Articles | Volume 12, issue 2
Research article
15 Mar 2016
Research article |  | 15 Mar 2016

Volume transport and mixing of the Faroe Bank Channel overflow from one year of moored measurements

Jenny E. Ullgren, Elin Darelius, and Ilker Fer

Abstract. One-year long time series of current velocity and temperature from eight moorings deployed in the Faroe Bank Channel (FBC) are analysed to describe the structure and variability of the dense overflow plume on daily to seasonal timescales. Mooring arrays were deployed in two sections: located 25 km downstream of the main sill, in the channel that geographically confines the overflow plume at both edges (section C), and 60 km further downstream, over the slope (section S). At section C, the average volume transport of overflow waters ( < 3 °C) from the Nordic Seas towards the Iceland Basin was 1.3 ±  0.3 Sv; at section S, transport of modified overflow water ( < 6 °C) was 1.7  ±  0.7 Sv. The volume transport through the slope section was dominated by mesoscale variability at 3–5-day timescales. A simplified view of along-path entrainment of a gravity current may not be accurate for the FBC overflow. As the plume proceeds into the stratified ambient water, there is substantial detrainment from the deeper layer (bounded by the 3 °C isotherm), of comparable magnitude to the entrainment into the interfacial layer (between the 3 and 6 °C isotherms). A time series of gradient Richardson numbers suggests a quiescent plume core capped by turbulent near bottom and interfacial layers in the channel. At section S, in contrast, the entire overflow plume is turbulent. Based on a two-layer heat budget constructed for the overflow, time mean vertical diffusivities across the top of the bottom layer and across the interfacial layer were (30  ±  15) × 10−4 and (120  ±  43) × 10−4  m2 s−1, respectively.

Short summary
One-year long moored measurements of currents and hydrographic properties in the overflow region of the Faroe Bank Channel have provided a more accurate observational-based estimate of the volume transport, entrainment, and eddy diffusivities associated with the overflow plume. The data set resolves the temporal variability and covers the entire lateral and vertical extent of the plume.