Eric Kunze

Global Abyssal Mixing Inferred from Lowered ADCP Shear and CTD Strain Profiles

Abstract Internal wave–wave interaction theories and observations support a parameterization for the turbulent dissipation rate e and eddy diffusivity K that depends on internal wave shear 〈Vz2〉 and strain 〈ξz2〉 variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 × 10−4 m2 s−1 within 2° of the equator to (0.4–0.5) × 10−4 m2 s−1 at 50°–70°. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 × 10−4 m2 s−1 between 20° and 30°, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation ∫e is contributed from the main pycnocline, hotspots in ∫e show some correlation with small-scale bottom roughness and near-bottom flow at sites where st...

Internal Tide Reflection and Turbulent Mixing on the Continental Slope

Abstract Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May–June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ϵ, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slopes corrugations or internal tide generation at the shelf break was found at this site.

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 κe ≅ 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...

Global Patterns of Diapycnal Mixing from Measurements of the Turbulent Dissipation Rate

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixingobtainedfrom(i)Thorpe-scaleoverturnsfrommooredprofilers,afinescaleparameterizationappliedto (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strainfromfull-depthloweredacousticDoppler currentprofilers (LADCP)andCTDprofiles. Verticalprofiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10 24 )m 2 s 21 and above 1000-m depth is O(10 25 )m 2 s 21 . The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variabilityin theratiobetweenlocal internalwavegeneration and local dissipation.Insomeregions,the depthintegrateddissipationrateiscomparabletotheestimatedpowerinputintothelocalinternalwavefield.Inafew cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However,atmostlocationsthetotalpowerlostthroughturbulentdissipationislessthantheinputintothelocal internal wave field. This suggests dissipation elsewhere, such as continental margins.

Estimating Internal Wave Energy Fluxes in the Ocean

Abstract Energy flux is a fundamental quantity for understanding internal wave generation, propagation, and dissipation. In this paper, the estimation of internal wave energy fluxes 〈u′p′〉 from ocean observations that may be sparse in either time or depth are considered. Sampling must be sufficient in depth to allow for the estimation of the internal wave–induced pressure anomaly p′ using the hydrostatic balance, and sufficient in time to allow for phase averaging. Data limitations that are considered include profile time series with coarse temporal or vertical sampling, profiles missing near-surface or near-bottom information, moorings with sparse vertical sampling, and horizontal surveys with no coherent resampling in time. Methodologies, interpretation, and errors are described. For the specific case of the semidiurnal energy flux radiating from the Hawaiian ridge, errors of ∼10% are typical for estimates from six full-depth profiles spanning 15 h.

Shear and strain in Santa Monica Basin

By injecting a cloud of SF6 into Santa Monica Basin at a depth of 790 m and observing its vertical spread, Ledwell and Watson (1991) estimate the average diapycnal diffusivity between 750 and 850 m as Kp = 2.5 × 10−5 m2 m−1 This diffusivity is higher than inferred from microstructure measurements in the open-ocean thermocline for typical internal wave levels (Gregg and Sanford, 1988). To examine shear and strain in the basin, we dropped expendable current profilers (XCPs) and calculated strain from conductivity, temperature, and depth (CTD) casts. Shear and strain are above the GM76 model spectrum (Garrett and Munk, 1975; Cairns and Williams, 1976), and comparisons with published parameterizations indicate more than enough shear variance to account for the high diffusivity reported by Ledwell and Watson. Thus, the eddy diffusivity in Santa Monica Basin is not at odds with lower values estimated from microstructure measurements in the open ocean, but is due to an elevated internal wave field. The spectral shapes, however, differ from typical open-ocean spectra.

