More extreme summertime North Atlantic Oscillation under climate change
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Extreme states of the North Atlantic Oscillation (NAO) in summer can lead to severe weather events such as heatwaves and floods in Europe. But how the summer NAO extremes evolve in response to climate change remains unexplored. Here we show that the statistical distribution of summer NAO index grows wider with increasing global warming in large ensembles of climate change simulations as well as reanalysis data. The amplified internal variability by global warming leads to a higher probability of internally generated summer NAO extremes --- for both positive and negative phases --- accompanied by an amplification of their impacts on surface temperature over northwestern Europe. This study highlights that global warming may counteract our predictive skill for the summer NAO by amplifying its internal variability, and thus exacerbate risks of the associated severe weather events in Europe.
