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Tidally generated internal waves
Stockholm University, Faculty of Science, Department of Meteorology .
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the internal tide in the deep ocean, which is generated by the barotropic tide flowing over the bottom topography. The energy flux from the barotropic tide to the internal-wave field at the bottom is calculated using a method based on linear-wave theory and the traditional WKB approximation valid for a slowing varying vertical stratification. The global distribution of the baroclinic plus barotropic tidal velocities near the bottom is thus obtained, which is then used to analyzed the deep-sea sediment resuspension. The calculated energy flux of the internal tide is then compared with the energy dissipation rate obtained from different data sets of microstructure measurements conducted in several regions of the world ocean. A good correlation is generally found between the model estimates and observations, giving us some confidence that the theory reasonably well predict internal tide generation. It is also found that the ratio of the averaged energy dissipation rate to the averaged energy flux is very different in different regions. A direct global calculation of the energy flux is done by projecting the internal tides onto vertical eigenmodes, so that the vertical density profile and the finite ocean depth are taken into account in a fully consistent way. The results of the modal energy flux is important for understanding the pathway from generation to dissipation of the internal tides, since the low-mode internal tides are less affected by local nonlinear processes responsible for degrading their energy to small-scale mixing. The agreement between this detailed method and the WKB-based method is found to be high, while this methods provides new information on the vertical mode distribution of internal tide generation.Finally, the bottom-trapped internal tides, which are generated when the tidal frequency is smaller than the Coriolis frequency, is examined. The energy density associated with these waves is computed using linear wave theory and vertical normal-mode decomposition. An emphasis is placed on the bottom-trapped internal tides in the Arctic Ocean, as yet, there is a lack of the comprehensive understanding of the mixing processes in this basin. Through the development of new methods to estimate internal tide generation, this thesis provides a valuable information to the problem of the better understanding of tidal mixing in the deep ocean and its role on the large-scale ocean circulation, with a possible applications to the improvement of ocean general circulation model.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology , Stockholm Univeristy , 2014. , 53 p.
Keyword [en]
Tidally generated internal waves in the deep ocean
National Category
Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-103164ISBN: 978-91-7447-931-7 (print)OAI: oai:DiVA.org:su-103164DiVA: diva2:716001
Public defence
2014-06-10, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2014-05-19 Created: 2014-05-07 Last updated: 2014-05-09Bibliographically approved
List of papers
1. Deep-sea fluid and sediment dynamics-Influence of hill- to seamount-scale seafloor topography
Open this publication in new window or tab >>Deep-sea fluid and sediment dynamics-Influence of hill- to seamount-scale seafloor topography
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2013 (English)In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, Vol. 127, 203-241 p.Article in journal (Refereed) Published
Abstract [en]

Deep-sea sediments play a central role in a wide range of subject areas. A number of important controls on the formation of sedimentary deposits have been studied. However, to date, the impact of submarine landscape geometry as a possible control has received comparatively little attention. This seems to be particularly true for intermediate-scale topographic features such as abyssal hills, knolls and seamounts that can be found in many regions of the global seafloor: recent estimates suggest that in the deep open oceans, away from continental margins, there might be as many as similar to 25 x 10(6) abyssal hills, knolls and seamounts. Despite this large number very little is known about how they influence environmental complexity and patchiness, biogeochemical fluxes and the formation of sedimentary records. This paper reviews the currently known types of fluid-flow interactions with abyssal hills, knolls and seamounts that could potentially influence the way sediments are formed. The main types of relevant flow components are: quasi-steady to eddying background flow; internal lee and near-inertial waves; barotropic and baroclinic tides; and seamount-trapped waves. Previous studies looking into systematic links between fluid dynamics and sediments at hills, knolls and seamounts are reviewed. Finally, a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid-flow components leaves a detectable imprint in the recent sediments on and around a short seamount. The main conclusions and implications are as follows. (1) Topographically generated flow-field geometries that are composed of a number of different prevailing fluid-flow components can be reflected and detected in properties of the underlying sediments. (2) Tidal and other higher-frequency (lee-wave, near-inertial) components of deep-ocean currents can be essential for locally driving total current velocities across threshold values for non-deposition/erosion/resuspension of freshly deposited deep-sea sediments. Moreover, there is evidence suggesting that not only maximum current speeds but also intensities of higher-frequency (tidal and/or (near-)inertial) current-direction variability might control sediment dynamics and sediment formation. This relativises the view that current speed is the main, or even only, controlling factor for sediment dynamics and sediment formation. (3) When it comes to the reconstruction of paleo-flows, these findings imply that certain sedimentary records may well reveal more about variability in the higher-frequency flow components than about variability in the basin-scale net flow component that often is the focus of paleoceanographic studies. (4) Single-core paleo-records from hill-, seamount- or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features. To arrive at unbiased paleo-records for non-fluid-dynamic parameters, the influence of the flow-field geometry would have to be removed from the record first (5) It seems the mechanistic understanding of hill- and seamount-related flow/topography interactions and their links to sediment dynamics is approaching a level that may (a) facilitate improved interpretation of topographically controlled sedimentary paleo-records, (b) help fill in the knowledge gap that exists for functional deep-sea biodiversity at intermediate space scales, and (c) improve predictive capabilities for exploration of economically relevant iron-manganese (Fe-Mn) crusts on seamounts.

Keyword
Abyssal hill, Seamount, Sediment, Tides, Erosion, Non-deposition
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-100115 (URN)10.1016/j.earscirev.2013.10.005 (DOI)000328871100010 ()
Note

AuthorCount:6;

Available from: 2014-01-28 Created: 2014-01-27 Last updated: 2017-12-06Bibliographically approved
2. Comparison of calculated energy flux of internal tides with microstructure measurements
Open this publication in new window or tab >>Comparison of calculated energy flux of internal tides with microstructure measurements
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(English)Manuscript (preprint) (Other academic)
National Category
Oceanography, Hydrology, Water Resources
Identifiers
urn:nbn:se:su:diva-103157 (URN)
Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2014-05-07Bibliographically approved
3. Global calculation of tidal energy conversion rate into vertical normal modes
Open this publication in new window or tab >>Global calculation of tidal energy conversion rate into vertical normal modes
(English)Manuscript (preprint) (Other academic)
National Category
Oceanography, Hydrology, Water Resources
Identifiers
urn:nbn:se:su:diva-103159 (URN)
Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2014-05-07Bibliographically approved
4. On the generation of bottom-trapped internal tides
Open this publication in new window or tab >>On the generation of bottom-trapped internal tides
(English)Manuscript (preprint) (Other academic)
National Category
Oceanography, Hydrology, Water Resources
Identifiers
urn:nbn:se:su:diva-103160 (URN)
Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2014-05-07Bibliographically approved

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