Frontal structure and transport in southern Drake Passage from ocean gliders

The southern boundary of Drake Passage hosts strong boundary currents, including the Antarctic Circumpolar Current (ACC) and outflows from the Weddell Sea. Recent work has shown that mesoscale and submesoscale dynamics are important for cross-shelf exchange and water mass modification. However, the local deformation radius and the scale of the fronts are smaller than the sampling resolutions in typical satellite observations or even ship-based CTD transects. For this reason, we carried out a 15-week field program with two Seagliders deployed west and east of the Shackleton Fracture Zone (SFZ) to continuously survey the continental shelf and slope regions. More than 1400 profiles of temperature, salinity, dissolved oxygen, fluorescence and optical backscatter were collected to a maximum depth of 1000 m during the mission. This enables us to examine the frontal structure in this region with unprecedented resolution and precision.

Using the hydrographic observations, we calculated baroclinic and full geostrophic velocity fields, the latter using the gliders’ depth-averaged currents, along 35 individual transects crossing the continental shelf and slope. Strong westward-flowing and eastward-flowing fronts are observed east and west of the SFZ, respectively. Fronts near the continental shelf-break appear in shallow water of depths about 800 m; geostrophic velocities peak near the bottom. These boundary currents reach a maximum velocity of 40 cm/s and are confined to horizontal scales of only 10-15 km, which accounts for about 4 Sv of transport. Generation of deep anomalies of potential temperature, salinity and potential density, possibly resulting from the frictional processes in the bottom boundary layer, are observed over the continental slope. The role of these deep anomalies in influencing the vertical structure of the fronts as well as subsurface variability will be discussed.