| The continuous, swift flow of cold water over the sill of the Faroe Bank Channel, the deepest
passage from the Nordic Seas to the North Atlantic Ocean, contributes to the ventilation of
the deep North Atlantic Ocean. The amount of bulk entrainment and mixing of the
overflow along its path was previously estimated from coarsely spaced currentmeters
and water-mass considerations; however remain poorly constrained. The spatial
distribution of mixing remains unknown due to lack of direct measurements. Here
we report from the first direct turbulence measurements conducted in June 2008,
supplemented by moored observations. The shipboard survey covers the first 120 km
downstream of the sill, and includes vertical profiles of hydrography and velocity (from a
conductivity-temperature-depth package equipped with lowered-acoustic Doppler current
profilers), and of turbulent dissipation rate from a vertical microstructure profiler (Rockland
Sci. Int.).
The dynamic properties and mixing of the overflow plume as it descends toward the
Iceland Basin are described. The plume has a vertical structure composed of a 70 ± 35 m
thick well-mixed bottom layer overlaid by a 120 ± 60 m thick stratified interfacial layer. The
vigorously turbulent plume is associated with intense mixing and enhanced turbulent
dissipation near the bottom and at the plume-ambient interface, but with a quiescent
core. The latter is due to weak shear production of turbulent kinetic energy near the
velocity maximum, located typically deeper than the interface. Enhanced mixing at the
stratified and highly-sheared interface is mostly due to coexisting shear instabilities and
internal wave-turbulence transition. Entrainment velocity inferred from dissipation
measurements show strong lateral variability. A pronounced transverse circulation
is observed consistent with rotating plume dynamics. The transverse circulation
actively dilutes the bottom layer of the plume. The bulk entrainment parameterizations
mainly devised for non-rotating, two-layer gravity current plume dynamics, and
the traditional turbulence closure models that lack the internal-wave/turbulence
transition will be inadequate in representing the mixing of the Faroe Bank Channel
overflow. |