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

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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)

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Atlantic Ocean Heat Transports
HadCEM
HadCM3
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Do atmospheric processes set ENSO period? (1)
(Guilyardi et al. 2002)
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Do atmospheric processes set ENSO period? (2)
(Guilyardi et al. 2002)
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Do ocean processes set ENSO amplitude? (1)
(Meehl et al. 2001)
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Do ocean processes set ENSO amplitude? (2)
(Meehl et al. 2001)
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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)
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Temperature drifts - coupled model
0m
147m
147m
289m
520m
AC PCM AD add KPP mixing GM Visbeck et
al. (Gent et al . 2002)
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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 - 10s km - regions of sea-ice
  formation

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Thermocline ventilation in two ocean models
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1/3
Tracer
PV
(Cox 1985)
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A systematic study of the effects of ocean
resolution on climate simulations
Results from 3 coupled models HadCM3 Ocean
resolution 1.25x1.25xL20 HadCEML Ocean
resolution 1.25x1.25xL40 HadCEM Ocean
resolution 0.33x0.33xL40 All ocean models are
coupled to the identical atmosphere, resolution
2.5x3.75xL19. No flux adjustments used
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Dependence of global heat balance on ocean
advection scheme and resolution
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HadCEM model drifts
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SST errors
HadCEM
HadCM3
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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
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HadCEM Atlantic Overturning
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HadCEM Ocean Heat Transports
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Atlantic Ocean Heat Transports
HadCEM
HadCM3
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Pacific Ocean Heat Transports
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Instability Waves in Tropical Pacific (HadCEM)
(Maybe these can be parameterised?) Does the
atmosphere model see them?
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Salinity and velocity at 260m in HadCEM
(Parameterise these!)
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Global mean SST response to increasing CO2 in 3
coupled models
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The future?
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Impact of inhomogeneous diapycnal mixing
(Hasumi Suginohara 1999)
Homogeneous
Inhomogeneous
Hor. Average (INHM)
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Mesh adaptivity flow over seamount
(Courtesy Matthew Piggott, Imperial College)
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Supercomputer performance
(Source Earth Simulator Centre)
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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

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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 doesnt 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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