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Spiritum sidus: The star of our life – On internal tides in the ocean
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-2866-8729
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tidal currents flowing over rough bathymetry generate internal tides. These internal waves with tidal frequency can be decomposed into vertical modes. Low modes generally travel thousands of kilometers, until they break due to shear flow instabilities, while high modes are believed to break close to the generation site. The power released by breaking internal tides is thought not only to shape the overturning circulation, but also to mix the upper ocean. Both processes have a large influence on the climate system, most notably for their key role in regulating the heat and carbon uptake by the ocean.

The generation of internal tides, or tidal conversion, can be calculated from the bottom topography, the ocean stratification and the tidal currents. Global computations of the tidal conversion have been based on linear wave theory. However, such linear calculations are only valid if the seafloor slope is subcritical, and it is not known how to treat supercritical slopes. This is especially true for the conversion decomposed into vertical modes, which, taken individually, behaves very differently from the total conversion (the sum of the contributions from all modes).

In the first paper of this thesis, we looked into the validity of linear theory in the supercritical limit. Specifically, we translated the critical slope condition, a notion defined for the superposition of all modes, into a mode-wise condition on the topographic height. The findings were applied to estimates of the global M2-tide conversion into the first 10 vertical modes (in the open ocean, excluding the continental shelves and slopes). The results unveil the rapid increase with mode number of the oceanic area where linear theory fails. In terms of conversion, this shows that linear theory is unadapted to quantify the role played by high modes in closing the internal wave energy budget.

Typically, continental slopes are supercritical, and hence locations where the linear theory fails. Because of their characteristic shape, they are also an important source of low-mode internal tides. In the second paper of this thesis, we constructed a computationally inexpensive method to compute the tidal conversion by continental slopes and applied it at the global scale. It uses the usual observational data as inputs but relies on a reduced-physics numerical model rather than on linear theory to estimate the tidal conversion.

Unveiling the global pattern of the dissipation of internal tides (i.e. where they break) has been a challenging objective for a few decades. This can be explained by the lack of suitable observations to compare theory with. Until recently, the only observational data of internal tides with global coverage were based on satellite altimetry. However, only the part of the wave field that is exactly phase-locked to the astronomical forcing can be identified from altimetry data.

In the third paper of this thesis we created a new observational data set of internal tides, with global coverage, based on Argo park-phase data. These data are recorded while the floats are adrift at 1000 m depth, between two vertical profiling sequences. Thanks to the high sampling rate of Argo floats, the records capture the full amplitude of the waves, including the non-phase-locked part. This component turned out to be several times larger than previously thought. In the fourth paper of this thesis, we validated the internal tides in a realistic global ocean simulation with Argo data. Incidentally, this also worked to validate the Argo observations.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2024. , p. 26
Keywords [en]
Internal waves, Tides, Argo floats, Time series analysis, Flow-topography interaction
National Category
Oceanography, Hydrology and Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-227098ISBN: 978-91-8014-697-5 (print)ISBN: 978-91-8014-698-2 (electronic)OAI: oai:DiVA.org:su-227098DiVA, id: diva2:1842347
Public defence
2024-04-22, Vivi Täckholmssalen, NPQ-husen, Svante Arrhenius väg 20 A, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2024-03-26 Created: 2024-03-04 Last updated: 2024-03-08Bibliographically approved
List of papers
1. Tidal conversion into vertical normal modes by near-critical topography
Open this publication in new window or tab >>Tidal conversion into vertical normal modes by near-critical topography
(English)Manuscript (preprint) (Other academic)
Keywords
Internal waves, Tides, Flow-topography interaction
National Category
Oceanography, Hydrology and Water Resources
Research subject
Oceanography
Identifiers
urn:nbn:se:su:diva-227189 (URN)
Funder
Swedish Research Council, 2017-04623
Available from: 2024-03-04 Created: 2024-03-04 Last updated: 2024-03-06Bibliographically approved
2. Tidal conversion into vertical normal modes by continental slopes
Open this publication in new window or tab >>Tidal conversion into vertical normal modes by continental slopes
(English)Manuscript (preprint) (Other academic)
Keywords
Internal waves, Tides, Flow-topography interaction
National Category
Oceanography, Hydrology and Water Resources
Research subject
Oceanography
Identifiers
urn:nbn:se:su:diva-227190 (URN)
Funder
Swedish Research Council, 2017-04623
Available from: 2024-03-04 Created: 2024-03-04 Last updated: 2024-03-06Bibliographically approved
3. Global Mapping of the Nonstationary Semidiurnal Internal Tide Using Argo Data
Open this publication in new window or tab >>Global Mapping of the Nonstationary Semidiurnal Internal Tide Using Argo Data
2022 (English)In: Journal of Geophysical Research - Oceans, ISSN 2169-9275, E-ISSN 2169-9291, Vol. 127, no 4, article id e2021JC018283Article in journal (Refereed) Published
Abstract [en]

