Strong, year-round eastward wind forcing drives the Antarctic Circumpolar Current (ACC), the Southern Ocean's dominant current, on an unbroken path around the Antarctic continent. This near-constant source of eastward wind stress momentum must find a sink, lest the current accelerate indefinitely. The path that this wind stress momentum travels from source to sink plays an essential role in setting the strength and structure of the upwelling branch of the meridional overturning circulation, a key nexus of heat and carbon exchange between the deep ocean and the atmosphere.

This thesis maps Southern Ocean momentum sinks and the pathways that momentum travels through the fluid from source to sink. Using the Southern Ocean State Estimate, a 1/6 degree, data-assimilating model of the Southern Ocean south of 24.7 S, we find that 95% of wind stress momentum exits the fluid via topographic form stress: 58% across submerged ridges, and 42% across the land masses that block the ACC (Chapter Two). We isolate the interfacial form stress field for the first time in a general circulation model, finding that interfacial form stress carries zonal momentum from source to sink, balancing wind stress, topographic form stress, and thermodynamic forcing to help set the structure of the meridional overturning circulation in the Southern Ocean (Chapter Three). We find that transient eddy interfacial form stress dominates the total interfacial form stress field and tends to concentrate along the six largest topographically steered currents in the ACC. Finally, we explore interfacial form stress via observational data from the cDrake experiment, a four-year deployment of current and pressure recording inverted echo sounders across a standing meander in the Subantarctic and Polar Fronts in the Drake Passage (Chapter Four). The cDrake observational estimate aligns closely with previous observational estimates in the region, and provides a strong ground-truth for the model interfacial form stress field.

The Antarctic Circumpolar Current (ACC) and the Southern Ocean meridional overturning circulation are dynamically linked through interactions between the mean flow, eddies, and mixing by breaking internal lee waves over rough topography. However, quantifying the time-mean and the spatio-temporal variability of the ACC transport, eddy fluxes, and small-scale mixing remains challenging as observations are scarce. This thesis work analyzes the mean eddy heat flux, finescale internal-wave-driven turbulence, and transport of the ACC in Drake Passage, and it examines the possible physical processes driving the spatial and temporal variability of these quantities.

First, the eddy heat flux as a function of ACC streamlines is quantified using a unique 20-year time series of upper ocean temperature and velocity transects with unprecedented horizontal resolution. Using the time-varying streamlines, the across-ACC eddy heat flux is maximum poleward in the south flank of the Subantarctic Front and it reduces towards the south, becoming statistically insignificant in the Polar Front. These results indicate heat convergence south of the Subantarctic Front.

Second, a unique four-year time series of stratification and near-bottom currents, and finestructure density and velocity profiles were employed to estimate the expected linear lee-wave energy and infer turbulent dissipation due to breaking internal waves. In contrast to idealized numerical predictions of 50$\%$ local dissipation of lee-wave energy, less than 10$\%$ dissipated locally regardless of the abyssal hill topographic representation.

Third, the high-spatial-resolution time series of temperature, salinity, and velocity are used to identify trends in the Drake Passage total and geostrophic transport in the upper kilometer. We uniquely found that the Subantarctic Front and Polar Front, the two major ACC fronts, have significantly accelerated during the last decade whereas the area between these fronts and between the Polar Front and the Southern ACC Front has decelerated. These opposite trends compensate such that no significant trend is discernible in the total and geostrophic transport integrated across Drake Passage. We suggest the acceleration of the fronts is driven by an increase in the eddy activity in between the fronts.