‘Fine’ structure in the Faroe Bank Channel Overflow CPT, Nov 04 Jim Price, WHOI
Faroe-Shetland Trough Faroe-Bank Channel Wyville-Thomson Ridge
section name, repeats I H,2 E D,4 G,2 F,3 C A,2 Latitude B,2 220 CTD/LADCP 105 XCP 17 sections RRS Discovery 247 Longitude
Tocn h T(x)
0.7< Fr < 0.9 0.3< Fr < 0.5
0.6< Fr < 0.7 0.8< Fr < 1.1
L Reykjanes Ridge climatology d
FBC Overflow, descent and mixing • The topography opens up beyond the narrow FBC, and the overflow spreads out as width/distance ~ 0.6. It descends the ISR slope at an angle of roughly 0.15. Transformation of T/S appears to occur over about 150 km downstream of the sill, more or less uniformly. Transport increases by almost 100% over this distance. • The entrainment rate is well-defined, We/U ~ 5x10-4 on average, or 4 – 6 x10-4, if the sections are sorted on transport (or time). Froude numbers in the core of the current were approx 1, though smaller values can be found. The estimated entrainment rates and Fr are not inconsistent with a putative entrainment law, E(Fr). • The net mixing, phi, also shows a significant sensitivity to source transport, being reduced when source transport was increased, roughly (Q / phi)(dphi / dQ) ~ -3/4 (when ¼ < phi < ½) • Inference from a very simple model is that a primary process driving the overflow toward larger Fr is the spreading of the current. • Some Open Questions: • Is a layered formulation of mixing sufficient? • Why this We(Fr)? Good question... what would constitute an answer?
CPT, Nov 2004 end
slope = 0.002 slope = 0.0005 • Water mass production • by a marginal sea presents two somewhat separate problems : • What is the exchange of source water with the open ocean? • How is the source water modified by mixing as it descends?
Qp = 3 Sv Qs = 1.8 Sv
N Atlantic inflow FBC overflow
D2 D3 D4 D1
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. Some Open Questions: 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. Is there an influence of the highly variable, upper layer inflow?
But as for certain truth, no man has known it, Nor will he know it; neither [the turbulent entrainment law]*, Nor yet of all the things of which I speak. And even if by chance he were to utter the final truth, He would himself not know it: For all is but a woven web of guesses. *[of the gods] Xenophanes, mid 6th to early 5th centrury, BCE
But as for certain truth, no man has known it, Nor will he know it; neither [the turbulent entrainment law]*, Nor yet of all the things of which I speak. And even if by chance he were to utter the final truth, He would himself not know it: For all is but a woven web of guesses. *[of the gods] Xenophanes, mid 6th to early 5th centrury, BCE The sea is the source of water and of wind, For without the great sea, there would be no wind nor streams of rivers, Nor rainwater from on high, But the great sea is the begetter of clouds, winds, and rivers. Xenophanes, the first oceanographer?
Qs = 2.3 Sv Qs = 1.4 Sv sill
outflowing total transport, Q sill source transport, Qs