Extreme temperature events in Europe under a reduced AMOC

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken by the end of this century across all future scenarios considered by the IPCC Sixth Assessment Report. Consequently, the climate system will likely be influenced not only by continued global warming but also by the effects of a reduced AMOC. In this study, we assess the impact of AMOC weakening on extreme cold events in winter and extreme warm events in summer over Europe, using targeted sensitivity experiments with the EC-Earth3 climate model. Starting from a fully coupled ocean-atmosphere simulation with an artificially weakened AMOC, we conducted a series of atmosphere-only integrations with prescribed sea surface temperatures and sea-ice cover to isolate the atmospheric response to both moderate and strong reductions in AMOC strength.

Our results show that during boreal winter, a weakened AMOC induces average cooling over Europe and intensified cold extremes. However, the cooling at the Northern Hemisphere’s high latitudes intensifies the near-surface temperature meridional gradient at high northern latitudes. The enhanced meridional temperature gradient strengthens the jet stream, which in turn reduces the frequency of atmospheric blocking over the North Atlantic and northwestern Europe. Since wintertime blocking is typically associated with prolonged cold spells, this mechanism leads to a paradoxical reduction in such events despite the overall cooling.

In boreal summer, the weakening AMOC causes widespread cooling across the Northern Hemisphere, especially over the North Atlantic. While most of Europe experiences a decrease in extreme warm events, Eastern Europe and western Russia emerge as exceptions, with an increased frequency of heatwaves. As the AMOC weakens, maximum cooling during boreal summer occurs over the North Atlantic, reducing the temperature gradient at higher latitudes. Consequently, the jet stream weakens which facilitates the development of atmospheric blocking patterns. These blocking, through mechanisms such as subsidence warming and increased shortwave radiation under clear skies, contribute to more frequent heatwaves in the region.

Our findings underscore the pivotal role of large-scale ocean circulation in shaping regional climate extremes. As the AMOC is expected to weaken in the coming decades, understanding its interaction with atmospheric dynamics is essential for improving projections of future climate risks, particularly the compound effects of global warming and ocean circulation changes on European weather extremes.