Assessing the reversibility of temperature and precipitation extremes under AMOC weakening and recovery

Most CMIP6 models simulate a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC), beginning around 1990 and persisting for decades after peak warming, with recovery requiring more than a century. This weakening is associated with reduced northward oceanic heat transport, pronounced winter cooling in the North Atlantic, a northward shift of the North Atlantic jet stream, and an increased risk of summer heatwaves in Europe, as well as a southward displacement of the Intertropical Convergence Zone (ITCZ). 

While the magnitude of AMOC weakening is broadly consistent across models and scenarios, its recovery shows large inter-model differences, particularly in overshoot scenarios. Here, we investigate the reversibility of the AMOC and its impact on large-scale circulation, with a focus on temperature and precipitation and associated extreme event indices. 

We analyze two Earth System Models with interactive carbon cycles (NorESM2-LM and MPI-ESM1.2-LR) under two overshoot scenarios: SSP5-3.4-OS (high overshoot) and SSP1-1.9 (low overshoot). The models exhibit contrasting AMOC responses to negative emissions. NorESM2-LM shows pronounced hysteresis and incomplete recovery, whereas MPI-ESM1.2-LR exhibits a largely reversible AMOC response with minimal path dependence. This contrast is reflected in the development of the top-of-atmosphere radiation balance, where NorESM2-LM has a pronounced hemispheric asymmetry and persistent energy imbalance during the cooling and stabilization phases, whereas MPI-ESM1.2-LR shows a largely symmetric and reversible response that closely follows global mean temperature. Results indicate the presence of Bjerknes Compensation in the northern hemisphere for NorESM2-LM, yielding a partial offset of the reduced oceanic heat transport by the atmosphere. We will further assess the reversibility of climate extremes using indices established by the Expert Team on Climate Change Detection and Indices, focusing on heat extremes, drought prevalence and precipitation intensity in regions sensitive to AMOC-induces circulation changes. 

Our results highlight the central role of the AMOC in governing regional climate responses on centennial timescales and underscore the importance of understanding AMOC hysteresis and reversibility when considering the long-term consequences of delayed action and subsequent large-scale carbon dioxide removal (CDR).