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 ecosystem

Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.