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Mixing processes in dense overflows with emphasis on the Faroe Bank Channel overflow
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The amount and distribution of mixing and entrainment that the overflows across the Greenland-Scotland Ridge encounter influence the ventilation of the deep North Atlantic. Constituting about 30% of the total overflow (about 6 Sv) across the Greenland-Scotland Ridge, the continuous, swift overflow through the deepest passage from the Nordic Seas to the North Atlantic Ocean, the Faroe Bank Channel, is a major overflow in the region.

The mixing processes of the Faroe Bank Channel overflow are explored by combining results from observations, including the first direct turbulence measurements, numerical simulations of the overflow, and an idealized process study. The observations show an overflow characterized by strong lateral variability in entrainment and mixing, a transverse circulation actively diluting the bottom layer, and a pronounced vertical structure composed of an about 100m thick stratified interface and a comparably thick well-mixed bottom layer. The turbulent overflow is associated with intense mixing and enhanced turbulent dissipation rate near the bottom and at the plumeambient interface, but with a quiescent core.

Results from numerical simulations of the overflow with second order turbulence closures are compared to the observations. Turbulent dissipation rate and eddy diffusivity profiles inferred from the observations are used in refining the parameters of the turbulence closure. In the bottom-most 50-60 m, where the Richardson number is small and the production of turbulent kinetic energy is well-resolved, the model reproduces the observed vertical structure of enhanced dissipation rate and eddy diffusivity exceptionally well. In the interfacial layer and above the plume-ambient interface, however, the model does not resolve the mixing. A further investigation of the observations, addressing the role of the transverse circulation and internal waves in mixing in the stratified interface, shows that the transverse circulation effectively contributes to mixing of the overflow plume. Dissipation rates are more than doubled in the interfacial layer due to the transverse flow. In the ambient above the overflow plume, internal wave breaking is the dominant mechanism for dissipation of turbulent energy. In the interfacial  layer the main mechanism of mixing is the shear-instability and entrainment associated with the swift gravity current, enhanced by the secondary circulation. However, the internal wave continuum is energetic in the interfacial layer and may contribute to mixing.

To investigate the influence of unresolved small scale topography on the flow of a stratified fluid, a 2-m resolution, non-hydrostatic, three-dimensional numerical model is used. The drag and associated mixing on the stratified flow over real, 1-m resolution, complex topography (interpolated to model resolution) are studied. The results show that a significant drag can be exerted on the flow of a stratified layer overlaying a well-mixed layer (resembling the bottom and interfacial layer of the Faroe Bank Channel overflow) over rough topography. A parameterization of the internal wave drag is developed and implemented, and provides satisfactory results in terms of the domain integrated turbulent kinetic energy levels.

Place, publisher, year, edition, pages
NTNU, 2011.
Series
Doctoral theses at NTNU, ISSN 1503-8181 ; 2011:35
Identifiers
URN: urn:nbn:no:ntnu:diva-12328ISBN: 978-82-471-2590-8 (printed ver.)ISBN: 978-82-471-2591-5 (electronic ver.)OAI: oai:DiVA.org:ntnu-12328DiVA: diva2:406640
Public defence
2011-02-10, 00:00
Available from: 2011-03-28 Created: 2011-03-24 Last updated: 2011-03-28Bibliographically approved
List of papers
1. Intense mixing of the Faroe Bank Channel overflow
Open this publication in new window or tab >>Intense mixing of the Faroe Bank Channel overflow
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2010 (English)In: Geophysical Research Letters, ISSN 0094-8276, Vol. 37, L02604- p.Article in journal (Refereed) Published
Abstract [en]

The continuous, swift flow of cold water across the sill of the Faroe Bank Channel, the deepest passage from the Nordic Seas to the North Atlantic Ocean, forms a bottom-attached dense plume (overflow). The amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep North Atlantic, however, remain poorly known due to lack of direct measurements. Using the first direct turbulence measurements, we describe the dynamic properties and mixing of the overflow plume as it descends toward the Iceland Basin. The vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume-ambient interface, but with a quiescent core. Our measurements show a pronounced transverse circulation consistent with rotating plume dynamics, a strong lateral variability in entrainment velocity, and a vertical structure composed of order 100 m thick stratified interface and comparably thick well-mixed bottom boundary layer with significant transport and entrainment. Citation: Fer, I., G. Voet, K. S. Seim, B. Rudels, and K. Latarius (2010), Intense mixing of the Faroe Bank Channel overflow, Geophys. Res. Lett., 37, L02604, doi:10.1029/2009GL041924.

Identifiers
urn:nbn:no:ntnu:diva-12323 (URN)10.1029/2009GL041924 (DOI)000274126300008 ()
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-28Bibliographically approved
2. Regional simulations of the Faroe Bank Channel overflow using a sigma-coordinate ocean model
Open this publication in new window or tab >>Regional simulations of the Faroe Bank Channel overflow using a sigma-coordinate ocean model
2010 (English)In: Ocean Modelling, ISSN 1463-5003, Vol. 35, no 1-2, 31-44 p.Article in journal (Refereed) Published
Abstract [en]

Gravity-driven overflow of cold, deep and intermediate water from the Norwegian Sea through the Faroe Bank Channel carries significant volume flux and contributes to the ventilation of the deep North Atlantic Ocean. Here we present results from regional simulations of the Faroe Bank Channel overflow using a terrain-following (sigma-coordinate) ocean model with fine resolution near the sea bed. The model results are compared with observations of hydrography, currents and turbulence conducted in 2008. Turbulent dissipation rate and eddy diffusivity profiles inferred from the observations are used in refining the parameters of the turbulence closure. The model reproduces the observed vertical structure of the enhanced dissipation and diffusivity in the bottommost 50-60 m exceptionally well. In this region, shear-induced mixing dominates and is found to be well-represented by the applied second order turbulence closure models. Farther away from the boundary, however, in the 100-m thick interfacial layer and above the plume-ambient interface, the model does not resolve the observed mixing. The contribution of turbulence from breaking internal waves is one of the processes not represented in the model with significant consequences for observed entrainment and mixing. Regular sub-inertial oscillations (eddies) at 4-4.5 day period develop downstream of the sill, consistent with the observations. When averaged over several eddy events, the evolution of section-averaged plume properties over the oscillation period shows that the eddies significantly affect mixing and the descent rate of the plume. At a section 60 km downstream of the sill, eddies lead to periodic and abrupt cross-isobath descent of the overflow plume and an increase in dissipation rate by one order of magnitude. (C) 2010 Elsevier Ltd. All rights reserved.

Keyword
Dense overflow, Gravity current, Entrainment and mixing, sigma-level model
Identifiers
urn:nbn:no:ntnu:diva-12324 (URN)10.1016/j.ocemod.2010.06.002 (DOI)000281472700003 ()
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-28Bibliographically approved
3. Mixing in the stratified interface of the Faroe Bank Channel overflow: the role of transverse circulationand internal waves
Open this publication in new window or tab >>Mixing in the stratified interface of the Faroe Bank Channel overflow: the role of transverse circulationand internal waves
2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, Vol. 116, no C07022, 1-14 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:no:ntnu:diva-12326 (URN)10.1029/2010JC006805 (DOI)
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2012-01-17Bibliographically approved
4. Stratified flow over complex topography
Open this publication in new window or tab >>Stratified flow over complex topography
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:no:ntnu:diva-12327 (URN)
Available from: 2011-03-24 Created: 2011-03-24 Last updated: 2011-03-28Bibliographically approved

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