Inter-model diversity and Its Drivers in Southern Ocean Meridional Overturning Circulation

The Southern Ocean Meridional Overturning Circulation (SO MOC) plays a critical role in redistributing heat, carbon and other tracers globally. Despite its importance, substantial inter-model diversity persists in climate model simulations. This study examines mechanisms underlying such diversity using pre-industrial control simulations from the Coupled Model Intercomparison Project (CMIP5/6). We find that discrepancies in the upper overturning cell of SO MOC are primarily governed by model-dependent sensitivity to wind stress, rather than differences in wind stress magnitude. Latitudinal position of maximum zonal wind stress, closely linked to the Southern Annular Mode-like mean state, determines sensitivity through both Eulerian and parameterized eddy-induced circulation. For the lower overturning cell, inter-model differences are largely driven by variations in surface buoyancy fluxes—primarily meltwater—and mean ocean stratification, particularly within key Antarctic Bottom Water formation regions. A mechanistic understanding of SO MOC diversity is essential for improving simulation realism and constraining future climate projections.