UC San Diego

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.