Data from Argo floats equipped with Iridium communications are used to obtain a global map of the total amplitude (or variance) of the semidiurnal internal tide at 1,000 dbar. The results are confirmed by a comparison with data from an historical collection of moored instruments. The obtained amplitude is in turn compared with the High-Resolution Empirical Tide (HRET) model, based on satellite altimetry. While HRET only contains the stationary component, with a fixed phase difference to the astronomical tide, the present results capture the total amplitude, including the nonstationary component. We estimate the global average ratio of total (Argo) to stationary (HRET) semidiurnal internal tide variance to be 6.5, and the amplitude ratio to be 3.6. Our estimate of the stationary fraction of the semidiurnal internal tide is subject to significant uncertainties. In particular, HRET is thought to mainly represent baroclinic mode-1 waves, while Argo data contain contributions from all modes.

Keywords
internal tides, internal waves, in situ measurements, remote sensing
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-203984 (URN)10.1029/2021JC018283 (DOI)000777613100001 ()
Available from: 2022-04-19 Created: 2022-04-19 Last updated: 2024-03-04Bibliographically approved
4. Validating the spatial variability in the semidiurnal internal tide in a realistic global ocean simulation with Argo and mooring data
Open this publication in new window or tab >>Validating the spatial variability in the semidiurnal internal tide in a realistic global ocean simulation with Argo and mooring data
Show others...
2023 (English)In: Ocean Science, ISSN 1812-0784, E-ISSN 1812-0792, Vol. 19, no 3, p. 811-835Article in journal (Refereed) Published
Abstract [en]

The autocovariance of the semidiurnal internal tide (IT) is examined in a 32 d segment of a global run of the HYbrid Coordinate Ocean Model (HYCOM). This numerical simulation, with 41 vertical layers and ∘ horizontal resolution, includes tidal and atmospheric forcing, allowing for the generation and propagation of ITs to take place within a realistic eddying general circulation. The HYCOM data are in turn compared with global observations of the IT around 1000 dbar, from Argo float park-phase data and mooring records. HYCOM is found to be globally biased low in terms of the IT variance and decay of the IT autocovariance over timescales shorter than 32 d. Except in the Southern Ocean, where limitations in the model cause the discrepancy with in situ measurements to grow poleward, the spatial correlation between the Argo and HYCOM tidal variance suggests that the generation of low-mode semidiurnal ITs is globally well captured by the model.

Keywords
Internal waves, Tides, Argo floats, Time series analysis, Flow-topography interaction
National Category
Oceanography, Hydrology and Water Resources
Research subject
Oceanography
Identifiers
urn:nbn:se:su:diva-227188 (URN)10.5194/os-19-811-2023 (DOI)001006713800001 ()2-s2.0-85163617207 (Scopus ID)
Funder
Swedish Research Council, 2017-04623Swedish Research Council, 2018-05973
Available from: 2024-03-04 Created: 2024-03-04 Last updated: 2024-03-06Bibliographically approved

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