Faroe Bank Channel Overflow: Mesoscale Variability*

Abstract

The Faroe Bank Channel is the deepest connection through the Greenland-Scotland Ridge, where dense water formed north of the ridge flows southward over the sill crest, contributing to the formation of North Atlantic Deep Water. The overflow region is characterized by high mesoscale variability and energetic oscillations, accompanied by a high degree of sea surface level variability. Here, 2-month-long time series of velocity and temperature from 12 moorings deployed in May 2008 are analyzed to describe the oscillations and explore their generation and propagation. The observed 2.5-5-day oscillations in velocity and temperature are highly coherent both horizontally and vertically, and they are associated with 100-200-m-thick boluses of cold plume water flowing along the slope. A positive correlation between temperature and relative vorticity and the distribution of clockwise/counterclockwise rotation across the slope suggest a train of alternating warm cyclonic and cold anticyclonic eddies, where the maximum plume thickness is located downslope of the eddy center. The along-slope phase velocity is found to be 25-60 cm s 21 , corresponding to a wavelength of 75-180 km, while the vertical phase propagation is downward. The oscillations are present already in the sill region. The observations do not match predictions for eddies generated either by vortex stretching or baroclinic instability but agree broadly with properties of topographic Rossby waves.

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

“…The observed temperature fluxes of the order of 0.1 K m s −1 were about 10 times larger than the values reported by Voet and Quadfasel (2010) from the region downstream of the Denmark Strait overflow and in agreement with values reported by Darelius et al (2011) and the modelled values by Guo et al (2014). Since density is largely determined by temperature in the FBC, the upslope temperature flux is equivalent to an upslope buoyancy flux, i.e.…”

“…This mesoscale variability is typical of the entire record. The plume takes the form of domes of cold water moving past the mooring array, as previously noted by Geyer et al (2006) and Darelius et al (2011). While the presence of cold water at moorings S2-S4 was associated with downslope velocities, it was associated with upslope movement at mooring S1.…”

“…B1). One method is to compute transport per unit width for each mooring, multiplying this by a representative width for each mooring, and then summing up the contribution of the individual moorings, as described in Darelius et al (2011). This will be referred to as Q moor .…”

“…The dense overflow takes the form of a train of 100-200 m thick boluses of cold water moving along the slope at periods of 2.5-6 days (Darelius et al, 2011). The cold water domes are associated with energetic mesoscale oscillations in the velocity field that extend throughout the water column and are linked to a wave pattern in sea surface height (Darelius et al, 2013).…”

“…Aside from the FBC sill mooring (Hansen and Østerhus, 2007), which provides one of the world's few long-term moored time series of a deep branch of the meridional overturning circulation, mooring measurements of the FBC overflow of more than a few months' duration are rare (e.g. Saunders, 1990;Darelius et al, 2011). In particular, few long-term observations have been made downstream of the sill, and none of them covers the whole vertical and lateral extent of the plume.…”