scholarly journals Internal Waves and Turbulence in the Antarctic Circumpolar Current

2013 ◽  
Vol 43 (2) ◽  
pp. 259-282 ◽  
Author(s):  
Stephanie Waterman ◽  
Alberto C. Naveira Garabato ◽  
Kurt L. Polzin
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Antarctic Circumpolar Current ◽  
Wave Radiation ◽  
Turbulent Dissipation ◽  
Internal Wave Field ◽  
Diapycnal Diffusivity ◽  
Circumpolar Current ◽  
The Antarctic

Abstract This 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 interior dissipation is related to a breaking internal wave field. Elevated turbulence occurs in association with downward-propagating near-inertial waves within 1–2 km of the surface, as well as with upward-propagating, relatively high-frequency waves within 1–2 km of the seafloor. While an interpretation of these near-bottom waves as lee waves generated by ACC jets flowing over small-scale topographic roughness is supported by the qualitative match between the spatial patterns in predicted lee wave radiation and observed near-bottom dissipation, the observed dissipation is found to be only a small percentage of the energy flux predicted by theory. The mismatch suggests an alternative fate to local dissipation for a significant fraction of the radiated energy.

scholarly journals Suppression of Internal Wave Breaking in the Antarctic Circumpolar Current near Topography

2014 ◽  
Vol 44 (5) ◽  
pp. 1466-1492 ◽  
Author(s):  
Stephanie Waterman ◽  
Kurt L. Polzin ◽  
Alberto C. Naveira Garabato ◽  
Katy L. Sheen ◽  
Alexander Forryan
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Antarctic Circumpolar Current ◽  
Energy Dissipation Rate ◽  
Background Flow ◽  
Energy Propagation ◽  
Topographic Roughness ◽  
Flow Speeds ◽  
Circumpolar Current ◽  
The Antarctic

Abstract Simultaneous full-depth microstructure measurements of turbulence and finestructure measurements of velocity and density are analyzed to investigate the relationship between turbulence and the internal wave field in the Antarctic Circumpolar Current. These data reveal a systematic near-bottom overprediction of the turbulent kinetic energy dissipation rate by finescale parameterization methods in select locations. Sites of near-bottom overprediction are typically characterized by large near-bottom flow speeds and elevated topographic roughness. Further, lower-than-average shear-to-strain ratios indicative of a less near-inertial wave field, rotary spectra suggesting a predominance of upward internal wave energy propagation, and enhanced narrowband variance at vertical wavelengths on the order of 100 m are found at these locations. Finally, finescale overprediction is typically associated with elevated Froude numbers based on the near-bottom shear of the background flow, and a background flow with a systematic backing tendency. Agreement of microstructure- and finestructure-based estimates within the expected uncertainty of the parameterization away from these special sites, the reproducibility of the overprediction signal across various parameterization implementations, and an absence of indications of atypical instrument noise at sites of parameterization overprediction, all suggest that physics not encapsulated by the parameterization play a role in the fate of bottom-generated waves at these locations. Several plausible underpinning mechanisms based on the limited available evidence are discussed that offer guidance for future studies.

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

2014 ◽  
Vol 44 (7) ◽  
pp. 1854-1872 ◽  
Author(s):  
Amy F. Waterhouse ◽  
Jennifer A. MacKinnon ◽  
Jonathan D. Nash ◽  
Matthew H. Alford ◽  
Eric Kunze ◽  
...  
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Wave Generation ◽  
Wave Power ◽  
Turbulent Dissipation ◽  
Turbulent Dissipation Rate ◽  
Diapycnal Diffusivity ◽  
Internal Wave Generation ◽  
O 10

Abstract 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 mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles 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(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. 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 variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Quantifying mixing from standard observations: revisiting finescale parameterization in the Arctic Ocean

2021 ◽  
Author(s):  
Till Baumann ◽  
Ilker Fer ◽  
Kirstin Schulz ◽  
Volker Mohrholz ◽  
Janin Schaffer ◽  
...  
Keyword(s):  
Energy Dissipation ◽  
Internal Wave ◽  
Arctic Ocean ◽  
Wave Field ◽  
Turbulent Mixing ◽  
Dissipation Rate ◽  
The Arctic ◽  
Depth Range ◽  
The Arctic Ocean ◽  
Internal Wave Field

