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Southern Ocean water-masses in climate-scale models

Southern Ocean water-masses in climate-scale models. Matthew England Centre for Environmental Modelling and Prediction The University of New South Wales www.maths.unsw.edu.au/~matthew M.England@unsw.edu.au. OUTLINE. Southern Ocean water masses SOWM in climate models: 1980s, 1990s

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Southern Ocean water-masses in climate-scale models

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  1. Southern Ocean water-masses in climate-scale models Matthew England Centre for Environmental Modelling and Prediction The University of New South Wales www.maths.unsw.edu.au/~matthew M.England@unsw.edu.au

  2. OUTLINE • Southern Ocean water masses • SOWM in climate models: 1980s, 1990s • Today’s climate models • The future?

  3. Southern Ocean water-masses

  4. Rintoul and England (2002) JPO

  5. Animation of seasonal cycle in MLD

  6. Why bother capturing Southern Ocean water-masses? * Linked to global ocean THC, wind-driven circulation, poleward heat transport, … * Tied to key climate indices such as oceanic CO2 uptake, rate of change of SST/TAIR due to atmos CO2 rise, … * Well-constrained by observations, unlike… (see above list)

  7. Why is it so hard to get right? • Depends on correct representations of… • * Air-sea and ice-sea fluxes (heat, freshwater and momentum) • * Ocean processes: • Downslope flows, entrainment fluxes • Open ocean and coastal convection, mixed layer physics • Isopycnal and diapycnal mixing • Eddy fluxes • Baroclinic flow • Ekman pumping Speer, Rintoul and Sloyan, JPO, 2000

  8. Back in the 80’s …. Annual-mean temperature change predicted for ~ the year 2050 in the GFDL coupled climate model experiment (Manabe et al. 1989).

  9. AAIW NPIW AABW Observed Pacific Ocean Salinity Climate model (MS, 1988) Climate model (MS, 1991)

  10. AAIW NPIW AABW Observed Pacific Ocean Salinity Poor representation of Antarctic sea-ice Excessive cross-isopycnal mixing

  11. Model tuning, circa 1990

  12. NADW AAIW AABW Observed Atlantic Ocean Salinity Modelled 60ºN 40ºN 20ºN EQ 20ºS 40ºS 60ºS 80ºS Latitude

  13. Toggweiler and Samuels (1995) JPO

  14. FW flux ~ (S – S*) Griffies (2004)

  15. FW flux (m yr -1) 45S 60S 75S

  16. Why is getting the correct T-S not enough?

  17. Radiocarbon – GEOSECS Pacific section Diagnostic Simulations Prognostic Simulation Toggweiler et al. 1989a,b JGR

  18. Geochemical tracers in ocean models World Ocean Circulation Experiment Bomb Radiocarbon

  19. AABW CDW Geochemical tracers in ocean models Spurious convection at 50-60°S Weak CDW upwelling Broad, sluggish AABW overturn

  20. Danabasoglu et al. 1994; England, 1995, 1999; Hirst and McDougall, 1996, 1998, …

  21. Observed and modelled radiocarbon England and Rahmstorf (1999) JPO

  22. OCMIP http://www.ipsl.jussieu.fr/OCMIP/

  23. Southern Ocean abyssal CFC-11 Doney and Hecht (2002) JPO

  24. Griffies (2004) Beckmann and Doscher, 1997, Campin and Goosse, 1999; Gnanadesikan, et al 2000; …

  25. Doney and Hecht (2002) JPO

  26. Today’s IPCC class of models…

  27. CSIRO Mk2 climate model JPO

  28. Interannual to Centennial Variability of Southern Ocean Water Masses … with Agus Santoso†, Steve R. Rintoul*, Tony Hirst*, Siobhan O’Farrell*

  29. * Rintoul and England [2002, JPO, 1308-1321] * Santoso and England (JPO, 2004) * Santoso, England, Hirst (JPO, accepted)

  30. Russell et al. (2005)

  31. Russell et al. (2005)

  32. Errors in interior model T-S are a result of at least one of the following: • erroneous surface T-S • (2) spurious rates of ocean overturn within the surface ML • (3) incorrect interior ocean circulation • (4) unrealistic mixing processes in the model. • Errors in surface T-S may themselves be a result of • incorrect air-sea heat / FW / momentum fluxes, • errors in surface circulation and mixing. •  Diagnosis of subsurface ocean model T-S against observations is not unambiguous: errors may be symptomatic of any number of problems in ocean model forcing, circulation and/or physics.

  33. Parameter sensitivity studies ought to still be in at the fore of our efforts…

  34. Parameter sensitivity studies ought to still be in at the fore of our efforts… Smax Gnanadesikan, Griffies, and Samuels (OM, 2005)  maximum eddy-induced advective transport of Agm * Smax

  35. “Effects in a climate model of slope tapering in neutral physics schemes” Gnanadesikan, Griffies, and Samuels (2005)

  36. Eddy-permitting and eddy-resolving models…

  37. Model assessment ‘metrics’ • EKE density • Baroclinic/barotropic flow • Poleward HT • Property transports in density classes • Variability of the above • Water masses????

  38. Webb et al. FRAM

  39. If water masses are analysed in high-resolution ocean models over short integration times (10-30 years) their T-S fields are very near initial conditions below ~300-m – hence using T-S to assess skill is meaningless. b) Model

  40. Global eddy resolving models (~ 1/10o ) are computationally expensive • Water-mass assessment requires tracers T, S, chemical tracers, age of water,…. • Tracer equation can adopt a relatively large time step, t • Hence high-resolution models can be assessed for their representation of geochemical tracers

  41. Off-Line Tracer Model OGCM Horizontal Velocity Fields Continuity Equation u , v w Source Terms Mixing Terms Tracer Conservation Equation Tracer Concentration T (x, y, z, t) T, S, CFCs, 14C,….

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

  43. Applications: hypothetical spread of radioactive traces Hazell and England, 2003, J. Enviro. Rad. Hazell and England, J. Enviro. Rad.

  44. Applications (cont’d): Moon jellyfish advective pathways Dawson, Sen Gupta, and England, 2005: Coupled biophysical global ocean model and molecular genetic analyses identify multiple introductions of cryptogenic species. Proc. Nat. Acad. Sciences, 102, 11968-11973.

  45. Geochemical tracer simulations derived from off-line models using eddy-resolving GCMs • Preliminary example: CFC uptake in the POCM simulation • Model includes POCM advective fields, vertical, isopycnal and biharmonic mixing, and a pre-run diagnosed convective mixed layer. All parameters include a seasonal cycle. • Run model for 100 – 200 years simulation time (NB: multi-1000 yr simulations become feasible on modest computing facilities – so applications to age and 14C are also possible, Sen Gupta and England, 2004)

  46. Global animation of ocean CFC spreading >2000m POCM: ¼-degree global ocean model Sen Gupta & England, JPO, 2004

  47. AABW flow pathways Sen Gupta & England, JPO, 2004

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