PNNL

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is projected to slow down in climate models due to greenhouse gas emissions. However, the physical mechanisms determining the rate of the projectedAMOCslowdown remain unclear. Accordingly, this study isolates the roles of oceanic advection and surface flux feedbacks that might accelerate or decelerate the AMOC’s weakening using CO2 quadrupling simulations in the CESM1.2 model. Surface flux feedbacks are isolated in partially coupled experiments in which either all surface flux components or the momentum flux response to AMOC’s weakening that might provide feedback are suppressed, while a tracer decomposition of ocean density anomalies isolates the advection feedbacks. Comparing the ocean density components in the experiments shows that the AMOC’s response is initially determined by CO2-induced anomalous surface heat fluxes, afterward feedbacks determine its response. In the fully-coupled case, surface heat flux feedback strongly promotes AMOC slowdown and causes its near shutdown, while a weaker but active AMOC is maintained when the surface flux feedback is inhibited in the partially-coupled case. The positive surface heat flux feedback works by canceling out the negative oceanic heat advection feedback on deep water formation in the subpolar North Atlantic (SPNA). With the heat advection feedback offset, the positive salinity advection feedback becomes the dominant contributor to SPNA density changes and deepwater formation. In the partially-coupled case, negative ocean heat advection feedback and the CO2-induced subtropical Atlantic saline anomalies import into the SPNA play a stabilizing role. The results highlight the importance of SPNA salinity gradients and gyre circulation strength in determining the AMOC’s response rate or recovery.