Seismic imaging of a large horizontal vortex at abyssal depths beneath the Sub-Antarctic Front.

The global ocean and climate systems are strongly influenced by physical oceanographic processess within the Southern Ocean1. In particular, the exchange of water between subtropical North Atlantic Deep Water and the Antarctic Circumpolar Current controls the rate at which the latter upwells and mixes2. Despite its significance, the details of this exchange are poorly understood. Acoustic imaging of the water column can reveal the detailed thermohaline structure3. Here we present a subsurface acoustic image, acquired in October 1998, that crosses the Sub-Antarctic Front in the South Atlantic Ocean, where the two water masses converge and shear past each other. We find that down to a depth of 2.5?km, the vertical boundary between the North Atlantic Deep Water and the Antarctic Circumpolar Current is sharp and mass exchange is negligible. Below this depth, where cross-track velocities converge, we detect a prominent swirling structure that is 500 m high and 10?km wide. We analyse prestack acoustic records, which suggest that this structure rotates at an average speed of 0.3±0.1?m?s?1 about a horizontal axis. We suggest that the structure could either be a thermohaline intrusion created by frontal instability processes, or—more speculatively—a localized and intermittent overturning even