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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 1 , W.M. Smethie Jr. 2 , 1 Bedford Institute of Oceanography, Canada 2 Lamont Doherty Earth Observatory, USA. Labrador Sea Workshop, BIO Nov. 10-11, 2008. 129 I Tracer Characteristics.

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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. Smith1, W.M. Smethie Jr.2, 1 Bedford Institute of Oceanography, Canada 2 Lamont Doherty Earth Observatory, USA Labrador Sea Workshop, BIO Nov. 10-11, 2008

  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

  6. Smith et al., JGR, 2005

  7. 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 .

  8. 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.

  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. 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

  11. u = 2 – 5 cm/s tmix = 1 – 8 y

  12. Conclusions • Boundary current model advection times (tadv) of 3–8 y (u = 5 and 2 cm/s, respectively) and mixing times (tmix) of 1–8 y (intermediate to low mixing regimes) are all consistent with 129I/CFC-11 results for the DWBC at Line W and the Labrador Sea. • Values of u = 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. However, other tracer studies imply lower spreading rates. • Line W 129I/CFC-11 results (2008-2012) will further constrain advection and mixing times.

  13. 1997 1999 2005 Transit time = 2 y to Labrador Sea;  8 y to Sta. W for DSOW

  14. Sta. W

  15. 4 1 3 2 Arctic ventilation DSOW

  16. 4 1 3 2 Arctic ventilation DSOW

  17. 4 1 3 2 Arctic ventilation DSOW

  18. 4 1 3 2 Arctic ventilation DSOW

  19. S  34.5

  20. Artic ventilation DSOW N. Atlantic ventilation

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