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Transit Times in the North Atlantic DWBC Measured Using the Tracer Pair 129 I/ CFC-11

Transit Times in the North Atlantic DWBC Measured Using the Tracer Pair 129 I/ CFC-11. J.N. Smith – Bedford Institute of Oceanography OSNAP Meeting: April 26, 2011. 129 I Tracer Characteristics. t 1/2 = 1.6 x 10 7 y Measured by AMS (IsoTrace: Univ. Toronto) 250 ml samples

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Transit Times in the North Atlantic DWBC Measured Using the Tracer Pair 129 I/ CFC-11

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  1. Transit Times in the North Atlantic DWBC Measured Using the Tracer Pair129I/CFC-11 J.N. Smith – Bedford Institute of Oceanography OSNAP Meeting: April 26, 2011

  2. 129I Tracer Characteristics • t1/2= 1.6 x 107 y • Measured by AMS (IsoTrace: Univ. Toronto) • 250 ml samples • $200/sample for AMS • Sample collection; cheap • Nuclear fuel reprocessing plant source – Sellafield (UK) & La Hague (France)

  3. BNFL Sellafield

  4. DSOW

  5. DSOW Input to DSOW

  6. Boundary Current Model Waugh and Hall, JPO, 2005 X=0 tmix Labrador Sea u X=L Line W db di Core has uniform flow, u; interior has zero flow; mixing between regions is parameterized as relaxation with time scale, tmix; a = db/di (= 0.1 in DWBC).

  7. Tracer continuity equations: Cb and Ci are tracer concentrations in boundary current and interior, respectively; S is tracer source or sink. Similar to Pickart et al. (1989); Rhein (1994); Doney and Jenkins, 1994.

  8. Transit Time Distributions: co(t) = time varying, tracer concentration at x = 0 c(x, t) = interior tracer concentration G(x, t) = TTD at location x: Greens function that propagates boundary condition on tracer concentrations from specified region into interior. = advective time to reach point x = Dirac delta function = Heaviside function

  9. Transit Time Distributions: co(t) = time varying, tracer concentration at x = 0 c(x, t) = interior tracer concentration G(x, t) = TTD at location x = advective time to reach point x = Dirac delta function = Heaviside function

  10. u = 2 – 3 cm/s tmix = 3.5 – 7 y a = 0.1

  11. Conclusions • Boundary current model advection times (tadv) of 5–7 y (m = 2-3 cm/s), mixing times (tmix) of 3.5–7 y (intermediate to low mixing regimes) and a = 0.1 are consistent with 129I/CFC-11 results for the DWBC. However, tadv = 3 y (m = 5 cm/s); tmix = 1.1 y; a = 0.08 also fits: must empirically constrain a; input hydrography to model. • Values of m = 5 cm/s; tmix = 1 y are consistent with best-fit values in models of Pickart et al., 1989; Doney and Jenkins, 1994; core velocity is similar to mean velocities from direct measurements. Previous tracer studies for which mixing is not included give lower spreading rates. • Line W 129I/CFC-11 results (2010-2012) will further constrain advection and mixing times.

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