Friction and mixing in the Faroe Bank Channel outflow

Three hydrographic sections made during RRS Discovery cruise 242, from 7 September to 6 October 1999, are used to study the Faroe Bank Channel overflow. Each section, made perpendicular to the plume direction, comprised 10–14 stations measuring the outflow properties and velocity. The overflow defined as water with potential temperatures below 3 °C has densities greater than 28.1 kg m^–3, salinities between 34.9 and 35.1, with velocities as high as 80 cm s^–1 approximately 20 km downstream from the sill. We use these measurements to describe the characteristics of the outflow and to estimate the amount of friction and mixing as the plume travels along the continental slope into the Iceland Basin. From Lowered Acoustic Doppler Current Profiler (LADCP) measurements we estimate stress in the bottom Ekman layer. Our stress results are best related to the overflow velocity with a drag coefficient of 0.5 × 10^–3 and to the height of the bottom Ekman layer with a von Karman constant of 0.75. Below a potential temperature of 8 °C, the isotherms sink with distance downstream. Calculating transport below different isotherms, we make an estimate for the amount of entrainment. In the region downstream of the sill, entrainment is initially maximum near the 0 °C isotherm just above the cold dense outflow and turbulent diffusivities reach as high as 500 cm² s^–1. Fifty kilometre downstream of the sill the depth of maximum entrainment has moved upwards to the 2 °C isotherm and turbulent diffusivity has decreased to 50 cm² s^–1. Froude numbers are found to exceed one in the centre of the overflow on all three sections. Calculating Richardson numbers we see evidence that mixing takes place above the fast flowing outflow core.