<p>Ocean mixing governs the vertical exchange of matter, heat and salt in the water column. In the Arctic Ocean, the vertical transport of heat due to turbulent mixing is ultimately coupled to the sea-ice cover, with immediate and far-reaching impacts on the climate and ecosystem. A detailed understanding and quantification of turbulent mixing is crucial to assess and predict the state of the changing Arctic Ocean. However, direct observations of turbulent mixing are complicated, expensive and sparse.</p><p>Finescale parameterization of turbulent energy dissipation allows for the quantification of mixing based on standard hydrographic observations such as velocity and density profiles. This method is based on the assumption that energy dissipation is achieved exclusively by cascading energy from large, observable scales to small scales by wave-to-wave interactions in the internal wave field, which in turn can be related to vertical diffusivity and hence turbulent fluxes. While the finescale parameterization is proved to be reliable at mid-latitudes, the Arctic Ocean internal wave field is distinct from the canonical mid-latitude spectrum and the applicability of the parameterization is not certain. Furthermore, in the historically quiescent Arctic, the application of finescale parameterization suffers from a generally low signal to noise ratio and processes violating the assumptions in the parameterization, such as double diffusion.  During the year-long MOSAiC expedition, both standard observations as well as specialized microstructure measurements were carried out continuously. We analyse dissipation rate and stratification measurements (from an MSS90L profiler) and 8-m vertical resolution current measurements (from a 75 kHz RDI acoustic Doppler current profiler) in the depth range from 70 -198 m, in the absence of thermohaline staircases or double-diffusive intrusions. Although the range of dissipation measurements is limited and spans 1e<sup>-11</sup> W kg<sup>-1</sup> to 8.8e<sup>-7</sup> W kg<sup>-1</sup>, direct comparisons between in-situ observations of dissipation rate and finescale parameterization provide a detailed insight into the capabilities and limitations of this method in various meteorological, oceanographic and geographic conditions. The aim is to provide guidance in how far standard oceanographic observations may be utilized to quantify mixing in past, current and future states of the Arctic Ocean.</p>

scholarly journals Diapycnal Mixing in the Antarctic Circumpolar Current

2011 ◽  
Vol 41 (1) ◽  
pp. 241-246 ◽  
Author(s):  
J. R. Ledwell ◽  
L. C. St. Laurent ◽  
J. B. Girton ◽  
J. M. Toole
Keyword(s):  
Antarctic Circumpolar Current ◽  
Austral Summer ◽  
Drake Passage ◽  
Meridional Overturning Circulation ◽  
Velocity Shear ◽  
Reference Spectrum ◽  
Diapycnal Mixing ◽  
Diapycnal Diffusivity ◽  
Circumpolar Current ◽  
The Antarctic

Abstract The vertical dispersion of a tracer released on a density surface near 1500-m depth in the Antarctic Circumpolar Current west of Drake Passage indicates that the diapycnal diffusivity, averaged over 1 yr and over tens of thousands of square kilometers, is (1.3 ± 0.2) × 10−5 m2 s−1. Diapycnal diffusivity estimated from turbulent kinetic energy dissipation measurements about the area occupied by the tracer in austral summer 2010 was somewhat less, but still within a factor of 2, at (0.75 ± 0.07) × 10−5 m2 s−1. Turbulent diapycnal mixing of this intensity is characteristic of the midlatitude ocean interior, where the energy for mixing is believed to derive from internal wave breaking. Indeed, despite the frequent and intense atmospheric forcing experienced by the Southern Ocean, the amplitude of finescale velocity shear sampled about the tracer was similar to background amplitudes in the midlatitude ocean, with levels elevated to only 20%–50% above the Garrett–Munk reference spectrum. These results add to a long line of evidence that diapycnal mixing in the interior middepth ocean is weak and is likely too small to dictate the middepth meridional overturning circulation of the ocean.

