Bifurcation behavior of the coupled SPG-AMOC system

Both the Subpolar Gyre (SPG) and the Atlantic Meridional Overturning Circulation (AMOC) have been labeled as separate climate tipping systems, because characteristic transitions in flow patterns have been found in a hierarchy of models of each system. However, the precise connections between the two tipping systems are still unclear, in particular how they can affect their individual tipping behavior. Here, we present bifurcation diagrams of the combined SPG-AMOC system in a spatially three-dimensional North Atlantic Ocean model. We show that SPG tipping is associated with what we call ‘Welander-snaking’ behavior and the AMOC tipping with a Stommel back-to-back saddle-node bifurcation. The details of the bifurcation diagrams are shown to be sensitive to the formulation of the convective parameterization and the horizontal diffusivity of the model.