Abstract

The results of 12 coupled climate models participating in the Coupled Model Intercomparison Project (CMIP2) are compared together with observational data and evaluated in order to investigate the following questions: 1) How does the current generation of climate models reproduce the major features of the winter North Atlantic Oscillation (NAO) and 2) How does NAO intensity and variability change in response to increasing atmospheric CO₂ concentration. Long-term changes in the intensity and spatial position of the NAO nodes (Icelandic Low and Azores High) are investigated. Different definitions of the NAO index and the Arctic Oscillation (AO) are analyzed. The spatial and temporal structure of the NAO in the models is considered. The observed spatial and temporal variability of the NAO is found to be reproduced well by the models. The observed temporal trend in the NAO in recent decades lies beyond the natural variability found in the model control runs. For the majority of the models, there is a significant increase in the NAO trend in the forced runs relative to the control runs. These results clearly suggest a greenhouse-gas forcing effect on the NAO intensity, though the dynamical mechanisms remain uncertain.