1. Discovery 247; A field study of the Faroe Bank Channel Overflow By Jim Price, WHOI, and the science party of Discovery 247, Tom Sanford, James Girton and John Dunlap, APL/UofW, Cecilie Mauritzen, Dan Torres, George Tupper, Deb West-Mack and Dicky Allison, WHOI, and Mark Prater, URI. Our thanks to Cpt Robin Plumley and the crew of RRS Discovery, to Jeff Benson and the UKOR technical staff and the CTD crew, Liz Hawker, Peter Huybers, Patricia Kassis, Heather Deese, Avon Russell and Laura Cornick. Supported by the US National Science Foundation and the US Office of Naval Research.

3. Faroe-Shetland Trough Wyville-Thomson Ridge Faroe-Bank Channel

4. Longitude Latitude A,2 B,2 C D,4 E F,3 G,2 H,2 I section name, repeats 220 CTD/LADCP 105 XCP 17 sections RRS Discovery 247

5. sill

6. FBC overflow N Atlantic inflow

22. Reykjanes Ridge climatology d L

29. the local, boundary layer effect: Bottom stress causes a significant shear in a bottom (Ekman) layer of O(50 m) thickness at section G; Current speeds are otherwise nearly geostrophic.

31. total transport, Q source transport, Qs sill outflowing

32. sill

35. this view is from the NW ctd D2 this view is from the SE

37. D1 D4 D3 D2

53. FBC Overflow; descent from the sill The topography opens up dramatically beyond the FBC narrows, and the overflow spreads out as width/distance ~ 0.6 as it descended the ISR slope and entrained overlying NAW. Bottom stress was significant, 1-2 Pa, and the Ekman number ~ 0.12 based upon stress or ~ 0.15 based upon descent rate. There was only modest veering in the bottom Ekman layer so that spreading appeared to be largely barotropic. The bottom and interfacial boundary layers began to merge, and mixing/entrainment and dissipation were important in the property and Bernoulli balances. Current speed was approximately geostrophic. -4. Froude numbers in the core of the current were 0.7-1.2, though smaller values can be found. The estimated entrainment rate and Fr are not inconsistent with a putative entrainment law, E(Fr). There is significant entrainment, E = We/U ~ 5x10 -4. Froude numbers in the core of the current were 0.7-1.2, though smaller values can be found. The estimated entrainment rate and Fr are not inconsistent with a putative entrainment law, E(Fr). The net entrainment shows a significant sensitivity to source transport, being reduced when source transport was increased. Inference from a very simple model is that a primary process driving the overflow toward larger Fr is the spreading of the current. What’s missing? The entrainment law, E(Fr), is itself in need of an explanation. The FBC would be an excellent site for a detailed study of turbulent entrainment, but, what would constitute an answer? Is a layered formulation of mixing sufficient? The effect of bottom bed forms on bottom stress and mixing is potentially large but not known here, or generally.

54. FBC Overflow; exchange The mean transport of source water was 1.8 Sv, computed over 15 sections, and about as expected from previous studies. Bernoulli conservation suggests that the approach flow is on the left side Faroe-Shetland Trough. Relative vorticity was well-measured at the sill and was approx –f/5, suggesting not much squashing. There is some hint of partial blocking in the deepest part of the approach flow. (Note that PV and Bernoulli are likely not conserved strictly due to bottom drag.) The variability of transport was approx +- 50%, and somewhat larger than expected. The time scale of transport variability was roughly 10 days, but not well-observed here. The mode of transport variability is that the overflow thickness near the sill varied from a minimum of 250 m to a maximum of 500 m. Thickness variation persists well downstream of the sill. What’s missing? The connection to the upstream reservoir -- even the Faroe Shetland Trough -- is not clear in this data set. Averaged or typical values of upstream depth give excessive exchange, as found before. The mechanism or cause of the variability in the exchange is unknown. What is the effect of overflow across Wyville- Thomson Ridge ? Is there an influence of the highly variable, upper layer inflow?