Variability and Mixing of the Filchner Overflow Plume on the Continental Slope, Weddell Sea

Abstract

A large fraction of Antarctic Bottom Water is produced in the Weddell Sea, through mixing between the cold and dense shelf water masses and the warm and saline off-shelf water. We present observations of the dense Filchner overflow plume from one mooring at the Filchner sill and two moorings located downstream, on the continental slope. The plume variability over the continental slope at a monthly time scale is related to upstream conditions at the Filchner sill, with a high correlation in density. Revised column-integrated volume transport calculations across the Filchner sill indicate 50% higher values in 2010 compared with the earlier estimates available from 1985. Over the continental slope, the plume thickness fluctuates strongly between less than 25 m and more than 250 m. Observations of elevated temperature variance and high Froude numbers at the plume interface imply high mixing rates and entrainment of ambient water masses. The mixing events typically coincide with shear spikes across the plume. The shear spikes appear quasi-periodically, when counterrotating oscillations with periods of 24 and 72 h align. The clockwise 24-h oscillation is related to diurnal, barotropic tidal currents and topographic vorticity waves, whereas the counterclockwise 72-h oscillation is related to vortex stretching or topographic vorticity waves.