The investigation of the role of basin-scale forcing on the circulation of the Faroe-Shetland Channel (FSC) is important to further understanding of the inter-annual variability of the Atlantic water (AW) fluxes in this region. The FSC plays a key role in the transfer of warm and saline AW towards the Nordic Seas that is an integral part of the Atlantic Meridional Overturning Circulation which is projected to decline over the twenty-first century and might reduce the oceanic heat and salt transports towards the Arctic. So far little attention has been paid to the mechanisms driving the AW fluxes in the FSC, reliable estimates of AW temperature and salt transports time series are lacking. This study presents a new time series of the AW fluxes based on the combination of hydrography and altimetry data. The mechanisms involved in driving the variability of AW fluxes are considered based on observational data and the output from a high-resolution ocean model (VIKING20). The hydrographic observations from 1993 to 2015 show an increase in temperature and salinity of AW. However, there is no evidence of trends in AW volume, temperature or salt transports during the observed period. This analysis confirms that the amount of heat and salt transported through the FSC is dominated by the volume transport. Moreover, this study identifies a bias in the standard deviation of the geostrophic velocity at a depth associated with referencing the geostrophic calculations to the sea surface geostrophic velocity from satellite altimetry. This finding does not strongly influence the AW volume transports in the AW layer, however, it has important implications for estimates of the geostrophic volume transport at depth. This study shows that the Ekman driven up/downwelling and the differential Ekman pumping mechanisms driven by the local wind forcing may influence sea surface height (SSH) and the displacement of isopycnals in the channel, leading to AW volume transport variabilit However, due to the large associated error bars on the surface and subsurface parameters, there is no clear evidence that these mechanisms are significantly responsible for the AW volume transport variability in the FSC. Lagrangian trajectories show evidence of two pathways from the North Atlantic to the FSC that may explain AW variability in the FSC: one pathway involves the flow of warm and saline waters from the Rockall Trough that corresponds to high temperatures and low AW volume transport in the channel, and the other pathway involves the flow of relatively cooler and less saline waters from the Iceland Basin that is linked to low temperatures and stronger volume transport in the FSC. Moreover, we show that the first (second) pathway is associated with the negative (positive) phases of the North Atlantic Oscillation (NAO) and the ocean gyre contraction (expansion). The changes of the NAO index phases explain 26 % of the AW volume transport variance in the FSC. Another important mechanism that leads to stronger (weaker) AW volume transport is stronger (weaker) pressure gradient across the Greenland-Scotland Ridge, reflected by the SSH changes. This mechanism explains 29 % of AW volume transport variance in the FSC.

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 majeur en empêchant la chaleur accumulée en surface d’atteindre les profondeurs de l’océan. La pycnocline permanente est donc d’un intérêt important dans un contexte de changement climatique. Pour la première fois et grâce au réseau de données Argo, nous avons été capables de déterminer les propriétés de la pycnocline permanente. L’objectif de cette thèse est de déterminer la structure de la pycnocline permanente et d’étudier sa variabilité au cours des dernières décennies. Une méthode de détermination objective de la pycnocline permanente a été développée. Cette méthode a d’abord été appliquée à l’océan Atlantique nord avec les données Argo puis à l’océan global. Une structure complexe de la pycnocline permanente a été mise en évidence avec de fortes différences d’un gyre à l’autre. La pycnocline permanente est la plus profonde et la plus épaisse dans le gyre subtropical nord Atlantique. Cela explique que le gyre subtropical nord Atlantique soit le plus grand réservoir de chaleur au monde. Ensuite, les relations entre la variabilité du contenu de chaleur et les propriétés de la pycnocline permanente ont été étudiées en s’appuyant sur des réanalyses océaniques. Au cours des dernières décennies, un réchauffement important de l’océan a été observé et particulièrement dans l’océan Atlantique nord. Ce réchauffement est principalement dominé par un approfondissement des isopycnes. Les déplacements verticaux des isopycnes induisent des changements dans la stratification et affectent les propriétés de la pycnocline permanente (profondeur et densité potentielle). / 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.