Enhanced decadal variability in Norwegian sea with AMOC weakening in CESM

The subpolar North Atlantic (SPNA) exhibits rapid cooling events on the decadal timescale in some climate model projections, but the processes driving these events, particularly their link to projected AMOC decline, are not fully understood. This study examines changes in decadal variability in the SPNA associated with AMOC weakening using 2200 years of output from a freshwater hosing experiment performed with the Community Earth System Model (CESM) under pre-industrial radiative forcing (Van Westen et al. 2024). We analyze North Atlantic sea-surface temperature (SST) variability during a long period of gradual AMOC weakening preceding its full collapse. Results show that, during a 500-year period before the AMOC collapse when the AMOC has weakened by about 15%, decadal SST variability in the Norwegian Sea increases by an order of magnitude. Evidence is shown that the enhanced variability is linked to a strengthened convection–salinity feedback driven by gradual changes in the ocean mean state associated with AMOC weakening and increased freshwater forcing. These findings align with a recent study showing similar mechanisms contribute to increased internal variability of SPNA SST under global warming (Gu et al. 2024), although the specific location of the enhanced variability differs. Our results suggest that relatively minor changes in long-term AMOC strength can be associated with major changes in SPNA variability and hence add to our understanding of AMOC–SPNA interactions, with implications for future climate impacts and potential early warning signals of AMOC collapse.