The AMOC Response to GHG Forcing and Its Fingerprint on Dynamical Sea Level

To understand the AMOC response to historical greenhouse gas (GHG) forcing including the role of resolution, new historical simulations with GHG forcing fixed at 1950s level (fixedGHG) are performed in EPOC project. We compare results from fixedGHG run with HighResMIP control run and historical run, to isolate the impact of GHG on historical changes in the AMOC. We assess AMOC variability and its associated dynamical sea level (SSH) fingerprint in two configurations of in MPI-ESM-1.2-HR (0.4º ocean) and MPI-ESM-1.2-ER (0.1º eddy-resolving). In MPI-ESM-1.2-ER, the historical run exhibits a significant negative trend of the AMOC, while fixedGHG run exhibits a significant positive trend of the AMOC. The results support the ideas that GHG forcing leads to slowdown of the AMOC and aerosol forcing lead to spin-up of the AMOC. The change of the AMOC is coherent across latitudes, with larger amplitudes of trend in the subpolar North Atlantic in MPI-ESM-1.2-ER. In MPI-ESM-1.2-HR, neither experiment shows a significant long-term trend, although a slight AMOC decline emerges after the mid-1990s in the historical run. We further evaluate the AMOC–SSH relationship at 26°N using AVISO altimetry and RAPID observations. Both observations and the ER historical run display a canonical Gulf Stream–related dipole: positive SSH anomalies south and negative anomalies north of the Gulf Stream SSH ridge, along with negative anomalies along the Labrador Current—consistent with a strengthened Gulf Stream and Deep Western Boundary Current during strong AMOC states. The HR configuration fails to reproduce this fingerprint, underscoring the importance of eddy-resolving simulations for capturing AMOC–SSH covariability. We are further analyzing SSH patterns in fixed-GHG simulations to isolate the effects of GHG forcing and to elucidate the underlying mechanisms.