Enhanced Decadal Variance in Nordic Seas With AMOC Weakening in CESM

Abstract The subpolar North Atlantic (SPNA) exhibits rapid, decadal‐scale cooling events in several climate model projections, but the mechanisms behind this variability, particularly the role of a weakening Atlantic Meridional Overturning Circulation (AMOC), remain uncertain. Here we analyze model output from a quasi‐equilibrium hosing experiment using the Community Earth System Model version 1 (CESM1) to assess SPNA variability under a gradually declining AMOC. During a 600‐year period preceding an AMOC collapse, the overturning strength weakens by ∼15% and decadal surface temperature variance in the Nordic Seas increases eightfold. Statistical analysis links this enhanced variability to feedbacks between ocean convection, salinity, and sea‐ice, which strengthen under cooler background conditions associated with reduced AMOC strength. Our results suggest that even modest AMOC weakening is associated with substantially enhanced SPNA variability, pointing to increased regional climate variability under AMOC decline and to potential early warning indicators of AMOC collapse.