Secondary Circulation in the Faroe Bank Channel Outflow

Abstract

Data from a CTD station and three expendable current profiler drops at the center of the sill of the Faroe Bank Channel are used to examine the structure of the northwestward outflow of cold, relatively fresh, dense water from the Norwegian Sea into the Atlantic Ocean. A bottom boundary layer is present and exerts a bottom stress estimated at 3.5 Pa using observations in the log-layer. The shear at the interface between the outflow water and the water above is sufficiently strong to overcome the stratification and generate shear instabilities. The large stress at the bottom boundary creates an Ekman layer and thus a secondary cross-channel flow to the southwest there. A flow of similar magnitude but to the northeast is found in the high shear region at the interface. Hence, these data suggest a spiral velocity pattern in the outflow, created by the Ekman flow in the bottom boundary layer and cross-channel flow at the interface. This proposed circulation scheme explains the pinching of the density field observed at the southwest channel wall in CTD sections across the channel.