Cross-scale interactions and turbulent cascades in the North Atlantic Ocean

• Main Author: Ajayi, Adekunle
• Other Authors: Institut des Géosciences de l’Environnement (IGE), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP ), Université Grenoble Alpes (UGA), Université Grenoble Alpes 2020-., Julien Le Sommer
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2020
• Subjects: Turbulence; Submesoscales; Fine-Scales; Energy cascade; Swot; Sous-Échelles; Échelle fine; Cascade d'énergie; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; North Atlantic
• Online Access: https://theses.hal.science/tel-02861906; https://theses.hal.science/tel-02861906/document; https://theses.hal.science/tel-02861906/file/AJAYI_2020_archivage.pdf

The dynamics of the ocean at a scale < 100km (fine-scales) is currently not well known. This is due to the lack of sufficient observational datasets at this scale-range in the ocean. There are suggestions from recent studies that classes of motions at this scale-range impacts the distribution and exchanges of kinetic energy in the ocean and that balanced submesoscale motions (<50km) and unbalanced internal gravity waves can play an active role in fluxing kinetic energy towards dissipative scales in the ocean. To better understand fine-scale motions, the Surface Water and Ocean Topography (SWOT) satellite with the task of providing an unprecedented view of the ocean down to a wavelength of 10-15km is been assembled and expected for launch in 2021. In anticipation of SWOT, numerical ocean models capable of resolving fine-scales has been designed and implemented. In this study, we use three of these simulations to investigate (i) the spatial and temporal variability of oceanic eddies at fine-scales down to 10km scale, (ii) cross-scale kinetic energy exchanges at fine-scales in a regime of active submesoscale motions and (iii) in a regime of externally forced internal tides. Our results show that the distribution of oceanic eddies at spatial scale < 100km undergo strong seasonality and that this seasonality is as a result of an increased population of submesoscales eddies (10 - 50km) in wintertime. We found that submesoscale turbulence (a class of oceanic turbulence at fine-scale) is responsible for the increase in the distribution of submesoscales eddies in winter. Further analysis showed that submesoscale turbulence also affects the kinetic energy cascade by providing a route towards dissipation both at the surface and in the interior of the ocean. Interestingly, this cascade in the presence of externally forced internal tides is increased by a factor of 3 in summertime due to enhanced wave activity by tidal motions. Our analysis also shows that not accounting for the ageostrophic flows in the calculation ...