Comments on “Cold Outflow from the Faroe Bank Channel”

Abstract

The exchange of water between the Norwegian Sea and the North Atlantic around the Faroe Islands was investigated in 1987 and 1988 combining CTD data with shipborne Acoustic Doppler Current Profiler (ADCP)measurements. Tests were devised to assess the accuracy of the ADCP measurements and the results are described; it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s−1. In the outflow of cold water from the Faroe Bank Channel, which reaches speeds of nearly 1 m s−1 the error is acceptably small but strong time dependent components are revealed. The discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature, salinity and potential vorticity on an isopycnal surface are attributed to intense mixing. An effective diffusivity near 100 cm2 s−1 is derived and K-H instability demonstrated as a plausible source of the turbulent energy. Six current meter moorings were deployed from spring 1987 to spring 1988 near the sill of the Faroe Bank Channel but only two were recovered. Striking time dependence is seen at periods between 3 and 6 days mostly confined to the warm upper layer, but the origin and nature of these motions is not found. Seasonal signals are sought and none found in the cold outflow. This surprising result encourages the author to combine long-term current measurements with CTD and ADCP data and determine the flux of water colder than 3°C from the Faroe Bank Channel as (1.9±0.4)×106m3s−1.