scholarly journals Dissipation Rate Estimates from Microstructure and Finescale Internal Wave Observations along the A25 Greenland–Portugal OVIDE Line

2014 ◽  
Vol 31 (11) ◽  
pp. 2530-2543 ◽  
Author(s):  
Bruno Ferron ◽  
Florian Kokoszka ◽  
Herlé Mercier ◽  
Pascale Lherminier
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Correction Term ◽  
Thick Layer ◽  
Vertical Shear ◽  
Buoyancy Frequency ◽  
Reykjanes Ridge ◽  
Dissipation Rates ◽  
Internal Wave Field

AbstractA total of 96 finestructure and 30 microstructure full-depth vertical profiles were collected along the A25 Greenland–Portugal Observatoire de la Variabilité Interannuelle et Décennale en Atlantique Nord (OVIDE) hydrographic line in 2008. The microstructure of the horizontal velocity was used to calculate turbulent kinetic energy dissipation rates εvmp, where vmp refers to the vertical microstructure profiler. The lowest dissipation values (εvmp < 0.5 × 10−10 W kg−1) are found below 2000 m in the Iberian Abyssal Plain and in the center of the Irminger basin; the largest values (>5 × 10−10 W kg−1) are found in the main thermocline, around the Reykjanes Ridge, and in a 1000-m-thick layer above the bottom near 48°N. The finestructure of density was used to estimate isopycnal strain and that of the lowered acoustic Doppler current profiler to estimate the vertical shear of horizontal velocities. Strain and shear were used to estimate dissipation rates εG03 (Gregg et al.) associated with the internal wave field. The shear-to-strain ratio correction term of the finescale parameterization εG03 brings the fine- and microscale estimates of the dissipation rate into better agreement as Polzin et al. found. The latitude/buoyancy frequency term slightly improves the parameterization for weakly stratified waters. Correction term εG03 is consistent with εvmp within a factor of 4.5 over 95% of the profiles. This good consistency suggests that most of the turbulent activity recorded in this dataset is due to the internal wave field. The canonical globally averaged diffusivity value of order 10−4 m2 s−1 needed to maintain the global abyssal stratification (Munk) is only reached on the flank of the Reykjanes Ridge and in the region around 48°N.

Influence of the Distortion of Vertical Wavenumber Spectra on Estimates of Turbulent Dissipation using the Finescale Parameterization: Eikonal Calculations

2021 ◽  
Author(s):  
ANNE TAKAHASHI ◽  
TOSHIYUKI HIBIYA ◽  
ALBERTO C. NAVEIRA GARABATO
Keyword(s):  
Internal Wave ◽  
Wave Energy ◽  
Mesoscale Eddy ◽  
Wave Interaction ◽  
Interaction Theory ◽  
Turbulent Dissipation ◽  
Energy Cascades ◽  
Circumpolar Current ◽  
Noise Contamination ◽  
The Antarctic

AbstractThe finescale parameterization, formulated on the basis of a weak nonlinear wave–wave interaction theory, is widely used to estimate the turbulent dissipation rate, ε. However, this parameterization has previously been found to overestimate ε in the Antarctic Circumpolar Current (ACC) region. One possible reason for this overestimation is that vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett-Munk spectrum and have a spectral “hump” at low vertical wavenumbers. Such distorted vertical wavenumber spectra were also observed in other mesoscale eddy-rich regions. In this study, using eikonal simulations, in which internal wave energy cascades are evaluated in the frequency-wavenumber space, we examine how the distortion of vertical wavenumber spectra impacts on the accuracy of the finescale parameterization. It is shown that the finescale parameterization overestimates ε for distorted spectra with a low-vertical-wavenumber hump because it incorrectly takes into account the breaking of these low-vertical-wavenumber internal waves. This issue is exacerbated by estimating internal wave energy spectral levels from the low-wavenumber band rather than from the high-wavenumber band, which is often contaminated by noise in observations. Thus, in order to accurately estimate the distribution of ε in eddy-rich regions like the ACC, high-vertical-wavenumber spectral information free from noise contamination is indispensable.

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