Implications of changes in the Northern Hemisphere circulation for the detection of anthropogenic climate change

The first principal component of Northern Hemisphere sea level pressure, kn own as the Arctic Oscillation (AO) index, has increased significantly in re cent winters, and this change is associated with similar to 30% of Northern Hemisphere January-March warming. We examine the AO in a model used to det ect anthropogenic influence on climate, and find that it exhibits no system atic trend in response to greenhouse gas, sulphate aerosol, or ozone forcin g. To test the significance of this discrepancy for anthropogenic climate c hange detection, we include the spatiotemporal pattern of temperature chang e associated with the observed AO in the set of forcing-response "fingerpri nts" used to account for observed changes, thus separating temperature chan ge associated with the AO from a residual. We find that the detection of a global response to both anthropogenic greenhouse gases and sulphate aerosol s is robust to this exclusion of AO-related warming.