Large inter-model variations in the future subpolar North Atlantic hydrodynamics from CMIP6 simulations

Driven by the ongoing debate on the potential consequences of a future Atlantic Meridional Overturning Circulation (AMOC) collapse for the Nordic Seas and their marine ecosystems, this study examines the relationship between different patterns in the North Atlantic across various CMIP6 models and scenarios. Under the high-emission SSP5-8.5 scenario, all models show a weakened AMOC associated with increased heat transport into the Nordic Seas across the Iceland-Scotland Ridge (ISR). While ISR temperature trends vary across CMIP6 models, with both positive and negative trends, volume transport overcompensates for negative temperature trends, resulting in consistently positive heat transport trends. In contrast to short-term variability in the present day climate where AMOC seems to have lagged response to atmospheric forcings such as wind and atmosphere-ocean heat fluxes, the different long-term AMOC responses in a warmer future appear to be decisive for different trends in subpolar temperatures and corresponding shifts in temperature gradients and wind stress curl patterns. The increased northward volume transport across the ISR ridge seems partly caused by this south-eastward shift in the wind stress curl, giving rise to stronger southerly winds west of the British Isles, and partly as a function of increased freshwater in the North Atlantic and along the eastern rim of the Nordic Seas Due to negative temperature trends of different strength in the subpolar gyre and ISR across the SSP scenarios, the Nordic Seas are projected to experience less warming in SSP5-8.5 than in SSP3-7.0 by the end of the century. The highly different results from models and scenarios with respect to climate exposures indicate large uncertainties, with additional uncertainties in risk reports about climate impacts on the marine ecosystem that complicate decision-makers' actions to minimize future damage.