Diagnosing the Atlantic Meridional Overturning Circulation in density space is critical in warmer climates

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) plays a crucial role in shaping the global climate system by redistributing heat and influencing large-scale climate patterns. Using the AWI-CM3 model, we compare the AMOC strength under pre-industrial and abrupt4xCO₂ scenarios derived in depth (z-AMOC) and density (ρ-AMOC) space. Water mass transformations are assessed to analyze the impact of background climate on surface-induced and interior-mixing-induced transformations. We find that both location and strength of AMOC maximum are directly affected by the framework choice. AMOC weakening is seen under 4xCO₂ forcing in both frameworks, but with only z-AMOC showing recovery, whereas ρ-AMOC oscillates around the minimum value that is reached after the spinup phase of 50 years. The variability of the z-AMOC maximum in the pre-industrial scenario correlates only with that at 26° N due to the flattening of isopycnals into constant depth levels in the subpolar North Atlantic. Diagnosing ρ-AMOC reveals a shutdown of interior mixing induced water transformations. This is absent in z-AMOC, which only shows weakened vertical fluxes. Despite different mechanisms, all timeseries are highly correlated under 4xCO₂, indicating major density shifts. This is evident from weakening of downward diapycnal velocities at deep convection sites. This shutdown is attributed to the decline (increase) of heat (freshwater) flux contribution to surface-forced diapycnal water transformations, as a result of sea-ice melt and reduced poleward ocean heat transport to the subpolar North Atlantic. In contrast, the analysis of vertical velocities, used to derive z-AMOC, only reveals a general weakening of vertical fluxes, both upward and downward. Thus, z-AMOC and ρ-AMOC weakens, but driven by different mechanisms. Our results highlight the importance of diagnosing AMOC in density space, particularly in warmer climates. We recommend broader adoption of ρ-AMOC diagnostics across models and timescales to improve the understanding of AMOC response to climate change.