A Report of Data and Preliminary Analysis from Discovery 247 A Process Study of the Faroe Bank Channel Overflow by James F. Price, WHOI and the scientific part of D247 sponsored by the National Science Foundation May, 2006 additional, associated files stored in WHOAS include 1) the complete set of CTD section plots from this cruise (only a small fraction of the total can be shown here) 2) the archive of the station data, XCP, LADCP, CTD and nutrients

Discovery 247; Snapshots 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.

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

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

sill

The following two slides show a sample of the data collected at most stations: 1) CTD cast, including oxygen concentration 2) LADCP velocity at every CTD station (less one) 3) XCP velocity at about half of the CTD stations, mostly those west of the FBC sill.

FBC overflow N Atlantic inflow

The next three slides show a synthetic section along the axis of the overflow current.

The following sequence of seven slides shows potential temperature sections made from section A, well upstream of the sill, to section H, which is well west of the sill. Notice the thin black line that marks the approx upper limit of the dense water at section A, and kept at that level on subsequent slides. The same is done for the left side of the dense water by a small arrow. Notice how the dense water descends the topography downstream of the sill, especially.

The next slide shows total transport, the directly measured, outflowing transport of water colder than 6 C; source transport is the inferred transport of source water, inferred from the changing T/S relation (several slides hence). Notice that source transport shows no particular trend with distance downstream, but considerable variability. Total transport clearly increases downstream, but also varies significantly.

total transport, Q source transport, Qs sill outflowing

The next slide shows the downstream evolution of the transport-weighted potential temperature and salinity. Notice that temperature increases, consistent with entrainment of warmer Atlantic water by the overflow, while salinity increases, also due to entrainment of more saline Atlantic water.

sill

The next two slides show the transport weighted T and S, one point per section, on a T/S diagram. The presumed endpoints are in the source water and the Atlantic water above the overflow. The second slide shows the inferred fraction of entrained Atlantic water, phi; phi = 0 is none, phi = 1/2 indicates the overflow is 50% source water and 50% Atlantic water as a function of distance downstream and of source water transport.

Reykjanes Ridge climatology d L

Typical profiles from the core of the overflow current and just downstream of the sill. These data were used to estimate a bulk Froude number, Fr.

h T ocn T (x)

The next two slides develop a simple model of an entraining (and broadening) density current, and in the second slide compares this with the observed phi.

Bottom stress, estimated by fitting a log layer to the deepest 10 m of each XCP profile.

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.

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

The next four slides show repeated sections run near the sill in Faroe Bank Channel. The thickness of the overflow layer and the transport of source water varies substantially, but the structure of the current, including the relative vorticity of the current, are not highly variable.

D1 D4 D3 D2

The next five slides show repeated hydrographic sections. Notice that there is substantial variation in the thickness of the overflow water layer.

The next seven slides show current and density measured on a single occupation of the sill section, D.

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 , 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 ~ 5x Froude numbers in the core of the current were , 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.

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?