The role of model background mean state and internal variability in modulating the AMOC response to abrupt CO2 forcing

The Atlantic Meridional Overturning Circulation (AMOC) is a critical component of the Earth’s climate system, playing a key role in global heat and carbon transport and influencing both regional and global climate patterns. While the AMOC’s importance for climate regulation is well-recognized, its future evolution under anthropogenic forcing remains highly uncertain due to incomplete understanding of how background mean states and internal variability influence its response. This study investigates the AMOC’s response to abrupt 4xCO₂ forcing compared to a control climate, isolating the roles of background mean state and internal variability. Using CMIP6 models, we classify models based on subsurface (2000 m depth) temperature and salinity characteristics. Models with warmer subsurface conditions exhibit a stronger initial response to abrupt CO₂ forcing within the first decade. Over the subsequent century (years 101–150), models with a warm-salty background mean state show a 50% AMOC decline, while those with a warm-fresh state experience a dramatic 90% decline. This highlights the influence of the background salinity mean state and how its influence shapes Arctic sea ice loss and deep convection in the North Atlantic, with most models indicating a northward shift toward the Barents Sea and Arctic regions—except in warm-fresh states. Single-model large ensembles further underscore the role of internal variability and model resolution in these dynamics. However, observational uncertainties in present-day subsurface salinity and temperature fields present challenges in precisely constraining the current mean state, complicating efforts to determine which classifications best align with reality. Our findings emphasize the influence of background mean states on AMOC’s response to extreme forcing, offering insights to refine projections of climate change using coupled models and inform future climate mitigation strategies.