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The ocean component of climate models: what resolution and parameterisations are needed?

The ocean component of climate models: what resolution and parameterisations are needed?. Richard Wood Hadley Centre for Climate Prediction and Research, Met Office, Bracknell, UK (Thanks to Malcolm Roberts). Atlantic Ocean Heat Transports. HadCEM. HadCM3.

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The ocean component of climate models: what resolution and parameterisations are needed?

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  1. The ocean component of climate models: what resolution and parameterisations are needed? Richard Wood Hadley Centre for Climate Prediction and Research, Met Office, Bracknell, UK (Thanks to Malcolm Roberts)

  2. Atlantic Ocean Heat Transports HadCEM HadCM3

  3. Do atmospheric processes set ENSO period? (1) (Guilyardi et al. 2002)

  4. Do atmospheric processes set ENSO period? (2) (Guilyardi et al. 2002)

  5. Do ocean processes set ENSO amplitude? (1) (Meehl et al. 2001)

  6. Do ocean processes set ENSO amplitude? (2) (Meehl et al. 2001)

  7. Temperature drifts - ocean only model 0m 147m 520m 289m A: PCM B: add KPP mixing C: add GM D: add Visbeck et al. (Gent et al . 2002)

  8. Temperature drifts - coupled model 0m 147m 147m 289m 520m AC: PCM AD: add KPP mixing + GM + Visbeck et al. (Gent et al . 2002)

  9. Do we need eddy resolving climate models? • Dynamical length-scale - baroclinic Rossby radius - 20-50 km in ocean at mid-latitudes, of order 1000 km in atmosphere • Most climate models have ocean resolution more coarse than 1° • Oceanic processes possibly important for climate: • boundary currents and ocean eddies - 30-50 km - heat and freshwater transport • flow through topographic channels (20-50 km) and impact on thermohaline circulation • tropical dynamics and representation of equatorial waves • convective regions often pre-conditioned by eddies • water mass ventilation may be resolution dependent • coastal polynyas - 10’s km - regions of sea-ice formation

  10. Thermocline ventilation in two ocean models 1° 1/3° Tracer PV (Cox 1985)

  11. A systematic study of the effects of ocean resolution on climate simulations Results from 3 coupled models: HadCM3 Ocean resolution 1.25°x1.25°xL20 HadCEML Ocean resolution 1.25°x1.25°xL40 HadCEM Ocean resolution 0.33°x0.33°xL40 All ocean models are coupled to the identical atmosphere, resolution 2.5°x3.75°xL19. No flux adjustments used

  12. Dependence of global heat balance on ocean advection scheme and resolution

  13. HadCEM: model drifts

  14. SST errors HadCEM HadCM3

  15. North Atlantic salinity drifts in 3 coupled models 0m 2000m 4000m 0m 2000m 4000m 0m 2000m 4000m HadCEM 0 yrs 150 yrs -0.45 psu +0.45 psu HadCEML HadCM3

  16. HadCEM: Atlantic Overturning

  17. HadCEM: Ocean Heat Transports

  18. Atlantic Ocean Heat Transports HadCEM HadCM3

  19. Pacific Ocean Heat Transports

  20. Instability Waves in Tropical Pacific (HadCEM) (Maybe these can be parameterised?) Does the atmosphere model see them?

  21. Salinity and velocity at 260m in HadCEM (Parameterise these!)

  22. Global mean SST response to increasing CO2 in 3 coupled models

  23. The future…?

  24. Impact of inhomogeneous diapycnal mixing (Hasumi & Suginohara 1999) Homogeneous Inhomogeneous Hor. Average (INHM)

  25. Mesh adaptivity: flow over seamount (Courtesy Matthew Piggott, Imperial College)

  26. Supercomputer performance (Source: Earth Simulator Centre)

  27. Current technological issues • Typical atmosphere GCM has about 6x the computational cost of typical ocean GCM per gridpoint • A multicentury ocean integration at 0.1° resolution is a significant cost, even on a 40 Tflop computer • Inclusion of biogeochemical processes can increase the cost of atmosphere and ocean dramatically • Mass data storage and methods of data sharing may become the limiting technologies

  28. Summary • Ocean-only modelling studies give valuable insights into model sensitivities - but coupling brings surprises! • Sensitivity to some parameterisations/model formulations can be resolution-dependent • Increasing resolution doesn’t automatically give a better climate simulation • Large scale climate response fairly insensitive to ocean resolution. Need compatible atmospheric and ocean resolutions? • Future: higher resolution (->parameterisations?), diapycnal mixing, biogeochemistry, more flexible numerical methods, (technological limiters) … • … and more integration with observations! • Balance between model diversity and spreading too thinly

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