be referred 1995). Recti®cation need not preserve potential vortic- to as the summit rim. The summit plain is elongated so ity. that the rim radius is about 6.5 km zonally and half that To better understand the impact of seamounts on tides, meridionally (Fig. 1). Beyond the rim, Fieberling Sea- internal waves and turbulent mixing, ®ne- and micro- mount can be described as a Gaussian of radial scale structure pro®les were collected over Fieberling Sea- 12 km (Codiga 1991). Motions above the summit plain mount during March 1991 (Montgomery and Toole discussed here appear to be dynamically isolated from 1993). Here we use pro®le time series and surveys (sec- the nearest neighbor, Fieberling II, 40 km to the south- tion 2) to characterize the temporal and spatial structure east. This is consistent with Roden's (1991) conclusion of motions on top of Fieberling's summit plain. We ®nd that the seamount chain affects impinging currents as a a vortex cap 200 m thick atop Fieberling Seamount group only at abyssal depths. (section 3) with a core vorticity z 52(0.50 6 0.15) f The freefall High-Resolution Pro®ler (Schmitt et al. and a core potential vorticity anomaly 2(0.3 6 0.1) fN 2. 1988) carries ®ne- and microstructure sensors. The ®- Coincident with the vortex are 615 cm s21 diurnal mo- nestructure sensor suite includes a CTD and a two-axis tions, which turn clockwise in time and counterclock- acoustic current meter, which provide vertical pro®les of wise with depth on a vertical wavelength of about 200 horizontal velocity relative to an unknown but depth- m (section 5). The presence of the anticyclonic vortex independent constant temperature, salinity, and pressure. and the proximity of the seamount to the diurnal turning Data from microstructure sensors provide estimates of latitude raise the possibility that these oscillations are microscale temperature variance and kinetic energy dis- vortex-trapped near-inertial internal waves rather than sipation rates.
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