Impacts of AMOC slowdown on European circulation patterns

Abstract

The Atlantic meridional overturning circulation (AMOC) is a vital component of the global climate system regulating heat, carbon, and freshwater distribution. Most models predict a weakening of the AMOC throughout the 21st century, although there is significant uncertainty about its magnitude and the related regional climate impacts. In particular, the response of large-scale atmospheric circulation to the AMOC slowdown is still largely unknown, with implications for weather extremes and associated societal risks. The purpose of this study is to enhance our understanding of the impacts of an AMOC slowdown on atmospheric patterns with a focus on the Euro-Atlantic region, where the influence of AMOC is particularly relevant.We analyse changes in an ensemble of idealised abrupt-4xCO2 climate model simulations from the CMIP archives with respect to the preindustrial climate. We split the models into groups according to their AMOC response to the 4xCO2. Through rigorous statistical testing, we attribute the differences in the simulated climate impacts to the difference in the AMOC response. Specifically, we find that models that simulate a larger AMOC decline feature minimum warming in the subpolar North Atlantic (North Atlantic Warming Hole or NAWH), a southward shift of the ITCZ, and a poleward strengthening of the mid-latitude jet stream. Instead, models that simulate a smaller AMOC decline feature enhanced North Atlantic warming, an intensification of the hydrological cycle but no southward shift in the ITCZ, and smaller displacements of the mid-latitude jet.  To better characterize the large-scale atmospheric response at daily timescales, we use k-means clustering and self-organising maps to assess the changes in weather regimes over the Euro-Atlanic sector, including the NAO.  We further compare weather regimes’ frequency of occurrence and persistence between the two groups, attributing the differences to the AMOC decline. Our results indicate that the AMOC is a key source of large inter-model uncertainty in the simulation of future climate change impacts. Further observational campaigns may thus help us alleviate model biases and provide constraints on a number of societally relevant climate change impacts.