Observations of the velocity profile of a fast and deep oceanic density current constrained in a gully

Abstract

The southwestern side of the Wyville Thomson Ridge transports cold Faroe‐Shetland Channel Bottom Water in a narrow cascade from a depth of 500 m down to 1700 m. An upward looking acoustic Doppler current profiler located at a depth of 1200 m measured its currents to a height of 500 m for 6 months. The westward flowing deep density current (mean thickness 343 m) extended well above the bottom Ekman layer (of order 20 m) and had a profile with a bullet nose shape that had a peak velocity at a height that was about of its thickness. The mean maximum speed was about 60 cm s−1, although there was significant variability in velocity with 90% of the variance explained by mode 1 that had a similar bullet nose shape. From the downstream momentum balance it is estimated that the vertical eddy viscosity in the overflow was of order 0.5 m2 s−1 through most of its depth but somewhat larger near the interface. A full description of the velocity profile requires an opposing surface slope and current, with zero net pressure gradient within the overflow. The transverse circulation (mean speed ∼3 cm s−1) had southward flows at the interface and seabed and a return flow at middepth. This circulation is driven by imbalances between Coriolis forcing from the downstream current and the transverse pressure gradient. Its overturning scales suggest a bulk eddy viscosity of order 2 m2 s−1 and may indicate a feedback between the downstream and transverse currents.