Small-scale ocean dynamics in the Cape Basin and its impact on the regional circulation

• Main Author: Capuano, Tonia Astrid
• Other Authors: Laboratoire d'Océanographie Physique et Spatiale (LOPS), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Université de Bretagne occidentale - Brest, Xavier Carton, Sabrina Speich
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2017
• Subjects: Meso- and submesoscale processes; Indo-Atlantic exchange; Water masses dynamics; Processus de méso- et sous-méso- échelle; Échange Indo-Atlantique; Dynamiques des masses d’eau; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Antarctic; Antarc*
• Online Access: https://theses.hal.science/tel-01760721; https://theses.hal.science/tel-01760721/document; https://theses.hal.science/tel-01760721/file/These-2017-SML-Oceanographie_physique-CAPUANO_Tonia_Astrid.pdf

This study addresses the role of oceanic small-scale processes in the formation and transformation of subsurface waters that participate in the Indo-Atlantic interocean exchange.We focus on the Cape Basin dynamics, characterized by a highly non-linear turbulence.We provide qualitative and quantitative evidence of the direct impact that meso- and submesoscale structures, their dynamical interactions and their seasonal variability have on the local thermocline and intermediate waters. A sequence of numerical simulations, ranging from ’eddy-permitting’ to ’submesoscale resolving’, underlines the importance of an adequate vertical resolution to correctly depict the water masses properties.We point out that Agulhas eddies are mainly generated through baroclinic instabilities and are marked by a clear seasonality. This is linked to the seasonal occurrence of distinct meso-submesoscale instabilities in the upper layers: symmetric instabilities are at play during summer, while mixed-layer instabilities prevail in winter.We also found that Charney baroclinic instability connects these two submesoscale regimes and plays a major role in the seasonal formation of a newly-identified type of mode waters: Agulhas Rings Mode Water. Finally, we show that eddies of both polarity advect, stir and mix Antarctic Intermediate Water, via the mesoscale strain field producing filaments and T-S fine-scale structures.Our results suggest the existence of two dynamical regimes affecting the upper and intermediate layers of the Cape Basin. Near the surface, the submesoscale-driven frontogenesis and their enhanced energetics lead to a predominance of ageostrophic dynamics. The intermediate depths are, instead, characterised by a quasigesotrophic regime due to the prevailing mesoscale effects. Nous étudions le rôle des processus océaniques à petite échelle dans la formation et transformation des eaux de surface et intermédiaires qui participent à l’échange Indo- Atlantique à travers le Bassin du Cap. La dynamique de cette région est ...