The GentMcWilliams parameterization of ocean eddies in climate models

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The Gent-McWilliams parameterization of ocean
eddies in climate models

• Peter Gent
• National Center for Atmospheric Research

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What is needed from ocean to get climate change
correctly?

• Need to get heat uptake correct.In future, need
  to get CO2 uptake correct.
• Need a very good representation of mixed layer
  depths, and of the meridional overturning.
• Need a very good vertical mixing scheme and to
  get deep water formation in small, localized
  regions, preferably as observed.

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Aghulas Retroflection
How are eddy effects to be parameterized in 1o
model?
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The Veronis (1975) Effect
Steady Western Boundary Current
Horizontal diffusion produces a false upwelling
that would not occur if diffusion was along
isopycnal surfaces. This reduces the North
Atlantic MOC and the associated northward heat
transport.
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McDougall and Church, JPO (1986)
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Isopycnal Diffusion
Following Montgomery (1930) and Solomon (1971),
Redi (1982) implemented diffusion along isopycnal
surfaces in MOM using small slope approximation.
However, in long climate runs the model would
fail, often near the ACC, and horizontal
diffusion with a small coefficient was used to
make the model stable.
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The GM Parameterization
They proposed an eddy-induced velocity in
addition to diffusion along isopycnal surfaces.
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Isopycnal Coordinates
Providing kappa is not a function of depth. GM is
sometimes called thickness diffusion.
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Deep Water Formation 3? 3
Horizontal Mixing
GM 1990
Danabasoglu et al. 1993
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Conclusions

• With GM, ocean models were stable for very long
  integration times.
• Big improvements in solutions, especially the
  MOC, ACC, and deep water formation.
• The CSM1 with GM was the first climate model to
  run without flux correction and maintain a
  reasonable present day climate.
• Other climate models with GM ran without, or with
  much smaller, flux corrections.