1. Tracers in Ocean and Climate Models* Matthew England CEMAP, School of Mathematics The University of New South Wales * See also www.maths.unsw.edu.au/~matthew/publications.html#MR98

3. Possible due to: GEOSECS, TTO, SAVE, WOCE, …..

5. Why bother with tracers in models? Ocean model “validation” Diagnosis of model circulation mechanisms Studies of the ocean carbon cycle Data assimilation studies Paleoceanographic considerations

6. Ocean model “validation” (e.g. CFCs, 14 C) Diagnosis of model circulation mechanisms (e.g. dye/age tracers, 39 Ar) Studies of the ocean carbon cycle (carbon compounds, oxygen, phosphate, nitrate,…) Data assimilation studies (e.g. CFCs, tritium) Paleoceanographic considerations (e.g. carbon-13, oxygen-18) Why bother with tracers in models?

7. 100-1000 year ventilation 10-100 year ventilation

8. Robust diagnostic: T-S restored to observed in the interior Robust diagnosticObserved 14C Prognostic Supressed convection and vertical motion Prognostic experiment: Interior T-S free to evolve Toggweiler et al. [1989]

10. Chlorofluorocarbons

12. Plate 2. Distribution of CFC-12 on isopycnal surfaces corresponding to maximum NADW outflow in 1988 in the Redler and Dengg [1999] simulations. (a) In the 4/3° model, and (b) in the 1/3° model. The color bar indicates CFC concentrations in pmol/kg, with iso­pycnal layer depths contoured (meters).

13. Ajax section in the South Atlantic CDW AAIW AABW

14. Forcing functions for tracers ? sea-ice CFC 14 C 3 He CFC 14 C 3 He CFC 14 C CFC 14 C CFC 14 C CFC Air-sea gas flux = f (k, ice,  ) k = piston velocity ~ wind speed, U 2 or U 3  = solubility ~ SST, (SSS)

15. How to compute gas uptake: Use model-generated ice, winds, T-S? Use observed ice, winds, T-S? Tracers in coupled climate models: Both approaches can give an apparently good tracer simulation but for the wrong reason (see England and Maier-Reimer 2001 for details)

16. Case 1:

20. Spurious convection Case 2:

23. Other tracer techniques: Age/Dye tracers Tracer data assimilation Off-line tracer models (Cox, 1989, England 1995, O’Farrell 2000….) (Haine 1999, Schlitzer 1996, …) (Aumont 1998, Sen Gupta & England 2003)

24. Off-Line Tracer Model Tracer Conservation Equation OGCM Horizontal Velocity Fields Source Terms Mixing Terms Tracer Concentration T (x, y, z, t) Continuity Equation u, vw Interannual Seasonal Intraseasonal Water-mass source regions CFCs, 14 C, 3 He Radioactive waste  T,  S Pollution, etc…. Eddy statistics Isopycnal mixing GM (1990) Convective ML Wind Driven ML T, S, CFCs, 14 C,….

25. PhD project: Alex Sen Gupta Example: CFC simulations in a ¼ degree model Integrated CFC content below 2000m Year = 1980

26. PhD project: Alex Sen Gupta Integrated CFC content below 2000m Year = 2000

27. PhD project: Alex Sen Gupta Integrated CFC content below 2000m Year = 2020

28. PhD project: Alex Sen Gupta Integrated CFC content below 2000m Year = 2040

29. Advantages: Independent assessment of model (complements T-S) Can validate within ocean-only component Complements GBC and carbon cycle modelling Drawbacks: Extra CPU (especially 14 C) Can manipulate outcome to some extent Uncertainties: Tracer source functions Sparse data How to formalise assessment? Summary – Tracers in climate models