Decadal variability in the North Atlantic Ocean impacts regional and global climate, yet changes in internal decadal variability under anthropogenic radiative forcing remain largely unexplored. This study uses the Community Earth System Model 2 Large Ensemble under historical and the Shared Socio-economic Pathway 3-7.0 future radiative forcing scenarios and shows that the ensemble spread in northern North Atlantic sea surface temperature (SST) more than doubles during the mid-twenty-first century, highlighting an exceptionally wide range of possible climate states. Distinct SST trajectories arise from differences in North Atlantic deep convection, stochastically triggered and amplified by positive feedbacks involving ocean-atmosphere-sea ice interactions. Freshwater forcing associated with global warming seems necessary for activating these feedbacks, accentuating the impact of external forcing on internal variability. Further investigation on seven additional large ensembles affirms these findings. Real-time monitoring of these mechanisms and extending dynamical model predictions after positive feedbacks activate may enable skillful long-lead North Atlantic decadal predictions.

Qinxue Gu, Melissa Gervais, Gokhan Danabasoglu, Who M. Kim, Frederic Castruccio, Elizabeth Maroon, Shang-Ping Xie