Effects of a dynamic ocean on simulated climate sensitivity to greenhouse gases

Abstract

The potential effects of a dynamic ocean on climate change are assessed by comparison of a simulation from 1880 into the future by the CSIRO (Mark 2) coupled atmosphere–ocean general circulation model with equilibrium results from a mixed-layer ocean (MLO) version of the model. At 2082, when the effective CO2 is tripled, the global warming in the coupled model is barely half the 3×CO2 MLO result, largely because of oceanic heat uptake, as diagnosed using an effective heat capacity. The effective ocean depth continues to increase during a further 1700 years with stabilized tripled CO2, by which time the mean ocean warming reaches the upper ocean value. Some reduction of the coupled model warming is due to the effective sensitivity (for 2×CO2), determined from the radiative response to the forcing, being persistently 0.2 K lower than the MLO model value. A regional energy and feedback analysis shows that this is largely due to an overall equatorward oceanic heat transport anomaly, which reduces the high-latitude warming in the coupled model. The global warming at 3800 is around 95% of the anticipated equilibrium value, which is matched by the result of a simple energy balance model for the approach to equilibrium. The geographical effect of the oceanic heat transport is confirmed using a mixed-layer model with perturbed oceanic heat convergence. The eastern equatorial Pacific warming is enhanced by over 1 K, and rainfall is perturbed in an ENSO-like pattern.