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Structure and dynamical balance of the Antarctic Circumpolar Current in Drake Passage


This thesis investigates the structure and dynamics of the Antarctic Circumpolar Current (ACC) in Drake Passage using observations that resolve spatial scales from 100 m to 1000 km and temporal scales from inertial to interannual. The structure and variability of the current, the eddy and mean contributions to the vorticity balance, and the patterns of internal wave activity are examined. The two primary sources of data are a long time series (2005- present) of upper ocean currents from the ARSV Laurence M. Gould (LMG) shipboard acoustic Doppler current profiler (SADCP), and a four-year process study (cDrake) providing time series of near-bottom currents, bottom pressures, and bottom-surface sound travel times as well as bathymetry, lowered ADCP, and CTD data from five yearly cruises. The vertical structure in the upper 1000 m is equivalent barotropic, with variable vertical length scale. The mean transport in the upper 1000 m is 95±2 Sv. Transport variability is approximately equally divided between shear and depth-mean components. Eddy kinetic energy decreases with depth faster than mean kinetic energy, reinforcing the view of the ACC as a barrier to mixing. Using empirical relationships determined from historical hydrography, travel time data from the cDrake array in the PFZ can be converted to baroclinic streamfunction. The near-bottom current and bottom pressure measurements provide the barotropic reference velocity. Streamfunction derivatives can be computed by objective mapping. We used independent measurements and simulated idealized fields to validate the objectively mapped fields and error estimates. Mean and eddy nonlinear vorticity advection and bottom pressure torque dominate the mean vorticity balance. The residual is first order. SOSE has the same balance and similar scales, with the residual accounted for by sub-grid-scale dissipation. In the southeastern Pacific a Rossby-wave-like balance between mean relative vorticity advection and planetary vorticity advection is observed. Downward-propagating internal wave energy and shear-strain ratios consistent with near-inertial frequencies predominate over deep waters and in the surface layer. Over shallower topography upward- propagating energy and supra-inertial frequencies dominate. The seasonal cycles in wind stress and internal wave energy south of the Polar Front are aligned; the seasonal cycle north of the Polar Front matches that in surface-layer stratification

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