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The Tides in a general circulation model in the Indonesian Seas

Abstract : In the Indonesian seas, large tidal currents interact with the rough topography and create strong internal waves at the tidal frequency, called internal tides. Part of them will eventually propagate and dissipate far away from generation sites. Their associated mixing upwells cold and nutrient-rich water that prove to be critical for climate system and for marine resources. This thesis uses the physical ocean general circulation model, NEMO, as part of the INDESO project that aims at monitoring the Indonesian marine living resources. Models not taking into account tidal missing are unable to correctly reproduce the vertical structure of watermasses in Indonesian seas. However, taking into account this mixing is no simple task as the phenomena involved in tidal mixing cover a wide spectrum of spatial scales. Internal tides indeed propagate over thousands of kilometres while dissipation and mixing occurs at centimetric to millimetric scales. A model capable of resolving all these processes at the same time does not exist. Until now scientists either parameterised the tidal mixing or used models which only partly resolve internal tides. More and more scientists introduce explicit tidal forcing in their models but without knowing where the energy is going and how the internal tides are dissipated. This thesis intends to quantify energy dissipation in NEMO forced with explicit tidal forcing and compares it to the dissipation induced by the currently used parameterization. This thesis also provides new results about the quantification of the tidal energy budget in NEMO. I first contributed to an INDESO study that aimed at validating the model against several observation data sets. In a second and third study, I investigated the mixing produced in the model by explicit tidal forcing and its impact on water mass. Explicit tides forcing proves to produce a mixing comparable to the one produced by the parameterization. It also produces a significant cooling of 0.3 °C with maxima reaching 0.8°C in the areas of internal tide generation. The cooling is stronger on austral winter. The spring tides and neap tides modulate this impact by 0.1°C to 0.3°C. The model generates 75% of the expected internal tides energy, in good agreement with other previous studies. In the ocean interior, most of it is dissipated by horizontal momentum dissipation (19 GW), while in reality one would expect dissipation through vertical possesses. This value is close to the dissipation induced by the parameterization (16 GW). The mixing is strong over generation sites, and only 20% remains for far field dissipation mainly in the Banda and Sulawesi Seas. The model and the recent INDOMIX cruise [Koch-Larrouy et al. (2015)], which provided direct estimates of the mixing, are surprisingly in good agreement mainly above straits. However, in regions far away from the energy generation sites where INDOMIX found NO evidence of intensified mixing, the model produces too strong mixing. The bias comes from the lack of specific set up of internal tides in the model. More work is thus needed to improve the modeled dissipation, which is a theme of active research for the scientific community. I dedicated the last part of my thesis to the quantification of tidal energy sinks in NEMO. I first worked on a simple academic case: the COMODO internal tides test case, which analyses the behaviour of a vertically stratified fluid forced by a barotropic flow interacting over an idealized abyssal plain/slope/shelf topography without bottom friction. The results of the finite element T-UGOm hydrodynamic model are compared with those of NEMO. The central issue in calculating tidal energy budget is the separation of barotropic and baroclinic precesses. To this aim we developed an original method based on the projection on vertical modes. At first glance, this method compares well with the classical method of separation using the vertically averaged current. However, when looking into more details at energy budgets, vertical modes allow a cleaner and more realistic separation between barotropic and baroclinic tides. This precision will be very useful for the future SWOT mission. Also this method of separation allows quantification of the energy associated with each mode. This allowed us to identify an important bias in NEMO: The higher the mode is the shorter the NEMO wavelengths become compared to the T-UGOm wavelengths. NEMO also produces a strong numerical mixing that erodes the barotropic tides on the abyssal plain, where there is no bottom friction. The best suspect might be the 2D/3D coupling scheme implied by the NEMO’s time splitting. This work pinpoints areas for reflection and investigation to improve how the model takes into account internal tides dissipation. Based on our separation method, work is in progress to better quantify tidal energy budget in the Indonesian Seas.
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Contributor : Williams Exbrayat <>
Submitted on : Friday, July 7, 2017 - 9:46:33 AM
Last modification on : Monday, April 5, 2021 - 2:26:10 PM
Long-term archiving on: : Friday, January 26, 2018 - 9:20:33 PM


  • HAL Id : tel-01556796, version 2



Dwiyoga Nugroho. The Tides in a general circulation model in the Indonesian Seas . Ocean, Atmosphere. Universite Toulouse 3 Paul Sabatier (UT3 Paul Sabatier), 2017. English. ⟨tel-01556796v2⟩



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