The ocean's Meridional Overturning Circulation (MOC) plays a central role Earth's climate, setting the stratification and large scale dynamics of the global ocean and controlling the abyssal storage of carbon and heat. However, fundamental questions remain regarding the overall structure and driving processes of the MOC. In this dissertation we take a step towards establishing a more comprehensive view of the global MOC that takes into account modern observation-based predictions of its driving processes and the role of complex topography in the Southern Ocean in influencing its structure. We begin by characterizing the volume transport and water mass transformation rates of the global overturning circulation using the Estimating the Circulation and Climate of the Ocean (ECCO) reanalysis product. The ECCO solution supports a large rate of exchange between the mid-depth and abyssal overturning cells, consistent with recent estimates. However, much of the upwelling in ECCO's deep ocean is not associated with irreversible water mass transformations as is typically assumed in theoretical models. Instead, a dominant portion of the abyssal circulation in ECCO is associated with isopycnal volume tendencies, reflecting a deep ocean in a state of change and a circulation in which transient tendencies play a leading role in the water mass budget. Although observational constraints are insufficient to unambiguously determine whether the simulated tendencies are real, there are indications that much of the trends in ECCO are spurious. Whether or not ECCO's tendencies are realistic, they are a key part of its abyssal circulation and hence need to be taken into consideration when interpreting the ECCO solution. We next address the role of Southern Ocean topography and wind stress in the deep ocean overturning and water mass composition using a suite of idealized global ocean circulation models. Specifically, we investigate how the presence of a meridional ridge in the vicinity of Drake Passage and the formation of an associated Southern Ocean gyre influences the water mass composition of the MOC's abyssal limb. Our results show that a Drake Passage ridge can significantly decrease the strength of the mid depth cell, provided the ridge is high enough to intersect the isopycnals along which NADW flows southward. Passive tracer experiments meanwhile show that a weakening mid-depth cell is linked to decreasing abyssal ventilation by North Atlantic water masses, and further that an increasing ridge height in Drake Passage and a concurrent gyre spin-up leads to a substantial decrease in NADW-sourced AABW (AABW forms from former NADW that surfaces in the Southern Ocean) in the abyssal ocean. We explain this behavior using a simple scaling relationship between the gyre and a mixing-driven exchange of NADW-origin waters with the surface layer north of the ACC, and in the final study of this dissertation we test this mixing pathway against more complex idealized setups. Our results show that the gyre-driven mixing pathway is robust in the presence of zonal extensions to the Drake Passage ridge and in the presence of additional submarine topographic barriers placed throughout the Southern Ocean domain.