Observations of near-inertial waves in a front

Abstract Near-inertial with horizontal scales ∼O(10 km) dominate profiles of velocity finestructure collected in the North Pacific Subtropical Front during January 1980. Considerable spatial variability is observed. Two features in particular contain most of the energy: a 20 cm s−1 amplitude (λz = 100 m) wave on the warm edge of the front propagating downward and away from the front, and a low wavenumber (λz = 500 m) wave reflecting off the surface. The propagating wavegroup is four times as energetic as the local downgoing near-inertial wave field. Its spatial structure is not consistent with propagation in a homogeneous medium, which suggests that it may be interacting with the front. Possible mechanisms for the existence and properties of the wavegroup are discussed, including baroclinic/barotropic instability, wind-forcing and enhancement by wave-mean flow interaction. a wave-mean flow interaction model that predicts trapping and amplification of near-inertial flow interaction. A wave-mean flow intera...

Abyssal Mixing: Where It Is Not

Abstract A parameterization based on internal wave/wave interaction theory, which infers turbulence production from finescale internal wave shear, is applied to 114 full-water-depth velocity profiles in the Sargasso Sea. An average eddy diffusivity of 0.1 × 10−4 m2 s−1, independent of depth, is inferred. This value is consistent with full-water-depth microstructure measurements from abyssal basins in the eastern North Atlantic and eastern North Pacific. It is an order of magnitude smaller than the values inferred from a simple vertical advection-diffusion balance or bulk budgets. Thus, the mixing needed to close deep global water-mass budgets does not appear to occur over midlatitude abyssal plains. This suggests that ocean mixing is either (i) confined to boundary layers as in ideal thermocline theory or (ii) localized to hotspots, such as over rough topography or restrictive passages. Abyssal diffusivities do not display any dependence on bottom slope for slopes less than 7 × 10−2 based on 5–10 km bathy...

Near‐boundary mixing above the flanks of a midlatitude seamount

Fine-scale velocity and density profile data with concurrent turbulent velocity and temperature dissipation estimates obtained above the flanks of Fieberling Guyot, a seamount in the eastern North Pacific Ocean, are examined for evidence of near-bottom boundary mixing. Fine-scale shear and strain spectral levels were elevated over the flanks of the seamount in a 500-m-thick stratified layer above the bottom. The velocity shear was horizontally isotropic, clockwise and counterclockwise-with-depth shear spectral levels were comparable, and no significant correlation between shear and strain was observed. Above the steepest bottom slopes near the seamount summit rim, excess vertical strain relative to shear was observed (as compared to the background internal wave field), suggesting the presence of high-frequency internal waves. These signals may have been the product of wave reflections from the steep flanks of the seamount and/or wave generation from tidal currents flowing over the rough bottom. Associated with the enhanced shears and strains were more frequent occurrences of low 10-m Richardson number events, increased overturning scales, and larger estimated turbulent eddy diffusivity relative to observations 15 km or more from the seamount. In particular, turbulent diffusivity estimates increased from O(0.1×10−4 m2 s−1) in the ocean interior to 1–5×10−4 m2 s−1 within 500 m vertically (1–3 km horizontally) of the seamount flank. A simple geometric scaling argument suggests that boundary mixing of this intensity has relevance to the large-scale circulation at abyssal depths where a large fraction of the ocean waters is in close proximity to the bottom.

Internal Tide Radiation from Mendocino Escarpment

Abstract Strong semidiurnal internal tides are observed near Mendocino Escarpment in full-depth profile time series of velocity, temperature, and salinity. Velocity and density profiles are combined to estimate the internal tide energy flux. Divergence of this flux demonstrates that its source is the barotropic tide interacting with the escarpment. A baroclinic energy flux of 7 kW m−1 radiates from the escarpment, corresponding to 3% of the 220 kW m−1 fluxing poleward in the surface tide. Energy and energy flux are concentrated in packets that emanate from the flanks of the ridge surmounting the escarpment and one site ∼90 km north of the escarpment. Coherent beamlike structure along semidiurnal ray paths remains identifiable until the first surface reflection. Beyond the first surface reflection north of the escarpment, the energy flux drops by 2 kW m−1 and beams are no longer discernible. Turbulence, as inferred from finescale parameterizations, is elevated by over two orders of magnitude relative to th...