On the incompatibility of ocean and atmosphere models and the need for flux adjustments

The surface heat and freshwater fluxes from equilibrium ocean (OGCM) and atmospheric (AGCM) general circulation model climates are examined in order to determine the minimum flux adjustment required to prevent climate drift upon coupling. This is accomplished by integrating an OGCM with specified surface fluxes. It is shown that a dramatic climate drift of the coupled system is inevitable unless ocean meridional heat and freshwater (salt) transports are used as constraints for tuning the AGCM present-day climatology. It is further shown that the magnitude of the mismatch between OGCM and AGCM fluxes is not as important for climate drift as the difference in OGCM and implied AGCM meridional heat and freshwater (salt) transports. Hence a minimum flux adjustment is proposed, which is zonally-uniform in each basin and of small magnitude compared to present flux adjustments. This minimum flux adjustment acts only to correct the AGCM implied oceanic meridional transports of heat and freshwater (salt). A slight extension is also proposed to overcome the drift in the surface waters when the minimum flux adjustment is used. Finally, it is suggested that the flux adjustments which arise from current methods used to determine them are all very similar, leading to adjustment fields which are significantly larger than both AGCM and climatological fields over large regions.