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

K Daae,E Darelius,I Fer

doi:10.1175/jpo-d-18-0093.1

Abstract

AbstractA 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.

Highlights

Antarctic Bottom Water (AABW) contributes to the lower limb of the global ocean thermohaline overturning circulation (Orsi 1999)
40%–50% of the AABW originates from the Weddell Sea region (Fig. 1; Jacobs 2004; Ohshima et al 2013), through a chain of processes occurring over the continental shelf, beneath the Filchner– Ronne Ice Shelf (FRIS) and along the continental slope (Foldvik and Gammelsrød 1988)
The intrusions are often accompanied by low transport values, which could indicate that the core of the Ice Shelf Water (ISW) is deflected away from the S2 site by lighter water masses

Summary

Introduction

Antarctic Bottom Water (AABW) contributes to the lower limb of the global ocean thermohaline overturning circulation (Orsi 1999). Recent studies indicate that AABW has freshened during the past 30 years, substantially affecting global sea level rise (Purkey and Johnson 2013; Jullion et al 2013). A better understanding of the mechanisms changing the AABW properties could lead to improved predictions of the rise in global sea level. 40%–50% of the AABW originates from the Weddell Sea region (Fig. 1; Jacobs 2004; Ohshima et al 2013), through a chain of processes occurring over the continental shelf, beneath the Filchner– Ronne Ice Shelf (FRIS) and along the continental slope (Foldvik and Gammelsrød 1988). Cold and saline High Salinity Shelf Water (HSSW) is formed through atmospheric cooling and brine rejection from sea ice formation (Nicholls et al 2009).

Results

Discussion

Conclusion