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SUMMARY: A budget for turbulent mixing and water mass transformation in a 
 partially enclosed deep basin - Carl Spingys\, University of Southampton\,
  National Oceanography Centre
DTSTART:20191203T110000Z
DTEND:20191203T120000Z
UID:TALK135250@talks.cam.ac.uk
CONTACT:Louis Couston
DESCRIPTION:Water-mass transformation by turbulent mixing in the deep ocea
 n has been shown to be vital in closing both regional and global circulati
 on budgets. In many cases\, these large-scale budgets do not match well wi
 th observed rates of mixing. Here we consider the transformation rate with
 in a small\, partially enclosed deep basin\, Orkney Deep\, to investigate 
 thismismatch. Orkney Deep\, a 3500 m deep and 45 km wide gap in the South 
 Scotia Ridge\, is a key bottle-neck in the transport of dense water from A
 ntarctica to the global ocean. Within the DynOPO project\, we collected a 
 series CTD/LADCP sections and VMP microstructure profiles within Orkney De
 ep. These sections show a focussing of the transport in density space as t
 he water flows northwards. Applying the Walin Framework shows a lightening
  of the densest waters and a densening of lighter waters. These transforma
 tions are consistent with a maximum buoyancy flux of 1.5x10-8W kg-1 at 28.
 32 kg m-3. Comparing this buoyancy flux with the microstructure estimates 
 of turbulent kinetic energy dissipation rate implies a dissipation ratio o
 f O(1)\, much larger than the commonly used 0.2. This result is supported 
 by estimates of dissipation ratio taken from the combination of turbulent 
 kinetic energy and temperature variance dissipation rates. The transformat
 ion can be decomposed into three terms which broadly depend on the gradien
 ts in density space of: the diffusivity\, the isopycnal area\, and the str
 atification.We combine the density field from a high resolution simulation
  of the region with a single realistic mixing profile applied throughout t
 he region to investigate these contributions. The analysis shows that the 
 reduced stratification near the boundary is the key process in driving the
  lightening of the densest water masses in Orkney Deep\, highlighting the 
 joint importance of both enhanced mixing and re-stratification on sloping 
 boundaries.
LOCATION:British Antarctic Survey\, Room 187
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