Enhanced evidence in climate models for changes in extratropical atmospheric circulation

We investigate changes in extratropical atmospheric circulation as derived from the most recent multi-model ensemble of global climate projections. This internationally coordinated systematic data base allows for an accurate assessment of climate change signals against the background of model uncertainty. The multi-model mean time series of the northern-hemisphere (NAM) and southern-hemisphere (SAM) annular modes and of the North Atlantic Oscillation (NAO) are in line with the observed positive trends during the second half of the 20th century and project a further strengthening until the end of the 21st century. For SAM and NAM the simulated changes are unambiguously related to anthropogenic forcing and outperform the level of model uncertainty. This result may imply an enhanced probability for some severe regional impacts of climate change, in particular over extratropical land masses. The climate change signals are noticeably weaker under the B1 mitigation scenario than under the A2 business-as-usual scenario. Ozone forcing has a significant impact on the amplitude of future circulation changes, whereas no systematic effect is found with respect to the models' top-of-the-atmosphere.