| Summary: | The large-scale Meridional Overturning Circulation (MOC) in the Atlantic Ocean involves surface northward transport to the northern North Atlantic and a southward return flow at mid-depth of the North Atlantic Deep Water (NADW) which forms convectively. In the Indian and Pacific Oceans, a northward bottom water flow is again compensated by a mid-depth return flow toward the Southern Ocean (SO). This zonally-averaged picture of the MOC hides a rich horizontal structure involving concentrated mid-depth flows toward the SO near the western and eastern boundaries, both carrying significant meridional transports. While deep western boundary currents have been understood for a while, the dynamics of their eastern boundary counterparts are still a mystery and the climate models do not simulate such currents at the right location or with the right magnitude. Furthermore, the MOC is an important part of the processes that determine the mid-depth ocean stratification, also involving Southern Ocean winds and eddies, interior vertical mixing and northern high-latitude convection. This thesis has two parts: the first explores the dynamics of mid-depth eastern boundary routes of the MOC; the second studies how the interplay between SO dynamics and interior vertical mixing determines the vertical structure of the mid-depth ocean stratification. The MOC has a complex horizontal structure, and its meridional transport has long been thought to be mainly due to near-surface and deep \textit{western} boundary currents, with a negligible contribution from eastern boundary flows. However, observations have robustly revealed the existence of poleward Deep Eastern Boundary Currents (DEBCs) between 1--4 km depth, in the South Atlantic, Indian, and Pacific Oceans. These deep eastern boundary currents are found to be important pathways of mass, tracer, and heat transports towards the SO. I find that the upstream part of these currents (away from the outflow into SO) is governed by an interior-like vorticity balance ($\beta v\approx ...
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