The Bransfield gravity current

Abstract

Using in situ data and laboratory experiments, we show that the circulation of the Bransfield Current (BC) around the South Shetland Islands (SSI) may be characterized in terms of a propagating buoyant gravity current. First, we describe the SSI hydrography and some drifter trajectories, paying special attention to the recirculation of the BC at the northeastern tip and northern slopes of the SSI. We observed that when the northeastward-flowing BC reaches the northeastern tip of the SSI, it recirculates around an anticyclonic mesoscale eddy that has not previously been reported in this region. Part of this recirculating water then proceeds southwest along the northern SSI shelf break as a narrow baroclinic jet and another part join the Antarctic Circumpolar Current. Consequently, the cross-slope gradients of properties strengthen, and the southern boundary of the Antarctic Circumpolar Current becomes a nearly submesoscale (~10 km) front. Second, we compare the observations with buoyant gravity current laboratory experiments in an open basin setup where the SSI topographic barrier is represented by a central wall. The resulting circulation of the buoyant gravity current around the wall mirrors our in situ observations. First, a narrow buoyant gravity current flows northeastward along the southern boundary of the wall. Once the head of the buoyant gravity current reaches the tip of the wall, a recirculating anticyclonic vortex is generated, and the buoyant gravity current then proceeds westward along the north side of the wall. This circulation of the BC around the SSI as a buoyant gravity current may contribute to the fertilization of the waters around the SSI, as suggested by previously reported distributions of nutrients and phytoplankton.