The Gent-McWilliams The Gent-McWilliams parameterization of ocean parameterization of ocean eddies in climate modelseddies in climate models

Peter Gent Peter Gent

National Center for Atmospheric National Center for Atmospheric ResearchResearch

What is needed from ocean to What is needed from ocean to get climate change correctly?get climate change correctly?

• Need to get heat uptake correct.Need to get heat uptake correct.In future, need to get COIn future, need to get CO22 uptake correct. uptake correct.

• Need a very good representation of mixed Need a very good representation of mixed layer depths, and of the meridional layer depths, and of the meridional overturning.overturning.

• Need a very good vertical mixing scheme Need a very good vertical mixing scheme and to get deep water formation in small, and to get deep water formation in small, localized regions, preferably as observed.localized regions, preferably as observed.

Aghulas Retroflection

How are eddy effects to be parameterized in 1o model?

The Veronis (1975) EffectThe Veronis (1975) Effect

xxzw 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.

DENSITY

DEPTH

Ocean Observations suggest mixing along isopycnals is ~107 times larger than across isopycnals. Horizontal mixing causes spurious diapycnal mixing.

McDougall and Church, JPO (1986)

Isopycnal Diffusion

TTut

T

2

.

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.

The GM Parameterization

TTuut

T

2

*).(

.0*.),/.(* uw z

They proposed an eddy-induced velocity in addition to diffusion along isopycnal surfaces.

Isopycnal Coordinates

hhut

h

zhThickness

2

).(

/,

Providing kappa is not a function of depth.

GM is sometimes called thickness diffusion.

Deep Water Formation 3˚ 3˚

Horizontal Mixing GM 1990

Danabasoglu et al. 1993

ConclusionsConclusions

• With GM, ocean models were stable for With GM, ocean models were stable for very long integration times. very long integration times.

• Big improvements in solutions, especially Big improvements in solutions, especially the MOC, ACC, and deep water formation. the MOC, ACC, and deep water formation.

• The CSM1 with GM was the first climate The CSM1 with GM was the first climate model to run without flux correction and model to run without flux correction and maintain a reasonable present day maintain a reasonable present day climate. climate.

• Other climate models with GM ran without, Other climate models with GM ran without, or with much smaller, flux corrections. or with much smaller, flux corrections.

Limerick 2000

There

There once was an ocean model called MOM,Which occasionally used to bomb,

But eddy advection, and much less convection,Turned it into a stable NCOM.

Limerick 2004

There

There once was an ocean model called POP,Which occasionally used to flop,

But eddy advection, and much less convection,Turned it into the cream of the crop.