The Gent-McWilliams parameterization of ocean eddies in climate models Peter Gent National Center for Atmospheric Research

What is needed from ocean to get climate change correctly? Need to get heat uptake correct. In future, need to get CO 2 uptake correct. Need to get heat uptake correct. In future, need to get CO 2 uptake correct. Need a very good representation of mixed layer depths, and of the meridional overturning. 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. Need a very good vertical mixing scheme and to get deep water formation in small, localized regions, preferably as observed.

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

The Veronis (1975) Effect 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.

McDougall and Church, JPO (1986)

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.

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

Veronis (1975)

Isopycnal Coordinates Providing kappa is not a function of depth. GM is sometimes called thickness diffusion.

Deep Water Formation 3˚  3˚ Horizontal MixingGM 1990 Danabasoglu et al. 1993

Conclusions Conclusions With GM, ocean models were stable for very long integration times. With GM, ocean models were stable for very long integration times. Big improvements in solutions, especially the MOC, ACC, and deep water formation. 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. 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. Other climate models with GM ran without, 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.