Kurt L. Polzin

Finescale parameterizations of turbulent dissipation

Fine- and microstructure data from a free fall profiler are analyzed to test models that relate the turbulent dissipation rate (e) to characteristics of the internal wave field. The data were obtained from several distinct been previously available. Observations from the ocean interior with negligible large-scale flow were examined to address the buoyancy scaling of e. These data exhibited a factor of 140 range in squared buoyancy frequency (N 2 ) with depth and uniform internal wave characteristics, consistent with the Garrett-Munk spectrum. The magnitude of e and its variation with N(∼N 2 ) was best described by the dynamical model of Henyey et al. A second dynamical model, by McComas and Muller, predicted an appropriate buoyancy scaling but overestimated the observed dissipation rates. Two kinematical dissipation parametrizations predicted buoyancy scalings of N 3/2 ; these are shown to be inconscient with the observations. Data from wave fields that depart from the canonical GM description are also examined an interpreted with reference to the dynamical models. The measurements came from a warm core ring dominated by strong near-inertial shears, a region of steep topography exhibiting high-frequency internal wave characteristics, and a midocean regime dominated at large wavelengths by an internal tide. Of the dissipation predictions examined, those of the Henyey et al. model in which eN − 2 scales as E 2 , where E is the nondimensional spectral shear level, were most consistent with observations. Nevertheless, the predictions for these cases exhibited departures from the observations by more than an order of magnitude. For the present data, these discrepancies appeared most sensitive to the distribution of internal wave frequency, inferred here from the ratio of shear spectral level to that for strain. Application of a frequency-based correction to the Henyey et al. model returned dissipation values consistent with observed estimates to within a factor of 2. These results indicate that the kinetic energy dissipation rate (and attendant turbulent mixing) is small for the background Garrett and Munk internal wave conditions (0.25eN −2 ∼ 0.7 × 10 − 5 m 2 s − 1). Dissipation and mixing become large when wave shear spectral levels are elevated, particularly by high-frequency waves. Thus, internal wave reflection/generation at steep topographic features appear promising candidates for achieving enhanced dissipation and strong diapycnal mixing in the deep ocean that appears required by box models and advection-diffusion balances

Estimates of Diapycnal Mixing in the Abyssal Ocean

Profiles of diapycnal eddy diffusivity to a maximum depth of 4000 meters were derived from ocean velocity and temperature microstructure data obtained in conjunction with separate experiments in the Northeast Pacific and Northeast Atlantic oceans. These profiles indicate that in the ocean interior where the internal wave field is at background intensity, the diapycnal eddy diffusivity is small (on the order of 0.1 x 10–4 meters squared per second) and independent of depth, in apparent contradiction with large-scale budget studies. Enhanced dissipation is observed in regions of elevated internal wave energy, particularly near steeply sloping boundaries (where the eddy diffusivity estimates exceed 1 x 10–4 meters squared per second). These results suggest that basin-averaged mixing rates may be dominated by processes occurring near the ocean boundaries.

Mixing in the Romanche Fracture Zone

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data. The finestructure of density profiles is described using the Thorpe scalesLT. It is shown from microstructure data taken in the bottom water that the Ozmidov scale LO is related to LT by the linear relationship LO 5 0.95LT, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water. Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4 8C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.88C isotherm, emphasizing the role of these passages for bottom-water property modifications.

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.

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.

Eddy stirring in the Southern Ocean

There is an ongoing debate concerning the distribution of eddy stirring across the Antarctic Circumpolar Current (ACC) and the nature of its controlling processes. The problem is addressed here by estimating the isentropic eddy diffusivity ? from a collection of hydrographic and altimetric observations, analyzed in a mixing length theoretical framework. It is shown that, typically, ? is suppressed by an order of magnitude in the upper kilometer of the ACC frontal jets relative to their surroundings, primarily as a result of a local reduction of the mixing length. This observation is reproduced by a quasi-geostrophic theory of eddy stirring across a broad barotropic jet based on the scaling law derived by Ferrari and Nikurashin (2010). The theory interprets the observed widespread suppression of the mixing length and ? in the upper layers of frontal jets as the kinematic consequence of eddy propagation relative to the mean flow within jet cores. Deviations from the prevalent regime of mixing suppression in the core of upper-ocean jets are encountered in a few special sites. Such ‘leaky jet’ segments appear to be associated with sharp stationary meanders of the mean flow that are generated by the interaction of the ACC with major topographic features. It is contended that the characteristic thermohaline structure of the Southern Ocean, consisting of multiple upper-ocean thermohaline fronts separated and underlaid by regions of homogenized properties, is largely a result of the widespread suppression of eddy stirring by parallel jets.

Statistics of the Richardson number: Mixing models and finestructure,

Abstract Parameterization of the dissipation rate of turbulent kinetic energy (ϵ) in terms of Richardson number (Ri = N2/S2) is examined for a variety in internal wave environments. Previous work with these data suggests a scaling of ϵ ∼ E2N2g(w) to within a factor of 2, where E represents a low-wavenumber shear spectral density, N the background buoyancy frequency, and g(w) a dependence upon average wave frequency content. The alternative Richardson-number-based parameterization of Kunze et al. is also shown to collapse the dissipation data to within a factor of 2. On the basis of a number of theoretical Richardson number probability distributions, however, the nominal (E, N) scaling of the Kunze et al. model is determined to be E2N3. The difference between the nominal (N3) and observed (N2) scaling is hypothesized to be an effect of turbulent momentum and buoyancy fluxes on the internal wave shear and strain profiles associated with the shear instability. For a statistically homogeneous subset of ...

Internal Waves and Turbulence in the Antarctic Circumpolar Current

AbstractThis study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution.The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that i...

Evidence in hydrography and density fine structure for enhanced vertical mixing over the Mid‐Atlantic Ridge in the western Atlantic

Received 24 August 2001; revised 22 February 2002; accepted 25 March 2002; published 15 October 2002. [1] Anomalous conditions exist in the salinity, oxygen, and nutrient fields over the western flank of the Northern Hemisphere Mid-Atlantic Ridge. We examine possible advective sources for this anomaly, but determine that vertical mixing is the most likely cause. We proceed to use knowledge gained from the Brazil Basin Tracer Release Experiment in the South Atlantic (where microstructure and fine structure were obtained to explore the intensity, spatial distribution, and mechanisms of mixing in the deep ocean) to interpret density fine structure from common conductivity-temperature-depth data in the North Atlantic. These data support the hypothesis that the anomalous hydrographic conditions are associated with enhanced levels of vertical mixing. The inferred levels of vertical diffusivity over the Northern Hemisphere Mid-Atlantic Ridge are as high as in the South Atlantic: 1–10 � 10 � 4 m 2 /s. INDEX TERMS: 4532 Oceanography: Physical: General circulation; 4536 Oceanography: Physical: Hydrography; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: diapycnal mixing, fine structure, recirculation, Mid-Atlantic Ridge (MAR), hydrography, silicate

The Finescale Response of Lowered ADCP Velocity Profiles

Lowered acoustic Doppler current profiler (LADCP) velocity profiles are compared with simultaneous higherresolution expendable current profiler (XCP) profiles to determine the lowered ADCP’s response at short wavelengths. Although lowered ADCP spectra are attenuated in comparison to XCP spectra for vertical wavelengths as large as 150 m, the signals are coherent for wavelengths between 50 and 1200 m. A model spectral transfer function based on the expected response for the lowered ADCP reproduces the observed attenuation. Spectrally corrected LADCP data can be used to infer turbulent eddy diffusivities to within a factor of 3‐4 using a finescale parameterization.