AMOC weakening across latitude and time in CMIP6 future scenarios

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). This study investigates the latitudinal coherence of this AMOC weakening and how it differs across depth- and density-space, future forcing scenarios and individual models. Percentage AMOC weakening is presented as a function of latitude and time between 1900 and 2100. By identifying the first year when specific percentage thresholds of weakening are reached at each latitude, contour lines of weakening are mapped as a function of latitude and time. Propagation of AMOC weakening is inferred from the gradient of these lines (defined as weakening gradients). Percentage weakening is found to originate at high latitudes (45–51° N in depth-space and 51–61° N in density-space) and propagate southwards. Because density-space weakening originates further north than depth-space, it has a longer lead time for propagation and our results suggest it could provide a more effective framework for early warning signal development. South of 40° N, propagation speeds in depth-space and density-space representations are similar. In the most extreme future forcing scenario, the multi-model mean weakening gradient reduces from ∼ 3.0 to ∼ 1.4° yr ⁻¹ , implying that as the AMOC weakens so too does the propagation of AMOC weakening. This behaviour is also seen for the majority of models (67 %). For a small fraction of models (13 %) the weakening gradients increase as the AMOC weakens and for the remainder (20 %) there is no discernable trend. The weakening gradients reduce more steeply in less extreme future forcing scenarios than in more extreme ones. The initial speed of propagation is found to be positively related to historical AMOC strength.