Faroe-Bank Channel overflow
Cold and dense water from the Nordic Seas is transported southwards as Faroe-Bank Channel overflow.
This water flows from the Arctic Ocean into the North Atlantic through the Faroe-Bank Channel between the Faroe Islands and Scotland.
[1] Faroe-Bank Channel overflow (FBCO) contributes to a large extent to the formation of North Atlantic Deep Water.
[5] Therefore, FBCO is important for water transport towards the deep parts of the North Atlantic, playing a significant role in Earth's climate system.
Its primary sill, located south of the Faroe Islands, has a width of about 15 km and a maximum depth of 840 m, with very steep walls at both sides of the channel.
[9] Therefore, it may be complicated to exactly define which water entering the FBC contributes to the actual overflow.
In the stratified layer at the top of the channel, velocities become negative (i.e., in southeastern direction), which makes these water no part of the overflow.
The overflow velocity, then, scales as follows with the pressure gradient between the basins north and south of the ridge:
[10] Lastly, overflow can also be defined on the basis of hydrographical properties: namely as water that flows through the FBC having a temperature lower than 3 °C,[10] or having a potential density higher than 27.8 kg/m3.
Temperature and salinity profiles as well as current speeds in the FBC vary strongly on a day-to-day basis.
The dense water forms domes that move through the channel with a period of 2.5 to 6 days.
[13] The resulting eddies are the consequence of baroclinic instabilities within the overflow water, which then induce the observed periodicity.
When the atmospheric circulation governing the Nordic Seas is in a cyclonic (anticyclonic) regime, the source of the deep water predominantly comes via a western (eastern) inflow path, and the FBCO will be weaker (stronger).
This transition from a cyclonic to an anticyclonic regime takes place on an interannual timescale, but the atmospheric forcing also shows a seasonal cycle.
Within the FBC, water always flows along its eastern rather than its western boundary, regardless the different inflow pathways from the Nordic Seas.
[5] Moreover, at times the eastern inflow path is dominant, overflow waters are denser and higher in volume.
After passing the primary Faroe-Bank Channel sill, the overflow bifurcates into two different branches that both flow with a maximum velocity of 1.35 m/s on top of each other.
The average thickness of the total outflow plume along its descent is 160±70 m, showing a high lateral variability, and yields a transport of ~1 Sv per branch.
The shallow, intermediate branch transports warmer, less dense outflow water along the ridge slope towards the west.
This branch mixes with oxygen-poor, fresh Modified East Icelandic Water.
Both branches ultimately contribute to the formation of North Atlantic Deep Water.
The Atlantic meridional overturning circulation (AMOC) is important for Earth's climate because of its distribution of heat and salinity over the globe.
[10] From 1995 onwards, FBCO has been monitored by a continuous Acoustic Doppler current profiler (ADCP) mooring, measuring volume transport, hydrographic properties and the density of the overflow.
[17] The kinematic overflow, derived from the velocity field, showed a non-significant positive linear trend of 0.01±0.013 Sv/yr between 1995 and 2015, whereas the coldest part of the FBCO warmed in that same period with 0.1±0.06 °C (which made density decrease), causing increasing transport of heat into the AMOC.
These processes compensate each other; as a result the pressure difference at depth does not show a significant trend over time.
