Nonlinear Response of the Stratosphere and the North Atlantic-European Climate to Global Warming

The response of the northern winter atmospheric circulation for two consecutive global warming periods of 2K is examined in a grand ensemble (68 members) of idealized CO2 increase experiments performed with the same climate model. The comparison of the atmospheric responses for the two periods shows remarkable differences, indicating the nonlinearity of the response. The nonlinear signature of the atmospheric and surface responses is reminiscent of the positive phase of the annular mode of variability. The stratospheric vortex response shifts from an easterly wind change for the first 2K to a westerly wind change for the second 2K. The North Atlantic storm track shifts poleward only in the second period. A weaker November Arctic amplification during the second period suggests that differences in Arctic sea ice changes can act to trigger the atmospheric nonlinear response. Stratosphere-troposphere coupling thereafter can provide for the persistence of this nonlinearity throughout the winter. Plain Language Summary The effects of global warming are experienced in terms of changes in meteorological phenomena of the Earth's atmosphere, such as the storms that reach Europe, from their region of origin in the western North Atlantic. In this study, we analyzed a large ensemble of numerical experiments for the prediction of the atmospheric response to an idealized scenario of global warming. We compare changes for an initial 2K warming with those of a follow-up 2-K warming. Of relevance to climate change in Europe, we found that changes in the preferred locations of the North Atlantic storms for the second 2K of global warming cannot be predicted from their respective changes to the first 2K of global warming. This result signifies that the atmospheric response does not scale with global warming.