Faroe Bank Channel overflow 2012 – 2013

Since November 1995, the overflow through the Faroe Bank Channel has been monitored by different kinds of instrumentation. A key element in the monitoring system is data from ADCPs that have been deployed at a fixed location on the sill of the channel, usually from early summer one year to early summer the next with 3 – 4 weeks servicing intervals. From the beginning, the ADCPs at this location have been RDI 75 kHz Broadband instruments, but an accident during the deployment in June 2012 resulted in loss of the instrument to be deployed. To maintain the monitoring, a new ADCP was deployed at the location in late September 2012 and this was an RDI Long Ranger instrument. When data from this instrument were analyzed, it became clear that the instrument showed a similar bias as when deployed earlier in the Denmark Strait, and that bias has also been seen in other Long Rangers with the same firmware version. The error in the Long Ranger data appears as a reduced current speed in a layer that may extend at least 200 m up from the bottom, that is most of the overflow plume. This would give erroneous volume transports, if used uncritically, but the accumulated knowledge about the velocity profile at the location may be used to adjust the measured Long Ranger profiles and derive transport values from them. Based on some - apparently realistic – assumptions, we have used this procedure to derive daily and monthly transport values for the Faroe Bank Channel overflow during the 2012 – 2013 deployment period. The overflow seems to have been weaker than average in this period, but not exceptionally so, and there is no indication of any persistent weakening.