Stratification structure in subtropical gyres and its decadal variability in the North Atlantic Ocean

• Main Author: Feucher, Charlène
• 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, Herlé Mercier, Guillaume Maze
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2016
• Subjects: Permanent pycnocline; Stratification; Heat content; Decadal variability; Argo; Ocean reanalyses; Subtropical gyres; North Atlantic; Pycnocline permanente; Contenu de chaleur; Variabilité décennale; Réanalyses océaniques; Gyres subtropicaux; Atlantique Nord; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere
• Online Access: https://theses.hal.science/tel-02054666; https://theses.hal.science/tel-02054666v2/document; https://theses.hal.science/tel-02054666v2/file/These-2016-EDSM-Oceanographie_physique-FEUCHER_Charlene.pdf

Subtropical gyres are central to the observed climate changes throughout the last decades. It is observed between the surface and the permanent pycnocline an intense increase in the ocean heat content. The permanent pycnocline delineates thus an important heat reservoir. The permanent pycnocline has a major role in preventing heat to reach the deep ocean and it thus of a relative importance in the context of climate change. For the first time and thanks to the development of the Argo array, we have been able to characterize the observed structure of the permanent pycnocline. The objective of this PhD thesis is to investigate the structure of the permanent pycnocline and its variability over the last decades. We developed an objective method to characterize the properties of the permanent pycnocline. This method has been first applied to the North Atlantic Ocean with Argo data and then to the global ocean. A complex structure of the permanent pycnocline emerges with strong differences from one gyre to another. The permanent pycnocline is found to be the deepest and the thickest in the North Atlantic subtropical gyre. It implies that the North Atlantic subtropical gyre is the largest heat reservoir on Earth. Then, ocean reanalyses have been used to investigate the changes in the permanent pycnocline properties in the North Atlantic subtropical gyre. Over the last decades, there is a strong warming of the upper ocean, especially in the North Atlantic subtropical gyre. The warming in the ocean is dominated by the heaving of isopycnal surfaces. This heaving strongly affects the depths of isopycnals and the stratification. This in turn affects the properties of the permanent pycnocline, especially its depth and potential density. Les gyres subtropicaux sont au coeur des changements observés au cours des dernières décennies. On y observe entre la surface et la pycnocline permanente une augmentation du contenu thermique de l’océan. La pycnocline permanente délimite un important réservoir de chaleur et joue un rôle ...