Joint Seminar: The role of model background mean state in modulating the AMOC response to abrupt CO2 forcing
The future evolution of the Atlantic Meridional Overturning Circulation (AMOC) under anthropogenic forcing remains uncertain, in part due to limited understanding of how the background ocean state shapes its response. This work investigates the AMOC response to abrupt 4×CO₂ forcing using CMIP6 models, with a focus on the roles of density compensation from temperature (alphaT) and salinity (betaS). Models are classified into thermally-dominated, salinity-dominated, and balanced-compensation regimes based on western subpolar North Atlantic density structure at 500 m depth. Models with background states further from balanced density compensation exhibit the most rapid and persistent AMOC decline, while those with more balanced contributions show slower or more spatially distributed weakening. During the adjustment period (years 11–40), the alphaT to betaS ratio and control AMOC strength together explain ~80 percent of the inter-model variance in AMOC decline.
The spatial structure of mixed layer depth shoaling, sea ice retreat, and surface water mass transformation strongly differs across regimes, with salinity-dominated models showing early loss of deep water formation and strong subpolar stratification. Observational and reanalysis datasets, including WOA18, EN4, and ORAS5, map closest to the thermally-dominated regime within the density compensation framework. This suggests the present-day climate state may be more stable than models with strong salinity control, but still susceptible to future warming and freshening. These results indicate that differences in background climate state and internal variability both shape the timing and magnitude of AMOC weakening, and emphasize that constraining the present-day subpolar ocean state is essential for narrowing uncertainty in future projections.
Date
17.06.2025
Time
15:15–16:15 h
Place
- Bundesstr. 53, room 022/023
- Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg
Organizers
- Hongdou Fan