Observations of Antarctic Circumpolar Current dynamics in the Drake Passage and small-scale variability near the Antarctic Peninsula

Recent high-resolution underway observations in Drake Passage are employed to study the surface-layer currents and momentum balance of the Antarctic Circumpolar Current (ACC). Ocean currents were measured with a ship-mounted acoustic Doppler current profiler and upper ocean temperatures surveyed with expendable bathythermographs deployed from the R/V Laurence M Gould, during every season of the annual cycle over a 5-year period. The finely-resolved mean flow estimated from observed currents shows the topographic control of the mean Subantarctic Front (SAF) and the along-stream convergence of multiple filaments of the Polar Front (PF) and Southern ACC Front (SACCF) into single mean jets. Mean cross-track transport above 250 m is 28.7 ± 1 Sv and accounts for 20% of the total Drake Passage transport. Subtracting geostrophic current anomalies inferred from altimetry from the instantaneous velocity observations markedly reduces the velocity variance and results in an improved estimate of mean currents. Variance is attributed to mesoscale eddies, inertial currents, and least of all, baroclinic tides. Horizontal-wavenumber spectra of velocity fluctuations peak at wavelengths of ?200 km and are significantly anisotropic in a manner consistent with aspects of geostrophic turbulence. Eddy kinetic energy is surface intensified and decreases poleward in Drake Passage; in situ estimates are significantly higher than altimetric estimates everywhere. A mean Ekman spiral observed in Drake Passage verifies the Ekman balance; the Ekman layer is approximately 100 m deep. Eddy momentum and heat fluxes are averaged along streamlines inferred from the improved mean flow. These indicate that eddies flux vorticity along the axis of the SAF in the vicinity of the Patagonian continental slope and accelerate the PF in Drake Passage. However, the eddy forcing is small, implying that eddies transmit the surface wind stress downwards undiminished via an interfacial form stress. Variability in specific dynamic height contours correlated with each ACC front is used to examine the meandering and topographic control of the Drake Passage fronts. Finally an analysis of the annual cycle of the circulation of Port Foster, Deception Island is presented. Moored observations of water temperature, currents and acoustic backscatter show that seasonal, tidal and diurnal processes dominate the variability of this ecosyste