The effect of ocean dynamics in a coupled GCM simulation of the Last Glacial Maximum

General circulation models (GCMs) of the climate system are powerful tools for understanding and predicting climate and climate change. The last glacial maximum (LGM) provides an extreme test of the model’s ability to simulate a change of climate, and allows us to increase our understanding of mechanisms of climate change. We have used a coupled high resolution ocean–atmosphere GCM (HadCM3) to simulate the equilibrium climate at the LGM. The effect of ocean dynamics is investigated by carrying out a parallel experiment replacing the dynamic three-dimensional ocean GCM with a static thermodynamic mixed-layer ocean model. Changes to the ocean circulation, and feedbacks between the ocean, atmosphere and sea ice have an important influence on the surface response, and are discussed. The coupled model produces an intensified thermohaline circulation and an increase in the amount of heat transported northward by the Atlantic Ocean equatorward of 55 N, which is at odds with the interpretation of some proxy records. Such changes, which the thermodynamic mixed-layer ocean model cannot produce, have a large impact around the North Atlantic region, and are discussed in the study.