North Atlantic Circulation Shifts under Historical Anthropogenic Forcing in CESM2-LE

Understanding human-induced transformation in the multi-decadal variability of ocean circulation is important for predicting and interpreting current system change. This study illustrates the role the Subpolar Gyre (SPG) plays in modulating the variability of the Atlantic Meridional Overturning Circulation (AMOC), emphasising the interplay between surface and deep circulation and their sensitivity to atmospheric fluxes. Despite its significance, the dynamics of the SPG and its interaction with the AMOC during the historical period remain poorly understood due to limited observational data and palaeo reconstructions. 

We investigate the mechanisms behind the historical variations in overturning using the CESM2 100-member ensemble (CESM2-LE). We focus on the density overturning strength at 55°N, which captures the combined effects of both overturning and gyre circulation. Employing change point analysis and composite analyses to various hydrographic and circulation metrics, we describe the mechanics of abrupt shifts in subpolar overturning. 

Our analysis complements earlier studies on the CMIP6 historical AMOC strengthening and provides additional details on the associated internal variability based on the single-model ensemble used. We describe the precursing hydrography and atmospheric forcing to abrupt changes in subpolar overturning and the propagation of density anomalies associated with circulation shifts. Furthermore, our results show how variability before 1985 differs from the simulated variability during the period of AMOC decline driven by greenhouse gas forcing in recent decades. These results improve our understanding of the climate system's interactions and its sensitivity to external forcing over time.