Global warming induces more internally generated extremes of North Atlantic Oscillation and East Atlantic pattern

Increased weather and climate extreme events are often attributed solely to either human-induced climate change or internal variability, under the assumption that external forcing does not influence the internal variability. However, with the development of single-model initial-condition large ensembles, recent research shows the impact of global warming on internal variability. This study investigates how global warming influences the North Atlantic Oscillation (NAO) and the East Atlantic (EA) pattern, which are the dominant large-scale circulation/teleconnection modes in the North Atlantic sector.

The study analyzes the geopotential height data of the Max Planck Institute Grand Ensemble (MPI-GE)  with 100 ensemble members. The internal variability is quantified as the deviation from the ensemble mean. The influence of global warming on the internal variability is checked with a 1pcCO2 experiment, where the  concertation is increased by 1% every year. This experiment provides a scenario for relatively strong global warming based on increasing greenhouse gas concentration alone. The extreme NAO and EA are defined as those years where the indexes are above (positive extremes) or below (negative extremes) 2 standard deviations.

The results show increases in extreme events, especially negative extremes, for both NAO and EA during wintertime, in a warmer climate. While NAO extremes increase consistently across the whole troposphere, EA extremes increase more at higher altitudes (500hpa-200hpa) than at lower altitudes. The warming effect of positive extreme NAO over northern Eurasia gets weaker, while the cooling effect of negative extreme NAO over northern Eurasia gets stronger. The effects of both, positive and negative extremes of EA, extend eastward till Eastern Asia. Overall, this study underlines the impact of global warming onto the internal variability of NAO and EA.