Why does the thermohaline circulation slow at similar rates in different climate scenarios?

Overview

The Atlantic Meridional Overturning Circulation (AMOC) is an ocean circulation system important in regulating the global climate and is projected to weaken due to human-driven climate change. The path-dependence of AMOC in a coupled global climate model is investigated between four Shared Socioeconomic Pathways (SSPs) with an emphasis on independent effects of temperature-driven and salinity-driven density changes. To provide a straightforward link between the physical drivers of AMOC and AMOC strength, a dynamic decomposition of AMOC is verified, constrained, then used to approximate AMOC. Separation of the geostrophic shear component into thermally-driven and salinity-driven components reveals that throughout all scenarios, AMOC weakening is driven primarily by temperature changes in intermediate depths while salinity changes act to oppose the temperature-driven decline. Further, scenarios with greater greenhouse gas forcing (e.g., SSP5-8.5) show greater temperature-driven weakening that is compensated by greater salinity-driven strengthening, such that the divergence between AMOC trends in the four SSP scenarios is surprisingly limited (within 0.7σ). This supports a growing body of evidence that suggests long term changes in the AMOC mean state significantly lag external forcings. Future analyses may apply this framework to a large-scale model ensemble to identify model discrepancies and projection uncertainties.